δ15N values in Lake Erie sediments as indicators of nitrogen biogeochemical dynamics during cultural eutrophication

We have measured the stable nitrogen isotope values (δ15N) in two sediment cores sampled 15 years apart (1988 and 2003) from the Eastern Basin of Lake Erie and compared them to the total phosphorus (TP) and biogenic silica (BSi) concentrations in the 2003 core. Changes in the TP, BSi and total nitrogen (TN) accumulations in the 2003 core correspond to three stages in the trophic history of the lake that include the onset (∼ 1910 to ∼ 1950), peak (∼ 1950 to ∼ 1970), and amelioration (∼ 1970 to 2003) of eutrophication owing to accelerated nutrient loading. Decreasing BSi:TP atomic ratios (BSi:TPat) from ∼ 1910 to ∼ 1970 suggest a gradual Si drawdown in the lake during its progressive eutrophication, which led to Si limitation and reduced diatom settling rates. δ15N values show pronounced variability in ∼ 1950 to ∼ 1970, which corresponds with rapid increases in N from runoffs from the lake catchment. A trend of gradually increasing δ15N values from ∼ 1910 to ∼ 1970 is likely the result of denitrification associated with the development of anoxic bottom waters in the Central Basin of the lake. This study illustrates the complexity of extracting environmental information from δ15N values in sediments and confirms the need for further studies of the multiple processes and their interactions that contribute to lacustrine δ15N records.