Does mercury biomagnification in a boreal lake food web vary year-round? – A comparison using bulk δ15N and compound-specific δ15N of amino acids

Seasonality characterises northern latitude lakes, however, limited knowledge exists regarding seasonal fluctuations in mercury (Hg) biomagnification. This study tested for these fluctuations year-round in the food web of a boreal humic lake (Lake Pääjärvi, in southern Finland) and for differences between two methods of trophic level (TL) determination. A food web sample was collected in three open-water seasons and one ice-covered season. TL was calculated using bulk nitrogen stable isotopes (δ¹⁵N) and compound-specific stable isotope analysis of δ¹⁵N in amino acids (CSIA-AA). Biomagnification of total Hg (THg), measured through the linear regression of THg (log₁₀[THg]) and TLs, was detected in all seasons using both isotope methods. No significant differences in seasonal trophic magnification slope (TMS), the slope of the linear regression, in- and between methods were found. However, [THg] baseline estimates, the intercept of the linear regression, were significantly different between methods when comparing the same seasons. [THg] baseline values were generally higher in all seasons in bulk δ¹⁵N than CSIA-AA. Results highlight relatively stable biomagnification among seasons in both methods, however, direct comparisons of bulk δ¹⁵N and CSIA-AA TLs require further methodological development.