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Nutrient enrichment and precipitation changes do not enhance resiliency of salt marshes to sea level rise in the Northeastern U.S.
Citation:
Watson, Elizabeth B., Autumn J. Oczkowski, C. Wigand, Alana R. Hanson, Earl W. Davey, S. Crosby, Roxanne L. Johnson, AND H. Andrews. Nutrient enrichment and precipitation changes do not enhance resiliency of salt marshes to sea level rise in the Northeastern U.S. CLIMATIC CHANGE. Springer, New York, NY, 125:501-509, (2014).
Impact/Purpose:
In this article, we examined the combined effects of eutrophication and climate change on the dominant coastal salt marsh plant, Spartina alterniflora, and associated soil structure and processes using data from field mesocosms deployed in Narragansett Bay, and laboratory mesocosms developed at AED, in addition to field surveys in New England and newly available geomatic data. Field results and system stressor-response relation functions were developed.
Description:
In the U.S. Northeast, salt marshes are exceptionally vulnerable to the effects of accelerated sea level rise as compensatory mechanisms relying on positive feedbacks between inundation and sediment deposition are insufficient to counter inundation increases in low turbidity tidal waters. Instead, future salt marsh survival depends primarily on biogenic processes, biomass production, and organic matter accumulation, which are impacted by existing anthropogenic stressors, including tidal restrictions, ditching, invasive species establishment, and poor water quality. Using a combination of field and laboratory mesocosm inundation experiments, we developed an elevation-productivity relationship for the U.S. Northeast, and using a combination of geodetic surveys and newly-released elevation models, situated current salt marsh orthometric heights on this curve. We determined that the majority of Northeastern salt marsh (89%) is located at elevations where growth is sub-optimal relative to productivity maxima, suggesting that productivity declines will accompany even small increases in tidal flooding. Using laboratory mesocosm experiments where we manipulated water column nutrient levels and precipitation receipt, and imaged end of season belowground biomass through analysis of CT scans, we further found that altered precipitation patterns predicted by climate change scenarios were associated with significant reductions in above (40 ± 8.5%) and belowground biomass (38 ± 11%), and that elevated nutrient levels were associated with changes in plant structure that likely compromise organic matter accumulation and peat formation. In addition, elevated nutrient levels were found to enhance the coupled process of carbon mineralization (+28 ± 8.5%) and sulfate reduction (+30 ± 14%), suggesting that high nutrient loads accelerate the breakdown of existing organic matter. These results provide evidence that U.S. Northeast salt marshes are vulnerable to the effects of accelerated sea level rise, and that neither precipitation changes, nor cultural eutrophication, will contribute positively to long-term salt marsh survival.
