Jane T Paul

N Ramaiah

S Sardessai

Nutrient regimes and their effect on distribution of phytoplankton in the Bay of Bengal

Keywords: Bay of Bengal, Phytoplankton, Diatoms, Community composition, Nutrients, N: P: Si ratio

The seasonal dynamics of nutrient ratios and abundance of phytoplankton cells from the central (CB) and western (WB) Bay of Bengal were studied during the Fall Intermonsoon (FIM; Sept-Oct 2002) and Spring Intermonsoon (SpIM; April-May 2003). The nutrient molar ratios of macronutrients such as nitrate to phosphate (N:P), nitrate to silicate (N:Si) and silicate to phosphate (Si:P) in the top 120 m were calculated for both FIM and SpIM. During both the seasons, the N:P ratios along the CB and WB were lower than 16, indicating nitrate deficiency. Whereas, along both transects the N:Si ratio was <1 and Si:P >3 in the top 20 and 40 m during FIM and SpIM respectively indicating Si enrichment. Relatively greater nutrient concentrations along the WB than the CB appear to contribute to higher phytoplankton abundance.

The preponderance of diatoms in the Bay could be attributed to rapid utilization of available nutrients in particular during FIM thus resulting in low N:Si ratios in the water column. Among diatoms, pennales were predominantly controlled by nutrients and their ratios. While, apart from nutrients, physical stratification, light and eddies also seem to influence the distribution and abundance of centrales.

Introduction

Stretching over a distance of ~2500 km between 22°N and the Equator [START_REF] Berner | Methane in ocean waters of the Bay of Bengal: its sources and exchange with the atmosphere[END_REF] enclosing the Andaman Sea surrounding the Andaman and Nicobar Islands, the Bay of Bengal (BOB) comprises the north east part of the Indian Ocean. It experiences seasonal changes in the circulation due to semi annual reversing monsoon winds [START_REF] Shetye | Wind-driven coastal upwelling along the western boundary of the Bay of Bengal during the southwest monsoon[END_REF]. The large amounts of fresh water (ca.1.6 x 10 12 m 3 yr -1 , [START_REF] Subramanian | Sediment load of the Indian Rivers[END_REF] discharged into the Bay give it an estuarine characteristic with low surface salinities over a large area. Thus, excess precipitation and river runoff over evaporation lead to very low salinities and highly stratified upper layers [START_REF] George | A study of the carbon dioxide system in the northern Indian Ocean during premonsoon[END_REF][START_REF] Kumar | Why is the Bay of Bengal less productive during SM as compared to the Arabian Sea[END_REF]. This hampers exchange processes between the atmosphere, surface and deeper water layers that consequently affect the biological and biogeochemical processes [START_REF] Ittekkot | The Bay of Bengal -An Introduction[END_REF]. A wide range of variations in the nutrient concentrations has been observed and, although many rivers drain in to BOB, the nitrate-depleted surface waters in the northern BOB suggest that there is no significant riverine input of nitrate and thus, nitrate is considered to be limiting the primary production [START_REF] De Sousa | Distribution of Nutrients in the Western Bay of Bengal[END_REF] in the BOB. Additionally, the runoff may also affect the distribution and composition of the phytoplankton by influencing the circulation and mixing processes and in turn redistribution of nutrients.

Phytoplankton succession and community composition reflect the environmental conditions of the ecosystem, among which the availability of nutrients play a significant role [START_REF] Dugdale | Nutrient limitation in the sea: dynamics, identification, and significance[END_REF][START_REF] Rhyther | Nitrogen, phosphorous and eutrophication in the coastal marine environment[END_REF][START_REF] Smayda | Phytoplankton species succession[END_REF]. If the supply of nutrients is less than the uptake by phytoplankton, nutrient concentrations decrease and limit additional growth of phytoplankton [START_REF] Tilman | Phytoplankton Community Ecology: The Role of Limiting Nutrients[END_REF]. The limiting nutrient concentrations vary with season, location and phytoplankton community structure [START_REF] Fisher | Nutrient limitation of phytoplankton in Chesapeake Bay[END_REF]. Generally, nitrogen (N) limitation prevails in most of the marine systems [START_REF] Fisher | Nutrient limitation of phytoplankton in Chesapeake Bay[END_REF][START_REF] Howarth | Nutrient limitation of net primary production in marine ecosystems[END_REF]. Principally, halfsaturation constant, K s , for a limiting nutrient is greater than its ambient concentration and therefore regulates the growth of phytoplankton community. Changes in nutrient supply are often reflected in their ratios [START_REF] Yin | Shift from P to N limitation of phytoplankton biomass across the Pearl River estuarine plume during summer[END_REF]. Thus, elemental ratios (nitrate, phosphate and silicate) from water samples can sometimes be used as indicators of the status of nutrient loading or to predict productivity [START_REF] De-Pauw | Nutrient induced competition between species of marine diatoms[END_REF]. When the nutrient levels are sufficient for a healthy growth of diatoms, the atomic nitrate to silicate to phosphate (N:Si:P) ratios within the cells is about 16:16:1 [START_REF] Redfield | The influence of organisms on the composition of sea water[END_REF][START_REF] Brzezinski | The Si:C:N ratio of marine diatoms: interspecific variability and the effect of some environmental variables[END_REF]. Deviations from these ratios either in the nutrient availability or uptake indicate a potential for nutrient limited phytoplankton growth [START_REF] Hecky | Nutrient limitation of phytoplankton in freshwater and marine environments: a review of recent evidence on the effects of enrichment[END_REF][START_REF] Dortch | Does nitrogen or silicon limit phytoplankton production in the Mississippi River plume and nearby regions?[END_REF].

However, studies in the past [START_REF] Parsons | On the chemical composition of eleven species of marine phytoplankters[END_REF][START_REF] Healy | Indicators of Phosphorous and nitrogen deficiency in five algae in culture[END_REF]Brzenzinski, 1985;[START_REF] Levasseur | Phytoplankton biomass and nutrient dynamics in a tidally induced upwelling: The role of NO 3 : SIO 4 ratio[END_REF] suggest that when the ambient ratios of dissolved N:P <10 and N:Si ratio <1 it is indicative of a potential N limitation whereas N:Si >1 and Si:P ratio <3 are indicative of Si limitation [START_REF] Harrison | Marine diatoms in chemostats under silicate or ammonium limitation. III. Cellular chemical composition and morphology of three diatoms[END_REF].

Nutrient limitation in natural phytoplankton communities is primarily identified from bioassays in which response of the phytoplankton community to N or P is measured by additions of one or both nutrients in micro/mesocosms or, inferred from elemental ratios [START_REF] Havens | Using trophic state index values to draw inferences regarding phytoplankton limiting factors and seston composition from routine water quality monitoring data[END_REF]. These ratios are therefore useful for predicting the phytoplankton abundance and assemblages. The ability to identify limiting nutrients thus becomes of considerable importance for our understanding of the ecology of phytoplankton. Moreover, measurements of elemental ratios in the water body and their effect on the phytoplankton community can provide evidences for possible growth limitations. One of the main objectives of this study was to identify the effects of ambient nutrients and their ratios on the phytoplankton composition and distribution. In addition, effects of seasonal differences in the nutrient concentrations and their ratios on the phytoplankton abundance were also examined during this study. With this aim data on nutrients and phytoplankton from BOB collected under a multidisciplinary programme (Bay of Bengal Process Studies, BOBPS) during the Fall Inter Monsoon (FIM) and Spring Inter Monsoon (SpIM) have been analyzed in this study.

Materials and Methods

Sampling was carried out during FIM (September to October, 2002) (Approximate Location of Figure 1) Water samples were collected at all stations using a CTD (Sea Bird Electronics) rosette from eight discrete depths (near surface, 10 m, 20 m and thereafter at 20 m interval till 120 m). Sub sampling was done for estimation of nutrients which was immediately analyzed onboard using SKALAR autoanalyser. For analyzing phytoplankton cell counts (PCC) and phytoplankton composition, aliquots of water samples from each of the above depths were fixed in acid Lugol's iodine (1 % w/v) and 3 % formaldehyde and stored in dark until taken up for analyses. A settling and siphoning procedure was followed to concentrate samples from 250 ml to 10 ml [START_REF] Utermohl | Zur Vervollkommung der quantitativen Phytoplankton-Methodik[END_REF]. For counting phytoplankton cells (size 5 µm) and identification of genera and species, two one-ml replicates of concentrated samples were transferred into a Sedgewick-Rafter plankton counting chamber. All the 1000 squares in the Sedgewick-Rafter were examined microscopically using 20X -40X objectives for the phytoplankton cells. Oil immersion 100X objective on a Zeiss (Axioskop, 2plus, Germany) microscope was also made use of for confirming the genera or species.

Generic and species identification was done according to various keys [START_REF] Lebour | The Plankton Diatoms of northern Seas[END_REF][START_REF] Desikachary | Marine fossil diatom from India and Indian Ocean Region[END_REF][START_REF] Desikachary | Diatoms from the Bay of Bengal[END_REF]Desikachary et al., 1987;[START_REF] Constance | Catalogue of Diatoms[END_REF][START_REF] Tomas | Identifying Marine Phytoplankton[END_REF].

Results

Hydrology

A detailed description of the sea surface temperature (SST) and sea surface salinity (SSS) for FIM (also referred as postmonsoon) and SpIM (or, premonsoon) are provided in [START_REF] Kumar | High new production in the Bay of Bengal: Possible causes and implications[END_REF] and Prasanna [START_REF] Kumar | Eddy-mediated biological productivity in the Bay of Bengal during fall and spring intermonsoons[END_REF]. Briefly, the mean sea surface temperature (SST) during the FIM in the CB and WB was 28.70 ± 0.28°C and 29.98 ±0.54°C, respectively. A shallow 28°C isothermal layer of 20 m thickness was observed to be warmer by a degree in the northernmost location. On the other hand, the isothermal layer along the WB was generally of ~10 m thickness and warmer (>30°C). The sea surface salinity (SSS) in the CB was around 34 in the south, which decreased to 29 in the northern locations. Along the WB too the salinity decreased from 33 in the south to 25 in the north.

During SpIM, the SST generally did not vary much along both the CB and WB averaging 29 ±0.50°C. Along the CB, a shallow 10 m thick isothermal layer of 30°C was observed south of 15°N 88°E which became thicker (~30 m) and a degree colder in the north. A 30 m thick isohaline layer having a salinity of 33 was observed along the CB with a few pockets of low salinity. A halocline was observed between 30 and 100 m below which there were minimal variations in the salinity, as was also seen during FIM.

The isothermal layer along the WB was thicker (~40 m) and warmer (> 30°C) south of 16°N and became colder by a degree and thinner (~10 m) towards the north.

