Nick Herrmann
Honors Thesis Proposal

The History and Consequences of Salt Marsh Creek Bank Die-off on Cape Cod, Massachusetts

INTRODUCTION

Salt marshes are one of the most ecologically and economically valuable components of coastal ecosystems due to the critical services they provide (Costanza et al 1997, MEA 2005).  Salt marshes serve as nursery grounds for fisheries species (Boesch and Turner 1984), buffer coastal regions from erosion and storm damage (Frey and Bason 1985), and filter terrestrial runoff (Todd and Todd 1984). Because of these ecosystem services, marshes have attracted heavy human settlement and development throughout human history (Lotze et al 2006).

A salt marsh die-off epidemic has spread throughout the western Atlantic over the past two decades, threatening the services that marshes provide humans and coastal ecosystems (Bertness and Silliman 2008). Due to recent media attention (e.g. Lewis 2007), salt marsh die-off events have become a growing concern on Cape Cod in New England. New England salt marshes have been studied exhaustively for over 75 years (Johnson and York 1915, Redfield 1965), making recent die-offs, which were first noticed in 2004 National Park Service surveys (Smith 2006), entirely unexpected. Salt marsh die-offs are characterized by large swaths of denuded and eroding substrate in the low marsh and most commonly occur along creek banks. They result in the extensive loss of the primary foundation species Spartina alterniflora (cordgrass), which builds New England salt marshes by binding sediment and ameliorating physical stresses that would otherwise limit other marsh species (Redfield 1965; Hacker and Bertness 1999).

Recent surveys of a dozen Cape Cod salt marshes found that creek bank die-offs were pervasive – they were found in every marsh surveyed and have already destroyed nearly 50% of the creek bank habitat of the marshes examined (Holdredge et al 2009). Furthermore, historic aerial photographs revealed that these creek bank die-offs have been expanding at an accelerated rate, particularly in the last decade (Holdredge et al 2009, Bertness et al 2009). Current research has also shown that Cape Cod salt marsh creek bank die-offs are being driven by populations of the nocturnal herbivorous crab Sesarma reticulatum (Holdredge et al 2009). Cordgrass transplanted into ongoing die-off areas were entirely consumed by crabs in less than two weeks unless they were protected in cages. Moreover, observed grazing pressure from Sesarma was tightly correlated with the extent of die-off across a range of marshes.

Bertness and Silliman (2008) suggest that recent consumer-driven salt marsh die-off events, including those taking place on Cape Cod, are being triggered by human disturbance. Persistent disruptions in coastal food webs caused by human activities, such as excessive harvesting of predatory crabs and fish, have been linked with the development and growth of marsh die-offs. By decreasing the abundance of predators, human exploitation of fish populations is likely triggering consumer-control through increased herbivory in marsh systems that have historically been controlled by bottom-up factors such as nutrient availability (Bertness and Silliman 2008). This apparent switch from bottom up control to consumer control is leading to the loss of marsh vegetation and the subsequent loss of important ecosystem services.

RESEARCH QUESTIONS

There are two main questions that will direct my research. First, which human development activities, if any, have triggered salt marsh die-off events on Cape Cod? An informed history of the circumstances surrounding this consumer-driven loss of marsh vegetation will help resource managers and conservationists minimize future risks to valuable salt marsh habitat. I will test the hypothesis that proximity to fishing pressure has driven die-off events at a variety of Cape Cod marshes throughout recent history. I will also assess the roles of residential development and anthropogenic alterations to flow regimes in initiating die-off events.

Second, what ecosystem service losses are associated with salt marsh die-off events? The values of salt marsh ecosystem services are generally assumed but rarely quantified despite their importance for resource management and for public education in conservation. Quantifying ecosystem services will convert ongoing marsh loss into metrics that can be easily understood by conservation groups, resource managers, and the general public, thus allowing these groups to make better-informed decisions regarding the future of New England marshes. I will quantify the loss of carbon storage in marsh vegetation due to creek bank die-off across a wide range of Cape Cod marshes. I will also evaluate how die-off events affect a marsh’s ability to buffer coastal regions from erosion and storm damage.

METHODS

This study will focus on 14 salt marshes that are dispersed throughout Cape Cod and that currently exhibit varying intensities of creek bank die-off.

Assessing Human Impacts

IN PROGRESS - COMING SOON

Quantifying Ecosystem Service Loss

Loss of marsh habitat and cordgrass primary production. At each site I will quantify the current marsh area loss with field surveys and recent aerial photographs. The surveys, which were completed this summer, entailed walking along a series of representative creek banks at each site and estimating the area of denuded low marsh habitat along a given creek length. Using ArcGIS software to analyze aerial photographs available on the Massachusetts GIS database (http://www.mass.gov/mgis), I will quantify total creek length at each site and use this information in conjunction with survey data to estimate total marsh area loss.

