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Stable Isotopes Reveal Complex Changes in Trophic Relationships Following Nutrient Addition in a Coastal Marine Ecosystem

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Abstract

Complex links between the top-down and bottom-up forces that structure communities can be disrupted by anthropogenic alterations of natural habitats. We used relative abundance and stable isotopes to examine changes in epifaunal food webs in seagrass (Thalassia testudinum) beds following 6 months of experimental nutrient addition at two sites in Florida Bay (USA) with different ambient fertility. At a eutrophic site, nutrient addition did not strongly affect food web structure, but at a nutrient-poor site, enrichment increased the abundances of crustacean epiphyte grazers, and the diets of these grazers became more varied. Benthic grazers did not change in abundance but shifted their diet away from green macroalgae + associated epiphytes and towards an opportunistic seagrass (Halodule wrightii) that occurred only in nutrient addition treatments. Benthic predators did not change in abundance, but their diets were more varied in enriched plots. Food chain length was short and unaffected by site or nutrient treatment, but increased food web complexity in enriched plots was suggested by increasingly mixed diets. Strong bottom-up modifications of food web structure in the nutrient-limited site and the limited top-down influences of grazers on seagrass epiphyte biomass suggest that, in this system, the bottom-up role of nutrient enrichment can have substantial impacts on community structure, trophic relationships, and, ultimately, the productivity values of the ecosystem.

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Acknowledgements

This research was funded by a grant from the Everglades National Park (ENP) under cooperative agreement 1443CA528001022 and by the Florida Coastal Everglades Long-Term Ecological Research Program funded by the US National Science Foundation (Cooperative Agreement #DEB-9910514). Doug Morrison and Bill Perry facilitated permit issuance and use of ENP facilities. US Geological Survey Biological Research Division East, especially Mike Robblee, Andy Davis, Dave Reed, and Travis Thyberg, assisted with throw trap operation and faunal identification. We are grateful to Melissa Gil, Bryan Dewsbury, Susie Escorcia, Tom Frankovich, Jeremy Stalker, and Pascale William for devoting field and laboratory time to this project. Stable isotope analyses were performed at the FIU Stable Isotope Laboratory under the direction of W.A. Anderson. Pursell Technologies Inc. and IMC Global generously donated the nitrogen and phosphorus fertilizers, respectively, for this study. This is contribution number 451 from the Southeast Environmental Research Center at FIU.

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  1. Department of Marine Biology, Texas A&M University at Galveston, 5007 Avenue U, Galveston, TX, 77551, USA

    Anna R. Armitage

  2. Department of Biological Sciences and Southeast Environmental Research Center, Marine Science Program, Florida International University, North Miami, FL, 33181, USA

    James W. Fourqurean

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Correspondence to Anna R. Armitage.

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Armitage, A.R., Fourqurean, J.W. Stable Isotopes Reveal Complex Changes in Trophic Relationships Following Nutrient Addition in a Coastal Marine Ecosystem. Estuaries and Coasts 32, 1152–1164 (2009). https://doi.org/10.1007/s12237-009-9219-z

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