Responses of epiphytes on eelgrass (Zostera marina L.) to nutrient enrichment

Eutrophication is recognized as an increasing pollution problem in eelgrass (Zostera marina L.) dominated coastal lagoons. It is widely believed that increased nutrient loading produces a shift toward an epiphyte dominated community. Yet, quantitative studies of epiphyte responses to nutrient enrichment have been limited. One of the main reasons is the difficulty of ascertaining the ages of eelgrass leaves. I compared the epiphytes on eelgrass leaves with those on morphologically similar plastic substrates to evaluate the suitability of artificial plants as substitutes for natural eelgrass. The artificial substrate appeared to be a useful tool for the study of epiphyte dynamics.;As parts of three larger mesocosm experiments, I examined the effects of separate and combined nitrogen (N) and phosphorus (P) enrichment, a gradient of enrichment with combined N and P, and relative effects of enrichment with combined N and P and flushing on eelgrass epiphytes. In contrast to the general paradigm, enrichment with separate and combined N and P did not result in greater epiphyte biomass. Epiphytes can develop and maintain a very high biomass even when nutrient inputs are very low. Epiphyte biomass and community composition in mesocosms enriched with P were comparable to those in the unenriched mesocosms. However, fertilization with N increased the relative abundance of autotrophic epiphytes. Different forms of inorganic N appeared to make little difference in epiphyte responses when added in combination with P.;No clear gradient in epiphyte biomass, accumulation rate, or nutrient content was found to correspond with the gradient of combined N and P. However, filamentous green algae and red algae constituted large percentages of the epiphyte community at higher levels of nutrient enrichment.;Slower lagoon flushing rates stimulated total and autotrophic epiphyte biomass in unenriched and enriched mesocosms. Combined N and P enrichment did not result in higher total epiphyte biomass and richer nutrient content at different levels of flushing, but did increase autotrophic epiphyte biomass. There was a marked positive relationship between mean autotrophic epiphyte biomass and nutrient input normalized for flushing rate, and there may have been important interactions between flushing rate and grazing pressure on the epiphytes.