Climate change and land use intensification are significant pressures jointly affecting lake processes via changes in seasonality, nutrient, and contaminant dynamics. Seasonality at northern latitudes shapes lake food web composition and dynamics, as open-water and ice-covered seasons influence these ecosystems. Mercury (Hg) is a ubiquitous toxic contaminant in northern ecosystems with properties to bioaccumulate in organisms and biomagnify in food webs, however, notable knowledge gaps remain regarding how seasonality, climate change, and land use influence these processes. The main aims of the thesis were to understand year-round fluctuations in Hg content and bioaccumulation in fish muscle tissue, assess year-round biomagnification patterns in boreal Lake Pääjärvi (next to Lammi Biological Station), and identify spatial patterns in biomagnification through food webs between boreal and subarctic regions.
Seasonal fluctuations in Hg content were detected particularly in perch, pikeperch and ruffe, with the highest bioaccumulation slopes in spring and summer, and the lowest in autumn and winter. Hg content in roach, bream and bleak, however, did not show similarly strong fluctuations. Combined biological and seasonal environmental variables explained Hg content in all six species, thus confirming importance of these variables for describing Hg in fish. Mercury content and trophic levels derived from nitrogen isotopes (δ15N) were used to model biomagnification using bulk stable isotope analysis (SIA) and compound-specific isotope analysis of amino acids (CSIA-AA). No significant difference in biomagnification was observed between the seasons, however, significant differences existed between the isotope methods.
Boreal and subarctic lake food webs were assessed for regional patterns in biomagnification. Subarctic lake biomagnification was primarily related to climate and productivity, while boreal lakes were more influenced by complex land use in lake catchments.
The highest Hg content of fish was measured in the winter and near the spawning period in spring is highly relevant to human consumers, and winter monitoring should be developed further. Variation in biomagnification may occur along climate-productivity gradients and is influenced by land use intensity. Increasing climate change and land use may potentially decrease biomagnification in lakes, however, high variability exists in the boreal region.
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