Modeling Geospatial Variation in Carbon and Nitrogen Isotopes Along the US Northeast Continental Shelf: Isoscape Development for Studies of Movement and Trophic Ecology.

Moore, Rhiannon B.*¹, Catherine Nowakowski², Lindsay Agvent², Kelton W. McMahon², ¹Department of Biology, Emory University, Atlanta, GA, ²Graduate School of Oceanography, University of Rhode Island, Narragansett, RI

The distribution, movement, and cycling of marine nutrients is key to understanding animal migrations, food web dynamics, and ecosystem health. However, the biogeochemical processes underlying nutrient transport are often difficult to observe directly. This study used bulk stable carbon and nitrogen isotopes of zooplankton collected during the NOAA EcoMon survey to develop geospatial isotope maps (isoscapes) along the continental shelf of the Northwest Atlantic Ocean, from the Mid-Atlantic Bight to the Gulf of Maine. We developed a generalized additive model (GAM) of the relationship between isotopic variation and latitude, longitude, and station depth. Our isoscapes revealed strong spatial variation in both carbon and nitrogen isotopes across the US NE shelf that highlighted distinct regional differences between geographically nearby regions. For example, we observed a strong divide in carbon isotope values between Georges Bank and the Gulf of Maine, and again in nitrogen isotope values between the New England and the Mid-Atlantic Bight. Incorporating ecological regions as a categorical variable improved the overall modeled fit (Carbon: 12.9% increase; Nitrogen: 26.5% increase), especially for the nitrogen isoscape, and substantially increased the amount of variance accounted for by our models. The addition of regions also revealed biogeochemical dynamics linked to oceanographic conditions and features, such as river plume effects from the Hudson River and strong cross-shelf gradients in Southern New England. Results from this project characterize the biogeochemical processes of the continental shelf of the Northwest Atlantic, which can be used to inform future research on animal foraging and migration ecology.