Long-term changes in macroalgal communities in a Danish estuary

Abstract

Long-term studies of the composition of marine macroalgal communities are rare because of the lack of quantitative investigations in the past. In Denmark, a single quantitative study was performed in 1941–1943 in the nontidal estuary Isefjord–Roskilde Fjord, at several sites of variable salinity (10–22 ‰) 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 attenuation 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 green 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 changes, the diversity of functional form groups declined. The inner parts of Roskilde Fjord, which were already nutrient-rich 50 years ago, did not show any significant changes in the composition of the macroalgal communities. The temporal changes during the 50 years are consistent with the spatial differences observed both then and now along the nutrient gradients. The results imply that quantitative measures of the abundance of macroalgal species having different thallus form, longevity and taxonomy are more sensitive and robust than qualitative measures in reflecting changes in the communities brought about by cultural eutrophication. The response of macroalgal communities gradually diminishes with progressively higher nutrient loadings, suggesting that the impact of eutrophication on coastal macroalgae will follow a geometric series similar to that established for lakes of different trophic levels.