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Coastal eutrophication as a driver of salt marsh loss

Abstract

Salt marshes are highly productive coastal wetlands that provide important ecosystem services such as storm protection for coastal cities, nutrient removal and carbon sequestration. Despite protective measures, however, worldwide losses of these ecosystems have accelerated in recent decades1. Here we present data from a nine-year whole-ecosystem nutrient-enrichment experiment. Our study demonstrates that nutrient enrichment, a global problem for coastal ecosystems2,3,4, can be a driver of salt marsh loss. We show that nutrient levels commonly associated with coastal eutrophication increased above-ground leaf biomass, decreased the dense, below-ground biomass of bank-stabilizing roots, and increased microbial decomposition of organic matter. Alterations in these key ecosystem properties reduced geomorphic stability, resulting in creek-bank collapse with significant areas of creek-bank marsh converted to unvegetated mud. This pattern of marsh loss parallels observations for anthropogenically nutrient-enriched marshes worldwide, with creek-edge and bay-edge marsh evolving into mudflats and wider creeks5,6,7. Our work suggests that current nutrient loading rates to many coastal ecosystems have overwhelmed the capacity of marshes to remove nitrogen without deleterious effects. Projected increases in nitrogen flux to the coast, related to increased fertilizer use required to feed an expanding human population, may rapidly result in a coastal landscape with less marsh, which would reduce the capacity of coastal regions to provide important ecological and economic services.

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Figure 1: Comparison photos of the marshes from the ecosystem nutrient-enrichment experiment.
Figure 2: Ecosystem attributes of reference and nutrient-enriched salt marshes after up to 7 years of nutrient enrichment.
Figure 3: The global relationship between nutrient loading and salt-marsh distribution and loss.

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Acknowledgements

We thank our TIDE (Trophic cascades and Interacting control processes in a Detritus-based Ecosystem), LENS (Landscape Evolution in a Nutrient enriched Saltmarsh) and PIE-LTER (Plum Island Ecosystems Long-term Ecological Research) colleagues for field assistance and comments. We thank the many research assistants, graduate and undergraduate students who maintained the nutrient enrichment and analysed samples. This work is supported by grants from the NSF (DEB0816963, DEB0213767, OCE0923689, OCE 0423565, OCE0924287), the NOAA and The Mellon Foundation.

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L.A.D., D.S.J., R.S.W., B.J.P. & J.W.F. designed the experiment and participated in sampling and data analysis. S.F. participated in geomorphic and geotechnical evaluation. W.M.W. estimated global N loading to coastal saltmarshes. L.A.D. and D.S.J. wrote the initial manuscript. All authors contributed to and approved the manuscript.

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Correspondence to Linda A. Deegan.

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The authors declare no competing financial interests.

Additional information

The data reported in this paper are archived in the Plum Island PIE-LTER database.

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Deegan, L., Johnson, D., Warren, R. et al. Coastal eutrophication as a driver of salt marsh loss. Nature 490, 388–392 (2012). https://doi.org/10.1038/nature11533

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