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, vasc ular plant detritus, bacteria, heterotrophic nanoflagellates and benthic di atoms. Quantitative information is available concerning how suspension-feed ing consumers, such as the ribbed mussel, Geukensia demissa (Dillwyn), util ize some of these components to satisfy their carbon demands. Despite this information there is still a limited understanding of how the relative nutr itive contribution of these different dietary items may shift during the ye ar associated with variations in both seston composition and the mussel's p hysiological condition. To investigate if the mussel's ability to use speci fic constituents of natural seston varies seasonally, we ran a series of pu lse-chase C-14 feeding experiments under ambient conditions in March, May, August and November 1996. Phytoplankton, cellulosic detritus, bacteria, het erotrophic nanoflagellates and benthic diatoms were radiolabeled and supple mented in small amounts to natural marsh water for feeding to mussels. The fate of C-14 in mussel tissues, feces, respiration and excretion was quanti fied and contrasted among the different diet types and seasons. Microcapsul es containing radiolabeled carbohydrate and protein were used as standards to differentiate possible between-experiment variations in seston compositi on 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 phys iology were only found for certain diets. The seasonal range of assimilatio n efficiencies for microcapsule standards (18-29%) and field-collected micr oheterotrophs (bacteria76-93% and heterotrophic nanoflagellates 87-94%) did not differ significantly during the year, whereas summer and autumn assimi lation efficiencies for cellulosic detritus (22-24%), phytoplankton (71-79% ) and benthic diatoms (89-93%) were up to twofold greater than those in win ter and spring (13%, 40-59% and 45-81%, respectively). We conclude that the digestive physiology (e.g., digestive enzyme production) of mussels respon ds to shifts in dietary components during the year. (C) 2001 Elsevier Scien ce B.V. All rights reserved.