Benthic animals

Benthic invertebrates are macroscopic animals that live at or near the sediment/water interface. Some benthic invertebrates, particnlarly mnssels, readily accnmnlate metals, prompting their use as biological indicators of mercnry contamination (Smith... 

It is emphasized that Cr+3, probably because of its very low solubility in seawater, appears to have a much lower bioavailability to most groups of marine animals than Ct 6, which is more water soluble (Carr et al. 1982). The clam Rangia cuneata appears to be an exception it accumulated up to 19 mg Cr/kg in soft parts, on a dry weight basis, during exposure for 16 days to chromium-contaminated muds, and retained most of it for an extended period the estimated biological half-time was 11 days (Carr et al. 1982). In general, benthic invertebrates rarely accumulate chromium from contaminated sediments (82 to 188 mg Cr+Vkg) only a few examples have been recorded (Neff etal. 1978). 

Ozretic, B., M. Krajinovic-Ozretic, J. Santin, B. Medjugorac, and M. Kras. 1990. As, Cd, Pd, and Hg in benthic animals from the Kvarner-Rijeka Bay region, Yugoslavia. Mar. Pollut. Bull. 21 595-597. 

The first consideration was the speciation and distribution of the metal in the sediment and water. Benthic organisms are exposed to surface water, pore water and sediment via the epidermis and/or the alimentary tract. Common binding sites for the metals in the sediment are iron and manganese oxides, clays, silica often with a coating of organic carbon that usually accounts for ca. 2% w/w. In a reducing environment contaminant metals will be precipitated as their sulfides. There is not necessarily a direct relationship between bioavailability and bioaccumulation, as digestion affects the availability and transport of the metals in animals, in ways that differ from those in plants. 

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. 

Wegmann and Hofstee [43] have developed a capillary gas chromatographic method for the determination of organochlorine insecticides in river sediments. Bottom soils from rivers, collected in slow current areas may contain high concentrations of organochlorine insecticides and polychlorobiphenyls. When the current moves more rapidly or benthic animals become more active, these compounds are stirred into the water along with suspended particles and become accessible to organisms that live in the bottom layer. 

The kinetics of desorption control bioaccumulation of historical (e.g., aged) contamination (e.g., PAHs in benthic animals [225]) and historically contaminated soils are less toxic and/or lead to lower body burdens than equivalent amounts of spiked soils [226,227]. 

A particularly important consequence of bioirrigation and bioturbation is the introduction of relatively 02-rich bottom water into the sediments. This enhancement in O2 supply is analogous to the aeration of soil by earthworms. Bioturbation can occur as deeply as 1 m below the sediment surface, but is most intense in the top 10 cm. The depth of O2 penetration is also strongly influenced by the flux of sedimenting POM. High accumulation rates of organic-rich particles can fuel bacterially mediated aerobic respiration supporting rates of O2 removal that exceed the benthic animals abilities to reaerate the sediments. In this case, anoxic conditions result. Since animals require O2, bioturbation does not occur in anoxic sediments. Thus, the effects of bioturbation are limited to the oxic portion of the sediments. 

In the surface sediments, the physical processes of compaction, bioirrigation, and bioturbation also influence downcore concentration profiles. The concentration minima and maxima tend to be broadened by the action of benthic animals, whereas compaction has a sharpening effect on the depth profiles of the solids. 

The tendency of a marine organism to bioaccumulate a toxic metal depends in part on its lifestyle, as this determines the degree to which it is exposed to elevated concentrations in seawater or the sediments. For example, benthic animals that burrow will be exposed to sediment pore waters rather than to the overlying seawater. Animals that use mucous feeding nets, such as larvaceans, will be more prone to sorb metals because of the large surfece area of their nets. 

Bioirrigation The movement of water through marine sediments caused by benthic animals. 

Macrofauna Benthic animals that are larger than 0.5 mm. 

Other animals, sponges lack a nervous system and have no true musculature. They are benthic and filter food particles suspended in the water. They have no specialized organ systems, often they are amorphous and asymmetrical animals. Only a few different cell types are encountered within sponges which are functionally independent to the extent that an entire sponge can be dissociated into its constituent cells. Special flagellated cells called choanocytes generate currents that help maintain water circulation within the sponge and capture food particles. 

Non-motile stages were extremely rare hence significant reduction in . catenella densities by settling was unlikely. Although many known and suspected predator species were present during the bloom, no data is available which would indicate the extent to which predation by zooplankton and benthic animals may have reduced the . catenella population. 

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