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Summary
Several lakes in Örebro County are affected by eutrophication that originates from human activity. Eutrophication can be caused by the water flowing to the lake carrying high levels of nutrients, but it can also be caused by internal processes that release nutrients from the sediments, so-called internal loading. This process can take place naturally. But when human activity has added nutrients to the sediments and this has led to an increase in the internal load, we have an increased internal load. Within the framework of the EU-funded LIFE-IP project ”Rich Waters”, existing data and new surveys have been carried out to create a situational picture of what the internal load looks like in Örebro County’s lakes. The focus has been on investigating lakes that may have an eutrophication problem. A total of 30 lakes are included in the report, of which 18 are certainly eutrophicated and another 8 may be.
To assess the lakes, methods have been used that have been developed in another project that has also been developed within ”Rich Waters” and which has led to the publication ”Handbook – a decision support tool for measures against internal phosphorus loading in lakes”. The handbook is divided into different stages, of which 16 of the lakes in this report are assessed with the support of step 1, where a risk assessment is carried out. A further 12 lakes have been investigated according to step 2 where calculations have been carried out to quantify the internal load. For the lakes Hjälmaren and Alsen, modelling has also been carried out according to step 3 of the handbook, to assess how the lake would be affected by different measures.
Of the investigated lakes, 14 are estimated to be affected by an elevated internal load, of which 5 are considered to be with certainty. 5 lakes are certainly not considered to have an elevated internal load.
The study shows that more environmental monitoring is needed to obtain sufficient data to assess further measures. In 6 of the lakes, it is appropriate to collect more data in order to make a more reliable risk assessment, and in about the same number of lakes, increased environmental monitoring is required to be able to quantify the internal load and proceed with modelling. The study also shows that in at least half of the eutrophicated lakes there is a continued need to reduce the external load.
