| Coastal ecosystems are particularly vulnerable to global change, by virtue of their proximity to human activity and exposure to associated stressors, and also as a result of their sensitivity to sea level rise. Understanding how coastal wetlands have responded to environmental change during the past century can inform predictions about wetland response to future stress. This dissertation presents three complementary research projects examining environmental change in coastal Connecticut and New York, USA.;The first research project focused on vertical marsh response to sea level rise. Using sediment cores and water level data from 14 marshes in Connecticut and New York, I evaluated how marsh accretion, mineral and organic sediment accumulation, carbon storage, and marsh hydroperiod changed from 1900 to 2012. I found that a regional acceleration of marsh accretion began around 1940, closely following an acceleration of sea level rise. Rates of marsh accretion lagged behind rates of sea level rise for several decades after these increases, but have since caught up. During the intervening years, the marsh surface deepened slightly relative to tidal datums, causing an increase in tidal flooding. As flooding increased, mineral and organic Sediment accumulation accelerated. Mineral and organic sediment accumulation co-limit accretion, with the limiting effect of mineral sediment more pronounced in areas of Spartina patens, where mineral sediment deposition was lower. Organic accumulation was comparable across vegetation types, and was more strongly correlated with marsh accretion than was mineral accumulation. This work indicated that marsh response to sea level rise in the region is sensitive to processes affecting rates of belowground production and decomposition.;The second research project examined lateral marsh change. Understanding the causes of vegetation loss, and prescribing management solutions, requires understanding patterns of vegetation change. To advance these goals I measured rates of vegetation loss in 12 marshes in Long Island Sound, and conducted field surveys to evaluate four competing hypotheses for marsh loss: nitrogen enrichment, sediment deprivation, herbivore activity, and submergence. My results indicate that regionally, vegetation loss increased dramatically after 1974, though individual marshes varied in their precise trajectory. Within individual marshes, areas experiencing vegetation change were concentrated in lower, wetter environments such as creekbanks and mosquito ditches. Rates of vegetation loss were higher in western Long Island Sound, where high tidal ranges insulate the marsh platform from submergence but may cause the lower boundary of vegetation to be closer to physical growth constraints and more vulnerable to submergence. Vegetation change was not linked solely to one stressor; there were indications that sediment regimes, herbivory, and submergence all have regional roles in marsh vegetation change. However, nitrogen enrichment had no relationship to marsh vegetation change.;Finally, I explored the use of marshes as records of environmental quality. Chapter 4 reports mercury depth profiles and accumulation chronologies for 14 salt marshes in Connecticut and New York. Substantial spatial variation existed in the timing and magnitude of peak Hg accumulation, occurring as late as the 1980s. Hg accumulation rates in salt marshes were dramatically higher than reconstructions of atmospheric deposition, providing evidence that direct atmospheric deposition of Hg was minor relative to other contributions. Although all sediment cores showed steadily declining Hg deposition in recent decades, surface concentrations in eight of the 14 marshes remained at concentrations with the potential to affect biota. |
