Tipping Points

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Results of bioassay experiments showing the end-of-season standing biomass of S. alterniflora vs relative elevation at Plum Island.
James Morris

PIE scientists have documented that salt marsh primary production responds to sea level anomalies at several locations along the east coast of the United States. At Plum Island, salt marsh primary production is nearly twice as great during high sea level years as opposed to low sea level years (Fig. 1). Many marshes are perched high in the tidal frame at an elevation that is super-optimal for the vegetation, and when sea level is anomalously high during summer months (it can vary by as much as 10cm), primary production responds positively.

Experimental studies, where marsh plants are grown at different elevations relative to mean sea level, support the findings of the long-term field measurements and show production is greater when plants are grown below the current marsh platform (photo).

PIE scientists are taking long term measurements of sediment accretion at a number of marshes along the east coast, including Plum Island. The measurements are made using marker horizons for recent sedimentation and "sediment elevation tables" (SETs) for total new sediment accretion. SET measurements incorporate the change in marsh elevation from recent sedimentation and belowground biomass as well as elevation losses from subsidence.

The research on salt marsh productivity, coupled with studies on sedimentation, have led to the development of a theoretical model that explains how marsh landscapes maintain equilibrium with sea level. This model and field experiments have demonstrated that marsh accretion rates vary with sea level, flooding frequency, sediment supply, and nutrients. The studies have shown empirically that 1) there is an optimum relative elevation for maximum primary production, 2) the relative elevation of Plum Island's marshes exceeds the optimum for vegetative growth, and 3) the equilibrium elevation is inversely related to the rate of sea-level rise.

PIE’s model predicts that there is a tipping point that will result in the irreversible loss of salt marsh habitat. The tipping point depends on factors such as sediment supply from surrounding watersheds and tidal range. The model predicts that Plum Island's marshes will not keep pace with rising seas, though their ultimate demise will play out over decades. Human alteration of watersheds, by affecting sediment erosion and transport, has the potential to enhance or compromise the marshes' ability to keep pace with accelerated sea level rise over the next century. Additionally, 14C analyses of sediments suggest that the existing peat marshes are cannibalizing themselves, in that eroded peat from creek banks is deposited on the remaining marsh surface, which can increase sedimentation on interior marsh surfaces, but at the expense of total marsh area.

Annual aboveground marsh production (Spartina alterniflora) vs mean high water level at Plum Island.
Morris (unpublished)
For further reading: 
Morris, J.T. 2005. Effects of changes in sea level and productivity on the stability of intertidal marshes. In: Lasserre P:, Viaroli P., Campostrini P. (eds) Lagoons and coastal wetlands in the global change context: Impacts and management issues Proceedings of the International Conference, Venice, 26-28 April 2004. ICAM Dossier No3, UNESCO, pp. 121-127.
Kirwan, M.L., G.R. Guntenspergen, and J.T. Morris. 2009. Latitudinal trends in Spartina alterniflora productivity and the response of coastal marshes to global change. Global Change Biology 15:1982-1989.
Mudd, S.M., S. Howell, and J.T. Morris. 2009. Impact of the dynamic feedback between sedimentation, sea level rise, and biomass production on near surface marsh stratigraphy and carbon accumulation. Estuarine, Coastal and Shelf Science 82:377-389.
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James Morris
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