High Mercury Concentrations of European Perch (Perca fluviatilis) in Boreal Headwater Lakes with Variable History of Acidification and Recovery

High dissolved organic carbon and low pH are often associated with elevated mercury content of fish in boreal lakes, but less is known about the fish mercury dynamics in lakes recovering from acidification stress. We measured total mercury concentration (THg) in muscle of European perch (Perca fluviatilis) and evaluated the overall correlation with environmental and growth variables in 24 boreal headwater lakes in the 2010s. We found negative correlations of length-corrected perch THg with lake pH and perch growth, but no correlation with dissolved organic carbon. The main emphasis in the present study was focused to a subset of ten lakes in southern Finland with known perch THg during severe acidification in the 1980–1990s. The comparison of perch THg concentrations in the 2010s with values determined in the 1980–1990s showed a sharp increase in most acidic lakes where the perch populations suffered from severe acid stress in the 1980s. This increase was attributed to growth condensation caused by sharp decrease in perch growth during recovery of reproduction capacity and the consequent increases in population densities of perch. Our results highlight the importance of perch growth rate and population density for understanding the variability of fish Hg in boreal headwater lakes, where recovery from acidification can lead to higher mercury concentration of fish in certain cases.

Quality transformation of dissolved organic carbon during water transit through lakes: contrasting controls by photochemical and biological processes

Dissolved organic carbon (DOC) may be removed, transformed, or added during water transit through lakes, resulting in changes in DOC composition and pigmentation (color). However, the process-based understanding of these changes is incomplete, especially for headwater lakes. We hypothesized that because heterotrophic bacteria preferentially consume noncolored DOC, while photochemical processing removes colored fractions, the overall changes in DOC color upon water passage through a lake depend on the relative importance of these two processes, accordingly. To test this hypothesis we combined laboratory experiments with field studies in nine boreal lakes, assessing both the relative importance of different DOC decay processes (biological or photochemical) and the loss of color during water transit time (WTT) through the lakes. We found that influence from photo-decay dominated changes in DOC quality in the epilimnia of relatively clear headwater lakes, resulting in systematic and selective net losses of colored DOC. However, in highly pigmented brown-water lakes (absorbance at 420 nm > 7 m−1) biological processes dominated, and there was no systematic relationship between color loss and WTT. Moreover, in situ data and dark experiments supported our hypothesis on the selective microbial removal of nonpigmented DOC, mainly of low molecular weight, leading to persistent water color in these highly colored lakes. Our study shows that brown headwater lakes may not conform to the commonly reported pattern of the selective removal of colored constituents in freshwaters, as DOC can show a sustained degree of pigmentation upon transit through these lakes.

Quality transformation of dissolved organic carbon during water transit through lakes: contrasting controls by photochemical and biological processes

Dissolved organic carbon (DOC) may be removed, transformed or added during water transit through lakes, resulting in qualitative changes in DOC composition and pigmentation (color). However, the process-based understanding of these changes is incomplete, especially for headwater lakes. We hypothesized that because heterotrophic bacteria preferentially consume non-colored DOC, while photochemical processing remove colored fractions, the overall changes in DOC quality and color (absorbance) upon water passage through a lake depends on the relative importance of these two processes, accordingly. To test this hypothesis we combined laboratory experiments with field studies in nine boreal lakes, assessing both the relative importance of different DOC decay processes (biological or photo-chemical) and the loss of color during water transit time (WTT) through the lakes. We found that photo-chemistry qualitatively dominated the DOC transformation in the epilimnia of relatively clear headwater lakes, resulting in selective losses of colored DOC. However, in highly pigmented brown-water lakes (absorbance at 420 nm > 7 m−1) biological processes dominated, and there was no systematic relationship between color loss and WTT. Instead in situ data and dark experiments supported our hypothesis of selective microbial removal of non-pigmented DOC, mainly of low molecular weight, leading to persistent water color over time in these lakes. Our study shows that individual brown headwater lakes do not conform to the commonly reported pattern of selective removal of colored constituents in freshwaters, but rather the DOC shows a sustained degree of pigmentation upon transit through these lakes.