Feedbacks Between Nutrient Enrichment and Geomorphology Alter Bottom-Up Control on Food Webs
Classic bottom-up theory predicts that increased resource availability (for example, nutrients) at the base of the food web will stimulate primary production and, in turn, secondary production. Recent studies, however, indicate that bottom-up controls on food web production can be modified by other factors, such as landscape configuration and continuity. As part of a 10-year ecosystem-scale experiment in a New England salt marsh, we investigated the response of secondary consumers, specifically a fish, the mummichog (Fundulus heteroclitus), to nutrient enrichment. In the first 6 years, we observed a classic bottom-up response of increased production of algae, invertebrate prey, and mummichogs. After the sixth year, however, mummichog biomass declined to below reference levels by the eighth year. This decline in mummichog biomass coincided with nutrient-induced collapse of the low-marsh habitat. Based on stable isotope analyses, field surveys, and small-scale experiments, we suggest that the geomorphic changes induced a trophic decoupling between creek and marsh habitats, thereby reducing mummichog access to prey in the intermittently flooded marsh. Thus, despite continued stimulation of algal and invertebrate prey production, fish abundances declined to below pre-enrichment levels. Our results demonstrate how geomorphic controls can override classic bottom-up control and emphasize the importance of long-term studies in detecting the response of slow-turnover phenomena (for example, changing landscapes).
Keywordsfood web theory nutrient enrichment landscape control saltmarsh estuary trophic subsidy spatially coupled geomorphology
We thank the many, many undergraduates who contributed to data collection over the decade of this research; Allison Hall for her work conducting the mummichog diet experiment. Special thanks to Chris Stallings for his consultation on the statistical analysis. Thanks to David Behringer for creating the map of the field sites. We thank the two anonymous reviewers and the editor for their constructive comments. Support was provided by NSF (DEB-1354494, OCE-1238212, OCE-1233678), the Northeast Climate Science Center (DOI G12AC0000), the US Fish and Wildlife Service, and Woods Hole Oceanographic Institution. This paper is Contribution No. 3749 of the Virginia Institute of Marine Science, College of William & Mary.
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