With a salinity of 33.5, the surface waters north of 15°N were fresher by 0.5. In general, isohalines shoaled from south to north in the upper 100 m. The SSS averaged 33 ±0.30 along both transects. The ratio of silicate to phosphate (Si:P) showed a progressive decrease with depth as illustrated in Fig. 2C and Fig. 3C. The surface waters showed a higher than 16:1 ratio in both seasons except in CB during FIM indicating a strong deficiency of P relative to Si. The highest Si:P ratio which was close to 72, was observed in surface waters in the SpIM along the CB transect. This was due to high surface silica (~2.13 µM; figure not shown) along this transect. There were large scale variations in the surface waters and, the Si:P ratio was >3 throughout the water column. In deeper waters below 40 m, the Si:P ratio was lower than 16:1 but >3 in both seasons and transects indicating low Si which was, in any case, not in limiting levels. The Si:P ratios showed a negative relation with the total phytoplankton cell counts (PCC) in both seasons and transects. But had a significant relation with PCC along the CB in FIM (r = -0.41, p ≤ 0.01; n = 39) and WB in SpIM (r = -0.54; p ≤ 0.00; n = 32).

Nutrient Ratios

Phytoplankton distribution

The abundance of phytoplankton (PCC; cells x 10 3 L -1 ) during FIM and SpIM along CB and WB is illustrated in Fig. 4A and Fig. 4B, respectively. During FIM, the abundance was higher mostly in the top 60 m along both transects. The concentration of the phytoplankton was more towards the south in the CB with the highest surface abundance observed at CB1 (3.2 x 10 3 L -1 ). While along the WB, the phytoplankton abundance was more in the northern locations viz. WB3 and WB4. The surface concentration of phytoplankton was in the range of 0.48 -22.76 x 10 3 L -1 increasing northwards with the highest abundance observed at WB4.

During SpIM (Fig. 4B) the preponderance of phytoplankton was lower in the CB compared to the WB. Along the CB, the surface PCC ranged from 0.16 to 0.64 x 10 3 cells L -1 with the highest abundance at CB4. While, the surface PCC ranged from 0.04 to 1.08 x 10 3 cells L -1 along the WB. However, the highest PCC was observed at station WB3 at a depth of 40 m (19.48 x 10 3 cells L -1 ).

(Approximate Location of Figure 4)

Phytoplankton Composition

Distribution of mean PCC and mean abundance of centrales and pennales in the top 120 m along both transects during FIM and SpIM is shown in Fig. 5A and5B.

Similarly, Figs. 6 and 7 depict the distribution pattern of the predominant species. In general diatoms dominated during these seasons. Centrales were predominant in the top 40 m along both CB and WB during FIM. In the deeper column, pennales were dominant (Fig. 5A). During SpIM, pennales dominated at all depths along the CB.

Along WB however, the centrales dominated in the top 80 m (Fig. 5B). Among the predominant phytoplankton (species that contributed 5% of the total PCC) recorded during FIM (Fig. 6), Thalassionema nitzschioides and Thalassiosira spp were present in all the stations along the CB. While Navicula spp was observed at all the nine stations, Rhizosolenia styliformis was observed only at three stations (CB1, CB5 and WB4) which were eddy influenced regions (Prasanna [START_REF] Kumar | High new production in the Bay of Bengal: Possible causes and implications[END_REF][START_REF] Kumar | Eddy-mediated biological productivity in the Bay of Bengal during fall and spring intermonsoons[END_REF]. (Approximate Location of Figure 6 and Figure 7)

Chaetoceros lorenzianus and

Discussion

In the BOB, the winds are weak and variable during FIM and SpIM, representing the primary and secondary heating seasons [START_REF] Narvekar | Seasonal variability of the mixed layer in the central Bay of Bengal and associated changes in nutrient and chlorophyll[END_REF].

The riverine influx peaks during FIM and is least during SpIM. As a result of the high amounts of fresh water influx, the surface salinity is reduced to as low as 29 and 25 along the CB and WB during FIM inducing a strong halocline and stratification in the top 50 m. Due to the minimal riverine input into the BOB during SpIM, the surface salinity is higher along both transects with the halocline deepening to 100 m. Moreover, higher concentrations of surface nitrate (~0.5 -1.5 µM) and silicate (~15 µM) observed during FIM along WB could be attributed to the riverine influx. A similar presence of the nutrients in the euphotic depth (60-80 m) during SpIM can be attributed to eddy pumping as suggested by Prasanna [START_REF] Kumar | Eddy-mediated biological productivity in the Bay of Bengal during fall and spring intermonsoons[END_REF]. However, the N:P ratios in the top 60 m (where light levels are usually adequate for photosynthesis), along both transects in both seasons were <10 suggesting nitrate as limiting to the phytoplankton. The N:Si ratios were <1 suggesting Si enriched nutrient regime in the top 40 m. Lower Si:P ratio (< 16:1)-an indication of poor silica waters [START_REF] Harrison | Marine diatoms in chemostats under silicate or ammonium limitation. III. Cellular chemical composition and morphology of three diatoms[END_REF], however apparently do not limit the phytoplankton since the Si:P ratio were always > 3 throughout the 0-120 m water column in the BOB both during FIM and SpIM. The high abundance of diatoms (>90%) vis-à-vis dinoflagellates could be attributed to the high Si:P or low N:Si ratios. Such realms are reported to support diatom dominated systems [START_REF] Bethoux | Temporal trends in nutrient ratios: chemical evidence of Mediterranean ecosystem changes driven by human activity[END_REF].

The phytoplankton abundance was in general higher along the WB during both seasons. However it was higher in the top 20 m during FIM and deeper during SpIM. This maybe attributed to higher concentrations of nitrate and silicate in the north [START_REF] Kumar | Eddy-mediated biological productivity in the Bay of Bengal during fall and spring intermonsoons[END_REF] during FIM. Among the phytoplankton, centric diatoms such as, Chaetoceros lorenzianus, C. curvisetus, C. didymus, Bacteriastrum furcatam and B. comosum were concentrated along the WB irrespective of the season (Figs. 6;7). With more cell volume it appears that centrales prefer relatively higher nutrient regions even within an oligotrophic Bay. [START_REF] Kobayashi | Distribution of diatoms along equatorial transect in the western and central Pacific during La Niña conditions[END_REF] also observed that this group prefers high nutrient regions vis-à-vis oligotrophic waters. Also, as suggested by [START_REF] Figueiras | Phytoplankton composition in the Ria of Pontevedra (NW Spain)[END_REF], abundance of centric diatoms that was observed in the northern stations along the WB during FIM seems to be associated with greater physical stratification. The northern stations are also comparatively more turbulent and richer in nutrients than the southern stations.

Increased occurrence of Chaetoceros spp has been indicative of higher nutrient concentrations in the western and central Pacific [START_REF] Kobayashi | Distribution of diatoms along equatorial transect in the western and central Pacific during La Niña conditions[END_REF].

Among various species of diatoms, C. lorenzianus and C. curvisetus were more abundant at WB4 (Fig. 6) where higher nutrient concentrations were observed during FIM. Interestingly, although the cells of Thalassiosira spp were ubiquitously distributed along the CB, they were observed to be more abundant in the top 20 m at WB4 where Si was ~15 µ during FIM. The demand for Si by diatoms appears to differ with species [START_REF] Mochizuki | Seasonal changes in nutrients, chlorophyll a and the phytoplankton assemblage of the western subarctic gyre in the Pacific Ocean[END_REF] and, such inter-species differences in silica requirements may be one reason for the observed differences in the distribution of Thalassiosira spp.

Higher amount of nutrients and a deeper euphotic depth (~80 m, [START_REF] Kumar | High new production in the Bay of Bengal: Possible causes and implications[END_REF] were observed to be conducive for the growth of C. didymus, B. furcatum and B. comosum during SpIM. Thus, in the BOB, in particular along the WB, ideal light conditions and high nutrient concentrations appear to be beneficial for the growth of these species at different depths.

Along the CB, the centric diatoms were either less than or just about the same in abundance as pennate diatoms. Rhizosolenia styliformis is an exception among the centric diatoms having been previously reported from warm, oligotrophic waters [START_REF] Venrick | The distribution and ecology of oceanic diatoms in the North Pacific[END_REF][START_REF] Venrick | Recurrent groups of diatom species in the North Pacific[END_REF][START_REF] Sournia | Les cyanophycees dans le plancton marin[END_REF][START_REF] Villareal | Nitrogen fixation by the cyanobacterial symbiont of the diatom genus Hemiaulus[END_REF]Villareal, , 1994a, b;, b;[START_REF] Guillard | The ecology of marine plankton diatoms[END_REF] and eddy influenced regions [START_REF] Vaillancourt | Impact of a cyclonic eddy on phytoplankton community structure and photosynthetic competency in the subtropical North Pacific Ocean[END_REF]. Rhizosolenia spp was one of the dominant species in the waters where the N:Si was <1 during FIM. Some of these species are known to contain endosymbiotic cyanobacteria (Richelia intracellularis), which can fix N 2 gas [START_REF] Venrick | The distribution and significance of Richelia intracellularis Schmidt in the North Pacific Central Gyre[END_REF][START_REF] Villareal | Nitrogen fixation, suspension characteristics and chemical composition of Rhizosolenia mats in the central North Pacific Gyre[END_REF].

Moreover, Rhizosolenia spp are also known to undergo vertical migration in search of nitrate and then come to the surface for photosynthesis [START_REF] Singler | Nitrogen inputs into the euphotic zones by vertically migrating Rhizosolenia mats[END_REF].

They are in a way thus independent of critical limits specifically nitrate and are more dependent on the light for photosynthesis. Hence, it can thrive even when the waters are nitrate depleted as observed in the CB. Due to the cosmopolitan nature and wide temperature tolerance of Coscinodiscus spp [START_REF] Horner | A taxonomic guide to some common marine phytoplankton[END_REF] this species was present at most of the stations along CB during SpIM and did not show any marked difference in its spatial distribution in the Bay.

Pennate diatoms have generally higher surface to volume ratio and thus are able to assimilate nutrients even when their concentrations are limited or, very low.

Therefore they can thrive in the Bay better than the centric ones. A pennate such as Thalassionema nitzschioides representing >10 % of the population in the BOB is a known cosmopolitan species [START_REF] Hasle | Identifying marine diatoms and dinoflagellates[END_REF] and surviving in wide nutrient regimes [START_REF] Abrantes | Diatom assemblages as upwelling indicators in surface sediments off Portugal[END_REF]. It was ubiquitously distributed at all the stations along CB during FIM. Usually, T. nitzschioides is reported from high productive regions [START_REF] Schrader | Spatial and temporal variation of Peruvian coastal upwelling during the last Quaternary[END_REF] and, also from low nutrient regimes [START_REF] Kobayashi | Distribution of diatoms along equatorial transect in the western and central Pacific during La Niña conditions[END_REF] similar to the observation in the present study where this species was dominant in the CB rather than the nutrient elevated waters along the WB in FIM.

