Collaborative Research: Reevaluating Pre-denitrification BNR for Low Molecular Weight Dissolved Organic Nitrogen and its Impact on Phytoplankton Bloom Dynamics in Coastal Waters

University Of North Carolina At Chapel Hill, Chapel Hill NC

Investigators

Abstract

Coastal waters are contaminated by nutrient-rich run-off that stimulates the dense growth of plant life. Algal blooms, a common damaging form of this plant growth, cause oxygen depletion and food web changes that threaten the sustainability of these waters. Significant reduction of this nutrient-rich run-off in coastal ecosystems is primarily achieved by advanced or upgraded wastewater treatment plants (WWTPs) that can effectively remove these contaminants. However, despite the successful decreases in nutrient releases through WWTP upgrades, these coastal systems still face the same contamination, possibly because the engineering practices used to decrease nutrient discharge from WWTPs lead to the production of different forms of these nutrients that still stimulate algal bloom formation. To examine this hypothesis, this research will investigate the potential linkage of advanced WWTP discharge and algal bloom formation in coastal waters. This research is significant because it could potentially provide an explanation for persistent oxygen depletion in coastal waters even after significant decreases in nutrient inputs from WWTP discharges. If successful, this research could help develop new strategies for WWTPs to mitigate nutrient effects in impaired coastal ecosystems to better protect the Nation's water security. The objectives of this research are: 1) to evaluate pre-denitrification biological nutrient removal (BNR) processes, the most common way to reduce inorganic nitrogen (N) discharge from WWTPs, for the formation of low molecular weight dissolved organic N (LMW-DON); and 2) to better understand the impact of BNR processes on suspended algal (phytoplankton) production and composition in coastal waters. The hypotheses are that: 1) the pre-denitrification BNR processes are prone to produce significantly larger amounts of LMW-DON than conventional activated sludge systems; 2) LMW-DON leads to unexpectedly larger phytoplankton biomass yield compared to inorganic N; and 3) LMW-DON selectively affects phytoplankton community composition, favoring harmful bloom taxa. To test these hypotheses, this research proposes four specific aims: 1) document LMW-DON in WWTP effluents; 2) investigate the production of LMW-DON in pre-denitrification BNR; 3) investigate phytoplankton community responses to effluents laden with different level of LMW-DON; and 4) investigate the effect of cation-enhanced bioflocculation on effluent LMW-DON. This collaborative research combines expertise in Environmental Engineering with Environmental Biology and Ecology. This research will employ in situ and laboratory-based bioassays on Neuse River estuary (NC) and Long Island Sound (NY-CT) water to investigate the qualitative and quantitative effects of N in effluents on phytoplankton production and community structure. This research will also explore the addition of cations in wastewater systems as a potential solution for minimizing production of effluent LMW-DON. This interdisciplinary collaborative research will have far-reaching impacts for the understanding of the role of WWTP-driven LMW-DON in both eutrophication and structuring of phytoplankton communities in coastal waters. Furthermore, the project will shed light on mechanisms coupling specific N inputs to coastal eutrophication and harmful bloom dynamics. The results of this research can transform how N is managed to mitigate eutrophication in coastal waters. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.