Ærtebjerg, G. (red.), 2007. Technical report no. 639, 95 pp.
Summary
The years 2005 and 2006 revealed a number of positive developments in the environmental quality in Danish waters. The national action plans for water quality have resulted in a substantial decrease in the loading of nitrogen and phosphorus to these waters, in years with an average runoff a 43% reduction in nitrogen and a 77% reduction in phosphorus during the period 1989-2004. Nitrogen loading is now approaching the levels observed in the 1960s. Atmospheric deposition of nitrogen is estimated to have decreased by 22% during the period 1989-2006. These factors have resulted in a significant decline in nutrient concentrations in Danish waters since 1989. However, it appears that since 1997 and 2002 the concentrations of phosphorus and nitrogen, respectively, have remained relatively constant. Phytoplankton biomass in these waters has also decreased from 1979 to 2006 resulting in generally clearer waters.
Oxygen conditions in the off-shore inner Danish waters in 2005 and 2006 were on the whole relatively good. However, low winds and warm temperatures resulted in severe oxygen depletion and mortality of benthic fauna in shallow estuaries in 2006. For example 20-30,000 tons of blue mussels were killed in the Isefjord.
Eelgrass beds with coverage of at least 5% has increased to deeper waters along the open coastlines and in the outer parts of the estuaries. In the same areas the coverage of nuisance algae has declined.
The phasing out of TBT in antifouling paint for ship-hulls has resulted in a decline in its concentrations in many areas and the consequent effect on marine gastropods imposex has also been reduced. However, TBT poses still a threat for marine life, especially in the coastal areas.
Despite positive trends in certain individual parameters, the general environmental conditions of the Danish waters have not improved notably. Phytoplankton production, Secchi depth and especially the oxygen conditions are far from the levels they were at in the 1960s. The absolute depth limits of single specimens of eelgrass have not increased. Algae coverage on the stone reefs in the <st1:place w:st="on">Kattegat</st1:place> has not changed during the period 1994-2006. The diversity of benthic fauna in the off-shore inner Danish waters during 2004-2006 has experienced a 50% reduction in comparison to the status in 1994 and the faunal density has fallen by 25-50%.
The lack of improvement in the general conditions, despite clear reductions in the nutrient loading, is due to a combination of issues. The coverage of eelgrass is halved compared to the beginning of the 20th century. From the 1960s to the1980s phytoplankton productivity doubled in the <st1:PlaceName w:st="on">Belt</st1:PlaceName> <st1:PlaceType w:st="on">Sea</st1:PlaceType> and parts of the <st1:place w:st="on">Kattegat</st1:place> and benthic fauna have been severely influenced by oxygen depletion events since the beginning of the 1980s. In addition fishing has not only changed the structure of the food web but also physically influenced the sea floor which may have harmed benthic fauna and plants. These are just few of the factors that have led to fundamental changes in the functioning of the marine ecosystem.
The effects of eutrophication have also been enhanced by climate change. Annual water temperatures have increased by 1° C in comparison to conditions in the 1970s and 1980s. The large year to year variability in precipitation and runoff has resulted in nitrogen loading varying annually by a factor of two, which presents an additional stress to estuarine and coastal ecosystems.
All these effects have resulted in an increased recirculation of nutrients between benthic and pelagic systems with greater phytoplankton production and subsequent oxygen consumption. Oxygen depletion events occur relatively easily and more often and hinder the ability of benthic fauna to re-establish their communities, and possibly also limit the extent of eelgrass coverage.
An additional reason for the limited improvements in the marine ecosystems may be the effects of contaminants. Across much of the region their levels are relatively elevated, particularly the heavy metals such as mercury, cadmium and copper, but also organic contaminants such as PAH’s (anthracene in particular), PCB’s and TBT. There are also elevated levels of dioxins, phthalates and polyfluorinated alkyl substances in specific areas and siloxanes have been detected in a screening investigation. The biological effects of these contaminants are evident in many areas, in particular the hormone disrupting effects of TBT in gastropods, but also changes in the lysosomal membrane stability of blue mussels due to organic contaminants (PAH, PCB), and the occurrence of deformed offspring and increased production of detoxifying enzymes in Eelpout.
The understanding of the effects of these contaminants on ecosystems and their interactions with eutrophication processes is expanding. Research over the past few years has shown phytoplankton species differ in their contaminant tolerance and the availability of contaminants is dependent on the local environmental conditions (i.e. trophic status). Contaminants have been shown to influence the long term plankton productivity and the diversity of marine flora and fauna. Additionally, contaminants inhibit the spreading and growth of eelgrass and benthic fauna and as a result change the cycling of energy and nutrients in a system.
Thus contaminants, like nutrients, are a threat to the marine environment, which like nutrients have to be taken into account and acted upon in order to improve the environmental conditions of the Danish marine waters.
In summary the alterations in the marine ecosystems and the still persisting pressures are so large that the system can not be expected to return to its original condition after reductions in just one of the anthropogenic pressures acting on it (e.g. reductions in nutrient loadings). Recent research has shown that complex marine ecosystems do not respond reversibly to reduction in pressures, but establish new stable states. This is currently underway and it may take several years before a new steady state is reached.
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