VARIATION OF ENDOSULFAN RESIDUES IN WATER AND SEDIMENT TAKEN FROM THE MOSS LANDING DRAINAGE OF MONTEREY COUNTY
Data from previous studies conducted by the State Mussel Watch Program (SMW) implied increasing endosulfan residues inmollusks used as indicator organisms of chemical contamination in the Moss Landing Drainage area of Monterey County. As a first step in confirming a chronological trend, studies were conducted by the California Departments of Food and Agriculture (CDFA) and Fish and Game (CDFG) to determine within-site variability of endosulfan concentrations, and to estimate sample size necessary for future research. Personnel from the Environmental Hazards Assessment Program (EHAP) of CDFA collected sediment and water samples while personnel from the Pesticide Investigations Unit (PIU) of CDFG collected mollusk and fish samples at the same sites in a coordinated effort. This report contains results from the EHAP study. Three sites previously monitored by SMW in Monterey County were selected for the collection of sediment and water samples. All samples were analyzed for endosulfan I, II and sulfate. Only 33, 33 and 25% of the sediment samples contained detectable residues of endosulfan I, II and sulfate, respectively. Endosulfan concentrations in sediment were not uniformly distributed in the Moss Landing Drainage area with residues occurring more frequently in agricultural areas, away from tidal action and dredging activities. Only 25% of the water samples contained detectable endosulfan residues in the form of endosulfan sulfate. However, it is conceivable that endosulfan residues exist in water (either dissolved or on suspended sediment) below the detection limit given the consistent find of residues in mollusks and fish in this area and the established bio-concentration factors for fish species. Minimum sample sizes of 22 and 11 for sediment and water, respectively, were estimated as necessary to calculate the true mean for a given site. Future study of temporal or spatial trends in endosulfan residues of this drainage area might best be concentrated on sediment and biological organisms since residues in water are mostly below the current detection limits.
Acute toxicity to and bioconcentration of endosulfan by estuarine animals
Acute (96-h) flow-through toxicity tests with endosulfan (Thiodan) were conducted with several estuarine animals. The test species and their 96-h lethal concentration for 50 percent of the organisms (LC 5 0 ) values were: pink shrimp (Penaeus duorarum), 0.04 μg/litre; grass shrimp (Palaemonetes pugio), 1.3 μg/litre; pinfish (Lagodon rhomboides), 0.3 μg/litre; spot (Leiostomus xanthurus), 0.09 μg/litre; and striped mullet (Mugil cephalus), 0.38 μg/litre. In a 56-day bioconcentration study (28-day uptake, 28-day depuration), striped mullet were exposed to 0.008 and 0.08 μg endosulfan/litre seawater. The two endosulfan isomers (endosulfan I and II) were rapidly metabolized to endosulfan sulfate; only trace amounts of each isomer were detected in edible tissue or offal of mullet exposed to 0.08 μg/litre (0.035 μg/litre measured) for 28 days. Maximum bioconcentration factors of endosulfan were 2249 in edible tissue and 2755 in whole-body analyses. After 48 h in pesticide-free seawater, endosulfan was not detected in the previously exposed mullet. Our studies suggest that endosulfan in the estuarine environment would be a hazard because of its acute toxicity and bioconcentration potential, but animals surviving exposure and moving to areas free of endosulfan would lose the chemical rapidly.
Lake Muzahi, Rwanda: limnological features and phytoplankton production
Lake Muhazi, a small lake of Rwanda (East Africa) was studied from 1986 to 1990. A dramatic decrease of the catch of Oreochromis niloticus (350 T y−1 in the fifties vs 30 T y−1 in 1982) suggested a loss of productivity or overfishing. In the same period, other ecological changes occurred: the submerged macrophytes regressed and there was a decrease in Secchi depth (0.65 m in 1987 vs 1.5 m in the fifties). Compared to other lakes of the same area, the plankton production seemed low. The results of the present study characterize lake Muhazi as a shallow lake with a rather unstable diurnal stratification and with slight differences in mixing regime between its eastern, deepest part and its western, shallowest part. Secchi disk depth does not vary seasonally to a large extent. The water has a rather high mineral content (conductivity of about 500 µS cm−1 at 25 °C) and low concentrations of dissolved N and P, except in the hypolimnion, where NHinf4sup+-N can be high.Two species, Microcystis aeruginosa and Ceratium hirundinella, account for most of the phytoplankton biomass, which is about 50–80 mg chlorophyll a m−2 in the euphotic zone, usually with little seasonal variation. Daily gross production estimates amount to about 6 to 9.5 g O2 m−2 d−1 with a significant difference between the two parts of the lake. Data on C:N and C:P ratio in the phytoplankton suggest that some N deficiency might occur in the eastern part. Moreover, the Zm:Zc ratio could also lead to rather low net production rates (0.21–0.25 d−1 for a mixed layer of 4 m)In conclusion, the primary production of lake Muhazi is medium for African lakes and the hypothesis that decreased planktonic production could account for a reduced fish production should be discarded. Whereas the present yield of the fishery is only 20 kg ha−1 y−1, the yield estimated from primary production ranges between 46 and 64 kg ha−1 y−1. This could be reached through proper management. Finally, some hypotheses are given to explain the ecological changes which occurred in the lake.