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GCE IV - Key Finding in 2019

    Saltwater intrusion releases N from freshwater marshes

    Freshwater marshes process and store N. But what happens when they are inundated with salt water, as is expected as a result of sea level rise? Widney et al. (2019) synthesized the results of biogeochemical studies done as part of the GCE SALTEx experiment to assess the response of a tidal freshwater marsh to saltwater intrusion. They found that continuous additions of dilute seawater resulted in increased porewater chloride, sulfate, and sulfide concentrations and lower soil oxidation-reduction potential relative to untreated plots. The concentrations of inorganic N increased dramatically, with a 50-fold increase in porewater NH4 and a 2-fold increase in NO3 relative to untreated plots. At the same time, plant biomass decreased by 50–90%, with almost complete loss of above-ground macrophytes and a concurrent decrease in plant N storage (from 27 to 11 g N/m2). These results, which are consistent with our original hypotheses (Fig. 1), suggest that sea level rise is likely to shift tidal freshwater marshes from a sink to a source of N by releasing ammonium-N from sorption sites, increasing nitrification and severely reducing N storage in macrophyte biomass. The SALTEx experimental manipulation is now complete and we are beginning to observe recovery of the system. However, these results suggest that one of the effects of sea level rise will be to release N and potentially exacerbate eutrophication of coastal waters.


    Fig. 1 Hypothesized responses of a tidal freshwater marsh to saltwater intrusion. Predicted changes are indicated as positive (+) or negative (-). Source: Widney et al. 2019.


This material is based upon work supported by the National Science Foundation under grants OCE-9982133, OCE-0620959, OCE-1237140 and OCE-1832178. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.