Dense beds of suspension-feeding brittlestars, in some cases extending over hundreds of square metres of sea floor and containing millions of individuals, represent significant concentrations of biomass on the sea floor. The removal of large amounts of suspended particulate matter from the water column, combined with the output of nitrogenous excretory products, can make an important contribution to the cycling of nutrients in the areas where beds occur. In the Bay of Brest, Brittany, benthic suspension-feeders, numerically dominated by Ophiothrix fragilis (local aggregations over 400 individuals m^-2) can filter the equivalent of 30% of the total water volume of the bay daily (Hily, 1991). The inflow of nutrient-rich stream water into the bay leads to very high primary productivity, but eutrophication does not occur, apparently because of the removal of particulate matter by the benthic community. Davoult et al. (1991) estimated the quantities of ammonium excreted by Ophiothrix beds in the Dover Strait and concluded that the beds made an important contribution to the total flux of ammonium into the water column in the area. This nutrient regeneration is a necessary component of the cycle of phytoplankton production on which most marine life ultimately depends. Further work (Davoult & Gounin, 1995b) showed that rates of ammonium release vary over the year in parallel with changes in the brittlestar biomass and feeding rate. All these parameters increase over the spring to a peak in May/June before declining again.

The possible role of brittlestar beds in other important biogeochemical cycles has been little-studied (Hutchins et al., 1996). Gounin et al. (1995) studied the transfer of heavy metals (iron, manganese, lead, copper and cadmium) through Ophiothrix beds. They concluded that heavy metals ingested or absorbed by the animals transited rapidly through the body and were expelled in the faeces. The brittlestars did not appear to accumulate metals in the tissues and so would not act to decontaminate the near-bottom water mass.

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