MARSH PLANTS ENHANCE COASTAL MARSH RESILIENCE BY CHANGING SEDIMENT REDOX CONDITIONS IN AN URBAN, EUTROPHIC ESTUARY

Coastal wetlands are subject to increasing threat from eutrophication, sea-level rise, and coastal development. Despite considerable efforts to restore wetlands and the many services they provide to coastal communities, the ecological mechanisms contributing to the success or failure of marsh restoration are rarely assessed. The accumulation of hydrogen sulfide in marsh sediments is one mechanism that may accelerate rates of marsh loss in eutrophic estuaries. We made use of a chronosequence of restored marsh islands in Jamaica Bay (New York, NY) to assess how sediment redox conditions, specifically oxygen and sulfide depth profiles, change among seasons and over the lifetime of young restored marshes (restored 2003-2012). We also examined an extant marsh that has remained stable, and one that has deteriorated in extent and sediment stability over the past 50 years. We collected sediment cores from each marsh during the winter, spring, summer, and autumn of 2015-2016, and used a motorized microprofiling system to measure the vertical distribution of oxygen and hydrogen sulfide. We fit a logistic function to each depth profile to estimate three parameters: (1) maximum concentration, (2) rate of increase/decline, and (3) depth of maximum increase/decline. For each core, we also quantified sediment density, porosity, organic matter content, and belowground plant biomass. To estimate differences in tidal inundation among sites, we deployed HOBO water level loggers at each marsh for at least one lunar phase cycle in autumn 2016. Our results indicated that minimum oxygen concentrations and maximum sulfide concentrations occur during the summer. Sulfide concentrations were highest in sites that experienced the greatest period of daily tidal inundation, which included the degraded extant marsh and the oldest restored marsh. Spatial patterns in oxygen and sulfide were both strongly related to belowground biomass of the plant community, indicating that root growth increases sediment oxygen and partially alleviates sulfide stress. Our data suggest that belowground plant growth enhances the resilience of extant and restored marshes to sea-level rise and eutrophication by increasing marsh elevation and facilitating oxygen diffusion into anoxic marsh sediments.