The pennate diatoms are generally found in greater abundance in calmer and least upwelling zones [START_REF] Pace | Phytoplankton mass culture in discontinuously upwelling water[END_REF]. Thus, calmer waters with low wind speed and increased stability (Varkey et al., 1996) during SpIM in the BOB could be attributed to the high concentrations of pennates along the CB. High temperatures (> 30°C), calm waters and low nutrients, which are prevalent in the BOB during SpIM are ideal conditions also for the growth of Trichodesmium sp. In such nitrate depleted conditions which other diatoms and dinoflagellates circumvent, this cyanobacteria is able to fix atmospheric N 2 [START_REF] Dugdale | High nitrogen fixation rates in the Sargasso Sea and the Arabian Sea[END_REF][START_REF] Goering | Estimates of in situ rates of nitrogen uptake by Trichodesmium sp in the tropical Atlantic Ocean[END_REF][START_REF] Qasim | Some Characteristics of a Trichodesmium bloom in the Laccadives[END_REF][START_REF] Ramos | Bloom of the marine diazotrophic cyanobacterium Trichodesmium erythraeum in the Northwest African Upwelling[END_REF] and form blooms. Interestingly, cell counts of Pseudo-nitzschia spp were found to be higher in deeper waters generally corresponding to higher nitrate levels. Our observations corroborated with the experimental results obtained by [START_REF] Carter | In situ microcosm experiments on the influence of nitrate and light on phytoplankton community composition[END_REF] highlighting that this species increased in abundance on additions of nitrate, suggesting it's preference for nitrate replete conditions. However, certain species which are observed in higher abundance in nitrate replete waters such as Navicula spp and Thalassionema nitzschioides, also showed sporadic appearances in the nitrate depleted waters suggesting the role of other factors such as light in the distribution of certain species. Light and nutrients [START_REF] Mur | Competition of the green alga Scenedesmus and the blue green alga Oscillatoria[END_REF][START_REF] Sommer | Nutrient competition between phytoplankton species in multispecies chemostat experiments[END_REF][START_REF] Sommer | Comparison of steady state and non-steady state competition. Experiments with natural phytoplankton[END_REF] play a major role apart from salinity [START_REF] Rice | Response of estuarine phytoplankton to environmental conditions[END_REF] in the phytoplankton competition. Competition of species regulates species diversity, coexistence and dominance. The competition could be indirect which is caused by the production of inhibitory substances [START_REF] Sharp | Interaction (allelopathy) between marine diatoms: Thalassiosira pseudonana and Phaeodactylum tricornutum[END_REF] or direct that is caused by one or more factors [START_REF] Taylor | Theoretical studies on the coexistences of competing species under continuous flow conditions[END_REF][START_REF] Tilman | Phytoplankton Community Ecology: The Role of Limiting Nutrients[END_REF] including syntrophy. Forced by nutrient limitations, many species of phytoplankton get redistributed within the euphotic column and accumulate at depths having nutrient regimes that suit their specific growth demands, resulting in their predominance in these regimes. A similar scenario in the BOB where certain species appear to be seasonally dominant appears to prevail along both transects while some of them appear to be ubiquitous in the Bay.

Conclusions

The phytoplankton abundance, in particular in the CB appears to be controlled by the ambient nutrient concentrations. Within the community, the abundance of pennales though predominantly controlled by nutrient concentrations, show species specific response to varying nutrient ratios. In case of centrales, nutrient availability, physical stratification, light penetration and the occurrence of eddies apparently influence their distribution and abundance. Among the different ratios, the N:Si ratio in particular appears to effect the phytoplankton community structure. This study has provided for the first time details on phytoplankton species specific responses to different nutrient regimes and ratios from the Bay of Bengal which can be applied to other areas as well. However, further studies are needed to understand phytoplankton community responses to the changing nutrient ratios including those of the trace elements such as Fe, Mn, Co and their affect on the higher trophic levels in the Bay of Bengal.

Figure captions:

  and SpIM (April to May 2003) on onboard ORV Sagar Kanya. As illustrated in Fig. 1, a total of nine stations on two transects, one in the Central Bay (CB, along 88 °E), and another along the Western Bay (WB, 81°-85°E) were sampled during both the cruises. Five stations were covered in the CB (CB1-CB5) and four along the WB (WB1-WB4).

Fig

  Fig.2Aand Fig.3Aillustrating the average nitrate to phosphate ratios (N:P ratio)

  Chaetoceros curvisetus were in higher abundance in the top 20 m during FIM along the WB. During SpIM, Trichodesmium spp, Coscinodiscus spp, and Pseudo-nitzschia spp, were the predominant forms in the CB (Fig.7). While Chaetoceros didymus, Bacteriastrum comosum and B. furcatum were in higher numbers along the WB at deeper depths at 40 -80 m coinciding with the ideal N:Si (1:1) ratios. However during SpIM similar to FIM, Navicula spp was observed at all the stations and was also one of the predominant species along both transects. (Approximate Location of Figure 5) Chaetoceros coarctatus, C. eibenii, C. lorenzianus, C. curvisetus, Rhizosolenia sp, R. cylindrus, R. shrubsolei, R. styliformis, Skeletonema costatum, Synedra henneydyna, Thalassiosira sp, T. gravida, Navicula messanensies, N. septentronalis dominated in the silica enriched waters (N:Si ratios <1) during FIM. Despite similar conditions during SpIM, Bacteriastrum comosum, B. furcatum, Biddulphia sinensis, Chaetoceros coarctatus, C. compressus, C. curvisetus, C. didymus, C. eibenii, C. messaneines, Coscinodiscus sp, C. concinnus, Hemisdiscus hardmanius, Lauderia annulata, Leptocylindrus minimus, Navicula directa, N. distans, N. messanenis, Pseudo-nitzschia sp, Rhizosolenia cylindrus, R. stolterforthii, R. styliformis, Guinardia striata, Stephanopyxis palmeriana, Thalassiosira sp and Trichodesmium sp dominated. Thus both the number of dominant phytoplankton species and their types differed between seasons in the BOB. Along both transects Rhizosolenia alata, R. styliformis, R. imbricata, Leptocylindrus danicus, L. mediterraneanus, Navicula spp, N. distans, N. messanensis, Nitzschia longissima, N. angularis, Pseudo-nitzschia spp, Thalassionema nitzschioides, Synedra spp, S. ulna, Biddulphia mobiliensis, Chaetoceros sp, C. curvisetus, Coscinodiscus spp, C. concinnus, Hemiaulaus hauckii, Fragilariopsis doliolus and Thalassiothrix fauenfeldii dominated in waters below 40 m that was rich in nitrate and where the N:Si ratios >1 during FIM and SpIM.

Figure 1 :Figure 2 :

 12 Figure 1: Sampling locations in the Bay of Bengal along Central Bay (CB) and

Figure 3 :

 3 Figure 3: Vertical structure of the nutrient ratios: A) N:P, B) N:Si and C) Si:P along

Figure 4 :

 4 Figure 4: Distribution of phytoplankton abundances (PCC, cells x 10 3 L -1 ) during A)

Figure 5 :

 5 Figure 5: Seasonal distribution of mean Phytoplankton (PCC), Centrales (Cent) and

Figure 6 :

 6 Figure 6: Distribution of predominant (>5%) phytoplankton in the BOB during FIM.

Figure 7 :Latitude

 7 Figure 7: Distribution of predominant (>5%) phytoplankton in the BOB during SpIM.

Paul et al. Figure6

Acknowledgements

We thank the Director, NIO for the support and encouragement. We thank the DOD for financial aid to conduct this study. This work was carried out as a part of BOBPS programme. The first author acknowledges the SRF fellowship from CSIR.

We thank Dr. S. Prasanna Kumar, Chief Scientist and all the participants and crew members of SK 182 and SK 191. This is NIO contribution number XXXX.


Miguel Angel Huerta-Diaz
email: mhuertadiaz52@yahoo.com

Francisco Delgadillo-Hinojosa

Martín Hernández-Ayón

José Antonio Segovia-Zavala

Zaúl García-Esquivel

Héctor López-Zárate

Arturo Siqueiros- Valencia

Salvador Galindo-Bect

Diagnosis of trace metal contamination in sediments: the example of Ensenada and El Sauzal, two harbors in Baja California, Mexico by

Keywords: enrichment factor, geochemistry, grain size, metals, sediment pollution, trace elements, Baja California

Total metal concentrations in sediments from within Ensenada and El Sauzal harbors are generally higher than at the mouths. Grain-size analyses suggested that this enrichment could be due to the presence of fine-grained sediments in the inner part of the harbors rather than to anthropogenic perturbations. The (Me/Al) sample ratios for Pb, Co, Ni and Fe were significantly higher for Ensenada Harbor relative to El Sauzal Harbor, whereas the ratios for Cd, Mn, Zn and Cu were statistically equivalent for both harbors. Calculated enrichment factors [EF Me = (Me/Al) sample /(Me/Al) shale ] indicated that the metals showing slight enrichment were those associated with anthropogenic contamination (Pb, Zn), or probably related to primary productivity in the water column (Cd, Co). The levels of most of the metals were not greatly enriched, a consideration that is of the utmost importance when contamination issues are at stake.

Introduction

Harbors are enclosed and low-energy water bodies where fine-grained sediment tends to accumulate. They are also prone to receive significant metal inputs from marinas, boat hull maintenance, wholesale fish markets, shipping activities, sacrificial anodes, and industrial, storm and urban discharges. Once in the water column, metals are quickly adsorbed onto particulates and eventually removed to bottom sediments [START_REF] De Groot | Standardization of methods of analysis for heavy metals in sediments[END_REF][START_REF] Santschi | Accumulation of sediments, trace metals (Pb, Cu) and hydrocarbons in Narragansett Bay, Rhode Island[END_REF][START_REF] Blake | Spatial and temporal variations in copper speciation in San Diego Bay[END_REF], thus producing conspicuous trace metal enrichments. Moreover, trace metal enrichments in sediments produced by normal harbor activities can be further enhanced by the larger surface area of the finer sediments that are naturally deposited in these areas of minimum hydrodynamic energy. It is therefore important to measure the extent of metal enrichment in harbor sediments since they can act as point sources of contamination during dredging operations or any other activity by which contaminated sediments can be transported out of the harbors and into neighboring bays or open ocean areas.

Ensenada and El Sauzal harbors are the main harbors on the Pacific coast of northern Mexico. They are located in Todos Santos Bay (Figure 1) and plans are well underway to modernize both harbors by increasing their extension and container handling capacity, especially in the case of El Sauzal Harbor, whose area will be increased approximately ten times.

