Nutrient over-enrichment is a critical driver of harmful cyanobacterial blooms world-wide. Point sources of nutrients have long been identified as management targets, while diffuse, non-point sources (e.g., agricultural sources, and onsite septic systems) are more challenging to assess and manage for nutrient reductions. In North Carolina, nitrogen levels have increased over time in many freshwater and estuarine systems, and atmospheric deposition of nitrogen has also increased, resulting in nitrogen hot spots; these are often associated with areas that have highly clustered confined animal operations. In addition to emissions to the airshed, these sources of nutrients can result in substantial hydrologic export of both nitrogen and phosphorus, particularly after extreme rainfall events mobilize nutrients stored in waste lagoons, pastures, and croplands. This session will bring together research that has been done in the Carolinas related to nutrient source assessments, including atmospheric deposition, monitoring of surface waters where nutrient levels have increased and phytoplankton blooms have developed, and nutrient management approaches. We also welcome submissions from researchers from a variety of disciplines (remote sensing, monitoring, modeling, etc.), who are working in other places where nutrient hot spots and climate interactions are impacting water quality and the formation of harmful blooms.
Lead Organizer: Diane Lauritsen, LIMNOSCIENCES (ddlauritsen@comcast.net)
Co-organizers:
Lorrayne Miralha, Ohio State University (miralha.1@osu.edu)
Daniel Obenour, North Carolina State University (drobenou@ncsu.edu)
Nathan Hall, University of North Carolina-Chapel Hill (nshall@email.unc.edu)
Astrid Schnetzer, North Carolina State University (aschnet@ncsu.edu)
Presentations
09:00 AM
DRIVERS OF CONTINENTAL SCALE EUTROPHICATION AND OXYGEN DECLINES IN U.S. ESTUARIES (8933)
Primary Presenter: Kaitlin Reinl, University of Wisconsin - Madison (kreinl@wisc.edu)
The National Estuarine Research Reserve System's (NERRS) System-Wide Monitoring Program (SWMP), established in 1995, aims to measure short-term variability and long-term trends in water quality, meteorological conditions, biological systems, and land-use/land-cover in estuarine ecosystems. This study analyzed SWMP water quality and meteorological data from 99 stations across 28 Reserves along U.S. coasts, including the Great Lakes, covering 10 to 23 years. The findings indicate widespread eutrophication in estuaries, with significant increases in chlorophyll-a at nearly 70% of sites. Additionally, almost half of the sites show significant declines in dissolved oxygen, and over 60% report significant temperature increases. To explore potential drivers of changes in chlorophyll-a and dissolved oxygen, we used model selection and averaging techniques on Linear Mixed Effects models for each response variable. The models suggest that increases in dissolved phosphorus and turbidity, as well as decreases in ammonia and trophic state, are linked to rising chlorophyll-a concentrations while increasing temperatures and chlorophyll-a trends likely contribute to decreases in dissolved oxygen. Notably, while dissolved oxygen concentrations are declining, there is little change in the extent of hypoxic conditions (DO < 2mg/L). These results highlight the importance of developing effective strategies to protect the ecological health of estuarine systems in the face of environmental change.
09:15 AM
Exploring algal-nutrient relationships along the freshwater-marine continuum (8870)
Primary Presenter: Daniel Obenour, NC State University (drobenour@ncsu.edu)
Cyanobacteria harmful algal blooms (CHABs) have become an increasing concern in coastal North Carolina over recent decades. While CHABs are typically thought of as a freshwater problem, these blooms and their associated toxins can be found in brackish and saline waters along the freshwater-marine continuum. In this project, we develop a Bayesian hierarchical (multilevel) model to explore how chlorophyll-nutrient relationships vary along the Pamlico River Estuary, and extend these relationships to cyanobacteria abundance using phytoplankton community data (biovolume, cell count). In particular, we explore the spatially varying associations of salinity, temperature, critical nutrient ratio (nitrogen: phosphorus), and hydrologic inflows with algal abundance. These relationships are tested individually, and then combined into a comprehensive model that is found to explain ~ 60% of chlorophyll variability across space and time. Further, results suggest nitrogen is the limiting nutrient ~ 90% of the time. We then apply the model to predict the sensitivity of chlorophyll and cyanobacteria concentrations to changes in nutrients and environmental variables. Modeling results are presented for different seasons, hydrologic conditions, and along the freshwater-marine continuum. Results can be used to forecast when CHABs are most likely, and when and where nutrient reductions will be most effective
09:30 AM
ESSENTIAL AND SUBSTITUTABLE RESOURCE RATIO THEORY PREDICTS PHYTOPLANKTON COMPOSITION AND METABOLITES (8752)
Primary Presenter: Zohreh Mazaheri Kouhanestani, University of Missouri (zmfmf@missouri.edu)
Phytoplankton competition response to stoichiometric nutrient ratios has been shown to affect phytoplankton biomass, composition, and toxin concentrations. This study examines the effect of essential (total nitrogen [TN] and total phosphorus [TP]) and substitutable (nitrate [NO3-] and ammonium [NH4+]) resource ratios on phytoplankton community composition, metabolites, and microcystin cell quotas. In a 21-day, large-scale mesocosm experiment, six nutrient treatments created high (NO3- and NH4+) and low (NO3- + P, NH4+ + P, and P) TN:TP ratios with differing N forms, effectively driving P- or N- deficient conditions throughout the experiment. We predict that cyanobacteria will dominate when the TN:TP and NO3-:NH4+ molar ratio is low. Our findings indicate that cyanobacteria dominated low TN:TP treatments by the end of the experiment. On day 14, the low TN:TP (~18) and NO3-:NH4+ (~0.12) molar ratios favored Sphaerospermopsis sp. and Microcystis sp. leading to a sixfold increase in microcystin production, particularly LA and LF congeners, in the NH4+ + P treatment. In the NO3- + P and P treatments, the TN:TP molar ratio of 29-31 and NO3-:NH4+ ratio of 0.07-0.09 on day 21 led to dominance of diazotrophic and toxin producing species, with microcystin concentrations peaking on day 14. These P-rich conditions also upregulated the sxtA saxitoxin gene. Our findings emphasize that essential and substitutable ratios can affect phytoplankton community composition highlighting their pivotal role in effectively managing cyanobacterial dominance and toxin synthesis in aquatic ecosystems
