Tipping Points

PIE scientists have discovered marshes have "tipping points" beyond which sediment accumulation fails to keep up with rising sea level and the marshes drown. These tipping points vary regionally and are influenced by human activities such as dam building and land clearing that affect sediment transport from the watershed.

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. 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.

Aboveground primary production at PIE as a function of mean high water level during the summer of measurement.
(Morris et. al. 2013)
Results of bioassay experiments showing the end- of- season standing biomass of S. alterniflora vs relative elevation at Plum Island (photo Morris)
Long-term measurements of sediment accretion are being made at a number of marshes along the east coast including Plum Island. Measurements of recent sedimentation are made using marker horizons and total new sediment accretion is measured using "sediment elevation tables" (SETs). SET measurements incorporate the change in marsh elevation from recent sedimentation and belowground biomass as well as elevation losses from subsidence.

Our 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. We have shown empirically that there is an optimum relative elevation for maximum primary production and, furthermore, that the relative elevation of Plum Island's marshes exceeds the optimum for vegetative growth. The equilibrium elevation is inversely related to the rate of sea-level rise.

Our 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 tide range. We predict that Plum Island's marshes will not keep pace with rising seas, though the ultimate demise of Plum Island marshes 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, i.e. 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.

For further reading:

  • Morris JT, Sundberg K, Hopkinson CS.  2013.  Salt marsh primary production and its responses to relative sea level and nutrients. Oceanography. 26:78-84.
  • 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.

For further information:

James Morris (morris [at] inlet [dot] geol [dot] sc [dot] edu)