S C Johannessen
email: johannessen@pac.dfo-mpo.gc.ca

G Potentier

C A Wright

D Masson

R W Macdonald

Water column organic carbon in a Pacific marginal sea (Strait of Georgia, Canada)

Marginal seas provide a globally important interface between land and interior ocean where organic carbon is metabolized, buried or exported. The trophic status of these seas varies seasonally, depending on river flow, primary production, the proportion of dissolved to particulate organic carbon and other factors. In the Strait of Georgia, about 80% of the organic carbon in the water column is dissolved. Organic carbon enters at the surface, with river discharge and primary production, particularly during spring and summer. The amount of organic carbon passing through the Strait (~ 16 x 10 8 kg C/yr) is almost twice the standing inventory (~9.4 x 10 8 kg C). The organic carbon that is oxidized within the Strait (~ 5.6 x 10 8 kg/yr) presumably supports microbial food webs or participates in chemical or photochemical reactions, while that which is exported (7.2 x 10 8 kg/yr) represents a local source of organic carbon to the open ocean. DOC, POC, organic carbon cycling, water column, Strait of Georgia

Introduction

Organic carbon enters the coastal ocean by in situ primary production and with river discharge. As emphasized by [START_REF] Liu | Exploring continental margin carbon fluxes on a global scale[END_REF], shelf seas are not passive receptors of this organic carbon but rather provide a globally important interface between land and interior ocean where organic carbon is metabolized, buried or exported. Metabolism of organic carbon in coastal waters provides a globally significant source of carbon dioxide [START_REF] Smith | Coastal metabolism and the oceanic organic carbon balance[END_REF], although marginal seas of the North Pacific generally export organic carbon [START_REF] Chen | Roles of continental shelves and marginal seas in the biogeochemical cycles of the North Pacific Ocean[END_REF]. Whether a marginal sea is a net source or net sink of CO 2 varies seasonally, depending on the strength of primary production and river inflow, whose effects are particularly pronounced near shore.

A large part of the fixed carbon is suspended in water as dissolved organic carbon (DOC), much of which is readily transformed back to carbon dioxide by microbial respiration and photochemical oxidation. In the global ocean, the inventory of DOC ( 680Gt) far outweighs particulate organic carbon (POC; 20 Gt) or biota (3 Gt), and a longstanding puzzle is how a large component of DOC manages to persist for years and even millennia in the ocean [START_REF] Eglinton | Organic matter in the contemporary ocean[END_REF]. Accordingly, the fate of the reactive pool of DOC helps to determine the ability of the biological pump to sequester organic carbon on time scales ranging from days to thousands of years. In addition to their role in the global carbon cycle, POC and DOC can affect the oceanic transport of contaminants, many of which have a strong affinity for organic matter (e.g. [START_REF] Brannon | Effects of sediment organic carbon on distribution of radiolabelled fluoranthene and PCBs among sediment, interstitial water and biota[END_REF][START_REF] Chevreuil | PCB pollution behaviour in the River Seine[END_REF], and can mediate their reactions [START_REF] Blough | Impact of photochemical processes in the hydrosphere[END_REF].

In some coastal areas, organic carbon can itself be the most important contaminant [START_REF] Turner | Gulf of Mexico hypoxia: Alternate states and a legacy[END_REF].