Additionally, Ensenada Harbor has a cruise terminal capable of handling at least two cruise ships simultaneously. All this expansion work and increase in shipping traffic are associated with dredging activities that may produce a net export of metals out of the harbors. Hence, understanding the relative contributions of the different trace metals is necessary in order to assess the impact of future developments in these harbors and their surrounding waters, and to develop effective strategies to protect them and other harbors elsewhere.

In this paper we will try to answer the following specific questions: Are sediments from Ensenada and El Sauzal harbors enriched in trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn) answer these questions, trace element levels (normalized with Al) will be compared with those reported for other sediments from the Southern California Bight and with average shale values of sedimentary rocks to evaluate the extent of anthropogenic perturbations in the sediments of these two important Mexican harbors. Additionally, pollution load indexes [START_REF] Tomlinson | Problems in the assessment of heavy-metal levels in estuaries and the formation of a pollution index[END_REF][START_REF] Angulo | Mussel-Watch' data: a useful index to assess coastal pollution[END_REF] and geoaccumulation indexes [START_REF] Müller | Schwermetalle in den sedimenten des Rheins -Veranderungen seit[END_REF] will be calculated for both harbors.

Finally, total concentrations will be compared with the U.S. NOAA's sediment quality guidelines (as proposed by [START_REF] Long | Incidence of adverse biological effects within ranges of chemical concentrations in marine and estuarine sediments[END_REF] to estimate the possible consequences of the metal levels analyzed in this study to the local biota. Because of the absence of similar studies in the region, this paper will also provide baseline data for future research on anthropogenic impacts in the region.

Materials and methods

Study area

Ensenada and El Sauzal harbors are situated in Todos Santos Bay, which is located in the northwestern part of the Baja California Peninsula, Mexico (Figure 1a), 100 km south of the US-Mexico border (31º 40 ', 31º 56' N; 116º 36', 116º 50' W). This 240-km 2 bay is limited to the south by Punta Banda, to the north by Punta San Miguel, to the southwest by two small islands located near the mouth of the bay, and to the northeast by the city of Ensenada (around 400,000 inhabitants). According to [START_REF] Pavía | La influencia de los ciclones tropicales en la lluvia del noroeste de Baja California y suroeste de California[END_REF], the climate of the region is Mediterranean, with scarce rainfall (annual average of 250 mm) that occurs mainly during the winter (average of 200 mm).

The regional surface circulation pattern is produced by a predominant northwest component of the wind and surface currents have been reported to average 15 cm s -1 and 5 cm s -1 during summer and winter, respectively [START_REF] Alvarez-Sánchez | Drift patterns of lagrangian tracers in Todos Santos Bay[END_REF]. This wind-driven circulation pattern promotes the transport of water from the adjacent Pacific Ocean towards the inner bay, generating a cell circulation system (dotted arrows; Figure 1b). Part of this flow is directed toward Ensenada harbor and the other part toward Punta Banda, in the southwestern part of the bay page 4 (Huerta-Diaz et al.)

(Figure 1b). Upwelling is intense all year round, but especially in spring and summer [START_REF] Gonzalez-Morales | Daily variation of physico-chemical characteristics, biomass, and phytoplankton primary production in an upwelling costal zone of Baja California[END_REF].

Ensenada Harbor, an international marine terminal, was built in 1956 and covers an area and 250 m long (Figure 1c). Water depths range from 1 to 9 m, with maximum depth occurring in the main navigation channel [START_REF] Flores-Vidal | Numerical simulation of the hydrodynamics of a port and effect of a wave-driven seawater pump[END_REF]. Inputs to this harbor are essentially from fishery companies dedicated to extraction, processing and production activities, as well as from blood and organic wastes discharged by docked fishing boats, and from the powder generated during the unloading of cement from large boats or vessels.

Field and laboratory methods

Sediment samples from Ensenada Harbor were obtained from one core (core 0E)

collected at approximately 6 m water depth on November 2, 1998 [START_REF] Carreón-Martínez | Mercury and silver concentrations in sediments from the Port of Ensenada, Baja California, Mexico[END_REF].

This core was collected within the dredged channel, close to Arroyo Ensenada and in front of the cruise terminal, which was still not constructed at that time. Six more cores (cores 1E to 6E) were collected on January 8, 2002 at water depths of approximately 9 m (Figure 1d) and beneath the docks, considered the areas least affected by sediment resuspension and dredging. Sediments had a hydrogen sulfide smell, were black along the length of the cores and did not show any oxidized portion close to the sediment-water interface. Cores from El Sauzal Harbor were collected on September 29, 2004 (cores 1S to 4S) at water depths ranging from 6 to 9 m (Figure 1c). A polycarbonate plastic core liner (7.2 cm internal diameter, 60 cm in length) was introduced into the sediment, retrieved and capped underwater by a diver who transferred it to a boat. Once on board, the core liner was capped with a plastic cap, sealed with electrical tape and transported to shore, where it was extruded and sliced every 1 cm with a plastic spatula. Each section was then transferred to a 50-mL centrifuge polypropylene tube and stored at -20ºC for further laboratory analysis. Only core 0E was sliced in a glove bag under nitrogen atmosphere to avoid precipitation of the reduced Fe and Mn dissolved in the anoxic portion of the interstitial water.

The 10-13 cm sediment depths of core 2S consisted almost entirely of fish scales.

All laboratory materials were washed with phosphate-free soap, rinsed three times with distilled water and left for 24 hours in a 5% HCl solution. The material was then rinsed three times with deionized water (Milli-Q grade) and left semi-closed to dry at room temperature. Total metal concentrations were obtained after complete digestion of 0.5 g of sediment in Teflon beakers with concentrated HNO 3 , HClO 4 and HF [START_REF] Carignan | The co-diagenesis of sulfur and iron in acid lake sediments of southwestern Québec[END_REF]. was used to ascertain the accuracy and precision of the total extraction procedure. Recovery percentages of the BCSS-1 CRM ranged from 90% for Cd to 104% for Cu (Table 1). Blanks were routinely run and analyzed in the same manner as the samples. Limits of detection for the different trace metals, calculated as three times the standard deviation of the procedural blanks, were (in µmol g -1

): 2.4 (Al), 0.0076 (Cd), 0.026 (Co), 0.024 (Cu), 0.014 (Fe), 0.0047 (Mn), 0.11 (Ni), 0.035 (Pb), 0.013 (Zn). In this paper, and for practical purposes, all metals associated with sediments will be designated as trace metals, regardless of their concentration level. Percentage of sediment grain size <62.5 µm (%GS) was measured using a Horiba laser scattering particle page 6 (Huerta-Diaz et al.)

size distribution analyzer model LA-910, with size interval of 0.02-1000 µm. The analytical efficiency of the particle analyzer was determined with 0.9-µm sieved polystyrene spheres (CRM NIST 8010D and NIST 1690). Organic carbon (org-C) concentrations in the sediments were estimated using the loss on ignition (LOI) method (550°C for 5 h). To reduce the complexity within the similarity matrix, the dataset (including metal concentrations, org-C content and %GS) was transformed into a simpler factor matrix by principal components analysis (PCA). For both Ensenada and El Sauzal harbors, the PCA was calculated based on the correlation matrix of the standardized data using the SYSTAT 8.0 statistical package. For this analysis, eigenvalues >1 were considered significant and, in addition, the orthogonal varimax rotation was chosen for factor rotation.

Results and discussion

Trace metal distributions

Trace metal profiles for both Ensenada and El Sauzal harbors showed no substantial changes in concentration with sediment depth (Figures 2 and3), except for Cu and Zn in core 1E, in which peaks of maximum concentration (229 µmol g -1 and 30 µmol g -1

, respectively; Figures 2c and2i) were observed at a depth of 5.5 cm for both elements. The concentration of Cu was also high in the first 6 cm of core 4E (2.66 ± 0.49 µmol g -1

), while Cd concentrations increased with depth (maximum value = 0.018 µmol g -1 at 19.5 cm) in the first 20 cm of core 0E. Significant trace metal enrichments close to the sediment-water interface are absent in both harbors, further proof that the cores were anoxic up to the sediment-water interface. The lack of significant features in all trace metal profiles is probably a consequence of the absence of the oxic layer ). Bioturbation can be discarded as the cause since all the cores were anoxic-sulfidic up to the sediment-water interface.

Overall, trace metals in Ensenada Harbor showed a wide range of average concentrations (Table 2), the lowest corresponding to Cd (overall average 32 ± 16 nmol g -1

) and the highest to Al (overall average 3.81 ± 0.75 mmol g -1

). On average, core 1E showed the highest values for Cu (15 ± 40 µmol g -1

), Fe (1.33 ± 0.16 mmol g -1

), Mn (12.7 ± 2.5 µmol g -1

) and Zn (5.8 ± 4.5 µmol g -1

); core 0E for Co (1.79 ± 0.45 µmol g -1

) and Ni (2.136 ± 0.046 µmol g -1

); core 2E for Cd (50.6 ± 2.5 nmol g -1

) and Al (4.94 ± 0.25 mmol g -1

); and core 6E for Pb (371± 14) (Table 2). Metals (Co, Cu, Ni, Zn, Fe and, to a certain extent, Al, Mn and Cd) and org-C generally decreased in concentration from the inner part of Ensenada Harbor to its mouth (Figure 4). This result is not unexpected since the inner portion of the harbor shows finer grain size (Figure 4), more shipping activity and less hydrodynamic energy than the outer parts. In this harbor, the deepest areas range from 8 to 12 m and the shallower parts from 2 to 6 m. Cores 3E, 4E and 5E

were taken just at the edge of the deep main channel and thus presented higher sand composition but also low metal concentrations (Figure 4). Sediments located at the mouth of the ). As in Ensenada Harbor, trace metal enrichments close to the sediment-water interface are conspicuously absent for practically all metals, suggesting that the cores were anoxic up to the sediment-water interface.

