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Geography, not human impact, is the predominant predictor in a 150-year stable isotope fish record from the coastal United States
Citation:
Oczkowski, A., B. Kreakie, M. Gutierrez, Peg Pelletier, M. Charpentier, E. Santos, AND J. Kiddon. Geography, not human impact, is the predominant predictor in a 150-year stable isotope fish record from the coastal United States. ECOLOGICAL INDICATORS. Elsevier Science Ltd, New York, NY, 111:106022, (2020). https://doi.org/10.1016/j.ecolind.2019.106022
Impact/Purpose:
Across the globe, nitrogen (N) inputs to land have approximately doubled since the 1940s. A portion of this N runs off of the land and into coastal waters. The negative effects of this N runoff are well documented for individual estuaries. Here we examine how this N impacts coastal food webs across the continental United States. We compared fish collected prior to the 1940s (mean year of collection was 1914) to those collected in 2015 and used stable isotopes of N and carbon (C) as a proxy for human impact. A model that incorporated collection year, known human-associated watershed characteristics (population density, fertilizer use intensity, animal husbandry, etc.), basic geographic information, and fish life histories was used to predict isotope values. The model was able to account for most of the variation in the isotope values, but the main predictors were geography (e.g., latitude, longitude, state) and the feeding (trophic) level of the fish species and not human-related variables. Overall, the results of this study underscore the importance of considering the broader geographic characteristics and offshore N contributions to a particular estuary when trying to understand how human sourced N has impacted coastal food webs.
Description:
Since the 1940s, anthropogenic nitrogen (N) inputs have grown to dominate global N cycles, particularly in fluvial systems. Negative impacts of this enrichment on downstream estuaries are well documented. Efforts at N reductions are increasingly successful but evaluating ecosystem response trajectories is difficult because of a lack of knowledge of historic conditions. To document continental-scale coastal food web N-dynamics prior to large increases in human N-loads, we sampled 208 fish from an archival collection, taken from coastal waters across the continental U.S., with a median collection year of 1904. The archival fish were compared with 526 samples collected in 2015 from 126 estuaries also along the U.S. coastline. We used stable isotopes of N (δ15N) and carbon (δ13C) as a proxy for human inputs and organic matter sources. Watershed attributes from 1910 and 2012, census data, fish life histories, and basic estuarine geography were used to develop random forest models that determined which variables were the best predictors of isotope values. State, latitude, and fish trophic level were consistently the most important predictors, while human impacts played a lesser role. When the fish were collected (~1914 vs 2015) was not an important predictor, rather where the fish was collected was the best predictor of N source. The model results illustrate the important role that geography plays in coastal food web dynamics and underscore the importance of offshore N-sources to coastal food webs.
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DOI: Geography, not human impact, is the predominant predictor in a 150-year stable isotope fish record from the coastal United States