The Strait of Georgia (Fig. 1) is a highly productive marginal sea on the west coast of Canada. Its physical and biological oceanography have been studied extensively (see reviews by [START_REF] Waldichuk | Physical oceanography of the Strait of Georgia[END_REF][START_REF] Thomson | Oceanography of the British Columbia Coast[END_REF][START_REF] Harrison | Review of the biological oceanography of the Strait of Georgia: pelagic environment[END_REF][START_REF] Leblond | The Strait of Georgia: functional anatomy of a coastal sea[END_REF][START_REF] Harrison | An assessment of nutrients, plankton and some pollutants in the water column of Juan de Fuca Strait, Strait of Georgia and Puget Sound, and their transboundary transport[END_REF][START_REF] Macdonald | Marine sediments in the Strait of Georgia, Juan de Fuca and Puget Sound: What can they tell us about contamination?[END_REF][START_REF] Thomson | Physical oceanography of the Strait of Georgia -Puget Sound -Juan de Fuca Strait system[END_REF], although little attention has been paid to chemical and geochemical processes, especially in the water column. Estuarine circulation in the Strait of Georgia is driven largely by the discharge of the Fraser River, which supplies about 65% of the fresh water to the Strait (e.g. [START_REF] Leblond | The Strait of Georgia: functional anatomy of a coastal sea[END_REF]. The Strait is open at both ends, exchanging with the Pacific Ocean through Johnstone Strait in the north and through the much larger Juan de Fuca Strait, via Haro Strait, in the south. Strong tidal currents in Haro Strait cause intense tidal mixing in that area (e.g. [START_REF] Leblond | The Strait of Georgia: functional anatomy of a coastal sea[END_REF]. The trophic status of the Strait of Georgia system remains uncertain, although preliminary budgets for particulate and dissolved organic carbon suggest that, unlike the other temperate Pacific marginal seas, the Strait likely provides a small net source for CO 2 [START_REF] Johannessen | A sediment and organic carbon budget for the greater Strait of Georgia[END_REF]. The data on which the seawater component of these budgets was based were sparse, illustrating the need for more measurements of organic carbon, particularly of DOC, which appeared to be sufficiently important to control whether the system was a net sink or a net source of CO 2 . Here, we describe the spatial and seasonal variability of organic carbon in the water column of the Strait of Georgia and discuss controls on its distribution. This overview will help to focus future, hypothesis-driven research into local carbon dynamics.

Methods

Sample collection and in situ measurements

In April, June, September and December, 2003, water samples for DOC and TOC analysis were collected along a cruise track from the mouth of Juan de Fuca Strait to the northern Strait of Georgia (Figs. 1 and2) aboard the CCGS Vector. These samples form part of a larger data set that has been collected quarterly since 1999. The full data set includes conductivity / temperature / depth (CTD), fluorescence, photosynthetically active radiation (PAR), and transmissivity profiles as well as nutrient and oxygen measurements made on discrete water samples. The electronic measurements were made at 76 stations (Fig. 1), including the 20 stations where bottle samples were collected. Some of the accompanying physical and chemical data have been analyzed and published elsewhere [START_REF] Johannessen | Distribution and cycling of suspended particles inferred from transmissivity in the Strait of Georgia, Haro Strait and Juan de Fuca Strait[END_REF][START_REF] Masson | Seasonal water mass analysis for the Straits of Juan de Fuca and Georgia[END_REF]. where r is the pathlength (m) and tr (%) is the percent transmission over that pathlength [START_REF] Baker | The effect of particle size on light attenuation coefficient of natural suspensions[END_REF]. Chlorophyll a fluorescence was measured with a Seapoint fluorometer also mounted on the frame of the rosette (excitation wavelength 470 nm, emission 685 nm).

Dissolved oxygen was measured by Winkler titration aboard the ship, using an automated Dosimat system, and nutrient samples were stored frozen until their analysis on return to the laboratory, according the protocol established by Barwell-Clark and Whitney (1996).

Calculation of inventories and fluxes

We calculated surface layer and deep layer organic carbon inventories at each station.

We defined the break between upper and lower layers as the top of the deep, relatively uniform layer, as determined by visual inspection of the salinity profiles. The Strait of Georgia does not display a classical density profile with a well defined upper mixed layer and a pycnocline overlying a deep mixed layer [START_REF] Masson | Chlorophyll distribution in a temperate estuary: the Strait of Georgia and Juan de Fuca Strait[END_REF]. The upper layer comprises a series of interfingered layers of water, and in some places the density decreases all the way to the surface, without any apparently well-mixed surface layer.

Consequently, choosing the top of the deep layer was the most consistent way to divide the water column throughout the study area. This depth also usually coincided with a sharp decrease in the concentrations of DOC and TOC between the upper and lower layers (shown in section 3.1 below). We assigned the organic carbon concentration measurements for each station to a small area of the surrounding basin (Fig. 1) and calculated the volume of water that that area represented, using a 1 km by 1 km depth grid (Table 1). The inventories for the upper and lower layers and for the whole water column in each area were calculated as the product of the water volume and the average concentration of TOC (or DOC or POC) in that section. Seasonal inventories for the northern and southern Strait of Georgia, Haro Strait and Juan de Fuca Strait were calculated by summing the inventories at all the stations included in the larger zones.

