Differences of u p to 5 % in bI3C and b15N were consistently observed across a size spectrum (< 3 to > 250 pm) of suspended particulate organic material (SPOM) sampled from a coastal Mediterranian site over a 10 mo period. On each of 4 sampling dates, lowest values of b13C and 615N (to -25.3 and -0.5 %o, respectively) were always found in size fractions < 8 pm, with higher values in larger particles Relationships among SPOM 6I3c, h'", and C/N suggest that isotope fractionation associated with plankton C and N cycling is responsible for the isotop~c var~ations observed. The relative I3C and 15N depletions present in the sn~allest but most abundant SPOM size fractions are consistent with the view that nano-and picoplankton constitute a sizable, low-trophic-level component of marine SPOM.
Particulate material collected in sediment traps at 80 m depth in coastal northwestern Mediterranean waters, and fresh fecal pellets of copepods harvested in the surrounding water, were examined for the presence and activity of ammonia-oxidizing and methanogenic bacteria. Results provide the first evidence for the coexistence of living and active cells of methanogenic and nitrifying bacteria in either fresh zooplankton fecal pellets or large settling particles containing numerous large size fecal pellets. In addition, the presence of these 2 bacterial types in both copepods and the fecal pellets produced by them suggests that the bacteria probably originate from the digestive tract of zooplankton, most likely as ingested species for nitrifiers and enteric species for methanogens.
Laboratory experiments employing gamma-emitting radiotracers assessed the retention efficiency of ingested metals in the calanoid copepod Anomalocera patersoni and the retention of excreted metals in fecal pellets. Adult copepods ingested the haptophyte Isochrysis galbana labeled with Io9cd , "Z n, 2 0 3~g , " Â ¥ ' "~ and '-"Pu at a rate of 3.79 mg algal C g-I ammal dry wt h-I. Average retention efficiencies were 30 % for Cd, 48 % for Zn, 21 O/O for Hg, 4.5 % for Am, and 0.8 % for Pu. Algal cells resuspended into unlabeled seawater retained metals to varying extents, with depuration curves conforming to a 2-compartment model; Cd was lost most rapidly. The fecal pellets produced by animals feeding on radioactive I. galbana lost essentially all of their l o g c d , "~n , ~m , and 2 3 7~u to unlabeled seawater within 1 d. ^Hg was lost most slowly, again conforming to a 2-compartment model; its retention half time was about 25 d in the slowly exchanging pool. Bacterial activity did not appear to have an effect on metal retention in the fecal pellets. The retention efficiencies suggest that Cd, Zn, and Hg should be recycled by copepods in surface waters as part of the organic cycle in the sea, while the transuranic elements should be defecated and removed from surface waters by sinking biogemc debris.
Analyses of pesticide residues in sediments, water and biota of the Altata-Ensenada del Pabellon coastal lagoon system in Sinaloa, Mexico, showed the presence of organochlorine and organophosphorus compounds. For all the compounds analysed, concentrations in sediments were higher near the points of water discharge from ponds and drainage from the agricultural area. Among the organochlorines, total DDTs displayed the largest sedimentary reservoir, followed by total endosulfans and chlorpyrifos. In sediments, as well as in biota, pp'-DDT concentrations were lower than those of DDT metabolites, which confirms the reduction in the previous massive use of this compound in agriculture. Endosulfan is currently used in the region and endosulfan residues in lagoon sediments attained levels considered to be toxic to meiofauna, therefore constituting an ecological risk to lagoon ecosystems. There was a large sedimentary reservoir of chlorpyrifos but its ecotoxicological risk is difficult to assess due to lack of adequate comparative data. Nevertheless, concentrations of chlorpyrifos which approach acute toxic levels for shrimp were recorded in lagoon water. This suggests that drainage from agricultural fields during high runoff may, on occasion, cause mass mortality of shrimp and fish Organophosphorus pesticides are widely regarded to degrade very rapidly in aquatic systems. Experimental research performed with 14C-labelled chlorpyrifos and parathion has shown that they may be stabilised for relatively long periods of time through sediment-water partitioning. This extension of their environmental half-lives increases their potential for impacting on coastal ecosystems. Since organophosphorus pesticides are highly toxic for aquatic organisms at concentrations generally lower than organochlorines, their presence in the lagoon water and sediment is a matter for much concern. The increased use of tropical coastal lagoons for shrimp and fish farming requires the implementation of environmental management practices to protect these ecosystems from the impact of agricultural activities. Environmental management aims to preserve the ecosystem integrity of these coastal lagoons and, through improving the water quality, to allow the development of aquaculture and reduce human exposure to pesticide residues in food.
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