Lake-specific responses in sedimentary sulphur, after additions of copper sulphate to lakes in Michigan, USA

Copper sulphate (CuSO4) is commonly added to lakes and reservoirs to manage nuisance and exotic species. Several studies have previously reported that CuSO4 is very useful for this purpose, and that the copper is ultimately stored in lake sediments. In contrast, there has been little study on the fate of the sulphate from CuSO4 additions. The purpose of this study was to elucidate the effects of CuSO4 additions on sedimentary sulphur. Concentrations, isotopes, and fluxes of total and reduced sulphur in sediment cores from four Michigan lakes were compared, including two reference lakes that have never received CuSO4 additions, and two treatment lakes that have received CuSO4 additions by lake managers totalling 1–3 kg sulphate ha−1 year−1 since 1940. The results of this study confirm that sediments do not consistently provide records of sulphate loading across lakes. Isotopic evidence indicates this inconsistency is caused, in part, by lakes with well‐mixed sediments, in which sulphate is reduced to sulphide, but then subsequently reoxidized to sulphate and remobilized to the overlying water column. One of the treatment lakes, however, exhibited a clear correlation between CuSO4 additions and an increased sulphur flux to the sediment. During any given year, however, the sulphate added from CuSO4 additions amounted to no more than 10% of the sulphate added from wet deposition. Based on this seemingly insignificant quantity of sulphate, ascribing any effect of CuSO4 additions on sedimentary sulphur is tenuous at best. One possibility is that the addition of CuSO4 at rates that do not overwhelm the natural sulphur cycle of a lake or reservoir is a reasonable management tool for nuisance and exotic species.