Investigating the Role of Microseira wollei mats in Nitrogen and Phosphorus Cycling at Lake Wateree, South Carolina

Harmful Cyanobacterial Blooms (HCBs) increasingly threaten human health and aquatic ecosystems in freshwater environments. Microseira (Lyngbya) wollei, a non-heterocystous cyanobacterium prevalent in the southeastern United States, forms perennial benthic and seasonal pelagic mats that can release toxins and fix atmospheric nitrogen. This study focuses on Lake Wateree, a hydroelectric reservoir characterized by extensive M. wollei mats, aiming to elucidate the role of this cyanobacterium in benthic nutrient cycling dynamics. From March to October 2023, excluding April and June, we sampled two sites: one with M. wollei mats (HCB) and one without. We established two pairs of sites, sampling them alternately over six months. Denitrification rates, representing the conversion of nitrate to N₂ gas, were measured alongside anammox and dissimilative nitrate reduction to ammonium, using ¹⁵N-labeled incubations in whole sediment cores. Our results showed that nitrogen removal via denitrification was 3.9 times higher in sediments associated with M. wollei mats (up to (0.96 mmol N/m²/day), establishing a correlation between mat thickness and denitrification rates (R² = 0.98). Anammox rates were generally below detection limits. Notably, we observed net negative N₂ fluxes from sediments to the water column in M. wollei cores, indicating biological nitrogen fixation. Additionally, elevated phosphorus and iron concentrations in sediments with M. wollei suggest active regulation of redox conditions, facilitating the release of legacy phosphorus from organometallic complexes. These findings are crucial for management strategies at Lake Wateree and enhance the understanding of nutrient dynamics associated with benthic HCBs, informing future mitigation efforts.

Presentation Preference: Oral

Primary Presenter: Archana Venkatachari, University of South Carolina (avenkatachari@seoe.sc.edu)

Authors:

Annie Bourbonnais, University of South Carolina  (abourbonnais@seoe.sc.edu)

John Hite, University of South Carolina  (JDHITE@email.sc.edu)