Søndergaard, M., Johansson, L.S. & Kjeldgaard, A. 2023. Sammenhænge mellem oplandskarakteristika og tilstanden i søer uden veldefinerede afløb. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 32 s. - Videnskabelig rapport nr. 548.
Many Danish lakes do not have well-defined inflows and outflows. This makes it difficult to calculate the supply of nutrients and thus to target the efforts for lakes that do not meet the environmental objectives in the water area plans. One of the main aims of this report was to estimate a catchment for these lakes and subsequently assess whether the state of the lake could be related to certain catchment characteristics, which could then form the basis for an effort.
On the basis of a dataset forwarded by the Danish Environmental Protection Agency, it has been possible to estimate a catchment area for a total of 418 lakes. For these lakes, the catchment is characterised on the basis of soil type, land use, point sources of phosphorus and nitrogen, potentially drained areas and calculated losses of phosphorus and nitrogen from cultivated areas. In total, 12 types of catchment characteristics were determined. The summer average concentrations of total phosphorus, total nitrogen, chlorophyll and the average degree of coverage of submerged macrophytes for the individual lakes were used as input data for determination of lake state.
Empirical relationships between lake state and catchment characteristics showed some statistically significant correlations, but in all cases the correlations were weak, i.e. only a small part (<10%) of the state could be explained on the basis of a certain catchment characteristic. A positive correlation was thus found between the proportion of cultivated area in the catchment area and the nutrient concentrations in the lakes. Correspondingly, there was a positive correlation between the potentially drained area in the catchment and the concentration of nutrients. Both correlations point to the fact that changes in the catchment can contribute to improving the state of the lakes.
The generally weak correlations between the characteristics of the catchment area and lake state indicate that the state of a lake largely depends on other conditions that have not been investigated in this project. One of these could be previous nutrient inputs that still affect the lake state. Another could be interactions with the groundwater, where a large contact with the groundwater and a high content of nutrients may possibly affect the state of the lakes. It may also be naturally nutrient-rich lakes – possibly via naturally high nutrient concentrations in the groundwater.
There is no information about the non-human-impacted state (the reference state) in lakes without inlets/outlets. An estimate of this state is given on the basis of the nutrient and chlorophyll concentrations in lakes where the cultivated area is <20% and where the lakes are assessed as being in a high or good ecological state. These lakes generally have low concentrations of both phosphorus (median total phosphorus = 0.020 mg/l) and nitrogen (median total nitrogen = 0.70 mg/l) and low chlorophyll concentrations (median chlorophyll = 5.3 µg/l) and must therefore expected to have clear water conditions with a rich flora and fauna.
In the report a flow diagram is presented, which can be used as a tool to qualify a decision on improving the state of lakes without well-defined inlets/outlets where the ecological state does not meet the target. One of these tools could be lake restoration.
