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

• Published in: Limnology and oceanography Vol. 59; no. 2; pp. 465 - 481
• Main Authors: Li, Jiying, Katsev, Sergei
• Format: Journal Article
• Language: English
• Published: Waco, TX John Wiley and Sons, Inc 01-03-2014; American Society of Limnology and Oceanography
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Marine and continental quaternary; Surficial geology; United States; Lake Superior; Great Lakes; models; coastal sedimentation; sediment sources; nitrogen; ammonium; oxidation; vertical distribution; nitrogen cycle; nitrates; North America; denitrification; ammonium ion; offshore; lake sediments; depth; nitrification; pore water; iron; dissolved materials; oxygen; marine sediments
• ISSN: 0024-3590; 1939-5590
• DOI: 10.4319/lo.2014.59.2.0465

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 ∼ 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.