The fluxes of TOC, POC and DOC through Juan de Fuca Strait were calculated as the product of their average surface (or deep) concentrations and the average annual outflow (inflow). The inflow and outflow were taken from a salt balance calculation made by [START_REF] Johannessen | A sediment and organic carbon budget for the greater Strait of Georgia[END_REF] and on previously published current measurements in Juan de Fuca Strait [START_REF] Godin | An analysis and interpretation of the current data collected in the Strait of Juan de Fuca in 1973[END_REF]. This ratio is similar to that observed in the East China Sea, where POC/DOC = 0.1 [START_REF] Gao | Main geochemical characteristics and key biogeochemical carbon processes in the East China Sea[END_REF], but POC is more important in this nearshore region than in the global ocean, where POC/DOC ~ 0.03 [START_REF] Eglinton | Organic matter in the contemporary ocean[END_REF]. A similarly high POC/DOC ratio was observed off the Oregon coast, to the south of the study area (POC/DOC ~ 0.3; [START_REF] Hill | Organic carbon and nitrogen in the northern California current system: comparison of offshore, river plume and coastally upwelled waters[END_REF]. The dissolved and particulate organic carbon are not significantly correlated to one another in any of the seasons (r 2 0.2), probably indicating that different mechanisms control their distributions.

Results

Distribution of organic carbon

In all four seasons, the concentration of dissolved organic carbon was highest at the surface, particularly in the central and southern Strait of Georgia, and lowest in the subsurface seawater flowing in through Juan de Fuca Strait (Fig. 3, Table 3). The concentration of DOC was inversely related to salinity (Fig. 4a), as similarly observed along a transect perpendicular to the Catalan coast in the Mediterranean Sea [START_REF] Doval | Dissolved and particulate organic carbon and nitrogen in the northwestern Mediterranean[END_REF], positively related to oxygen and inversely related to nutrients (Figs. 4b,4c).

The variation in POC through the year differs from that of DOC. Like DOC, POC was highest in surface water in April and June, but in September and December it was generally low throughout the study area, with only small, local maxima, possibly related to particle sinking or resuspension (Fig. 5, Table 3). There was also a relatively high concentration of POC in the deep water of the northern Strait of Georgia in April and June. The extremely high POC concentration indicated at mid-depth in the far northern Strait of Georgia in June is based on only one measurement, so we cannot be certain of exactly what it represents. However, the high concentration in nearby bottom waters is based on multiple measurements. While the wintertime concentration of DOC in the deep, uniform layer was higher than that in the summer and spring, there was essentially no POC in the study area in December.

Inventories and fluxes

The water column inventory of TOC in the Strait of Georgia did not vary much with season (average seasonal concentrations in Table 3 multiplied by water volume in Table 2) and was generally about 9.5 x 10 8 kg C (9.1 x 10 8 -9.8 × 10 8 kg C). Over the surface area of the Strait of Georgia considered here (7464 km 2 , see Methods) the inventory implies an average of 130 g C m -2 , with the highest value occurring in December.

However, the distribution of TOC among surface and deep layers, the northern and southern Strait, and particulate and dissolved forms, did vary considerably (Fig. 6, Table 4). The proportion of the TOC held in the surface layer varied depending both on the concentration of organic carbon and on the depth of the surface layer.

The inventory of organic carbon held in the water column of the Strait of Georgia is low, relative to the fluxes into the Strait. The annual net primary production in the Strait is estimated at 90 -120 g C m -2 yr -1 [START_REF] Mackas | Nitrogenous nutrient sources and sinks in the Juan de Fuca Strait / Strait of Georgia / Puget Sound estuarine system: assessing the potential for eutrophication[END_REF].. Extrapolating that water column primary production rate to the surface area of the Strait of Georgia considered in this study (7464 km 2 ), the annual net primary production is 6.7 × 10 8 -9.0 × 10 8 kg C (Table 1). The Fraser and other rivers add an additional 7.5 ×10 8 kg organic C annually, and anthropogenic sources add up to 1.2 × 10 8 kg C [START_REF] Johannessen | A sediment and organic carbon budget for the greater Strait of Georgia[END_REF]. Thus, the Burial and advection come to 11 ×10 8 kg C yr -1 . By difference, this leaves 4.0 × 10 8 -6.2 × 10 8 kg C yr -1 , about half of the inventory, to be oxidized in the Strait of Georgia and its sediments each year. The preliminary carbon budget constructed by [START_REF] Johannessen | A sediment and organic carbon budget for the greater Strait of Georgia[END_REF], indicated that about 0.9 × 10 8 kg C/yr was oxidized in sediments. Using this sediment oxidation value, about 3.1 x 10 8 -5.3 x 10 8 kg of organic carbon is oxidized in the water column each year. About 1.1 × 10 8 kg C/yr POC is exported through Juan de Fuca Strait.

