| Summary: | Climate change and intense land-use activities are promoting lake eutrophication and browning, affecting community structure and food web processes. In this thesis, space-for-time approach was used to study the environmental change impacts on food web structure, energy pathways, and organism nutritional quality (defined with fatty acids and mercury content) in subarctic and boreal regions. Only specific algal taxa can synthesize eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are important polyunsaturated fatty acids (PUFA) for consumer growth and reproduction. Mercury is a toxic heavy metal that, bioaccumulates to organisms via diet. In subarctic lakes, increasing temperature and productivity negatively affected EPA+DHA content of seston and zooplankton. However, no such changes were observed from boreal regions. Seston and cladoceran PUFA contents were uncorrelated in the boreal zone where cladoceran preferred to feed on high-quality algae. European perch (Perca fluviatilis (L.)) showed slight decreasing trends in their muscle DHA content from more transparent lakes towards shallower and murkier ones in the boreal areas. In subarctic climate–productivity gradient, the decreasing prey item quality (zooplankton and profundal benthos) did not affect fish muscle EPA+DHA content at community level. Perch in boreal low pH lakes and highly forested catchments rely more on terrestrial energy sources than eutrophic lakes with neutral pH. This likely resulted in higher mercury and omega-6 PUFA content in perch muscle. Increasing temperature and productivity fundamentally alter subarctic lake communities' structure and function, resulting in an increasing share of cyanobacteria, smaller-bodied zooplankton, smaller benthos taxa, and warmer-water-adapted cyprinid fishes. Positive trends in biomass at each second trophic level (phytoplankton and invertivorous fish) were observed along with climate-productivity gradient shaping biomass pyramids. Food web processes and PUFA dynamics seem to differ between ...
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