Markager, S., Carstensen, J., Krause-Jensen, D., Windolf, J. & Timmermann, K. 2010: Effekter af øgede kvælstoftilførsler på miljøet i danske fjorde. National Environmental Research Institute, Aarhus University. 54 pp. - NERI Technical Report no. 787.
This report contains an analysis of plausible effects of increasing the nitrogen loads to selected Danish fjords by up to 10%. The analyses are based on a series of empirical and dynamic models which describe the chain of mechanisms that nitrogen load initials: increased nitrogen concentration, which stimulates phytoplankton growth and causes increased chlorophyll concentration, which contributes to attenuate the light through the water column and shade eelgrass so that the depth limit is displaced towards shallower water. The results show that a 10% larger nitrogen load results in about 10% larger concentrations of inorganic nitrogen, about 6% larger concentrations of total nitrogen, an increase in chlorophyll of about 3%, and a 1% reduction in water clarity followed by about 1% shallower depth limits of eelgrass. The effect thus declines along the chain of mechanisms. This is partly because not only chlorophyll but also suspended particles and dissolved organic matter contribute to light attenuation in the nutrient rich Danish fjords where the sea floor in many places is devoid of vegetation and, thus, easily resuspended.
The report concludes that an increase in nitrogen load will push the ecosystems further in the negative direction and away from the aim of reaching a good ecological status. This is the case whether or not it is possible to actually measure the change with sufficient accuracy in the fjord in question.
The report also provides a conceptual model for the status of the fjords in relation to eutrophication. In the later years the environmental conditions of a number of fjords have improved as a response to reductions in nutrient loads, but a set of mechanisms, caused by eutrophication, act to maintain the fjords in impoverished conditions. For example, resuspension of the bare sea floor counteracts improvements in water clarity. Moreover, frequent occurrence of anoxia that kills flora and fauna imply that nutrients are maintained in the system. These effects delay a return to a situation with expanded eelgrass beds and rich populations of fish and bottom invertebrates. It is not clear exactly how much nutrient loads need to be reduced in order to shift the state of the ecosystem but additional reductions represent a step in the right direction.
A displacement of existing livestock and thus a redistribution of nitrogen loads within the catchment area of a fjord should not cause negative environmental effects. However, loads should not be displaced ‘inwards’ in fjords that contain separate basins. A redistribution that moves nitrogen loads ‘outwards’ in a given fjord or from a fjord to an open coast could have positive implications for the fjord environment.
The analyses of this report include primarily Skive Fjord, Horsens Fjord and Odense Fjord but also comprise results for the entire Limfjord, Mariager Fjord and Randers Fjord. They, thus, cover a broad range of Danish fjords and the conclusions should be valid for other Danish fjords as well.
Full report in pdf (1,73 MB)