The area-weighted inventory of organic carbon was lower in Haro and Juan de Fuca Straits than in the Strait of Georgia and was relatively stable year-round in Juan de Fuca Strait, ranging from 76 ±3 kg C m -2 in June to 88 ± 2 kg C m -2 in April (Fig. 6). The seasonal variation in the carbon that was held in the surface layer was due principally to changes in the depth of that layer throughout the year (Fig. 6). In Haro Strait the water column inventory of TOC varied from 37 g C m -2 in June to about double that in April.

Discussion

The correlations shown in Figure 4 are consistent with both algal and terrigenous sources of DOC to the Strait of Georgia. Phytoplankton release DOC in the daytime as they grow (e.g. [START_REF] Maranon | Significance and mechanisms of photosynthetic production of dissolved organic carbon in a coastal eutrophic ecosystem[END_REF]2005). During an estuarine phytoplankton bloom, [START_REF] Gobler | Cycling of colloidal organic carbon and nitrogen during an estuarine phytoplankton bloom[END_REF] measured production rates of POC and DOC that were nearly equivalent to one another. In the Strait of Georgia the growth rate of Chaetoceros and other algae is greatest during the late spring bloom (e.g. [START_REF] Harrison | Nutrient and plankton dynamics in the Fraser River plume, Strait of Georgia, British Columbia[END_REF]. However, we cannot use timing alone to apportion sources of organic carbon as either terrigenous or marine.

Terrigenous DOC discharged by the Fraser and other rivers also enters the Strait of Georgia at the surface, where the concentration of oxygen is high and those of the nutrients are low. Of course, some of the DOC carried by the river is undoubtedly produced by algae in the river. Unlike the situation in some Arctic rivers, where the DOC is highest during the spring freshet [START_REF] Rember | Increased concentrations of dissolved trace metals and organic carbon during snowmelt in rivers of the Alaskan Arctic[END_REF], DOC in the Fraser River usually peaks in the late spring, about two months before the peak water discharge (Fig. 7). It is not possible from the data collected for this study to discriminate among the three sources of DOC (terrigenous, riverine algal, marine algal). Future work could use stable isotopes of carbon and nitrogen, 14 C radiodating, degree of aromaticity or biomarkers to investigate the distribution and fate of DOC from the three sources. For example, in the Mississippi River estuary, δ 13 C measurements indicated that most of the terrigenous DOC was removed from the water column in the salt -fresh water mixing zone (salinity ~ 25, [START_REF] Wang | Sources and transport of dissolved and particulate organic carbon in the Mississippi River estuary and adjacent coastal waters of the northern Gulf of Mexico[END_REF]. Also, in the Mid-Atlantic Bight, rivers have been shown to export new, high-14 C DOC to the coastal ocean [START_REF] Mortazavi | Use of Keeling plots to determine sources of dissolved organic carbon in nearshore and open ocean systems[END_REF].

The distribution of POC was not tied directly to that of total suspended particles. Using data collected during [START_REF] Liu | Exploring continental margin carbon fluxes on a global scale[END_REF]-2003[START_REF] Johannessen | Distribution and cycling of suspended particles inferred from transmissivity in the Strait of Georgia, Haro Strait and Juan de Fuca Strait[END_REF] showed that the beam attenuation coefficient (c), a proxy for suspended particle concentration, was relatively high throughout Haro and Juan de Fuca Straits in the winter, while the POC was low there at that time (Fig. 5). There was no significant relationship between POC and c for most of the year and only a weak one (r 2 =0.5) in April, indicating that the organic content of particles varies in time and space. The distinction between particulate and dissolved organic carbon is operational -for this project organic carbon that passed through a 0.2 µm filter was considered dissolved -so the lack of a correlation between POC and c is not surprising. However, the ratio of POC to c describes in a relative way how much of the particulate matter is organic. The ratio is low throughout the year in Juan de Fuca and Haro Straits (Fig. 8), implying that most of the suspended particles in that area are inorganic. This could reflect their source or that all the available organic matter had been consumed or broken down into DOC by the time the particles had reached that area. In the Strait of Georgia POC/c is high at the surface in April, June and September (Fig. 8), indicating a high proportion of organic particles near the surface. This is consistent with primary production in the surface layer. Near-surface particulate organic matter is likely largely consumed, oxidized or broken down into DOC within the Strait, consistent with previous observations of the rapid consumption of organic carbon after a phytoplankton bloom [START_REF] Gobler | Cycling of colloidal organic carbon and nitrogen during an estuarine phytoplankton bloom[END_REF].

