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No. 399: The significance of climate change for the need for nutrient supply for lakes

Trolle, D., Nielsen, A., Andersen, T.K. & Søndergaard, M. 2020. Klimaændringernes betydning for indsatsbehov for næringsstoftilførsel til søer. Forsknings- og udviklingsprojekt vedr. anvendelse af dynamiske modeller til estimering af klimaeffekter på søer. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 22 s. - Videnskabelig rapport nr. 399. http://dce2.au.dk/pub/SR399.pdf

Summary

 

The climate influences the dynamics and the state of the environment in lakes all over the world, and several studies suggest that climate change generally exacerbates the symptoms of eutrophication. A warmer climate can lead to more widespread oxygen depletion and increased phytoplankton concentrations, and typically also increased dominance of bluegreen algae. In order to be able to maintain a given environmental state in a warming climate, it may therefore be necessary to further reduce today’s external nutrient load.

The purpose of this report is to assess the connections between the environmental state of Danish lakes and the nutrient load in today’s climate and a climate representing the period around 1900. On this basis, it is possible to assess whether - and how much - the nutrient target load and the effort to reduce the nutrient load to lakes have changed as a consequence of a changing climate. The report does not examine how the state of the environment will be affected in the future as a result of various climate projections.

The climate scenario, which represents the year 1900, was generated using a delta-change method, in which the air temperature and the water supply in today’s climate (based on the baseline period 2001-2005) were  corrected by monthly factors  based on the development of the Danish climate from the year 1900 until today. A comparison of today’s climate with the climate in the year 1900 shows that the changes are generally most pronounced in the winter months where the water flow as well as the air temperature are higher in today’s climate. For the summer period, however, the differences are small, i.e. the water supply and the air temperature are more or less similar in today’s climate and the climate around the year 1900.

The analyses are based on process-based models that were set up and adapted to four Danish lakes: Lake Søholm, Lake Bryrup Langsø, Lake Arreskov and Lake Hinge. The lakes exhibit various characteristics from deep and summer-stratified to shallow and fully mixed conditions, making it likely that they will respond differently to a changed climate.

Overall, the model simulations show considerable differences between the lakes as to how the calculated target load changes from the year 1900 to today’s climate. However, the differences in the target load, and thus potentially also the need for reduction efforts associated with the impact of climate change, are generally small, around 5%, which is significantly less than the natural year-to-year variation between load and environmental state. The analyses also show that the target load has increased slightly in today’s climate in two of the lakes, declined in one lake and remained unchanged in the last lake. The fact that the target load may increase could possibly be ascribed to the slightly increased water retention time in the lakes during summer as a result of the changed flow pattern. Empirical studies generally show that increased retention time results in reduced nutrient and chlorophyll concentrations in lakes.

We conclude that there is a difference between the lakes responses to a changed climate and that the calculated changes in reduction efforts as a result of climate change in the period from approx. 1900 to today are minor and significantly smaller than the natural year-to-year variations between load and environmental state.