Effect of continuous nutrient enrichment on microalgae colonizing hard substrates

In order to understand the effect of changing nutrient conditions on benthi c microalgae on hard substrates, in-situ experiments with artificial substr ates were conducted in Kiel Fjord, Western Baltic Sea. As an extension of p revious investigations, we used artificial substrates without silicate and thus were able to supply nutrient media with different Si:N ratios to porou s substrates, from where they trickled out continuously. The biofilm develo ping on these substrates showed a significant increase in biovolume due to N + P enrichment, while Si alone had only minor effects. The stoichiometric composition of the biomass indicated nitrogen limitation during most of th e year. The C:N ratios were lowered by the N + P addition. The algae were d ominated by diatoms in most cases, but rhodophytes and chlorophytes also be came important. The nutrient treatment affected the taxonomic composition m ostly at the species level. The significance of the results with regard to coastal eutrophication is discussed.