Abstract :
Spatio-temporal variation in the supply of suspended chlorophyllain the Eel River, Nova Scotia, Canada was examined in relation to the feeding regime of bivalve molluscs. Estuarine circulation patterns were deduced from semi-synoptic profiles of tidal height, salinity and chlorophyllaconcentration. Steep horizontal gradients in salinity and chlorophyllawere found to be generated from tidally-driven circulation and autochthonous algal production. Measurements of the flux of chlorophyllathrough the mouth of the Eel River demonstrated a substantially greater supply of material past sedentary organisms during ebb than during flood flow. A large net export of chlorophyllafrom the estuary to the near-shore zone occurred during all tidal cycles. These spatio-temporal gradients in food supply resulted in large quantitative differences in ration to benthic organisms living at different sites and tidal elevations within the estuary. To investigate the effect of this variation in food supply on possible seston depletion, a simple depletion model was developed and the results compared to a field experiment. The model links physical advection to biological filtering, and the results suggest that, during half of the tidal cycle, the retention of water over the bivalves was shorter than that required for clearance of a significant fraction of the suspended load by the filter-feeders. Relatively large depletion of seston was likely to occur only during low volume transport at low water, at which time bivalves were potentially food-limited. In the field experiment, measurements of algal concentration and flux were made over a dense bed of the infaunal bivalveMya arenaria( 103 ind. m−2, 240 g ADW m−2). Evidence supporting depletion was found in only one of seven comparisons of algal concentration before and after passage over the bivalve bed. The clams did not measurably affect the flux of chlorophylla. Depletion was not observed during two of three periods of favorable conditions, a result which may be explained by bivalve feeding behavior. In this small estuary, spatio-temporal variation in food resources, generated by hydrodynamic forcing, the source of algal production and the location of bivalves, results in a large range of feeding regimes within a single system.