Because of the estuarine nature of its circulation and the presence of several sills, the Strait of Georgia does not export POC to the open ocean along the bottom, as occurs in some other Pacific marginal seas (e.g. [START_REF] Chen | Roles of continental shelves and marginal seas in the biogeochemical cycles of the North Pacific Ocean[END_REF]. Rather, POC in bottom waters is predominantly buried in the Strait itself [START_REF] Johannessen | A sediment and organic carbon budget for the greater Strait of Georgia[END_REF], although some may be broken down to DOC and then consumed by bacteria, as occurs in the deep Pacific Ocean [START_REF] Hansell | Bacterioplankton distribution and production in the bathypelagic ocean: directly coupled to particulate organic carbon export?[END_REF]. The observation in this study that POC is exported in surface water through Juan de Fuca Strait contrasts with Johannessen et al.'s (2003) conclusion that it was not. The discrepancy probably results in part from the in situ production of POC in the surface waters of Juan de Fuca Strait that is of necessity included in the exchange estimate made in this paper but which was excluded from the 2003 work. However, in situ production is unlikely to account for the whole calculated export. There is no estimate of net primary productivity available for Juan de Fuca Strait, but the Strait of Georgia rate (90-120 g C m -2 yr -1 ; [START_REF] Harrison | Review of the biological oceanography of the Strait of Georgia: pelagic environment[END_REF]) applied to Juan de Fuca implies a net annual production of 2.0 x 10 8 -2.6 x 10 8 kg C, or about a third of the estimated export. Possibly much of the export occurs on very small particles that did not contribute much to the total sediment budget. Some of the POC also sinks to the bottom in the northern Strait, particularly in April. This seasonal pulse of organic-rich particles in late spring / early summer may explain the high concentration of organic carbon observed in the bottom sediments of the northern Strait (3.5- 5% Johannessen, 2003;3.5% Bornhold, 1978) relative to that measured in the southern Strait and Fraser River (~1% \Johannessen, 2003). The isotopic composition ( 13 C, 15 N) of the organic matter measured in a northern Strait sediment core implies the organic carbon to be predominantly recycled material of algal origin [START_REF] Macdonald | Sediment redox tracers in Strait of Georgia sediments -can they inform us of the loadings of organic carbon from municipal wastewater?[END_REF], which is consistent with the pulse of high-POC particles to the bottom observed in April, at the end of the spring bloom season.

The correlation between DOC and salinity (Fig. 4a) suggests that about 70% of the variation in DOC is related to conservative mixing. This does not imply that all of the DOC is terrigenous; rather, both algally-derived and terrigenous DOC enter the system at the surface and are associated with the fresher water. However, the scatter about the regression line is not randomly distributed. DOC residuals calculated as the vertical distance to the regression line in Figure 4a represent non-conservative variations in the concentration of DOC, with higher values implying production of DOC in situ. The residuals are generally lower in April and June (study area average -5.1 and -6.8 µM, respectively) than in September and December (average +3.1 and +2.9 µM). In the Strait of Georgia, the residual DOC is high in the surface water in April and June and low in the deep water at that time, while in September, it is closer to the value expected from mixing alone (Fig. 9). By December, a pool of high-DOC water appears in the deep Strait of Georgia. Overall in the study area, DOC is positively correlated with dissolved O 2 , because most of the time the two are produced or consumed together. However, in the deep Strait of Georgia in the winter, the increase in DOC in the winter is accompanied by a decrease in O 2 . Since the deep water of the Strait is renewed only a few times a year, in the late spring and late summer [START_REF] Masson | Deep water renewal in the Strait of Georgia[END_REF][START_REF] Pawlowicz | The circulation and residence time of the Strait of Georgia using a simple mixing-box approach[END_REF], it becomes stagnant in the winter, and oxidation of organic matter uses up oxygen. For DOC to be produced at the same time, it must be derived from the breakdown of POC. Since the residuals represent the non-conservative portion of the variation in DOC, the winter increase is not from the advection of DOC from outside the Strait. Indeed, the deep ocean water that intrudes periodically into the Strait has a lower concentration of DOC than that already in the Strait (Fig. 3). The POC that is broken down could derive from algal material that sinks into the deep northern Strait in late spring, or from resuspended bottom material. Similarly, the high DOC residual observed in Haro Strait / Victoria Sill and extending into Juan de Fuca Strait in September and December might result from the breakdown of POC resuspended from the bottom sediments during storms. The exact source of the winter DOC cannot be determined from these data.

