Zebra mussels mediate benthic–pelagic coupling by biodeposition and changing detrital stoichiometry

1. The zebra mussel ( Dreissena polymorpha ) is one of the most successful invasive species; it has colonised many aquatic systems in Europe and North America with strong impacts on various ecosystem processes. The effect of D. polymorpha filtration on pelagic seston concentrations has been quantified in several studies, but the magnitude and stoichiometry of the transfer of sestonic biomass into benthic detritus by D. polymorpha and the accompanying enrichment of the benthic habitat is still under-investigated.
2. We studied biodeposition by zebra mussels in two series of laboratory experiments with the food algae Cryptomonas erosa and Scenedesmus obliquus . We also measured the year-round biodeposition rate under natural conditions in the oligotrophic Lake Constance.
3. In all experiments, zebra mussel biodeposition was linearly related to seston concentration. In the field, the relationship changed with a seasonal shift in algal composition and lower biodeposition rates during the spring algal bloom.
4. For both algal species in laboratory experiments, biodeposited material was depleted in phosphorous at an algal concentration ≤0.6 mg ash-free dry mass L⁻¹, but not at higher concentrations. This effect was not observed in the field, probably because of high variation in C : N : P stoichiometry.
5. By mediating the transfer of pelagic resources into the benthos zebra mussels provide a sufficient amount of detritus for benthic invertebrates, especially during summer. Thus, material biodeposited by the mussels might increase benthic secondary production from pelagic resources, and zebra mussels are important mediators of this flux of organic matter from the pelagic zone into the benthos.     

Comparing grazing by Dreissena polymorpha on phytoplankton in the presence of toxic and non-toxic cyanobacteria

SUMMARY 1. The feeding behaviour of the zebra mussel ( Dreissena polymorpha ) was studied in the laboratory on different combinations of food, including a green alga ( Chlamydomonas reinhardtii ) and toxic and non-toxic strains of the cyanobacterium Microcystis aeruginosa .
2. The highest clearance rate of phytoplankton by zebra mussels was found when the mussels were feeding on a mixture of Chlamydomonas and non-toxic Microcystis , the lowest on a mixture of Chlamydomonas and toxic Microcystis .
3. The differences found in the clearance rates between food combinations can be partly explained by the production of pseudofaeces containing live phytoplankton cells. Zebra mussels expelled significantly more live phytoplankton cells in the presence of toxic Microcystis than in the presence of non-toxic Microcystis . The pseudofaeces contained predominantly live Chlamydomonas cells. Proportionately much less live Microcystis cells were encountered in the pseudofaeces.
4. Consequently, grazing of zebra mussels on a combination of Chlamydomonas and Microcystis may finally result in a dominance of Chlamydomonas over Microcystis . The presence of toxic Microcystis may even strengthen this shift.