Nitrogen cycling in deeply oxygenated sediments: Results in Lake Superior and implications for marine sediments

Abstract

To understand the nitrogen (N) cycle in sediments with deep oxygen penetration, we measured pore‐water profiles to calculate N fluxes and rates at 13 locations in Lake Superior in water depths ranging from 26 to 318 m. Sediments with high oxygen demand, such as in nearshore or high‐sedimentation areas, contribute disproportionally to benthic N removal, despite covering only a small portion of the lake floor. These sediments are nitrate sinks (average 0.16 mmol m−2 d−1) and have denitrification rates (average 0.76 mmol m−2 d−1) that are comparable to those in coastal marine sediments. The deeply oxygenated (4 to > 12 cm) offshore sediments are nitrate sources (average 0.26 mmol m−2 d−1) and generate N2 at lower rates (average 0.10 mmol m−2 d−1). Ammonium is nitrified with high efficiency (90%), and nitrification supports > 50% of denitrification nearshore and µ 100% offshore. Oxygen consumption by nitrification accounts for 12% of the total sediment oxygen uptake. About 2% of nitrate reduction is coupled to oxidation of iron, a rarely detected pathway. Our Lake Superior N budget indicates significant contributions from sediment—water exchanges and N2 production and is closer to balance than previous budgets. Our results reveal that sediment N cycling in large freshwater lakes is similar to that in marine systems. They further suggest that denitrification rates in slowly accumulating, well‐oxygenated sediments cannot be described by the same relationship with total oxygen uptake as in high‐sedimentation areas; hence, global models should treat abyssal ocean sediments differently than coastal and shelf sediments.