BIODEPOSITION BY THE HORSE MUSSEL MODIOLUS-MODIOLUS (DILLWYN) DURING THE SPRING DIATOM BLOOM

The production and nutritional quality of faeces and pseudofaeces from the horse mussel Modiolus modiolus (Dillwyn) was measured during the spring diatom bloom in Logy Bay, Newfoundland. Chloropigments, organic carbon, organic nitrogen and biogenic silica were determined in faece s and pseudofaeces, and compared with values from seston samples. The biodeposition rate of a horse mussel (faeces and pseudofaeces) of 5 g dry meat weight varied from 40.9 mg dry weight . day(-1) at the peak o f the bloom to 4-8 mg . day(-1) at the end. The concentrations of orga nic carbon and organic nitrogen were lower in the faeces and pseudofae ces than in the seston, but chlorophyll a levels were greater in the p seudofaeces than in the seston or faeces. In terms of dry weight, faec es production was five or more times greater than pseudofaeces product ion, but the latter were more important in recycling chlorophyll a, es pecially during the peak of the diatom bloom. Microscopic analysis of the biodeposits showed that chains and large diatoms were very abundan t in the pseudofaeces, whereas the faeces contained only frustules of small cells and of short chains, indicating that M. modiolus concentra tes the large diatoms contained in the seston and rejects them in the pseudofaeces. The rejection of organic-rich diatoms by M. modiolus dur ing the spring bloom provides a mechanism for the recycling of nutrien ts to suspension-feeders and deposit-feeders. Copyright (C) 1997 Elsev ier Science B.V.