Stable Nitrogen Isotopes of Plankton and Benthos Reflect a Landscape-level Influence on Great Lakes Coastal Ecosystems

As populations and human activities increase in coastal watersheds, an understanding of the connections of aquatic ecosystems to the adjacent terrestrial landscape is necessary to identify, monitor, and protect vulnerable coastal habitats. This study investigates the relationships between land-use patterns and δ 15N values of aquatic organisms in coastal ecosystems, across a defined watershed gradient for the U.S. portion of the Great Lakes shoreline. δ 15N measured in plankton and benthic invertebrates reflects a range of basin wide land-use gradients and demonstrates a strong connection between watershed-based anthropogenic activities and exposure in aquatic biota. For example, benthos δ 15N values range over 12‰ across sites in our study, but regression analyses suggest that over 50% of the variability is explained by the regional landscape. Further, multiple taxa at comparable trophic position showed similar patterns in relation to watershed-scale land use. Our results suggest that within the coastal environment, the expression of landscape in aquatic biota is stronger in habitats such as embayments and wetlands than open nearshore. These results support the use of δ 15N in Great Lakes coastal biota as an exposure indicator of watershed-scale N loading. INDEX WORDS: Great Lakes, coastal habitat, stable nitrogen isotope, landscape disturbance gradient, plankton, benthos.