Using continental-scale monitoring data to develop nitrogen and phosphorus thresholds to assess United States estuaries

Nutrient pollution continues to impact water quality and degrade coastal ecosystems. Coastal eutrophication assessments are useful tools to track the effects of nutrient over-enrichment at large-scales and over time. These assessments often use response variables including chlorophyll a, dissolved oxygen, and submerged aquatic vegetation to quantify nutrient effects or susceptibility to nutrient pollution in estuaries. Currently, the United States (U.S.) Environmental Protection Agency’s (EPA) National Coastal Condition Assessment (NCCA) program uses dissolved inorganic nutrients to assess the potential for eutrophication in U.S. estuarine resources, which offers limited interpretability. There is a lack of published nutrient threshold values that can be applied at national and regional scales to assess the extent of probable eutrophic conditions in estuaries. To overcome this information gap, we used nationally consistent data collected during the 2010 and 2015 NCCA surveys to explore developing assessment thresholds using stressor-response relationships between observed nutrient and chlorophyll a levels. Results showed that total nitrogen (TN) and phosphorous (TP) explained more variance in chlorophyll a across regions than dissolved inorganic nutrients. We used binomial logistic regression models to develop regionally optimized TN and TP thresholds.  logistic models were used to predict the probability of chlorophyll a exceedances using 5 and 20 µg l-1 as boundaries between good to fair and fair to poor conditions, respectively, to establish upper and lower TN/TP thresholds. These thresholds were used to transform chlorophyll a values to binomial values. Our analysis was more robust for the lower nutrient thresholds than the upper thresholds, due to data limitations associated with less frequent exceedance of the upper chlorophyll a threshold. Except for South Florida estuaries, there was similarity in the TN and TP thresholds across regions. We will include a comparison of TN and TP thresholds developed in our analysis to those in the literature calculated using various methods and for a variety of ecological endpoints. As a next step, we are applying similar analyses to water clarity and dissolved oxygen. Our analyses demonstrate the value of large-scale assessment surveys such as the NCCA in developing nutrient thresholds using consistent methodologies. 

Impact/Purpose

Nutrient pollution continues to impact water quality and degrade coastal ecosystems. Coastal eutrophication assessments are useful tools to track the effects of nutrient over-enrichment at large-scales and over time. These assessments often use response variables including chlorophyll a, dissolved oxygen, and submerged aquatic vegetation to quantify nutrient effects or susceptibility to nutrient pollution in estuaries.  There is a lack of published nutrient threshold values that can be applied at national and regional scales to assess the extent of eutrophication in our nations estuaries.  To overcome this information gap, we used nationally consistent data collected during the 2010 and 2015 National Coastal Condition Assessment surveys to explore developing assessment thresholds using stressor-response relationships between observed nutrient and chlorophyll a levels.  Results of our analyses, suggests that there is similarity in total nitrogen and phosphorous thresholds across regions, except for South Florida estuaries.  The nutrient thresholds were also consistent with site-specific nutrient targets developed for estuarine systems using a variety of methods.  Our analyses demonstrate the value of large-scale assessment surveys such as the NCCA in developing nutrient thresholds using consistent methodologies.

Citation

Brown, Cheryl A., Peg Pelletier, A. Oczkowski, AND L. Harwell. Using continental-scale monitoring data to develop nitrogen and phosphorus thresholds to assess United States estuaries. Coastal and Estuarine Research Federation, Portland, OR, November 12 - 16, 2023.