Summing the DOC residuals in the deep Strait of Georgia using the same procedure as for the inventory calculation described in Section 3.2 and dividing the inventory by the sea level surface area of the Strait as calculated for this work (7464 km 2 ), gives a change in the DOC residual inventory between June and December of about 15 g C m -2 (Table 5). This represents about 5 % of the total annual primary production [START_REF] Harrison | Review of the biological oceanography of the Strait of Georgia: pelagic environment[END_REF], so there is easily enough carbon entering the system annually to support this ingrowth. The residual is also smaller than, but a significant portion of, the flux of sinking organic carbon measured using sediment traps moored in the southern and central Strait in 1996-1999 (31 -365 g C m -2 yr -1 , [START_REF] Johannessen | Seasonal and spatial variations in the source and transport of sinking particles in the Strait of Georgia, British Columbia, Canada[END_REF]. If the observed seasonal pattern is repeated annually, then there must be a mechanism to remove the winter DOC from the Strait by the spring. Likely processes include oxidation, flocculation and settling to the sediments, or advection as part of the annual replacement of Strait of Georgia water, which occurs with a time scale of about 65 days, according to a recent model [START_REF] Pawlowicz | The circulation and residence time of the Strait of Georgia using a simple mixing-box approach[END_REF].

Quarterly measurements such as these cannot capture variability on daily, weekly or monthly time scales, so the difference between the June and December measurements represents a minimum change in residual inventory over that time period. The spatial scale of sampling also misses patchiness on the scale of hundreds of meters or smaller, but this is the first study to provide the requisite data to quantify the spatial and seasonal variability of water column organic carbon in the Strait of Georgia, and to provide a general depiction of the seasonal and spatial patterns.

Unfortunately, since we have organic carbon data for 2003 only, we cannot judge the range of interannual variability in the observed patterns. A useful extension of this work would be to characterize the source and age of the DOM in the Strait. [START_REF] Davis | Quantitative estimates of labile and semi-labile dissolved organic carbon in the western Arctic Ocean: a molecular approach[END_REF], for example, used carbon-normalized yields of amino acids to assign proportions of labile, semi-labile and refractory organic matter to the DOC pool in the western Arctic Ocean. They were able to discriminate terrigenous from marine-derived organic matter, which would be helpful in understanding the observed seasonal variations in the distribution of organic carbon in the Strait of Georgia.

Conclusions

Organic carbon enters the Strait of Georgia mainly at the surface, from a combination of in situ primary production and river discharge. The seawater that flows in below the surface of Juan de Fuca Strait and through Haro Strait into the Strait of Georgia brings with it very little organic carbon. In the spring and summer, there is a relatively high concentration of particulate organic carbon at the surface and throughout the water column of the northern Strait, with a pulse of probably marine-derived organic particles to the bottom associated with the spring bloom. However, most of the organic carbon in the study area is dissolved, particularly in the autumn and winter, when more than 80% of the TOC is dissolved.

The Strait of Georgia is dynamic with respect to organic carbon. About twice as much organic carbon passes through the Strait each year as is present in the year-round water column inventory, which implies sub-annual residence times for the carbon. Much of this carbon is advected out of the Strait or buried in the bottom sediments, but an amount equivalent to about half of the standing inventory is oxidized in the water column or sediments. This carbon may sustain microbial ecosystems or undergo photochemical reactions, producing carbon dioxide, nutrients, free radicals and other photoproducts.

Conservative mixing explains about 70% of the observed variability in the concentration of DOC, but there are also non-conservative seasonal variations, probably related to the transport and breakdown of POC. In the deep Strait of Georgia, this non-conservative variation amounts to a difference of about 15 g C m -2 between December and June, when the concentration is low.

To understand fully the effects of organic carbon cycling in this environment, future studies should examine the sources and fluxes of organic matter to the Strait and their variability on time scales of days to seasons. and2). (gC m -2 ; calculated as inventory divided by total surface area for comparison with average rates of primary production). Horizontal lines represent the average depth of separation of the upper and lower layers at the stations included in each zone in each season (see Tables 1 and2). The water column is relatively well-mixed in the Haro Strait / Victoria sill zone, and there was no clear separation of layers at most stations in that zone. By propagation of errors from the zone average concentrations shown in Table 3, and assuming no error in the defined volume of each zone, the standard deviation in these inventories is 1-5 gC m -2 .

