Freshwater sediments in Sea Level Rise Microcosm

As sea level rise impacts coastal wetlands, saltmarsh will overtake freshwater marsh in many areas, but changes in the sediment microbiome in response to saltwater intrusion are difficult to predict. Freshwater marsh sediment was exposed to ambient, brackish, and saline conditions as well as to elevated nitrate and phosphate to model the combined stresses of saltwater intrusion and coastal eutrophication. Initially, sediment bacterial and archaeal composition was similar to prior studies of freshwater marsh but diverged over time proportionally to the increase in salinity. There was no observed effect of nutrient amendment. Although the previously described loss of methanogenic populations and promotion of sulfate reducers in response to saltwater exposure was observed, taxonomic distribution was not similar to typical meso-polyhaline wetlands, representing instead a transitional microbial community. Without colonization by marine taxa, such a community may be short-lived naturally, ultimately equilibrating with more common saltmarsh species. However, such a transitional community may be informative for freshwater marsh not adapted to salinity pulses as marsh migration progresses.