Median concentration values were consistently high in cores 2S, 3S and 4S from the inner stations and low in core 1S from the harbor mouth, a reflection of the coarser grain size that characterizes this last core (Figure 4). The bathymetry for this harbor [START_REF] Flores-Vidal | Numerical simulation of the hydrodynamics of a port and effect of a wave-driven seawater pump[END_REF] shows that cores 3S and 4S were located at approximately 9 m depth (i.e., the deepest inner area), whereas cores 1S and 2S were collected at water depths of approximately 6 m. Judging from the grain size composition, station 1S was located where the hydrodymic energy was higher. (Cauwet, 1987;[START_REF] Grant | Multivariate statistical analyses of sediment geochemistry[END_REF][START_REF] Ergin | Effects of fine grain size on distribution of Mn in shallow and deep water Black Sea sediments: A comparison between oxic and anoxic depositional environments[END_REF][START_REF] Lin | Influence of the Yangtze River and grain size on the spatial variations of heavy metals and organic carbon in the East China Sea continental shelf sediments[END_REF]. It is widely accepted that metals are enriched in the fine (<62.5 µm) fraction of sediments [START_REF] Gibbs | Transport phases of transition metals in the Amazon and Yukon rivers[END_REF][START_REF] Helmke | Determination of trace element contamination of sediments by multielement analysis of clay-size fraction[END_REF][START_REF] De Groot | Standardization of methods of analysis for heavy metals in sediments[END_REF][START_REF] Förstner | Accumulative phases for heavy metals in limnic sediments[END_REF]Cauwet, 1987;[START_REF] Horowitz | The prediction of aquatic sediment-associated trace element concentrations using selected geochemical factors[END_REF][START_REF] Zhang | Grain size effect on multielement concentrations in sediments from the intertidal flats of Bohai Bay, China[END_REF] and that metals exhibit positive linear correlations with the fine-grained sizes [START_REF] De Groot | Standardization of methods of analysis for heavy metals in sediments[END_REF][START_REF] Ackermann | Monitoring of heavy metals in coastal and estuarine sediments -A question of grain size: <20 µm versus <60 µm[END_REF][START_REF] Ergin | Effects of fine grain size on distribution of Mn in shallow and deep water Black Sea sediments: A comparison between oxic and anoxic depositional environments[END_REF][START_REF] Meador | Elements in fish and sediment from the Pacific coast of the United States: Results from the national benthic surveillance project[END_REF][START_REF] Charlesworth | An assessment of metal contamination in northern Irish coastal sediments[END_REF][START_REF] Zhang | Grain size effect on multielement concentrations in sediments from the intertidal flats of Bohai Bay, China[END_REF]. Although grain size analysis is not suitable for differentiating natural baseline versus anthropogenic enrichments of trace elements, such an analysis may help to account for the natural diluting effect of sand.

Trace metals and grain size

The

Organic carbon

The concentrations of org-C in Ensenada Harbor showed considerable variation with depth (except core 2E, Figure 2k), but without any noticeable trend. In contrast, El Sauzal Harbor showed no substantial changes with depth, except core 2S. This core had the highest concentrations of the four cores, probably because it was collected where docked fishing boats wash their decks, discharging in the process blood and organic wastes into the water and underlying sediment. The concentrations of org-C in core 2S were especially high between 8 and 13 cm (Figure 3k), probably reflecting past high inputs of organic wastes, whose degraded remains are represented by the 10-13 cm sediment depth, consisting almost entirely of fish scales. Additionally, this core showed anomalously high org-C concentrations with respect to the %GS µm (Figure 5i), suggesting that anthropogenic contributions are considerable in the west corner of the harbor (core 2S, Figure 1). There was a significant correlation between %GS and org-C for both El Sauzal (r = 0.54, p 0.001, n = 74) and Ensenada (r = 0.71, p 0.001, n = 139) harbors (Table 3); however, the correlation for El Sauzal increased substantially when core 2S

was eliminated (r = 0.84, p 0.001, n = 61; data not shown), further supporting the contention of the anthropogenic origin of the sedimentary org-C.

Correlations between the different sediment components

Results of the PCA analysis (Table 4) show that two components explained 79.9% and 81.4% of the total variance for Ensenada and El Sauzal, respectively. In the case of Ensenada Another approach by which trace element enrichments can be evaluated consists of the geochemical normalization of metal concentrations with a conservative element. This approach is based on the assumption that there are certain elements that represent proxies for the clay mineral concentration [START_REF] Kersten | Normalization procedures for sediment contaminants in spatial and temporal trend monitoring[END_REF]. Geochemical normalization has several advantages. For example, it compensates for the mineralogical as well as the natural grain size variability of trace element concentrations in sediment [START_REF] Loring | Normalization of heavy-metal data from estuarine and coastal sediments[END_REF][START_REF] Birch | A test of normalization methods for marine sediment, including a new postextraction normalization (PEN) technique[END_REF]. Aluminum has been widely used as a normalizing agent for evaluating metal concentrations in estuarine and coastal sediments (e.g., [START_REF] Goldberg | Pollution history of the Savannah River Estuary[END_REF][START_REF] Schropp | Interpretation of metal concentrations in estuarine sediments of Florida using aluminum as a reference element[END_REF][START_REF] Summers | Normalization of metal concentrations in estuarine sediments from the Gulf of Mexico[END_REF][START_REF] Weisberg | Comparison of sediment metal:aluminum relationships between the eastern and gulf coasts of the United States[END_REF]. Some of the main advantages of using Al as a geochemical normalizer are its high natural concentration (second most abundant metal in the Earth's crust), minimal anthropogenic contamination, it is a structural element of clays, and the metal to Al proportions in the crust are relatively constant [START_REF] Schropp | Interpretation of metal concentrations in estuarine sediments of Florida using aluminum as a reference element[END_REF][START_REF] Summers | Normalization of metal concentrations in estuarine sediments from the Gulf of Mexico[END_REF]. Although Fe has also been used to normalize metal concentrations (e.g., [START_REF] Schiff | Iron as a reference element for determining trace metal enrichment in California coastal shelf sediments[END_REF], Al is a better agent because it is more strongly associated with the aluminosilicate matrix (one of the most important metal-bearing phases of sediments). In contrast, Fe can be significantly affected by diagenetic processes (redistribution, anthropogenically induced formation of Fe sulfides and Fe oxides; [START_REF] Summers | Normalization of metal concentrations in estuarine sediments from the Gulf of Mexico[END_REF], which can distort total Fe concentrations [START_REF] Loring | Normalization of heavy-metal data from estuarine and coastal sediments[END_REF].

Metal/aluminum (Me/Al) ratios for both Ensenada and El Sauzal harbors increase in the following order: Cd/Al < Pb/Al < Ni/Al Co/Al Cu/Al < Zn/Al < Mn/Al << Fe/Al (Figure 6). The Me/Al ratios of Pb, Co, Ni and Fe were significantly higher for Ensenada Harbor relative to El Sauzal Harbor, whereas the ratios for Cd, Mn, Zn and Cu were statistically equivalent for both harbors. Hence, geochemical normalization suggests that metal concentrations in sediments from El Sauzal show some similarities with those from Ensenada Harbor, but that the latter sediments are more enriched in some elements. Proximity to a larger urban settlement (City of Ensenada) may explain the higher Pb/Al ratio for Ensenada; however, explaining the higher values for the other three metal ratios is more difficult. It can be hypothesized that the presence of more intense reducing conditions and greater flushing times (due to its smaller size) in El Sauzal Harbor may explain the deficiency in Fe, Pb, Co, Ni and, to a certain extent, Mn. Fishing boats are routinely cleaned inside this harbor and blood and organic waste end up accumulating in the sediments. For example, the last 6 cm (7-13 cm interval) of core 2S were almost entirely made up of fish scales, an indirect indication of the magnitude of organic matter contribution to the sediments. All this waste could conceivably drive the sediment redox conditions toward a reducing condition, which is probably not so intense in Ensenada Harbor due to the absence of this type of wastes. Fe and Mn are dynamic participants in the redox cycle and their more active reductive dissolution in the El Sauzal sediments with subsequent diffusion to the water column, coupled with a rapid flushing out of this harbor may explain the differences between Fe/Al and Mn/Al in El Sauzal and Ensenada sediments. However, since the reaction of Mn with oxygen is several orders of magnitude slower than that for reduced Fe [START_REF] Jørgensen | Diagenesis and sediment-water exchange[END_REF]),

Mn will be preferentially lost to the water column due to its slower oxidation kinetics of precipitation. Cobalt enrichment could be associated with primary productivity since this element is associated with vitamin B12 (cobalamin), an essential component for the growth of many phytoplankton species [START_REF] Okbamichael | A new method for the determination of Vitamin B 12 in seawater[END_REF]. Furthermore, high concentrations of dissolved B12 are generally found in shallow harbors and bays with limited flushing [START_REF] Okbamichael | A new method for the determination of Vitamin B 12 in seawater[END_REF], characteristics that should favor the accumulation of Co in Ensenada Harbor.

It is generally accepted that for the calculation of metal enrichments it is preferable to use regional pre-industrial background concentrations. However, since no such information was available for the study area, the values of [START_REF] Turekian | Distribution of the elements in some major units of the Earth's crust[END_REF] The high EF Me values of Cd may be a consequence of the Cd-enriched plankton (10 to 20 ppm; [START_REF] Martin | Cadmium in plankton: Elevated concentrations off Baja California[END_REF] found in Baja California waters, which probably transfer this element to the sediments after their death and subsequent sedimentation. Additional Cd enrichment in the sediments can be produced by upwelling processes [START_REF] Segovia-Zavala | Cadmium in the coastal upwelling area adjacent to the California -Mexico Border[END_REF][START_REF] Muñoz-Barbosa | Trace metal enrichment in surficial sediments of the northwest coast of Baja California, Mexico[END_REF], which are common off the northwest coast of Baja California during spring and summer [START_REF] Alvarez-Borrego | Temporal and spatial variability of temperature in two coastal lagoons[END_REF]. The metals showing slight enrichment were those generally associated with anthropogenic contamination (Pb, Zn), or probably related to primary productivity and its relationship with vitamin B12, as was previously discussed for Co. However, it is surprising that Cu was among the metals neither enriched nor impoverished, since granular copper mine tailings were routinely employed to sandblast ships in a dry dock facility located adjacent to Ensenada Harbor. It is highly probable that the high enrichment value (EF Cu = 13) obtained for core 1E (Figure 7) was due to the use of this sandblasting tailing. Impoverishment of Mn and Ni can be the result of the slow kinetics of precipitation of the former [START_REF] Jørgensen | Diagenesis and sediment-water exchange[END_REF], and the known association of the latter with Mn oxides [START_REF] Murray | The interaction of metal ions at the manganese dioxide-solution interface[END_REF][START_REF] Sclater | On the marine geochemistry of nickel[END_REF][START_REF] Balistrieri | The surface chemistry of sediments from the Panama Basin: The influence of Mn oxides on metal adsorption[END_REF]Tessier et page 15 (Huerta-Diaz et al.) [START_REF] Angulo | Mussel-Watch' data: a useful index to assess coastal pollution[END_REF][START_REF] Kay | Processes of nickel and cobalt uptake by a manganese oxide forming sediment in Pinal Creek[END_REF][START_REF] Trivedi | Predicting divalent metal sorption to hydrous Al, Fe, and Mn oxides[END_REF], at least for the case of Ensenada Harbor, where a significant correlation was found between these two elements (Tables 3 and4). Hence,

Ni is probably released into the interstitial water upon reductive dissolution of the Mn oxides and, most likely, diffuses towards the sediment-water interface and is lost to the water column together with the dissolved Mn.

