Markager, S., Bassompierre, M. & Petersen, D.J. 2010. Analyse af miljøtilstanden i Horsens Fjord fra 1985 til 2006. Empirisk modellering. National Environmental Research Institute, Aarhus University. 84 pp. – NERI Technical Report No. 733.
In this report, the state of the environment in Horsens Fjord during the period from 1985 to 2006 was analysed. Horsens Fjord is a shallow eutrophic estuary on the east coast of Jutland, Denmark. The study was initiated by the Environment Centre Århus who financed the report, together with the National Environmental Research Institute at Aarhus University, Denmark. The aim was to establish a scientific background for the implementation of the water action plan for the estuary according to the Water Framework Directive. Thus, a key issue was to identify the maximal allowable nutrient loadings to the estuary in order to establish good ecological status in 2015. This is the forth report using empirical models to establish quantitative relationships between loadings and the environmental state of Danish estuaries.
Data for nutrients, chlorophyll concentrations, and light attenuation were analysed and quantitative relationships were established to nutrient loadings, taking into account effects of inter annual variations in climate.
The analysis showed that there has been a significant reduction in loadings of phosphorous (71% corrected for inter annual variations in runoff). The corresponding value for nitrogen was 40%, but due to high runoff late in the period, the actual reductions in nitrogen loadings were substantially lower.
The results showed that concentrations of nutrients have been reduced over the period as a consequence of reductions in nutrient loadings. However, for most of the growing season, concentrations of inorganic nutrient concentrations are still above values limiting for phytoplankton growth. This can explain why the response for chlorophyll concentrations and light attenuation are weak and in some cases absent. The results indicate that until now the reductions in loadings have mostly removed a surplus of nutrients in the estuary.
Observations of depth limits for occurrence of eelgrass (Zostera marina) over a period of hundred years showed that macrovegetation was widespread also at great depth. Our models for light attenuation are unable to predict values low enough to simulate this, even at zero loadings. This indicates that the present state of the estuary is very different from the situation before loadings increased in the 1950s. Most likely the present state, without widespread macro vegetation, will change into a non-linear way when loadings are reduced. However, a considerable time lag must be expected due to the pool of nutrients stored in the sediments and the time lag for re-growth of eelgrass.
Based on an overall assessment of parameters it is concluded that the maximal allowable loadings are about 13 tons of phosphorous and 560 tons of nitrogen per year in order to re-establish good ecological status of the estuary. There is a considerable uncertainty in these numbers. Our judgement is that the uncertainty is about ± 2 tons for phosphorous and ± 150 tons for nitrogen. However, even applying these reductions, it will probably take about 20 to 30 years before the good ecological status is reached. Hence, it is probably not possible to fulfil the requirements in the water framework directive by 2015. Greater reductions in nutrient loadings or the use of alternative measures will probably speed up the process.
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