  Water samples for DOC and TOC analysis were collected at depths of 0, 20, 75 and 150 m (depth permitting) and at the bottom of each cast ( 1-5 m off the bottom), with occasional replicates, for a total of 472 samples collected during the four cruises. TOC samples were collected from the Niskin bottle spigots directly into acid-washed, 40 mL glass vials with EPA septa, after three rinses with the sample seawater. DOC samples were gravity-filtered into the same type of vial through a 0.2 µm, Millipore, Opticap cartridge filter attached to the rosette spigot with a short length (~10 cm) of acid-washed Teflon tubing. Before each cruise, a new Opticap filter was flushed with 3 L of milli-Q water. Preliminary experiments had shown that flushing a new filter with 2 L of milli-Q water was sufficient to prevent contamination of the samples by the filter. Before sampling began at each station, the filter was rinsed with 1 L of seawater from the Niskin bottle collected at the greatest depth at that station. The filter and tubing were emptied after each sample was filtered and then flushed with 250 mL of each sample before any of the filtrate was collected. Samples were frozen immediately after collection and stored frozen until analysis. Nitrile gloves were worn at all times during sample collection and analysis. The concentrations of DOC and TOC were measured by high temperature combustion in an Apollo 9000 TOC analyzer set to 700˚C and calibrated with standard additions of potassium hydrogen phthalate (KHP) to filtered open ocean seawater, with a precision of 0.2 µM (average standard deviation of replicates analysed from the same vial.) The concentration of POC was calculated as the difference between TOC and DOC. The standard deviation of DOC and TOC samples collected in 7-12 separate vials from the same Niskin bottle during each cruise was 1-4 µM and 3-7 µM, respectively. By propagation of errors, the standard deviation of POC values was 4-8 µM. Transmissivity at 660 nm was measured using a Sea Tech transmissometer attached to the bottom of the frame of the sampling rosette, beside the CTD. Transmissivity was converted to the beam attenuation coefficient, c, according to the equation

  Most of the organic carbon in the water column of the Strait of Georgia, Haro Strait and Juan de Fuca Strait is dissolved. Except at the surface and in the deep, northern Strait of Georgia in April and June, DOC makes up at least 80% of the total (POC/DOC 0.25).

  total sources of TOC to the Strait are approximately 15 × 10 8 -18 × 10 8 kg C yr -1 , about twice the inventory measured year-round. The sinks are burial in the sediments (4.3 × 10 8 kg C yr -1 , Johannessen et al., 2003) advection through Juan de Fuca Strait (and possibly Johnstone Strait) (7.2 ×10 8 kg C yr -1 , this study, see Methods), and oxidation.

Fig. 2 .Fig. 4 .

 24 Distribution of sampling stations and depths along a vertical transect from the mouth of Juan de Fuca Strait (station 102) to the northern end of the Strait of Georgia (station 16). The bottom shape (grey) actually shows the cast depth (usually < 5 m off the bottom); all the samples were taken in the water column. The inset map shows the transect route in plan view. Plot generated in Ocean DataView, Schlitzer (2006). Fig.3. Vertical profile of DOC concentration in April, June, September and December along the transect shown in Figure1, overlain with salinity contour lines The vertical lines represent rosette cast locations. Plot generated in Ocean Data View withVGgridding, Schlitzer (2006). Relationships between [DOC] and (a) salinity, (b) the concentration of dissolved oxygen, (c) the concentration of nitrate for all cruises (n=267). All three fits were somewhat improved by omitting the December data (see section 3.1), but for completeness all data are shown and included. Fig. 5. Vertical profile of POC concentration in April, June, September and December along the transect shown in Figure 1, overlain with salinity contour lines The vertical lines represent rosette cast locations. Plot generated in Ocean Data View with VG gridding, Schlitzer (2006). Fig. 6. Inventories of DOC (bold) and POC (italic) in the upper and lower layers of the northern and southern Strait of Georgia, Haro Strait / Victoria Sill and Juan de Fuca Strait

Fig. 7 .

 7 (a) Monthly average discharge of the Fraser River and (b) monthly average [DOC] in the river, measured at Hope. Environment Canada, unpublished data, 2007. The discharge and concentration measured in 2003 were comparable to the average observed over the past 10 years.

Fig. 8 .Fig. 9 .

 89 Fig. 8. Vertical profile of the ratio of POC concentration to beam attenuation coefficient ([POC]/c) in April, June, September and December along the transect shown in Figure 1, overlain with salinity contour lines. The vertical lines represent rosette cast locations. Plot generated in Ocean Data View with VG gridding, Schlitzer (2006). Fig. 9. Vertical profile of DOC residual (see text section 41) in April, June, September and December along the transect shown in Figure 1. The residuals represent the vertical distance of each DOC point from the regression line of [DOC] on salinity in Figure 4a. A high value indicates that there is a high concentration of DOC relative to the conservative mixing line.

  Fig.1

Table 2 .

 2 Areas and volumes of large zones used in Table3. The station locations are shown in Figure1.

	Zone	Stations / boxes included	Area (km 2 )	Volume (10 9 m 3 )
	N. Strait of Georgia	16, 14, 12, 9, 6, Malaspina	3058	451
	S. Strait of Georgia	2, 27, 39, 42, 46, 56	4408	701
	Haro Strait / Victoria Sill	59, 62, 65, ADCP	2239	204
	Juan de Fuca	69, 72, 75, 102	2185	290
	Strait

Table 4 .

 4 Sources, sinks and inventory of organic carbon in the Strait of Georgia

		DOC	POC	TOC	Reference
	Sources
	Fraser and other river flux into Strait (10 8 kg C yr -1 )	5.3	2.2	7.5	Johannessen et al., 2003
					Total production from
					Mackas and Harrison,
					1997 (See Johannessen
	Net primary production (10 8 kg C yr -1 )	4.9	2.4	7.3	et al., 2003, for calculation of new
					production; multiplied
					by surface area used
					for this study)
	Anthropogenic fluxes into Strait (10 8 kg C yr -1 )	< 0.83	< 0.37	< 1.2	Johannessen et al., 2003
	Total sources of organic C (10 8 kg C yr -1 )	11	5.0	16	Sum of above
	Sinks
	Burial in sediments of Strait (10 8 kg C yr -1 )	0	4.3	4.3	Johannessen et al., 2003
	Net outflow through Juan de Fuca Strait (10 8 kg C yr -1 )	6.1	1.1*	7.2*	this study
	Sinks: burial + outflow (10 8 kg C yr -1 )	6.1	4.3 (5.4*)	10 (12*)	Sum of above
	Sink: oxidation, by difference (10 8 kg C yr -1 )	4.9	0.7	5.6	Sum of above
	Strait of Georgia water
	column inventory (10 8 kg C):	8.1	1.3	9.4	this study
	averaged over four seasons
	* Apparent outflow of particulate organic carbon may reflect in situ production in Juan de
	Fuca Strait, and not outflow from the Strait of Georgia

Table 5 .

 5 Inventory of DOC residuals (10 7 kg C) calculated, as described in Section 4, from the vertical distance to the mixing line in Fig.4a:[DOC] vs. salinity. "SoG" = Strait of Georgia; "HS" = Haro Strait/ Victoria Sill; "JdF" = Juan de Fuca Strait. The study area. Numbered, circled dots show locations of rosette and CTD casts, where water samples were collected for DOC and TOC analysis. The dots without numbers represent additional CTD casts. The small polygons marked around the rosette stations are those used in the carbon inventory calculations (see section 2.2 and Tables1

	Area	April	April	June	June	Sept.	Sept.	Dec.	Dec.
		surface	deep	surface	deep	surface	deep	surface	deep
	N. SoG 0.30	-1.4	0.70	-3.8	-0.29	-0.39	0.99	1.0
	S. SoG 1.1	-4.6	-0.35	-3.8	0.52	0.12	-0.81	3.2
	SoG Total	1.4	-6.0	0.35	-7.6	0.23	-0.27	0.18	4.2
	HS	0	0.76	-0.09	-0.56	0.24	1.0	0	1.3
	JdF	-1.0	-0.06	0.10	-0.05	0.83	0.06	0.04	0.18

Fig.2

December

Fig.5

Fig.6

Fig.7

Acknowledgements

The authors are grateful to Nes Sutherland for her painstaking analysis of the total and dissolved organic carbon and to Dr. Lisa Miller for the use of her TOC analyzer. We thank the officers, crew and scientific party of the CCGS Vector for assistance with the water sample collection. Lynne Campo and Andrea Ryan of Environment Canada provided Fraser River flow and DOC data free of charge. We thank two anonymous reviewers for their helpful comments. Funding for this work was provided by Fisheries and Oceans Canada and by Metro Vancouver (formerly the Greater Vancouver Regional District).