Seasonal variation in food utilization by the suspension-feeding bivalve molluscs Mytilus edulis and Placopecten magellanicus

Seston utilization by adult Mytilus edulis and Placopecten magellanicus coh orts was measured using an in situ method over a total period of 139 d duri ng the spring, summer and fall of 1995 in Bedford Basin and Mahone Bay, Nov a Scotia, Canada. Daily seston utilization measurements were combined with extensive water sampling to construct predictive empirical models of bivalv e ingestion based on environmental variables. Particle concentrations were highest in May in Bedford Basin (similar to 5 mg 1(-1)) and remained below 2 mg 1(-1) in Mahone Bay. Seston quality during the study varied between 30 (summer) and 90% (spring) organic content. Large seasonal changes in the r ates and efficiencies of feeding and absorption were observed, but only 28% of the variance in daily ingestion rates of both species could be explaine d by a wide range of potential environmental influences (temperature, sesto n abundance and composition and vertical particle flux). Ingestion and abso rption rates of scallops and mussels were highest during the spring, when d iet quantity and quality were high, and during late autumn, when quantity a nd quality were low. These data indicate that changes in seston utilization and related growth were not caused solely by seasonal food and temperature fluctuations, but imply phys iological regulation of feeding and digestion . Both species displayed a large capacity for controlling clearance and abs orption rates. Clearance rates during October and November were at least tw ice as high as observed at other times of the year, and absorption efficien cy gradually decreased at high diet quality and increased when quality was low. Temporal variations in food utilization by both species may be explain ed by the combined constraints on maximizing net energy gain of relatively low food availability and the seasonally changing energy demands of reprodu ction. The accuracy of various bivalve clearance (filtration) rate models w as assessed by comparing predicted responses with average in situ clearance rate estimates. Only those models based on natural seston rations provided adequate predictions of observed clearance behaviour. Clearance rate predi ctions based on algal cell rations greatly overestimated in situ clearance at all times of the year and appear to be of limited application for predic ting feeding activity in nature. Current theories on the ecological role of bivalve communities in coastal regions are questionable as they commonly d epend on the assumption that clearance capacity is fully exploited.