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Sea Level Change

marsh community
Salt marsh ecology is highly sensitive to changes in sea level. Investigating past changes in marsh flora through sediment cores may give us insight into the future of our wetlands. Enlarge »
sea level rise
Spatial patterns of sea level rise over the past 7000 years along the Northeastern US coastline. Enlarge »
barn island sea level
Sea Level Reconstruction from Barn Island, CT (Donnelly et al., 2004). Enlarge »

Sea level change dictates the long-term evolution of coastal systems, ecologically and geologically. Furthermore, in the modern world coastal systems are home to a large segment of the world’s population. In order to more accurately project future changes in sea level and mitigate socio-economic impacts on human populations, we need to better understand the causes of sea-level change and how these changes may be linked to climate variability. Namely, we must distinguish between natural and human-induced climate and sea-level changes. To accomplish this, it is necessary to extend the relatively short instrumental record of sea level change (~120 years) with paleoclimate information deduced from sediments. The coastal group at WHOI has several projects aimed at quantifying the direction and rate of sea-level change and its influence on coastal evolution on a variety of temporal and spatial scales.

Marsh Community Dynamics
Salt-marsh communities are sensitive areas that respond dynamically to changes in sea level. Coastal sediments have the potential to preserve plant macrofossil evidence of past changes in marsh community-composition, thus providing insight into how sea level rise in historic times has affected the ecology of the marsh.
New England provides an ideal setting for such studies, where salt-marsh communities are linked to the magnitude, frequency and duration of tidal inundation. For example, from our sediment cores we are able to document recent migration of low-marsh cordgrass (Spartina alterniflora) into the high marsh. Isotopic dating indicates that the initiation of cordgrass migration occurred in the late 19th century and is coincident with a recent acceleration in the rate of sea-level rise. If current rates of sea-level rise continue or increase slightly over the next century, New England salt marshes will be dominated by cordgrass. If climate warming causes sea-level rise rates to increase significantly over the next century, these cordgrass marshes will likely drown, resulting in extensive losses of coastal wetlands.

Marsh Drowning
The recent expansion of water-logged pannes (potholes) in salt marshes throughout the northeastern U.S. is attributed to increased tidal flooding associated with accelerated rates of sea-level rise. In fact, the significant expansion and coalescing of pannes has been invoked as a mechanism leading to extensive salt marsh loss in some areas. However, very little work has examined the long-term evolution of pannes. Core samples obtained from non-panne areas of salt marshes from all over the northeastern U.S. show little evidence of past panne occurrence and may indicate that extensive panne development is a relatively recent phenomenon. Paleo-environmental reconstructions of modern pannes can help answer the following fundamental questions: 1) when did modern pannes develop? 2) are they changing in aerial extent? 3) if they are expanding or contracting what is the rate and direction of change? 3) has the rate and direction of expansion and/or contraction changed over time? and 4) is there a correlation between panne expansion and warming climate and increased rates of sea level rise? In the face of these ongoing and potential alterations we need to understand how the productivity of coastal wetlands may change, how changes in plant communities will impact migratory bird and other faunal populations, and how the loss of coastal wetlands will impact other coastal ecosystems.

For more information please contact:

Jeff Donnelly (jdonnelly@whoi.edu)

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