Environmental change effect on quality of production in boreal lakes

Phytoplankton occupy a key position in aquatic food webs and their response to environmental changes can significantly impact the availability of energy and nutrients for the higher trophic levels. In this thesis, laboratory experiments, together with environmental sampling, were used to study how browning, eutrophication, and warming of boreal lakes affect the quality of phytoplankton production and its ramifications for food webs. Quality was centred on polyunsaturated fatty acids (PUFA), with an emphasis on eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), as these PUFAs are required for the growth and reproduction of consumers and are not synthesized by all phytoplankton species. Browning-driven decreases in light availability led the mixotrophic cryptophyte Cryptomonas sp. to increase reliance on bacterial-derived organic carbon over photosynthesis to sustain growth. However, this change in carbon utilization did not impact Cryptomonas sp. PUFAs, suggesting that browning will decrease primary production without affecting phytoplankton quality. Warming and eutrophication had opposing effects on the PUFA proportion of ten boreal phytoplankton species (from six different groups). Moreover, EPA and DHA production had a species-specific response to these environmental changes. Consequently, lakes with different nutrient levels may respond differently to warming with phytoplankton quality being mostly determined by phytoplankton community composition. In Finnish boreal lakes, eutrophication led to higher phytoplankton, zooplankton, and fish biomasses while altering the whole food web community. Volumetric primary production saturated at high phytoplankton biomass while EPA volumetric production responded logarithmically to eutrophication. Primary and EPA productivity (production per seston biomass) had unimodal responses to eutrophication. DHA volumetric production and productivity varied largely with eutrophication but were best described by unimodal models. Overall, eutrophication impaired EPA and DHA transfer from phytoplankton into zooplankton and fish.