The average ratios of the Ensenada:El Sauzal enrichment factors (defined as EF Me(Ensenada) /EF Me(El Sauzal) ) ranged from 0.63 ± 0.48 for Zn to 2.9 ± 12.4 for Cu (Table 5).

According to these results, only Zn and Cd concentrations were higher in El Sauzal Harbor, whereas the rest of the metals were more concentrated in Ensenada Harbor, especially Cu with a ratio of 2.9 ± 12.4. However, if the anomalously high EF Me results for some of the cores are removed from the calculation of the average ratio (e.g., Cd and Ni without core 0E and Cu without core 1E), then the values are very close to unity for most of the metals (Table 5). Since a value of 1.0 would indicate that total metal concentrations in the sediments of both harbors are equal, these results suggest that Zn is the only metal whose concentrations are generally higher in El Sauzal, while Co is higher in Ensenada (0.63 ± 0.48 and 1.7 ± 0.8, respectively; Table 5).

Furthermore, the EF Me values calculated from reported concentrations of pristine (Strangford and Carlingford loughs; [START_REF] Charlesworth | An assessment of metal contamination in northern Irish coastal sediments[END_REF] and anthropogenically impacted areas (Belfast Lough and PLOO;[START_REF] Charlesworth | An assessment of metal contamination in northern Irish coastal sediments[END_REF]City of San Diego, 2005) were approximately equal or generally higher for Cu, Fe, Mn, Ni, Pb and Zn than the values measured for Ensenada and El Sauzal harbors (Figure 7).

Pollution Load Index

The pollution load index (PLI) for a given core or zone is calculated from the contamination factors (CF Me = concentration in sediment/base value for that metal) of each of its constituent samples, according to the following equation [START_REF] Tomlinson | Problems in the assessment of heavy-metal levels in estuaries and the formation of a pollution index[END_REF]:

PLI = (CF Cd x CF Co x CF Cu x CF Ni x CF Pb x CF Zn ) 1/6
(2)

where the metal base value represents its average concentration in shale [START_REF] Turekian | Distribution of the elements in some major units of the Earth's crust[END_REF][START_REF] Li | Chemical composition and mineralogy of marine sediments[END_REF]. According to [START_REF] Tomlinson | Problems in the assessment of heavy-metal levels in estuaries and the formation of a pollution index[END_REF], PLI values of zero, one, or larger than one suggest absence of baseline pollutants, presence of them, or progressive deterioration of sediment quality, respectively. Figure 8 shows that for Ensenada

Harbor, the core closer to the value of baseline pollutants (1.24) was 5E (at the harbor mouth), whereas the innermost core 1E had the highest PLI value (4.29). The other cores showed values between 1.97 and 2.89, with an overall value for the zone of 2.47 ("Ens" black bar in Figure 8).

The PLI values calculated for El Sauzal Harbor suggest that this zone is appreciably less impacted by metal pollutants than Ensenada, ranging from 1.01 (core 1S at the harbor mouth) to 1.99 (core 2S), with a zonal value of 1.51 ("Sauz" black bar in Figure 8), equivalent to 61% of the overall value for Ensenada Harbor. Hence, as a rule, the cores collected at both harbor mouths had the lowest PLI values, whereas the inner cores had the highest values.

Geoaccumulation index

The geoaccumulation index (I geo ), introduced by Müller (1981), has been used to quantitatively measure metal pollution in aquatic sediments (e.g., Santos [START_REF] Bermejo | Spatial variations of heavy metals contamination in sediments from Odiel river (Southwest Spain)[END_REF], based on a pollution intensity classification (I geo class) that consists of 7 grades or classes (0 to 6, Table 6), the highest grade reflecting a 100-fold enrichment above baseline values. The I geo can be calculated using the equation [START_REF] Müller | Schwermetalle in den sedimenten des Rheins -Veranderungen seit[END_REF]

: baseline area studied 2 geo ] Me [ 5 . 1 ] Me [ log I = (3)
where [Me] baseline represents the metal concentration in average shales taken from [START_REF] Turekian | Distribution of the elements in some major units of the Earth's crust[END_REF] and [START_REF] Li | Chemical composition and mineralogy of marine sediments[END_REF], with a "1.5" factor included because of possible variations in the background data due to lithogenic effects [START_REF] Salomons | Metals in the Hydrocycle[END_REF]. Results from this exercise indicated that 79.3% (Ensenada Harbor) and 86.1% (El Sauzal

Harbor) of the elements belonged to I geo classes 0 and 1 (unpolluted to moderately polluted), with only 6.3% (Ensenada Harbor only) positioned in I geo class 4 (0% for El Sauzal Harbor; Table 6).

The only element that consistently ranked high in the I geo classes (3 or 4), for both harbors and all cores (except 0E), was Cd (Table 6). It is clear that Ensenada has more metals in the "polluted" I geo classes (2 to 6; Table 6) than El Sauzal (20.6% vs. 13.9%, respectively). None of the analyzed elements were positioned in the last two, more polluted classes (5 and 6).

Sediment quality guidelines

Comparison of our total concentrations with the NOAA's sediment quality guidelines [START_REF] Long | Incidence of adverse biological effects within ranges of chemical concentrations in marine and estuarine sediments[END_REF] can be used to evaluate the possible biological consequences of the levels of metals in Ensenada and El Sauzal harbors. [START_REF] Long | Incidence of adverse biological effects within ranges of chemical concentrations in marine and estuarine sediments[END_REF] matched biological and chemical data compiled from numerous modeling, laboratory, and field studies performed in marine and estuarine sediments. Using these data, effects range-low (ERL: lower 10th percentile of the effects data) and range-median (ERM: the median, or 50th percentile, of the effects data) guideline values were determined for a number of metals (Table 7: guideline values available only for Cd, Cu, Ni, Pb and Zn). These two guideline values (ERL and ERM) delineate three concentration ranges (<ERL, ERL-ERM, >ERM) for a particular metal. Hence, concentrations below the ERL value will represent a minimal-effects range, a range intended to estimate conditions in which biological effects would be rarely observed. Concentrations equal to and above the ERL, but below the ERM, will represent a possible-effects range within which effects would occasionally occur. Finally, concentrations equivalent to and above the ERM value will represent a probable-effects range within which effects would frequently occur [START_REF] Long | Incidence of adverse biological effects within ranges of chemical concentrations in marine and estuarine sediments[END_REF].

Table 7 shows that there is a considerable difference between the two harbors in the allocation of Cu, Ni and Pb levels among the different concentrations ranges. In El Sauzal Harbor (n = 74), Cu was distributed between the <ERL (29.7%) and ERL-ERM (70.3%) ranges, whereas in Ensenada

Harbor (n = 147), it occurred between the <ERL (10.4%), ERL-ERM (75.3%) and >ERM (14.3%) ranges (Table 7). In El Sauzal, Ni was also distributed between the <ERL and ERL-ERM ranges (17.6 and 82.4%, respectively), whereas in Ensenada it was between the ERL-ERM and >ERM ranges (47.8 and 52.2%, respectively) (Table 7). Finally, Pb was distributed only between the first two concentration ranges in both harbors, though the proportion of this metal in the <ERL range was only 14.3% in Ensenada Harbor, but 98.6% in El Sauzal Harbor (Table 7). The rest of the metals (Cd and Zn) showed similar behaviors in their distribution ranges (Table 7).

Conclusions

Despite the high total metal concentrations measured in the sediments of Ensenada and El Sauzal harbors, their Al-normalized values generally correspond to what would be expected from their aluminosilicate content. Results from EF Me , PLI, I geo and effects range (Figures 7 and8, and Tables 6 and7, respectively) clearly demonstrate that the inner parts of both harbors contain higher levels of metals relative to the outer parts (with cores 1E and 2S generally the most enriched), and that Ensenada is the more trace metal-enriched harbor of the two. In terms of particular elements, Cd ranked among the most enriched in both harbors (e.g., Figure 7, Tables 6 and7); however, Cu and Ni in Ensenada and, to a much lesser extent, Zn in both harbors, are the metals that can potentially cause adverse effects to local living resources.

Overall, PLI values for both harbors were reasonable ( 2.47), with an important percentage (79.3-86.1%) of the analyzed metals belonging to I geo classes 0 and 1 (unpolluted to moderatelly polluted). Hence, despite the apparently high total trace element concentrations measured in both Mexican harbors, the levels of most of the metals are not greatly enriched in these sediments and do not represent a threat to the local biota, a consideration that is of the utmost importance when contamination issues are at stake. , 1986), northern Irish coastal sediments (NICS; [START_REF] Charlesworth | An assessment of metal contamination in northern Irish coastal sediments[END_REF], Southern California Bight (SCB), Santa Monica Bay (SMB), outfalls of publicly owned treatment works (POTWs) and ¢¡ £ ¥¤ §¦ ©© ¡ ¥¤ areas (STW) [START_REF] Schiff | Sediment chemistry on the mainland shelf of the Southern California Bight[END_REF], are also included for comparison purposes The average composition of shale was taken from [START_REF] Turekian | Distribution of the elements in some major units of the Earth's crust[END_REF] and [START_REF] Li | Chemical composition and mineralogy of marine sediments[END_REF]. Concentrations (± one standard deviation) reported for Point Loma Ocean

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT 4. Results of PCA applied to sedimentary geochemical data gathered at Ensenada and El Sauzal harbors. Core 0E was not included in this analysis because the percentage of grain size <62.5 µm (%GS) and organic carbon (org-C) were not available. Cu and Zn concentrations at 5.5 cm depth in core 1E were also not considered because they represent outliers. [START_REF] Bermejo | Spatial variations of heavy metals contamination in sediments from Odiel river (Southwest Spain)[END_REF]: 0 = unpolluted, 1 = unpolluted to moderately polluted, 2 = moderately polluted, 3 = moderately to strongly polluted, 4 = strongly polluted, 5 = strongly to very strongly polluted, and 6 = very strongly polluted.

Harbor

  ? If so, to what extent and with which metals? Which harbor presents the highest metal enrichments? If present, are these trace metal enrichments anthropogenically derived? To page 3 (Huerta-Diaz et al.)

  of 1.95 km 2 (i.e., approximately 13 times bigger than El Sauzal Harbor). The mouth of the harbor is formed by a 1,640-m-long rock breakwater that is connected to the coast and by a 855-m-long jetty (El Gallo; Figure 1d). Water depths range from 1.5 to 11 m, with the deepest parts located adjacent to the loading and terminal docks and the main navigation channel. In addition to the presence of fine-grained sediments (approximately 80% <62.5 µm), this harbor tends to accumulate metals due to inputs from (a) a wholesale fish market, (b) marine vessels normally docked in the port and marina areas, (c) urban runoff discharged through Arroyo Ensenada (Figure 1d) during the winter rains, and (d) vessel reparation and maintenance activities (liberation of antifouling paint, sand blasting operations) within the port zone. El Sauzal Harbor, built in the mid-1980s and located 10 km north of Ensenada in the town of El Sauzal de Rodriguez (approximately 7,500 inhabitants), has an area of 0.15 km 2 (Flores-Vidal et al., 2005) and a 100-m-wide mouth formed by two breakwaters that are 500 m

  These measurements were transformed to org-C concentrations through a calibration curve (org-C % = (0.206 ± 0.010)*LOI; r 2 = 0.929, p<0.001, n = 33) using org-C values from Todos Santos Bay sediments measured by a LECO model CHNS-932 elemental analyzer. Due to the lack of available samples, org-C and grain size were not measured in core 0E.

(

  and probably the anoxic non-sulfidic layer as well) and of the homogeneity of the %GS (Figure page 7 (Huerta-Diaz et al.) 2j

  harbor presented low %GS (reflecting a high level of hydrodynamic energy) and, consequently, high sand contents that diluted trace metal concentrations. Core 0E, however, usually presented anomalous concentrations when compared with the other cores from Ensenada Harbor (Figure4); for example, it had considerably higher median concentrations of Co, Fe and Ni, and lower median concentrations of Cd and Pb relative to the other cores. This anomalous behavior can be attributed to the location of the core, which was collected inside the dredged channel and close to the mouth of Arroyo Ensenada. Sediment resuspension produced by continuous shipping traffic, repeated dredgings (which can alter the distribution of some key components of sediments, like nutrients and carbon;[START_REF] Nayar | Environmental effects of dredging on sediment nutrients, carbon and granulometry in a tropical estuary[END_REF] and occasional particulate and dissolved contributions from the seasonal stream have evidently altered the distribution and levels of trace metals in core 0E. These alterations are more noticeable for Cd, Co, Fe, Mn, Pb and Zn, as reflected in the variability (spread) of their concentrations(Figures 2a,b,d,f,h and i, respectively) and by the increase in length of the box plots shown in Figure4. page 8 (Huerta-Diaz et al.)Average metal concentrations in El Sauzal Harbor (Table2) were consistently high in cores 2S (Cd = 28.2 ± 1.5 nmol/g, Cu = 1.24 ± 0

Figure 4

 4 Figure 4 shows that total concentrations of Cd, Co, Fe, Mn, Ni and Pb are generally lower in El Sauzal Harbor than in Ensenada Harbor. These results probably cannot be ascribed to differences in org-C concentrations for Ensenada (1.50 ± 0.38 mmol g -1; n = 139) and El Sauzal

  size-class distribution in sediments is intimately related to the hydrodynamics of the systems. Hence, comprehension of this process in combination with coastal topography can help to understand the behavior of sediments in terms of flocculation, adsorption or precipitation.Based on textural parameters, Pérez-Higuera and Chee-[START_REF] Pérez -Higuera | Sediment transport in Todos Santos Bay[END_REF] inferred the existence of a southward dominant transport in the littoral zone of Todos Santos Bay. In the case of Ensenada Harbor, the water flow across the mouth produced by the tide effect is considered to be the main hydrodynamic forcing in the movement of particulate material, sediments and pollutants[START_REF] Coronado-Méndez | Actualización de la batimetría del puerto de Ensenada[END_REF]. The high sand contents (and low metal concentrations) are just a reflection of the high level of hydrodynamic energy at both harbor mouths.

Figure 5

 5 Figure 5 suggests that for sediments showing similar %GS, the metal concentrations measured in Ensenada and El Sauzal harbors were higher than those measured at the Point Loma Ocean Outfall (PLOO) discharge point and its surroundings, off San Diego Bay (City of SanDiego, 2005). This is surprising, considering that sediments from the PLOO area have been found to be contaminated with a number of trace metals (Sb, As, Ba, Be, Cd, Cr, Cu, Fe, Pb, Mn, Hg, Ni; City of San Diego, 2005). Furthermore, data reported for the Southern California Bight (SCB;[START_REF] Schiff | Sediment chemistry on the mainland shelf of the Southern California Bight[END_REF] show that average Cu, Ni and Zn values for the whole SCB (3,420.3 km 2 )

  , a firmly related group (PC1) was formed by Al, Cd, Co, Fe, Mn, Ni, Pb and %GS (loadings >0.63), while the second group (PC2) was formed by Cu, Zn and org-C (loadings >0.75). For El Sauzal Harbor, Cd, Cu, Zn and org-C (loadings >0.89) constituted the first group (PC1), and Al, Co, Fe, Mn and %GS the second (PC2; loadings >0.76). The strong correlation among Cu, Zn, Cd and org-C found for both harbors reflects the complex nature of organic matter as well as the importance of the role played by the diagenetic reactions driven by its degradation, which, in turn, regulates the behavior of these reactive metals. The strong association among Al, Fe, Mn, Co and %GS suggests that the concentrations of these metals are strongly controlled by the sediment grain size of these two Mexican harbors. The PCA results are essentially in agreement with the Pearson correlation coefficient matrix (Table3), which probably indicates the association of Cd, Cu and Zn with organic matter and the existence of a strong lithogenous component (Al, Fe and %GS) controlling the distribution of trace metals in sediments.3.5. Enrichment factorspage 12 (Huerta-Diaz et al.)

Figure legends Figure 1 .

 legends1 Figure legends

Figure 2 .

 2 Figure 2. Profiles of total trace metal concentrations (on a dry weight basis), percentage of grain size <62.5 µm (%GS) and organic carbon (org-C) for each of the seven cores collected in Ensenada Harbor. Gray symbols represent concentrations below the detection limit.

Figure 3 .

 3 Figure 3. Profiles of total trace metal concentrations (on a dry weight basis), percentage of grain size <62.5 µm (%GS) and organic carbon (org-C) for each of the four cores collected in El Sauzal Harbor.

Figure 4 .

 4 Figure 4. Box plots of total trace metal concentrations, organic carbon (org-C) and percentage of grain size <62.5 µm (%GS) for each one of the sediment cores collected in Ensenada and El Sauzal harbors (open and gray boxes, respectively). Cd concentrations below the detection limit in core 0E were not considered for the calculation of basic statistics. In this figure, the boundary of the box closest to zero indicates the 25th percentile, a line within the box marks the median, and the boundary of the box farthest from zero indicates the 75th percentile. Bars above and below the box indicate the 90th and 10th percentiles, respectively.

Figure 5 .

 5 Figure 5. Plots of percentage of grain size <62.5 µm (%GS) vs. metal concentration and organic carbon (org-C) for sediment cores collected in Ensenada and El Sauzal harbors. Concentrations (± one standard deviation) reported for Point Loma Ocean Outfall (PLOO; City of San Diego, 2005), Mazatlan Harbor (MH; Osuna-López et al., 1986), northern Irish coastal sediments (NICS;

Figure 6 .

 6 Figure 6. Box plots and average values (± one standard deviation) of total metal (Me) concentrations and Me/Al ratios for sediments collected in Ensenada and El Sauzal harbors. Metals were arranged in increasing order of average values of Ensenada Harbor. Note the log scales in the y-axes.

Figure 7 .

 7 Figure 7. Enrichment factors, EF Me = (Me/Al) sample /(Me/Al) shale , for each one of the sediment cores collected in Ensenada (E) and El Sauzal (S) harbors (black and gray bars, respectively).

  Outfall (PLOO; City of San Diego, 2005), Belfast Lough (B), and Strangford and Carlingford loughs (S&C) (Charlesworth and Service, 2000) are also included for comparison purposes. Dashed lines represent the value of 1.0 [Me/Al) sample = (Me/Al) shale ]. Error bars for E and S harbors were calculated by error propagation analysis.

Figure 8 .

 8 Figure 8. Pollution load index for each one of the sediment cores (gray bars) and for Ensenada (Ens) and El Sauzal (Sauz) areas (black bars). PLI values of zero, one (dashed line), or larger than one suggest absence of baseline pollutant, presence of them, or progressive deterioration of sediment quality, respectively.

  Figure 1

  0.66*** 0.63*** 0.77*** 0.44*** 0.60*** Ni 0.92*** 0.77*** 0.93*** 0.50*** 0.86*** 0.73*** Pb 0.72*** 0.60*** 0.73*** 0.36*** 0.79*** 0.43*** 0.73*** Zn 0.75*** 0.57*** 0.89*** 0.74*** 0.82*** 0.67*** 0.87*** 0.72*** %GS 0.63*** 0.61*** 0.75*** 0.35*** 0.79*** 0.53*** 0.62*** 0.63*** 0.62*** Org-C 0.49*** 0.37*** 0.69*** 0.50*** 0.74*** 0.30*** 0.58*** 0.69*** 0.72*** 0.71*** ***p ≤ 0.001 Table

*

  Percentage for each I geo class [number of metals]. **Overall percentage for each I geo class [number of metals].

  average shale values and, probably, subjected to a diagenetic process that reduces its concentration in the sample. Finally, if EF Me = 1, then the metal concentration has the same value as the reference value (normalized with Al). The results of this exercise are shown in

	and Li and Schoonmaker (2005) for the average composition of shale were used instead. The enrichment factor (EF Me ) was simply calculated as: = shale sample Me (Me/Al) (Me/Al) EF corresponding EF Me value. Values ranged from 0.41 (EF Cu , core 5E) to 13 (EF Cu , core 1E), and (1) from 0.36 (EF Ni , core 4S) to 12 (EF Cd , core 2S). Based on the EF Me values, the elements were arbitrarily divided into four groups: (1) those highly enriched (EF Me > 2.5) in the harbor sediments, represented solely by Cd (average EF Me value of 10 ± 3); (2) those slightly enriched (1.5 > EF Me > 2.5), represented by Zn, Co and Pb (average EF Me values of 1.7 ± 0.9, 2.1 ± 0.9 and 2.3 ± 0.5, respectively); (3) those neither enriched nor impoverished (0.6 > EF Me > 1.5), represented (excluding core 1E) by Fe and Cu (average EF Me values of 0.9 ± 0.2 and 1.2 ± 0.6, respectively); and (4) those slightly impoverished (EF Me < 0.6), represented by Mn and Ni (average EF Me values If EF Figure 7, where each core from Ensenada and El Sauzal harbors was plotted against the of 0.50 ± 0.06 and 0.6 ± 0.3, respectively).

Me > 1, the metal concentration in the sample is enriched relative to the average shale values and, maybe, anthropogenically impacted. If EF Me < 1, then the metal is impoverished relative to the

Table 1 .

 1 Measured (n = 21) and certified concentrations ( two standard deviations), and percentages of recovery of Beaufort Chemistry Standard Sediment (BCSS-1; National Research Council of Canada).

