LONG ISLAND SOUND SALT MARSHES: ACCRETION AND RELATIVE SEA LEVEL RISE

The resilience of coastal salt marsh environments at times of rising sea levels may be evaluated through reconstruction of past rates of relative sea level rise (RSLR). Foraminifera (eukaryotic, unicellular, heterotrophic organisms), are abundant in salt marshes, and their species distribution in intertidal settings is primarily controlled by the fraction of time that the salt marsh surface is covered by sea water, a function of the elevation of the marsh surface and the local tidal range. Foraminiferal assemblage composition in peat samples dated using Hg-pollution profiles, ²¹⁰Pb, ¹³⁷Cs and ¹⁴C, can thus be used to reconstruct the position of each sample in a core with regard to mean sea level, so that we can infer past rates of RSLR. The Jarvis Creek coastal salt marshes (Branford, CT) were divided in an inner and outer part by a tide gate, built in the 1890s but largely destroyed in 1979. We used one core outside and one inside the tide-gate location to reconstruct rates of RSLR as well as marsh environments. Before the construction of the tide gate both core sites were in a high-marsh setting, with vegetation dominated by Spartina patens and high-marsh foraminifera (e.g., Trochammina macrescens, Trochammina inflata and Tiphotrocha comprimata). This high-marsh environment persisted in the outer marsh through today. In the inner marsh, building of the tide gate resulted in establishment of Phragmites, dominating the inner marsh in 1979. Core samples with Phragmites roots have high-marsh foraminiferal assemblages, but when tidal flow was restored in 1979 low marsh vegetation (Spartina alterniflora) became established, and foraminiferal assemblages became characterized by low marsh species such as Ammotium salsum and Miliammina fusca, showing that marsh accretion had not kept up with RSLR. Accretion rates in the newly established low marsh averaged 5.5 mm/yr, thus outpacing recent rates of relative sea level rise of about 3 mm/yr. As a result, the abundances of the lower marsh species have decreased since 1979. Coastal salt marshes thus are resilient and able to recover from human activities that restrict tidal flow; accretion rates in such restricted marshes decline, so that high-marsh environments may be replaced with low marsh.