Conclusions

Eutrophication effects have been documented in all Danish marine waters every year since the beginning of the 1980s. At present it can be concluded that quality criteria has only been fulfilled in a restricted number of Danish estuaries, coastal and open water areas. The few coastal areas that meet the criteria are generally non-stratified shallow water areas with a relatively restricted load from land based sources. The causes for not fulfi lling the criteria are primarily the effects of nutrient load and oxygen depletion.

With corrections for variations in precipitation, run off and other climatic related conditions it can be concluded that:

NUTRIENT LOADS
Since 1990 land based inputs of nitrogen and phosphorus to estuaries and coastal areas have been reduced by 35% and 60%, respectively. The reduction in nitrogen (21%) is mainly caused by reduced loses from agricultural soils while the reduction in phosphorus is due to extension of sewage treatment. Atmospheric nitrogen deposition to Danish marine waters seems to have declined about 15% since 1989.

NUTRIENT CONCENTRATIONS
Nitrogen concentrations in estuarine and coastal areas have decreased signifi cantly after 1997 and since 1989 in the open waters. Concentrations of phosphorus have stabilised after a significant decrease in the beginning of the 1990s.

DIRECT EFFECTS
Water clarity increased and phytoplankton biomass and production decreased significantly in estuaries and coastal areas until the mid 1990s. The changes are less pronounced in open waters, although water clarity was higher and diatom biomass as well as phytoplankton production were lower in the 1990s than in the 1980s.

During the period 1989-2001 eelgrass cover at water depths above 2 meters increased in the outer part of the estuaries, but was signifi cantly reduced at shallow depths in inner estuaries and along open coasts. The depth distribution has not increased, but in the inner estuaries actually decreased, possibly due to oxygen depletion. The amount of nuisance macroalgae has been reduced (though this is not always noted in the cover) at shallow depths. Macroalgae cover at open water reefs shows no trends since 1994.

INDIRECT EFFECTS
The biomass of macrozoobenthos in open waters has decreased through the 1990s following a reduction in the biomass of diatoms and phytoplankton production. Macrozoobenthic communities in estuaries and coastal areas show large inter annual variations, which seem to be caused by recurrent oxygen depletion.

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Danish Environmental Protection Agency & National Environmental Research Institute • updated: