In the 1980s, Danish coastal waters suffered from eutrophication and several nutrient management plans have been implemented during the years to improve ecological status. This study aims at giving a holistic ecosystem perspective on 25 years of mitigation measures. We report trends of nutrient inputs and the responses to these in various chemical and biological components. Nutrient inputs from land were reduced by~50 % for nitrogen (N) and 56 % for phosphorus (P) since 1990. These reductions resulted in significant and parallel declines in nutrient concentrations, and initiated a shift in the dominance of primary producers towards reduced phytoplankton biomass (chlorophyll a concentration) and increased cover of macroalgae in deeper waters. In the last 5 years, eelgrass meadows have also expanded towards deeper waters, in response to improving water clarity. An expected improvement of bottom water oxygen conditions has not been observed, presumably because more frequent stratification and higher water temperatures have counteracted the expected positive effects of reduced nutrient inputs. The biomass of the benthic macrofauna decreased as expected, but it was composed of a drastic decline of filter feeders paralleled by a more moderate increase of deposit feeders. This shift was most likely induced by increasing stratification. The reduced benthic filtration along with the limited eelgrass cover probably kept relatively more particles in suspension, which can explain why improvements in the Secchi depths were modest. Overall, several ecosystem components demonstrated clear signs of improvement, suggesting that at least partial recovery is attainable. On this basis, we propose a conceptual scheme for recovery of shallow coastal ecosystems following marked reductions in nutrient inputs.

No abstract

Stony reefs in shallow water support abundant and species-rich animal communities, and may thus attract top predators such as the harbour porpoise. In summer 2008, the nature restoration project Blue Reef re-established 45 000 m 2 of cavernous stony reef at Laesø Trindel in the northern Kattegat, Denmark. To investigate whether the re-established reef attracts harbour porpoises, the acoustic activity of porpoises was monitored by static acoustic data loggers, T-PODs, before and after the restoration project. T-PODs were placed at the Laesø Trindel reef and at a ref- showed that porpoises not only appeared more often, but also stayed longer at Laesø Trindel. Furthermore, there was a striking diel pattern in porpoise activity at Laesø Trindel, with significantly higher activity during the night. This pattern became increasingly apparent over the study period. At the reference station, in contrast, most activity took place during the day throughout the study. The results suggest that these changes reflect a new food source which occurs at night on the re-established stony reef and is exploited by the porpoises.

No abstract