Hydrodynamic effects on the origin and quality of organic matter for bivalves: an integrated isotopic, biochemical and transplant study

Different hydrodynamic conditions can affect both the origin and the quality of organic matter available to bivalve molluscs. I chose 2 environments with very different hydrodynamics (a Mediterranean lagoon open to flow and a closed pond), but similar with regard to temperature, salinity, depth, wind exposure and algae coverage, to investigate this. The lagoon was characterised by active flow and bivalve molluscs, whereas the pond was closed off from the lagoon’s main flow but characterised by mussel beds of the highest density ever observed in the western Mediterranean. Biochemical features, 13C and 15N contents of particulate and sedimentary organic matter, and isotopic signatures of dominant organisms, were used as descriptors of the trophodynamics in both systems. In the lagoon, intense lateral drifting forces moved organic matter far from production sites, thereby depleting the water column of resuspended sedimentary organic matter. In the pond, winddriven conditions enhanced the coupling between the water column and the sediments, enriching the pond with resuspended sedimentary products. Integrating results, it was deduced that mussels in the pond could exploit, over time, organic matter produced in loco, continually rearranged in loco by means of wind-driven resuspension and consumed in loco with an efficiency able to sustain a massive secondary production. In contrast, under lagoon conditions, the benthic and pelagic habitats were uncoupled, so that each had its own independent status with its own dynamics and production. This reduced the suitability of this habitat for high levels of secondary production.