Marine microbenthic community structure regulated by nitrogen loading and grazing pressure

Generalisations on the combined effects of consumers and resources on autot rophs in aquatic food webs largely rely on freshwater studies. In this stud y, we tested these general concepts with marine benthic microalgae, which a re important components of coastal food webs. We manipulated nitrogen avail ability and herbivore presence in a factorial field experiment in the Weste rn Baltic Sea. Moreover, we investigated how herbivore control varied among 3 sites and 2 seasons and tested for trophic cascades by enhancing demersa l fish density at 2 sites. Nitrogen availability and herbivore presence had strong and antagonistic effects on microalgal biomass, species composition and diversity. Herbivores significantly reduced algal biomass, whereas nut rient enrichment led to an increase in biomass. Herbivore effects on microa lgal biomass increased with increasing nitrogen availability, indicating a functional response of herbivores to nutrient enrichment. The response of m icroalgae at the species level suggested a trade-off between nutrient use a nd grazing resistance which appeared to be Linked to algal growth form. Com pared to other growth forms, large erect species were most responsive to bo th nitrogen loading and herbivory. Grazing reduced microalgal diversity at low nutrient supply, but enhanced it at high nutrient supply. Herbivore eff ects varied considerably among different sites and were stronger in spring than in summer. Manipulations of fish density during summer did not have an y effects on microalgal community structure. In conclusion, our results dem onstrate that herbivores and nutrients have strong and balancing effects on marine microbenthic community structure.