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Opponent Professor Peter Eklöv (Uppsala University, Sweden) and Custos Docent Sami Taipale (Ģ��ֱ��). The doctoral dissertation is held in English.

Ossi Keva studied in his PhD thesis how environmental change, such as warming, eutrophication and browning affect lake ecosystem structure, energy sources and nutritional quality. During the PhD project, Keva determined fatty acid and mercury content from primary producers to top predators in boreal and subarctic Finnish lakes.

Environmental changes were found to have a major impact on lake food web structure as well as the fatty acid composition of algal and benthos communities. According to the study, omega-3 fatty acid contents in fish are relatively stable to environmental changes. However, the nutritional value of perch muscle to human consumption was the lowest in lakes with low pH and highly forested catchments, which was clearly driven by the elevated mercury content in fish muscle.

Only specific algal taxa can synthesize eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are important omega-3 polyunsaturated fatty acids for growth and reproduction of many organisms. Mercury is a bioaccumulative toxic metal, which enters to lakes via atmospheric deposition and catchment area runoff.

Keva’s study found that warming and eutrophication in subarctic lakes decreased algal and zooplankton community EPA+DHA content. On the other hand, similar trends in boreal lakes were not observed. Dietary models revealed that cladocerans fed preferably high-quality algae such as diatoms and cryptophyte and avoided low-quality cyanobacteria.

Fish muscle fatty acid composition seems to be relatively stable to environmental changes

DHA concentration of perch muscle was found to decrease slightly from deep clear water to shallow murky water lakes in boreal region. In subarctic region, however, warming and increasing lake productivity did not impact the EPA and DHA concentrations of fish communities.

“According to my study, warming and eutrophication of northern lakes increases the relative contributions of cyanobacteria in algal communities as well as cyprinids and perch in fish communities” Keva describes. “At the same time, the relative contribution of salmonids, which are valuable for fishing, decreases. The study results show that warming and eutrophication increases especially the biomasses of algal communities and herbivorous fishes.”

The results from Keva’s PhD thesis help ecologists to understand how lake ecosystems and aquatic organisms react to environmental changes. Food web processes and PUFA dynamics seem to differ between boreal and subarctic lakes. “Northern Finnish lakes are not getting clearer in future, but instead the change is more likely towards warmer and murky lakes, modifying the food web communities as well”, Keva describes.

The thesis is published in the JYU Dissertations as a number 525, Jyväskylä 2022, ISSN 2489-9003, ISBN 978-951-39-9168-5 (PDF). Link to publication: