Dynamic response of a mud snail Nassarius sinusigerus to changes in sediment biogeochemistry

Abstract

It has been widely documented that point sources of organic enrichment elicit abun- dance peaks in the distribution of benthic opportunistic species. However, the temporal dimension of such phenomena has not been examined. In this study, we explored the dynamic relationship between the seasonal distribution pattern of the mud snail Nassarius sinusigerus and the geochemi- cal conditions of the sediment along an organic enrichment gradient adjacent to a commercial fish farm. Cluster analysis of total dissolved sulfides (TDS), dissolved oxygen (DO) and organic matter (OM) revealed 3 distinct groups: 'highly', 'moderately' and 'slightly' impacted sediments. The sea- sonal distribution of N. sinusigerus along a transect away from the fish farm indicated that a peak in snail abundance was associated with the moderately impacted sediments, occurred between 20 and 80 m from the center of the farm, and corresponded to (DO) > 0.3 ppm and (hydrogen sulfide) < 1 µM. When the sediment conditions near the farm deteriorated during summer, the peak in snail abun- dance shifted away from the farm. An improvement in sediment conditions during the following winter enabled the migration of N. sinusigerus toward the farm. The factor drawing N. sinusigerus toward the farm was probably food availability, as suggested by the strong attraction of the snails to the sediments and to annelids below the fish cages. This study suggests that the distribution of N. sinusigerus around fish farms is determined by the balance between the attraction of the gastropod to the organically enriched sediments below the fish cages and deterrence due to deleterious sediment geochemistry (mainly anoxia and sulfides). As such, the distribution of N. sinusigerus may serve as an indicator of sediment conditions.