The impact of biotic interactions (competition and herbivory) on the structure of marine periphytic communities on hard substrates was analyzed with in situ and laboratory experiments. The in situ nutrient enrichment experiments in Kiel Fjord (Baltic Sea) revealed two important aspects: the presence of nutrient limitation and the competitive shifts in dominance of algal species due to changed nutrient supply. Nutrient limitation was indicated by the C:N:P ratios of benthic microalgae and by the increased biovolume of benthic microalgae following the experimental nutrient enrichment. The limitation was characterized by a phase of P-limitation in spring and a major influence of N-limitation from late spring to autumn. For diatoms, an Si-limitation was also indicated. Limitation occured although the nutrient pool in the water column was never totally depleted, indicating a limited access of periphytic algae to nutrients from the overlying water column. The nutrient-limited situation resulted in shifts in the competitive dominance due to the nutrient supply. On higher taxonomic level, a consistent dominance of diatoms was evident, which was overcome only in summer, mainly by filamentous red algae. Within the diatoms, a distinct separation was observed between species profiting from N-addition (Melosira moniliformis, Tabularia fasciculata, Berkeleya rutilans), those profiting from P-addition (Proschkinia complanata) and those most abundant in unenriched (Licmophora sp.) and low N-enriched (Melosira nummuloides, Haslea crucigera) treatments. Mainly species of erect growth form profited from the nutrient enrichment. Moreover, the nutrient supply resulted in a decrease of diversity of the microbenthic community, based on the increased dominance of single species (i.e. reduced evenness), whereas species numbers were constant within experiments. The response of biomass stoichiometry of benthic microalgae to nutrient limitation was further analyzed in laboratory experiments with natural species assemblages. Microalgae with optimal growth rates showed a C:N:P ratio of 119:17:1, which is close to the Redfield ratio. C:N ratios increased with increasing nutrient limitation irrespective of the limiting nutrient, whereas C:P ratios increased only under P-limitation. The N:P ratio decreased under N-limitation and increased under P-limitation. C:N:P ratios are thus a useful tool to determine nutrient limitation in benthic microalgae. The combined influence of herbivory and nutrient supply was investigated in field experiments, conducted in shallow embayments of the Western Baltic Sea (Maasholmer Breite, Geltinger Noor and Wackerballig). In these experiments, the natural herbivore population was excluded by a cage with 1 mm mesh size, and nutrients were added in one grazing experiment by slow release fertilizers, enhancing the ambient N concentration significantly. Strong and antagonistic effects of both factors were observed: Nutrient enrichment led to increased microalgal biovolume and favoured mainly large, erect species. Herbivores (mainly gastropods) removed substantial parts of the algal biovolume and reduced especially these erect species. Thus, herbivores were more effective under enriched conditions, since they had more access to loosely attached species. Highest microalgal biovolume was found in ungrazed and enriched treatments. The microalgae faced a trade-off between access to water column nutrients (erect growing, but loosely attached species) and mechanical resistance (adnately growing species). Species richness was lowered by the herbivores, whereas the diversity of benthic microalgae was unimodally related to enrichment in the grazed communities. The monopolizing effect of nutrient enrichment and the counteracting effect of herbicory seems to be an universal pattern for plant communities. The comparison of the results for different sites and different parts of the year showed a high temporal and spatial variability of herbivore control on benthic microalgae. In conclusion, these experimental results revealed a pattern of interacting biotic forces structuring the periphyton in terms of microalgal biomass, biomass stoichiometry, species composition and diversity.