Phytoplankton food quality effects on gammarids: benthic-pelagic coupling mediated by an invasive freshwater clam

Benthic-pelagic coupling mediated by bivalves has been shown to increase the flow of energy towards the benthos. To assess the capability of clams to process and therewith modify the quality of pelagic food sources for subsequent use by benthic invertebrates, we conducted a growth experiment in which juvenile Gammarus roeselii were raised either directly on sedimented pelagic autotrophs (algae, cyanobacteria) or on the same autotrophs biodeposited by the invasive freshwater clam Corbicula fluminea either as feces or pseudofeces. We show that growth and survival of G. roeselii are significantly improved when autotrophs are offered as biodeposition material and suggest that this clam-mediated upgrading of food quality is due to both an increased bioavailability of pelagic food particles, which are packed in mucus during clam processing, and an increased dietary provisioning with essential lipids (sterols and (or) polyunsaturated fatty acids) originating from the clams. Hence, filter-feeding bivalves provide a crucial link between the pelagic and benthic food web not only by deflecting energy fluxes, but also by processing and upgrading pelagic food (increased bioavailability, improved biochemical composition) for benthic invertebrates.