09:45 AM
The legacy effect of changing nutrient inputs to coastal ecosystems (9150)
Primary Presenter: Jacob Carstensen, Aarhus University (jac@ecos.au.dk)
Nutrient management plans have been successful in reducing nutrient inputs to many coastal ecosystems, but ecosystem responses have been unanticipatedly weak. This lack of recovery has been attributed to a legacy effect of past nutrient inputs, possibly sustaining sediment nutrient release and eutrophication over longer periods. We analyzed sediment pools of carbon, nitrogen and phosphorus sampled over 25 years (1999-2023) across 27 Danish estuaries, following substantial reductions in inputs of nitrogen (>50%) and phosphorus (>90%) from land, the majority of these occurring from 1985 to 1997. Variability among sediment cores was high, both on spatial and temporal scale, although it was reduced by normalizing total N and P to loss of ignition. All sediment pools changed significantly with depth, but there was no significant difference among estuaries despite large differences in area-specific nutrient loading, highlighting the large spatial variability within estuaries. No significant changes in sediment pools were observed over the study period, with an almost even distribution of directions of change. Given the relatively large sampling effort (>100 cores), we estimated that it should be possible to detect changes of 20% with a probability of 80%. Combining this with the lack of consistent trends suggests that the legacy effect of nutrient reductions was within a few years rather than decades or that the legacy effect is small. Hence, the lack of coastal ecosystem recovery is most likely due to other factors.
10:00 AM
Nitrous oxide from wastewater treatment plants – Eutrophication is more than nutrient loading (9199)
Primary Presenter: Weiyi Tang, University of South Florida (weiyitang@usf.edu)
Nitrous oxide (N2O), a potent greenhouse gas and ozone-destroying agent, is produced during nitrogen transformations in both natural and human-constructed environments. Wastewater treatment plants (WWTPs) remove nitrogenous nutrient compounds, but produce and emit N2O into the atmosphere during the process. Reducing nutrient inputs to coastal waters, therefore, is a double-edged sword. By measuring N2O concentrations at a monthly resolution over a year in the Potomac River Estuary, a tributary of Chesapeake Bay in the eastern United States, we found a strong seasonal variation in N2O concentrations and fluxes: N2O concentrations were larger in fall and winter but fluxes were larger in summer and fall. Hotspots of N2O emissions were revealed downstream of WWTPs. N2O concentrations were higher at stations downstream of WWTPs compared to other stations (median: 21.2 nM vs 16.2 nM) despite the similar concentration of dissolved inorganic nitrogen, suggesting the direct discharge of N2O from WWTPs into the aquatic system or a higher N2O production yield in waters influenced by WWTPs. Meta-analysis of N2O measurements associated with WWTPs globally revealed variable influence of WWTPs on downstream N2O concentrations and emissions. Other waste management systems, e.g., agriculture and animal husbandry, likely cause analogous hotspots and variable N2O emissions. Since wastewater production is projected to rise with the growing population, efficient N2O removal, in addition to dissolved nitrogen removal, should be an essential part of wastewater quality control.
10:15 AM
DAM REMOVAL EFFECTS ON GASEOUS CARBON FLUXES IN A MOUNTAINOUS MEDITERRANEAN STREAM (8868)
Primary Presenter: João Miguel Merces Bega, University of São Paulo (joaobega@usp.br)
The global prevalence of obsolete or unsafe old dams necessitates the development of effective restoration approaches and expanded knowledge in this field. Here, we investigated the effects of dam removal on gaseous carbon fluxes in a mountainous Mediterranean stream. This ecosystem function was compared among three reaches: one where a dam was removed (restored), one with an intact dam (impacted), and one in natural conditions (reference). In total, four sampling campaigns were conducted, one in each season: autumn (October 2022), winter (February 2023), spring (April 2023), and summer (June 2023). Gaseous carbon fluxes were measured by using the headspace equilibration method. The fluxes of CO2 (FCO2) in the restored reach (mean ± standard deviation: 88.77 ± 62.80) was similar to those in the reference reach (88.78 ± 108.18) (ANOVA test; p = 1.000) and higher than those in the impacted reach (66.18 ± 59.42 mmol m-2 d-1) (p = 0.017), whereas the fluxes of CH4 in the restored reach (0.47 ± 0.56) was higher than those in the reference (0.10 ± 0.06) (p < 0.001) and impacted (0.18 ± 0.12 mmol m-2 d-1) (p < 0.001) reaches. Lotic waters are expected to present greater FCO2 than impounded waters as shallower streams with higher water velocities are often associated with higher interfacial turbulence. Since the dam removal occurred only six years before the sampling, residual organic material from the dam's operational period likely continues to affect CH4 emissions. These findings are important for guiding future dam removal and restoration efforts.
SS34A - Reducing Nutrient Inputs to Coastal and Inland Waters: How Hard Can It Be?
Description
Time: 9:00 AM
Date: 31/3/2025
Room: W205CD