	Figure 8

Table 2 .

 2 Average total concentrations (Avg) and range of values for sediments from Ensenada (cores 0E to 6E) and El Sauzal (cores 1S to 4S) harbors. Numbers in parentheses refer to the number of samples. Cd concentrations below the detection limit in core 0E were not considered for the calculation of basic statistics.

		Al (mmol g	-1 ) Cd (nmol g	-1 ) Co (µmol g	-1 ) Cu (µmol g	-1 ) Fe (mmol g	-1 ) Mn (µmol g	-1 ) Ni (µmol g	-1 ) Pb (nmol g	-1 ) Zn (µmol g	-1 )
	Avg 0E (43)	3.49  0.47	6.8  4.2	1.79  0.45	1.32  0.19	1.31  0.16	9.3  1.3	2.136  0.046	231  62	2.50  0.78
	Range 0E	2.84-4.56	<7.6-18		1.11-2.81		0.92-1.59		1.03-1.68	6.5-12	2.069-2.293	135-423	1.34-4.22
	Avg 1E (32)	4.53  0.28	41.8  2.4	1.169  0.085 15  40		1.33  0.16	12.7  2.5	0.948  0.044	366  38	5.8  4.5
	Range 1E	3.99-5.07	37.7-46.6		1.052-1.411	4-229		1.11-1.82	10-20	0.892-1.10	295-429	3.5-30
	Avg 2E (27)	4.94  0.25	50.6  2.5	1.006  0.043 1.09  0.21	1.200  0.046 12.61  0.35 0.907  0.040	353  32	3.36  0.38
	Range 2E	4.43-5.52	45.5-55.2		0.918-1.083	0.91-1.58		1.09-1.26	11.87-13.15	0.832-1.02	307-490	2.99-4.18
	Avg 3E (36)	3.36  0.22	35.3  3.4	0.733  0.059 0.79  0.12	0.931  0.089	9.28  0.38 0.593  0.054	288  26	1.73  0.23
	Range 3E	3.00-3.99	30.2-41.4		0.637-0.919	0.53-0.94		0.742-0.126	8.27-10.0	0.518-0.744	233-337	1.33-2.17
	Avg 4E (21)	3.46  0.23	38.9  3.7	0.823  0.074 1.51  0.90	0.900  0.089 11.2  1.9	0.645  0.055	285  40	1.92  0.58
	Range 4E	2.93-3.81	34.8-47.8		0.695-0.963	0.37-3.01		0.739-1.084	9.2-16.4	0.560-0.750	223-336	1.13-3.03
	Avg 5E (14)	2.64  0.20	25.0  2.4	0.518  0.028 0.262  0.087 0.56  0.14	7.12  0.62 0.492  0.042	257  16	0.89  0.14
	Range 5E	2.26-2.90	19.6-29.3		0.470-0.574	0.148-0.418	0.42-0.97	6.14-8.19	0.428-0.600	241-298	0.70-1.2
	Avg 6E (9)	3.84  0.18	37.8  2.0	0.918  0.043 1.76  0.13	1.175  0.011	8.41  0.41 0.671  0.035	371  14	2.72  0.10
	Range 6E	3.52-4.07	35.1-42.0		0.870-0.999	1.57-1.99		1.16-1.19	7.73-9.07	0.637-0.742	350-400	2.57-2.85
	Overall
	Avg (182):	3.81  0.75	32  16		1.10  0.48	3.5  17.2		1.11  0.26	10.4  2.3	1.07  0.62	299  66	2.9  2.4
	Overall
	range:	2.26-5.52	<7.6-55		0.470-2.81	0.15-229		0.419-1.82	6.14-19.9	0.428-2.29	135-490	0.70-30
	Avg 1S (13)	2.33  0.08	19.2  1.2	0.348  0.015 0.251  0.031 0.483  0.036	5.21  0.37 0.363  0.032	129.5  8.5 1.33  0.19
	Range 1S	2.18-2.44	17.7-21.6		0.327-0.382	0.210-0.320	0.423-0.536	4.58-5.68	0.327-0.411	112-146	1.08-1.62
	Avg 2S (13)	2.61  0.12	28.2  1.5	0.431  0.020 1.24  0.13	0.570  0.026	6.06  0.24 0.476  0.017	183  18	4.75  0.70
	Range 2S	2.30-2.81	25.9-30.6		0.390-0.463	1.09-1.54		0.527-0.622	5.75-6.53	0.453-0.504	156-231	4.07-6.66
	Avg 3S (29)	2.81  0.24	23.8  3.5	0.495  0.037 0.72  0.23	0.668  0.085	7.6  1.1	0.448  0.041	158  10	2.92  0.64
	Range 3S	2.49-3.44	18.4-32.1		0.409-0.592	0.41-1.2		0.561-0.848	6.2-9.6	0.361-0.507	139-181	1.90-4.75
	Avg 4S (19)	2.60  0.14	23.8  3.4	0.444  0.044 0.75  0.17	0.565  0.069	6.55  0.59 0.370  0.031	154  82	3.15  0.75
	Range 4S	2.36-2.90	19.1-28.8		0.380-0.549	0.52-1.0		0.485-0.729	5.91-8.04	0.310-0.437	133-167	2.30-5.23
	Overall
	Avg (74):	2.64  0.24	23.8  3.9	0.445  0.062 0.74  0.34	0.592  0.095	6.6  1.2	0.418  0.057	157  19	3.0  1.2
	Overall
	range:	2.18-3.44	17.7-32.1		0.327-0.592	0.21-1.5		0.423-0.848	4.6-9.6	0.310-0.507	112-231	1.1-6.7

Table 3 .

 3 Pearson correlation matrix for the sedimentary geochemical data from Ensenada and El Sauzal harbors. Core 0E was not included in the analysis because the percentage of grain size <62.5 µm (%GS) and organic carbon (org-C) were not available. Cu and Zn concentrations at 5.5 cm depth in core 1E were also not considered because they represent outliers.

		Al	Cd	Co	Cu	Fe	Mn	Ni	Pb	Zn	%GS
					El Sauzal Harbor (n = 70)
	Cd	0.14
	Co	0.85*** 0.19
	Cu	0.20	0.87*** 0.26
	Fe	0.90*** 0.08	0.89*** 0.17
	Mn	0.51*** -0.10	0.72*** -0.04	0.56***
	Ni	0.53*** 0.53*** 0.57*** 0.63*** 0.57*** 0.24
	Pb	0.43*** 0.52*** 0.47*** 0.74*** 0.43*** 0.21	0.61***
	Zn	0.21	0.84*** 0.25	0.94*** 0.14	0.00	0.57*** 0.70***
	%GS 0.76*** 0.45	0.83*** 0.57*** 0.77*** 0.46*** 0.68*** 0.59*** 0.55***
	Org-C 0.32	0.69*** 0.31	0.84*** 0.28	0.07	0.70*** 0.77*** 0.84*** 0.54***
					Ensenada Harbor (n = 120)
	Cd	0.85***
	Co	0.84*** 0.71***
	Cu	0.31*** 0.16	0.56***
	Fe	0.84*** 0.74*** 0.88*** 0.46***
	Mn

Table 5 .

 5 Average ratios of Ensenada (n = 182) to El Sauzal (n = 74) enrichment factors (EF Me(Ensenada) /EF Me(El Sauzal) ) for the different elements analyzed. The associated errors were calculated by error propagation analysis. For the special case of Cd, Ni and Cu, results are presented with and without some of the cores that showed anomalously elevated EF Me values.

		Component Eigenvalues	Explained variance (%)	Accumulated Variance (%)
	Ensenada	1	5.40	49.1	49.1
	Ensenada	2	3.39	30.8	79.9
	El Sauzal	1	4.80	43.6	43.6
	El Sauzal	2	4.16	37.8	81.4
		Component loadings
	Geochemical	El Sauzal Harbor	Ensenada Harbor
	factor	PC1	PC2	PC1	PC2
	Al	0.17	0.90	0.92	0.25
	Cd	0.89	-0.03	0.94	0.04
	Co	0.20	0.95	0.77	0.58
	Cu	0.97	0.06	0.07	0.87
	Fe	0.13	0.94	0.77	0.53
	Mn	-0.11	0.76	0.71	0.28
	Ni	0.66	0.50	0.83	0.45
	Pb	0.75	0.36	0.64	0.50
	Zn	0.95	0.05	0.58	0.75
	%GS	0.52	0.76	0.63	0.48
	Org-C	0.89	0.18	0.35	0.78

Table 6 .

 6 Metals distributed according to their geoaccumulation index (I geo ). The I geo classes (in parentheses) consist of 7 grades of pollution intensity (

Table 7 .

 7 Effects range-low (ERL) and range-median (ERM) guideline values for trace metals (on a dry weight basis) and percent incidence of sediment concentration values in concentration ranges defined by the two guideline values. Percentage and number of data entries within each concentration range divided by the total number of entries for all ranges.

	Harbor	Element	ERL (µmol/g)	ERM (µmol/g)	Percent (ratios) incidence for each b concentration range
			Guidelines	a	<ERL	ERL-ERM	>ERM
	Ensenada	Cd	0.011	0.085	19.2 (35/182)	80.8 (147/182)	0.0 (0/182)
	El Sauzal	Cd	0.011	0.085	0.0 (0/74)	100 (74/74)	0.0 (0/74)
	Ensenada	Cu	0.54	4.25	10.4 (19/182)	75.3 (137/182)	14.3 (26/182)
	El Sauzal	Cu	0.54	4.25	29.7 (22/74)	70.3 (52/74)	0.0 (0/74)
	Ensenada	Ni	0.225	1.05	0.0 (0/182)	47.8 (87/182)	52.2 (95/182)
	El Sauzal	Ni	0.225	1.05	17.6 (13/74)	82.4 (61/74)	0.0 (0/74)
	Ensenada	Pb	0.356	0.879	14.3 (26/182)	85.7 (156/182)	0.0 (0/182)
	El Sauzal	Pb	0.356	0.879	98.6 (73/74)	1.4 (1/74)	0.0 (0/74)
	Ensenada	Zn	2.29	6.27	47.3 (86/182)	51.6 (94/182)	1.1 (2/182)
	El Sauzal	Zn	2.29	6.27	24.3 (18/74)	74.3 (55/74)	1.4 (1/74)

a Taken from Long et al. (1995); b

Acknowledgments

We thank Rafael Blanco-Betancourt, who collected all cores for this study, and two anonymous reviewers, whose insightful comments greatly improved the content of the manuscript. This research was partially supported by grants from Fondo Sectorial SEMARNAT-CONACYT (SEMARNAT-2002-C01-0016) and Fondo CONACYT-BC (SINVE 001-EC) to M.A. Huerta-Diaz.