Seasonal hypoxia and nitrate depletion in water chestnut ( Trapa natans ) beds: Implications for nitrogen cycling

Abstract

Dense beds of floating macrophytes can alter water quality by modifying dissolved oxygen (DO) and nitrogen (N) dynamics. Water chestnut ( Trapa natans L.), a non‐native annual macrophyte, forms extensive seasonal beds in freshwater systems in North America. The species is intensively managed to reduce negative impacts on native species, yet its effects on DO and nitrate (NO ₃ ⁻ ) remain poorly studied. We quantified DO and NO ₃ ⁻ ‐N concentrations inside and outside of three riverine T. natans beds in New York over three growing seasons. In addition, we assessed denitrification using field‐collected water and sediment in laboratory microcosm incubations. During summer months, DO of water from within T. natans beds was 76% lower on average (1.28 mg L ^{− 1} ) compared to outside beds (5.43 mg L ^{− 1} ). For NO ₃ ⁻ ‐N, concentrations averaged 0.45 (2017) and 0.11 (2018) mg L ^{− 1} outside beds, but were depleted to 0.10 (2017) and 0.01 (2018) mg L ^{− 1} within beds, 77%–88% lower than outside beds. In 2018, 60% of samples, mostly from within beds of T. natans , were below the analytical detection limit of NO ₃ ⁻ ‐N (0.02 mg L ^{− 1} ). Laboratory microcosm incubations did not detect consistent N ₂ production, despite experimental nitrate enrichment. Together, these results show that T. natans beds create seasonally hypoxic, nitrate‐depleted conditions. However, weak and inconsistent N ₂ production in our incubation experiments indicates that additional studies are needed to further assess the fate of nitrogen within T. natans beds and whether management may involve potential tradeoffs between habitat quality and nutrient cycling.