Animals living in marine sediments (the second largest habitat on earth) play a major role in global biogeochemical cycling. By feeding on organic matter from settled phytoplankton blooms they produce food for higher trophic levels and nutrients that can fuel primary production. In the Baltic Sea, anthropogenic stresses, such as eutrophication and introductions of invasive species, have altered phytoplankton dynamics and benthic communities. This thesis discusses the effects of different types of phytoplankton on the deposit-feeding community and the importance of benthic biodiversity for fate of the phytoplankton bloom-derived organic matter. Deposit-feeders survived and fed on settled cyanobacterial bloom material and in doing so accumulated the cyanobacterial toxin nodularin. Their growth after feeding on cyanobacteria was much slower than on a diet of spring bloom diatoms. The results show that settling blooms of cyanobacteria are used as food without obvious toxic effects, although they do not sustain rapid growth of the fauna. Since all tested species accumulated the cyanotoxin, negative effects higher up in the food web can not be ruled out. Both species composition and richness of deposit-feeding macrofauna influenced how much of the phytoplankton bloom material that was incorporated in fauna or retained in the sediment. The mechanism behind the positive effect of species richness was mainly niche differentiation among functionally different species, resulting in a more efficient utilization of resources at greater biodiversity. This was observed even after addition of an invasive polychaete species. Hence, species loss can be expected to affect benthic productivity negatively. In conclusion, efficiency in organic matter processing depends both on pelagic phytoplankton quality and benthic community composition and species richness.