Long-term changes in macroalgal communities in a Danish estuary

Long-term studies of the composition of marine macroalgal communities are r are because of the lack of quantitative investigations in the past. In Denm ark, a single quantitative study was performed in 1941-1943 in the nontidal estuary Isefjord-Roskilde Fjord, at several sites of variable salinity (10 -22 parts per thousand) and nutrient availability. This study was used as a reference for evaluating changes in the richness and relative abundance of macroalgal species from different taxonomic groups and functional forms at sites experiencing increasing concentrations of nitrate and light attenuat ion during the 50 years elapsed since then. Qualitative measures showed no, or few, changes over the period. In contrast, quantitative measures showed a significant increase in the relative abundance of small, opportunistic g reen algae (e.g. species of Chaetomorpha, Cladophora, Enteromorpha and Ulva ) and a significant decrease in the relative abundance of large, perennial brown algae (e.g. Fucus serratus and F. vesiculosus). Along with these chan ges, the diversity of functional form groups declined. The inner parts of R oskilde Fjord, which were already nutrient-rich 50 years ago, did not show any significant changes in the composition of the macroalgal communities. T he temporal changes during the 50 years are consistent with the spatial dif ferences observed both then and now along the nutrient gradients. The resul ts imply that quantitative measures of the abundance of macroalgal species having different thallus form, longevity and taxonomy are more sensitive an d robust than qualitative measures in reflecting changes in the communities brought about by cultural eutrophication. The response of macroalgal commu nities gradually diminishes with progressively higher nutrient loadings, su ggesting that the impact of eutrophication on coastal macroalgae will follo w a geometric series similar to that established for lakes of different tro phic levels.