You are here:
Patterns in nitrogen isotopes from fish at the continental scale
Citation:
Pelletier, M., A. Oczkowski, AND Jim Hagy. Patterns in nitrogen isotopes from fish at the continental scale. Coastal & Estuarine Research Federation Conference 2023, Portland, OR, November 12 - 16, 2023.
Impact/Purpose:
Nitrogen is an essential element, but elevated concentrations in our coastal areas have caused adverse impacts on water quality and aquatic life, including algal blooms and fish kills. EPA’s National Coastal Condition Assessment assesses the nation’s estuaries every 5 years. One of the measures is contaminant levels in whole fish. For this study we also measured stable isotopes of nitrogen in these same fish as a potential indicator of eutrophication. Nitrogen has 2 stable isotopes, 14N (99.6%) and 15N (0.4%). Although the ratio of the two isotopes 15N/14N, expressed as δ15N, has been used as a measure of anthropogenic nitrogen input it also varies depending on the position of an organism in a food web. For example, all things being equal, a top predator will have higher δ15N than an organism that feeds on algae. In this study we used a machine learning algorithm to examine how δ15N in whole fish related to a variety of explanatory variables including nutrients, fish contaminants, site characteristics, land use and fish life history characteristics. We determined that there were differences in response based on location, and that fish life history was important to account for the wide range of fish species collected by NCCA. Whole fish δ15N isotopes indicated nutrient status (N or P limited) as well as the degree of anthropogenic impact. Few studies have examined stable isotopes in fish in so many coastal areas. Therefore, one possible impact of this research is that it could provide a useful reference, providing valuable context for more localized research and monitoring.
Description:
Nitrogen isotopes are frequently used as an indicator of anthropogenic nitrogen loading at the local or regional scale. We were interested in assessing nitrogen isotope response in estuaries across the continental United States. In the summer of 2015, the EPA’s National Coastal Condition Assessment (NCCA) collected fish in 136 coastal waterbodies throughout the U.S. Whole fish were analyzed by NCCA for metals, organic contaminants and lipids. For this study we also analyzed these fish for isotopes of nitrogen and carbon. NCCA collected water quality, nutrients, chlorophyll, and sediment chemistry at each site. We used these data, along with fish life history and watershed land use to examine how whole fish δ15N was related to these environmental variables using random forest regression at the national and ecoregional scale. At the national scale, TN:TP was negatively related to δ15N of fish and reflected differences between the P-limited, δ15N depleted sites in the Floridian ecoregion as well as lower δ15N on the Atlantic relative to the Pacific coast. Phosphorus concentration was a stronger predictor of δ15N than nitrogen concentration, likely because water column nitrogen was utilized in primary production for most estuaries in this study. At the ecoregional scale, TN:TP was an important predictor of fish δ15N in 4 of 9 ecoregions, but other nutrients were also included in specific ecoregions. Fish life history is also an important predictor of fish δ15N at the national and ecoregional scale, reflecting the wide range of fish species collected in this study. Surprisingly, lipid content was not an important variable. Whole fish δ15N was also related to land use, with increased measures of human development being positively related to δ15N, and natural land use being negatively related. Selected fish contaminants were related to δ15N but reflected specific local conditions. This large-scale survey allowed us to describe ecological patterns on a continental scale. Inclusion of fish life history characteristics allowed us to account for the differences in fish species from the Atlantic, Pacific and Gulf coasts. Whole fish nitrogen isotopes indicated nutrient status (N or P limited) and the degree of anthropogenic impact. Overall, while this work is ongoing, we were able to identify some continental scale patterns that can be used to inform local or regional research using stable isotopes.
