Deciphering patterns in whole fish nitrogen isotopes on a continental scale

Area of Science:

• Environmental Chemistry
• Isotope Geochemistry
• Marine Ecology

Background:

• Nitrogen isotopes (δ¹⁵N) serve as indicators of anthropogenic nitrogen loading.
• Estuarine ecosystems are sensitive to nutrient inputs and land use changes.
• Understanding nitrogen cycling in coastal waters is crucial for environmental management.

Purpose of the Study:

• To examine the relationship between fish nitrogen isotopes (δ¹⁵N) and environmental variables across U.S. estuaries.
• To assess the influence of nutrient ratios, water quality, and land use on fish δ¹⁵N at national and ecoregional scales.
• To investigate the role of fish life history and contaminant loads in modulating δ¹⁵N values.

Main Methods:

• Analysis of whole fish δ¹⁵N from 136 U.S. coastal waterbodies sampled by the National Coastal Condition Assessment (NCCA).
• Utilized NCCA data on water quality, nutrients, chlorophyll a, and sediment chemistry.
• Employed random forest regression models incorporating fish life history and watershed land use data.

Main Results:

• Fish δ¹⁵N showed a negative relationship with surface water total nitrogen:total phosphorus (TN:TP) ratios nationally.
• δ¹⁵N values were lower on the Atlantic coast compared to the Pacific coast.
• TN:TP was a significant predictor in 4 of 9 ecoregions, with higher δ¹⁵N linked to increased nitrogen limitation (lower TN:TP).
• Fish life history and bioaccumulative contaminants (PCBs, mercury) were positively associated with fish δ¹⁵N.
• Land use impacts on δ¹⁵N were location-specific.

Conclusions:

• Nitrogen stable isotopes in fish effectively reflect ecological conditions at both regional and continental scales.
• Nutrient limitation, indicated by TN:TP ratios, is a key driver of fish δ¹⁵N in U.S. estuaries.
• Fish δ¹⁵N integrates environmental factors, including nutrient status, geography, life history, and contaminant exposure.