First lacustrine application of the diatom-bound nitrogen isotope paleo-proxy reveals coupling of denitrification and N2 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 (δ¹⁵N) of organic matter in the sedimentary record. To verify, and eliminate, potential biases of bulk sedimentary δ¹⁵N (δ¹⁵N_bulk) signatures by diagenetic alteration and external N inputs, we applied, for the first time, the diatom-bound N isotope (δ¹⁵N_db) paleo-proxy to lake sediments. By comparing δ¹⁵N_bulk and δ¹⁵N_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 δ¹⁵N_bulk in paleolimnological studies. Furthermore, in combining δ¹⁵N_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, δ¹⁵N_db, and the concentration of heterocyst glycolipids (a biomarker for N₂-fixing cyanobacteria) increased simultaneously. Together, these data suggest that the rise in δ¹⁵N_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 N₂ fixation in surface waters.