Summary

Only a few of the Danish coastal areas and open waters in 2004 fulfilled the goal for the marine environment of being only weakly influenced by man’s activities. Despite implementation of the Action Plans on the Aquatic Environment, the environmental conditions and nature conditions are still not markedly better. The reasons given are that it takes some years for the full effects of the Action Plans to be fulfilled through nutrient loading reductions to coastal areas, and that the period 1998-2002 was very wet and more nitrate was washed out than normal. In addition, the effects of the extreme hypoxia in 2002 probably have delayed the recovery of the marine environment. There are positive developments in the pelagic environment with lower concentrations of nutrients and algae than in the 1980s, and the oxygen depletion in 2004 was less wide spread and of shorter duration than in previous years. However, previous hypoxia events hamper the reestablishment of benthic fauna and probably also eelgrass. Therefore, there should be lower nutrients to the Danish waters before we will have a better environment. In some coastal areas, TBT and other environmentally harmful substances are also a problem, which must be considered.

 

Year 2004

The year 2004 differed markedly from an average year. The most noticeable was the low Secchi depth and high phytoplankton concentration and production in most areas, which could not be explained by variations in climatic factors such as runoff, temperature, radiation, wind or NAO index, which normally dominates the governance of the pelagic system. One reason was a very unusual and large bloom of the silico-flagellate Dictyocha speculum in the Little Belt area in April-June. The bloom was probably stimulated by nutrients in the bottom water due to a shallow pycnocline followed by mixing of nutrient rich bottom water to the surface. Similar hydrographic conditions can also have enhanced the phytoplankton concentration in the Kattegat, showing that hydrographic year-to-year variations can strongly influence the pelagic system.

 

As during a similar bloom of D. speculum in 1983 caged fish died, but a connection to the bloom was not established. The bloom was probably the main reason for the development of an early and widespread oxygen depletion in the Belt Sea in July-August. Strong wind creating inflow of oxygen rich water from the Skagerrak in the last half of September more or less ended the oxygen depletion, and generally oxygen depletion in 2004 was less severe, less widespread and shorter lasting than in the previous years.

 

In 2004 bottom fauna was characterised by low abundance, diversity and biomass in all areas. The abundance has significantly declined in estuaries and coastal waters since 1998, and the abundance and diversity in the open Belt Sea and the Kattegat since 1994. The decline in the estuaries, coastal waters and the Belt Sea is an effect of the extreme hypoxia in 2002, while the reasons for the decline in the Kattegat is more uncertain, as large parts of the Kattegat are not subjected to severe oxygen depletion.

 

The weather in 2004 was characterised by a very low frequency of high wind speeds, except in June and September-October, a cold period in June-July and high precipitation in January, June-August and October. The runoff was a little above normal, and was higher than average in February and October-December. The washout of nutrients from land was relatively low because of the nearly normal runoff and the implementation of the Action Plans on the Aquatic Environment. Therefore nutrient concentrations in the marine environment were relatively low, and all except silicate show decreasing trends.

 

However, in the estuaries and coastal waters the chlorophyll concentration and primary production have generally remained unchanged since 1993, and the Secchi depth has significantly decreased, as has the coverage and depth distribution of eelgrass in the inner parts of the estuaries. The environmental conditions improved in the estuaries from the 1980s to 1993 parallel to the reduction in the phosphorus load from sewage. Since then the phosphorus load has generally remained unchanged. The nitrogen load has varied much from year to year depending on the runoff, which was especially high in the period 1998-2002, and actually the nitrogen load has not exhibited a steady significant decline, which the biological community could respond to.

 

Further, there are unexplained year-to-year variations in the ecological system, and in the interactions between the components of the system. Among these, we do not know the effects of the changed bottom fauna on the conditions in the pelagic system. The reduced bottom fauna after the 2002 oxygen depletion might have changed the nutrient cycling in the system. Therefore, it has to be emphasised that it is unknown, if the conditions in 2004 were an adverse year-to-year variation, or the start of an adverse development.

 

Nutrient loading

 

·       Freshwater runoff in 2004 was 6% above average in 1989-2003, but 39% higher than in 2003, which was a very dry year.

 

·       Nitrogen loading to coastal areas in 2004 was 14% lower than average 1989-2003, but 57% higher than in 2003.

 

·       Phosphorus loading was 68% smaller than in 1989-90, but 37% higher than in 2003.

 

·       Development of the actual nitrogen load in tons without respect to freshwater inflow shows that N loading during the period 1989 to 2004 has not been significantly reduced, except in the Sound, where loading is dominated by point sources.

 

·       If one takes into account the variation in freshwater runoff, N and P loading to estuaries and coastal marine waters has been reduced by 43% and 77%, respectively, since 1989.

 

·       The atmospheric nitrogen deposition to Danish waters has been reduced by ca. 20% since 1989.

 

Nutrient concentrations

·       In 2004 nutrient concentrations in estuaries and coastal areas were among the lowest compared to the period 1989-2003, because of the about normal freshwater inflow and implementation of the Action Plans on the Aquatic Environment.

·       Concentrations of nitrogen in estuaries and coastal areas have been significantly reduced since the mid 1990s. In the open inner waters there has been a significant reduction in concentrations since 1989.

·       Concentrations of phosphorus are at a stable level after a significant fall in the beginning of the 1990s.

 

Effects of nutrient loading

·       Generally the pelagic conditions were poor in 2004 compared to previous years.

·       From the early 1980s until 1993 increases in transparency were observed and the concentrations and production of algae have decreased in estuaries and coastal waters. However, since 1993 the Secchi depth has decreased, while chlorophyll concentrations and primary production showed no trends, possibly because P loading has not changed since 1993, and the N load vary much from year to year without a significant decreasing trend.

·       In inner open water areas Secchi depth has increased since the end of the 1980s and the concentrations of algae have decreased.

·       Concentrations of oxygen in bottom waters during the months of July-November decreased significantly since 1977 in the estuaries of Funen, including the Archipelago south of Funen. In addition, oxygen concentrations in all of the open inner waters and in the Arkona Basin have been significantly reduced since the 1960s.

·       However, distribution of oxygen depletion in 2004 was smaller and lasted shorter than in 2002 and 2003.

·       Following oxygen depletion in 2002 and 2003 there was still large areas in estuaries and the Belt Sea where benthic fauna abundance and diversity are strongly reduced.

·       Also in the Kattegat the diversity was very low, and has been halved since 1994.

·       Along the open coasts eelgrass grew to the same depth as in the previous years, but both the depth limit and percent coverage of the bottom has declined in the inner parts of the estuaries through the period 1989-2004.

·       The percent coverage of eutrophication tolerant algae at 1-2 m depth was reduced significantly since 1994 in the inner parts of the estuaries.

·       On stone reefs in the open Kattegat the coverage of algal vegetation in 2004 was higher than average observed for the period 1994-2001.

 

Heavy metals, organic contaminants and xenobiotics

·       The concentrations of heavy metals in mussels in 2004 were evaluated to generally lightly to moderately polluted.

·       Generally the concentrations of PCB and PAH in mussels were at a level where the effects on the environment cannot be ruled out. The concentrations of other organic chlorine substances such as HCH and DDT were of no risk.

·       The concentrations of TBT in mussels were in 2004 lower than in 2003, but still exhibit a significant risk for severe effects on the environment. In certain coastal areas fish and mussels showed significant signs of effects of environmentally harmful contaminants in the form of increased enzymatic activity (EROD activity), decreased lysosomal stability and effects on reproduction and development of gonads.

 

Full report in PDF-format (4,993 kB).