Summary

In this report we analyse the state of the environment in the inner part of Mariager Fjord, on the east coast of Jutland, Denmark. Data for concentrations of nutrients, chlorophyll, water clarity and primary production were analysed with emphasis on consequences of nutrient loadings to the estuary. The study was carried out on the initiative of the Environment Centre Aalborg who financed the report together with the National Environmental Research Institute at the University of Aarhus. The primary aim was to establish the maximum allowable nutrient loadings in order to comply with the Water Framework Directive for good ecological status in 2015.

The analysis shows that there has been a significant reduction in loadings of phosphorous. The reduction is estimate to 72% from 1985 to 2008 when corrected for runoff. The runoff corrected loadings of nitrogen are only reduced by 13% in the same period and the actual loadings are only marginally reduced due to a number of years with high runoff late in the period.

Concentrations of nutrients are decreasing, particularly for phosphorus.

Concentrations of inorganic phosphorous are reduced to below 0.2 ?M l-1 which is considered to be limiting for phytoplankton growth. Concentrations below this level are measured for a significant part of the growth season in the last five years. Until now reductions in hosphorous loadings have primarily reduced the surplus of inorganic phosphorous. Further reductions will most likely course a significant reduction in chlorophyll concentration. For nitrogen a considerable surplus of inorganic nitrogen

is still present in the growing season, so reductions in nitrogen loadings will probably have a limited effect until the concentration of inorganic nitrogen in the growing season is reduced to a level where nitrogen becomes limiting for phytoplankton growth.

A somewhat surprising result of the analysis was a positive coefficient for

phosphorous loadings on Secchi depth, i.e. a reduction in loadings will reduce Secchi depth in the second half of the growing season. A similar effect is recorded in other estuaries and might be due to a lower removal of nitrogen earlier in the growing season when phosphorous is reduced.

Water clarity is significantly reduced from mid 90s and onwards. The analysis does not offer an explanation but a number of hypotheses are discussed. The decrease in water clarity is not related to the external factors

investigated in this report, e.g. loadings.

The report recommends measures which will reduce phosphorous loadings to about 6 to 8 tons per year or about 53% of the present loading. For nitrogen the recommendation is a reduction in loadings to between 200 and 400 tons per year or about 36% of the present loadings. The recommendation is based on an overall assessment of all the parameters  used in the report to assess the environmental status of the estuary.

 

 Full report  i pdf-format (3.137 KB)