First lacustrine application of the diatom‐bound nitrogen isotope paleo‐proxy reveals coupling of denitrification and <scp>N₂</scp> fixation in a hyper‐eutrophic lake

Past changes in the input/output, and internal cycling, of bioavailable nitrogen (N) in marine and lacustrine environments can be reconstructed by analyzing the N isotopic composition (δ 15 N) of organic matter in the sedimentary record. To verify, and eliminate, potential biases of bulk sedimentary δ 15 N (δ 15 N bulk ) signatures by diagenetic alteration and external N inputs, we applied, for the first time, the diatom‐bound N isotope (δ 15 N db ) paleo‐proxy to lake sediments. By comparing δ 15 N bulk and δ 15 N db in a sedimentary record from eutrophic Lake Lugano (Switzerland), we demonstrate that changing redox conditions influence the degree of N‐isotopic alteration of the bulk sediment, emphasizing the need for caution when interpreting δ 15 N bulk in paleolimnological studies. Furthermore, in combining δ 15 N db measurements with X‐ray fluorescence scanning and state‐of‐the‐art molecular biomarker analyses, we reconstruct nutrient cycling and paleoenvironmental conditions in the lake over the past ~ 125 yr. Coeval with the period of severe eutrophication in Lake Lugano in the 1960s, our proxy data indicate that export production, δ 15 N db , and the concentration of heterocyst glycolipids (a biomarker for N 2 ‐fixing cyanobacteria) increased simultaneously. Together, these data suggest that the rise in δ 15 N db is likely the result of enhanced water‐column denitrification in response to increased phytoplankton productivity. We hypothesize that greater export production during eutrophication led to anoxic conditions in the hypolimnion as a result of enhanced organic matter remineralization, raising water‐column denitrification. Enhanced N loss and remobilization of phosphorous (P) from the sediments under anoxic conditions lowered the N : P ratio in the lake, fostering cyanobacterial N 2 fixation in surface waters.