Nitrous oxide in the Great Lakes: insights from two trophic extremes

Freshwaters are a significant yet understudied component of the global nitrous oxide (N2O) budget. Despite the potential importance of the Laurentian Great Lakes in the freshwater N2O budget, studies have been extremely limited to date. This study evaluated the production pathways, concentrations, and atmospheric emissions of N2O across the two trophic extremes of the Great Lakes: the deep oligotrophic waters of Lake Superior and shallow eutrophic zones of western Lake Erie. Production pathways via denitrification and nitrification were evaluated through stable isotope analysis, and atmospheric emissions were determined from surface concentrations and wind speed. Across all sites and dates, N2O saturation ranged from 98 to 129% in Lake Superior and 93 to 676% in Sandusky Bay, Lake Erie, indicating these lakes are net sources of N2O to the atmosphere. Isotopic site preference values (S-P) suggest a mix of production pathways, with nitrification dominating most time periods and denitrification occurring under conditions of high nutrient availability and microbial activity. N2O atmospheric emissions were strong but highly variable in Lake Erie, and emissions in Lake Superior were consistently low (- 0.26 to 33.03 and - 0.14 to 1.41 mu mol N m(-2) day(-1), respectively). Our findings highlight two paradigms of N2O production and emissions: low, wind-driven rates in deep oligotrophic zones and temporally dynamic rates driven by N loading in shallow eutrophic zones. Offshore regions likely make up the majority of the N2O budget for the Great Lakes, yet nearshore regions have a greater capacity for increased N2O emissions in the face of increased nutrient loading.