Deciphering patterns in whole fish nitrogen isotopes on a continental scale.

Nitrogen isotopes (δ15 N) have been used as an indicator of anthropogenic nitrogen loading at local and regional scales. We examined δ15 N in fish from estuaries across the continental United States. In the summer of 2015, the U.S. Environmental Protection Agency's National Coastal Condition Assessment (NCCA) collected fish in 136 coastal waterbodies throughout the United States. Whole fish were analyzed by NCCA for metals, organic contaminants, and lipids. For this study, we also analyzed these fish for isotopes of nitrogen (N). NCCA collected water quality, nutrients, chlorophyll a, and sediment chemistry at each site. We used these data, along with fish life history and watershed land use, to examine how whole fish δ15 N was related to these environmental variables using random forest regression models at national and ecoregional scales. At the national scale, fish δ15 N were negatively related to total N:total phosphorous (P) ratios (TN:TP) in surface water and reflected differences between the P-limited, δ15 N depleted sites in the Floridian ecoregion to sites in other regions. δ15 N was lower on the Atlantic relative to the Pacific coast. When considered by region, TN:TP was an important predictor of fish δ15 N in 4 of 9 ecoregions, with higher δ15 N observed with increasing N limitation (lower TN:TP) Fish life history was also an important predictor of fish δ15 N at both the national and ecoregional scale. Whole fish δ15 N was positively associated with bioaccumulative contaminants such as PCBs and mercury. Although land use was related to δ15 N in fish, it was location specific. This study showed that N stable isotopes reflected ecological conditions at both regional and continental scales.