Benthic cyanobacterial mats enhance nitrogen turnover in a eutrophic reservoir

Abstract. Benthic cyanobacterial mats are widespread in eutrophic lakes, but their effects on sediment phosphorus (P), iron (Fe), and nitrogen (N) cycling are poorly quantified. We studied Microseira wollei, a mat-forming cyanobacterium capable of N₂ fixation and toxin production, in Lake Wateree, South Carolina (USA). We sampled paired sites with and without mats at two locations in March and monthly from June through October 2023 (three dates per location), and measured P, Fe, and N concentrations in mat interstitial water and porewater, and N transformation rates and fluxes using ¹⁵N-labelled and unlabelled whole-core incubations. Porewater P and nitrate (NO₃⁻) were higher beneath benthic mats in June and July and Fe was higher in June, September, and October, whereas ammonium (NH₄⁺) was lower beneath mats in June. Potential denitrification rates did not differ on average between paired sites (24.2 ± 13.9 vs. 19.3 ± 12.2 µmol N m⁻² h⁻¹ with and without mats), but increased with mat thickness (r² = 0.62). Potential dissimilatory nitrate reduction to ammonium (DNRA) rates, measured as ¹⁵NH₄⁺ production from added ¹⁵NO₃⁻, were 2.3 times higher in mat-covered sediments (13.1 ± 4.4 vs. 5.8 ± 3.3 µmol N m⁻² h⁻¹) but remained smaller than denitrification, accounting on average for 39 % of total NO₃⁻ reduction in mat sediments and 29 % in bare sediments. The net N₂ flux in unamended mat cores was directed from the water column to the sediments in all ten cores (−23.9 ± 32.2 µmol N m⁻² h⁻¹), suggesting that N₂ fixation likely exceeded denitrification beneath mats, although the mean was not significantly different from zero or from that in cores without mats. M. wollei mats are therefore hotspots of N turnover: they enhance denitrification with increasing mat thickness, more than double DNRA, may support N₂ fixation, and release P, Fe, and NH₄⁺ from the sediments, complicating the management of eutrophic lakes.