Benthic molluscs

Fig. 4.5. Distribution of early middle Miocene benthic molluscs and paleoposition of Japan as restored on the basis of paleomagnetism. Abbreviations w., water assoc., association (Chinzei, 1991).

The zoogeographic history of the Japan Sea proposed by Chinzei (1991) (Fig. 4.5 and Fig. 4.6) indicated that (1) from 17 to 15 Ma, before the spreading of the Japan Sea, tropical and subtropical molluscs invaded the area (2) the benthic and planktonic faunas changed sharply into low temperature faunas at about 15 Ma (3) from about 10 to 5 Ma, sedimentary deposits formed that were barren of benthic molluscs as a result of the stagnation of the Japan Sea basin (4) cold-water molluscan fauna reappeared in the coastal areas at about 5 Ma and (5) this has been followed by a period of marked cyclicity of cold and warm water faunas since about 1.3 Ma. 

Fig. 4.6. Distribution of benthic molluscs associations during the middle Miocene and presumed paleogeogra-phy of the Japan Sea (Chinzei, 1991).

The high bioavailability of Cd in the marine environment from Terra Nova Bay was ascribed to upwelling of deep nutrient rich water nearshore (70). Cadmiun has a nutrient-like concentration gradient in the water column, lower close to the surface and increasing with depth (75). In the water column, Cd would be absorbed by the phytoplankton, during the summer bloom, thus becoming available for benthic molluscs in which the Cd is bioaccumulated. Ecological studies undertaken in the inner and outer part of Potter Cove showed that after seasonal ice retreat no phytoplankton bloom occured (35). In spite of the low planktonic production, rich benthic animal comunities are found in the area (29). 

Tributyltin antifouling paints were banned for use in tin-based paints in the U.K. in 1987. Field work on the River Crouch Estuary showed that sediments from areas most contaminated with TBT in 1987 contained 0.16 mg TBT/kg DW however, by 1992 this had declined to 0.02 mg TBT/kg DW. TBT declines were accompanied by increases in abundance and diversity of benthic fauna, especially bivalve molluscs and amphipod crustaceans (Rees et al. 1999 Waldock et al. 1999). 

The bioavailability of selenium to a benthic deposit-feeding bivalve, Macoma balthica from particulate and dissolved phases was determined from AE data. The selenium concentration in the animals collected from San Francisco Bay was very close to that predicted by a model based on the laboratory AE studies of radiolabelled selenium from both particulate and solute sources. Uptake was found to be largely derived from particulate material [93]. The selenium occurs as selenite in the dissolved phase, and is taken up linearly with concentration. However, the particle-associated selenium as organoselenium and even elemental selenium is accumulated at much higher levels. The efficiency of uptake from the sediment of particulate radiolabelled selenium was 22%. This contrasts with an absorption efficiency of ca. 86% of organoselenium when this was fed as diatoms - the major food source of the clam. The experiments demonstrated the importance of particles in the uptake of pollutants and their transfer through the food web to molluscs, but the mode of assimilation was not discussed. 

Pathways 1-4 are known in various bivalve molluscs 2, 3, 8, and 9 are often utilized by helminths 5, 6, and 7, while theoretically possible in bivalve molluscs, do not appear to be utilized to any significant extent. Pathway 10, or sulfate oxidation of organic substrates, is well known to occur in the highly reduced layers of benthic silt, but the distribution of this activity between bacteria and lower invertebrate ani-... 

Everaarts ° detected the presence of PCBs and cyclic pesticides in benthic organisms from the Kenyan coast. They reported higher concentrations of PCB congeners and cyclic pesticides at the mouth of Sabaki River than the mouth of Tana River. The bivalve molluscs from the mouth of Sabaki River and Kiwaya Bay had the highest levels of PCBs. Residues of p,p -DDE were detected in all the samples at levels ranging from 15 to 48 ng/g of lipid in both bivalve and gastropod molluscs. 

The next groups were recorded downstream from Novi Sad Oligochaeta (four species), Bivalvia (one species). Gastropoda (one species) and Chironomidae. The density of the community was 7,020 Ind. m . The oligochaetes were dominant, L. hoffmeisteri being the most abundant. Benthic samples taken at this location contained a lot of empty molluscs shells. 

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