Since cordgrass production is a good proxy for many of the ecosystem services that marshes provide (i.e. nursery grounds, chemical filters of terrestrial runoff), I will convert marsh loss area into lost cordgrass biomass production by quantifying site-specific cordgrass production per square meter in undamaged areas and then multiplying this value by the area of each site that has been lost to die-off. When the cordgrass began to flower in early August, I harvested above-ground biomass in eight 1m2 plots of creek bank cordgrass at each site and recorded wet weight and dry weight for each plot. The plots were randomly chosen within areas where the substrate was hard enough to support Sesarma burrows (Bertness et al 2009). Although cordgrass flowering indicates a peak in the ratio of above-ground vs. below-ground biomass allocation, my calculations for lost biomass production at each site will provide conservative estimates for total loss of carbon storage due to creek bank die-off.

Loss of shoreline integrity function.  Since aboveground cordgrass promotes sedimentation (Redfield 1965), the ability of marshes to build and maintain coastal shorelines by trapping and binding sediment is another major function of salt marshes that is compromised by die-offs. This loss of shoreline integrity threatens to reduce the buffering capacity of coastal marshes against erosion and storm damage, particularly in light of projected sea-level rise associated with climate change (IPCC 2007).

I will estimate the loss of sediment accumulation due to salt marsh die-off by sampling sediment deposition in and out of die-off areas. In May I sunk tube-like containers nearly flush with the marsh surface to act as passive sediment collectors in (n=10) and out (n=10) of die-off areas. Each of the 14 sites received at least 10 collection tubes, while marshes with both vegetated and unvegetated creek banks received a total of 20 tubes (10 per treatment). Sediment from the tubes will be dried and weighted, and sedimentation rates (milligrams/week) will be analyzed to estimate sedimentation loss due to die-off. The goal will be to estimate the impact that creek bank die-offs have on the loss of the sedimentation function of Cape Cod salt marshes in terms of percent potential sediment loss, which can then be converted to a conservative estimate of total sediment loss.

To capture differences in both sedimentation and erosion between vegetated and unvegetated creek banks, I will also measure changes in marsh sediment elevation using a device designed after the surface-elevation table (SET) (see Cahoon et al 2002). In late May I drove 10-foot long pieces of PVC pipe to refuse along vegetated (n=10 per site) and unvegetated (n=10 per site) creek banks across all 14 marshes. I then cut the pipes so that they extended only about two inches above the marsh surface. One week later, I measured four distinct points of surface elevation at each replicate using the adapted SET. By returning to these measurement points in late November, I will quantify the localized change in marsh surface elevation over a period of six months in die-off areas and in vegetated areas. This composite measure of erosion and sedimentation processes will further illuminate the effect of creek bank die-off on salt marsh shoreline buffering capacity.

ANTICIPATED RESULTS, SIGNIFICANCE OF RESEARCH COMING SOON

LITERATURE CITED

Bertness, M.D. & Silliman, B.R., 2008. Consumer Control of Salt Marshes Driven by Human Disturbance. Conservartion Biology 22:618-623

Boesch, D.F., & Turner, R.E. 1984. Dependence of fishery species on salt marshes: the role of food and refuges. Estuaries 7:460–468.

Bertness, M.D., Holdredge, C. & Altieri, A.H. 2009. Substrate mediates consumer control of salt marsh cordgrass on Cape Cod, New England. Ecology 90: 2108-2117.

Cahoon, D.R., Lynch, J.C., Perez, B.C., Segura, B., Holland, R.D., Stelly, C., Stephenson, G. & Hensel, P. 2002. High-Precision Measurements of Wetland Sediment Elevation: II. The Rod Surface Elevation Table. Journal of Sedimentary Research 72: 730-733.

Costanza, R et al. 1997. The value of the world’s ecosystem services and natural capital. Nature 387:253–260

Frey, R.W., & Basan, P.B.. 1985. Coastal salt marshes. Pages 225–301 in R. A. Davis, editor. Coastal sedimentary environments. Springer-Verlag, New York.

Hacker S.D. & Bertness M.D. 1999. Experimental evidence for factors maintaining plant species diversity in a New England salt marsh. Ecology 80:2064–2073

Holdredge, C., Bertness, M.D., & Altieri, A.H. 2009. Role of Crab Herbivory in Die-Off of New England Salt Marshes. Conservation Biology.

IPCC. 2007. Climate change 2007: the physical science basis, summary for policymakers. Contrib.Work. Group I Fourth Assess. Rep. Intergov. Panel Climate Change, Cambridge, UK

Johnson, D.S. & York, H.H. 1915. The relation of plants to tide levels. Carnegie Institute, Washington, D.C.

Lewis, R. C. 2007. Cape salt marsh decline linked to native crab. The Boston Globe 19 November: D1.

Lotze, H. K. et al. 2006. Depletion, Degradation, and Recovery Potential of Estuaries and Coastal Seas. Science 312:1806-1809.

Millennium Ecosystem Assessment. 2005. Ecosystems and Human Well-being: Biodiversity Synthesis. World Resources Institute, Washington DC.

Redfield, A.C. 1965. Ontogeny of a Salt Marsh Estuary. Science 147:50 – 55.

Smith, S. M. 2006. Report on salt marsh dieback on Cape Cod (2006). National Park Service, North Truro, Massachusetts.

Todd, N.J., & Todd, J. 1994. From eco-cites to living machines. North Atlantic Books, Berkeley, California.