Seasonal utilization of different seston carbon sources by the ribbed mussel, Geukensia demissa (Dillwyn) in a mid-Atlantic salt marsh.

Seston in salt marshes contains a temporally and spatially complex mixture of natural microparticulate organic material, including phytoplankton, vascular plant detritus, bacteria, heterotrophic nanoflagellates and benthic diatoms. Quantitative information is available concerning how suspension-feeding consumers, such as the ribbed mussel, Geukensia demissa (Dillwyn), utilize some of these components to satisfy their carbon demands. Despite this information there is still a limited understanding of how the relative nutritive contribution of these different dietary items may shift during the year associated with variations in both seston composition and the mussel's physiological condition. To investigate if the mussel's ability to use specific constituents of natural seston varies seasonally, we ran a series of pulse-chase 14C feeding experiments under ambient conditions in March, May, August and November 1996. Phytoplankton, cellulosic detritus, bacteria, heterotrophic nanoflagellates and benthic diatoms were radiolabeled and supplemented in small amounts to natural marsh water for feeding to mussels. The fate of 14C in mussel tissues, feces, respiration and excretion was quantified and contrasted among the different diet types and seasons. Microcapsules containing radiolabeled carbohydrate and protein were used as standards to differentiate possible between-experiment variations in seston composition from seasonal changes in the mussel's feeding and digestive physiology. Mussel clearance rates for all diets were highest in summer and autumn and lowest in winter and spring. In contrast, seasonal shifts in digestive physiology were only found for certain diets. The seasonal range of assimilation efficiencies for microcapsule standards (18-29%) and field-collected microheterotrophs (bacteria 76-93% and heterotrophic nanoflagellates 87-94%) did not differ significantly during the year, whereas summer and autumn assimilation efficiencies for cellulosic detritus (22-24%), phytoplankton (71-79%) and benthic diatoms (89-93%) were up to twofold greater than those in winter and spring (13%, 40-59% and 45-81%, respectively). We conclude that the digestive physiology (e.g., digestive enzyme production) of mussels responds to shifts in dietary components during the year.