Salt Marsh Dieback on Cape Cod

A healthy saltmarsh in Pleasant Bay (left) bears little resemblance to the West End Marsh (right), where dieback has been extensive. 
Photos/Stephen Smith, NPS

 

In some affected marshes, large swaths of marsh vegetation have disappeared. The empty areas of bare peat continue to expand as more of the saltmarsh plants perish.

Photo/Stephen Smith, NPS

What is Salt Marsh Dieback?

The word “dieback” was originally used on Cape Cod to describe areas experiencing an unexplained loss of salt marsh vegetation.

In these areas, its often just relic shoot stubble or pieces of roots that are left, with much of the soil around them having been eroded away. In other places, the decayed plant matter in the marsh called “peat” is completely gone as well, and all that remains is a smooth, bare surface.

 

Major losses of cordgrass around the edges of Saquatucket Harbor by 1991. Color infrared aerial photos show that vegetation began disappearing at this site before 1984.

Photo/Ed Lilley

When did it start?

On Cape Cod, salt marsh dieback was thought to be a recent, sudden phenomenon. It was reported by Ron Rozsa (CT DEP) and Scott Warren (CT College) in 2002 in Harwich and Chatham and independently by Stephen Smith and John Portnoy (NPS) in 2003 within Cape Cod National Seashore.

It turns out, however, that vegetation losses on Cape Cod have been happening for decades.

Both low and high marsh vegetation, losses are clear in aerial and ground-level photography as early as 1984. In some marshes, significant widening of creeks without any tidal inlet changes - an indication of vegetation loss from the edges of creeks - can be seen as far back as the 1950s.

While individual patches of vegetation may disappear during a single growing season, overall losses are generally cumulative over the course of many years. The dying plants themselves are also difficult to see.

 

The purple marsh crab is a main culprit of saltmarsh dieback on Cape Cod.

Photo/Robert Bachand

What causes it?

The reason people do not see the dying process, such as seeing dead or brown plants, is because the plants are being eaten down to stubble during the night by a species of native, nocturnal herbivorous crab called the purple marsh crab (Sesarma reticulatum), whose populations have reached very high levels.

As such, salt marsh vegetation losses on Cape Cod are distinct from dieback events in other parts of the country, where marshes are succumbing to other kinds of herbivores (including snails) and sea level rise.

 

Plants within cages grow vigorously in dieback areas while those unprotected are quickly eaten down to stubble and eventually die.

Photo/Mark Bertness, Brown University

It is primarily cordgrass (Spartina alterniflora) losses that are attributed to the intense grazing by the purple marsh crab. Since 2006, Dr. Mark Bertness, Christine Holdredge, and other Brown University students have been researching the overgrazing hypothesis.

The research team used cages around the vegetation that the crabs could not get through. The plants protected by the cages, and were thus not fed on, grew up tall and healthy, whereas those that were not protected were frequently grazed down to stubble and died.

Other studies revealed that purple marsh crab populations are very high and that they are not often preyed on in Cape marshes. The lack of apparent predators, in addition to other variables such as climate warming and sea level rise, is what has presumably allowed them to become so abundant in this area.

 

How widespread is dieback?

Other areas of southern New England are being impacted to various degrees by the purple marsh crab. In fact, marsh losses have occurred at numerous sites along the southern coast of New England and New York.

 

As the peat washes out to sea, the marsh itself disappears with it. Climate change is only expected to intensify the storms that wash away massive amounts of peat at once.

Photo/Benjamin Weihbrecht

How does vegetation loss affect salt marshes?

The vegetation in marshes grows in peat, which is partially decomposed plant material (mostly roots) that looks like soil. It is hugely important for its role in storing carbon, which is one reason marshes are so important for combatting climate change.

However, when vegetation is grazed away by the purple marsh crab, there’s nothing to hold the peat together.

Furthermore, crab burrowing behavior causes the peat to become riddled with holes, which destabilizes it further. Flow through creekbanks in the marsh causes the destabilized peat along the banks to wash out to sea. When storms hit the coast, thousands of years’ worth of peat that comprises the marsh platform and contains abundant carbon (mainly from root production) can be swept away in a single event.

 

As marsh elevation sinks due to soil subsidence and the sea level rises due to climate change, marshes are in danger of drowning in place.

Photo/NPS

Areas of marsh stripped bare of vegetation are also subject to a process called soil subsidence, meaning that the ground is sinking. This situation combines poorly with the rates of sea level rise and causes the marsh race even faster towards disappearing entirely.

 

What can be done?

Sea level rise is happening now. To try and give our marshes as much of a fighting chance as we can, scientists at Cape Cod National Seashore have experimented with several crab exclusion methods, the first being a mesh laid horizontally over the bare surface. The mesh contains holes that are large enough that fiddler crabs can go in and out, but purple marsh crabs cannot. If the mesh is put down in an area that is already bare of vegetation, not much happens.

However, if it is placed next to a patch of saltmarsh vegetation that has not been grazed away, the healthy plants will grow offshoots sideways underground that will sprout through the mesh. These plants are safe from grazing by the purple marsh crab but are still subject to the threats surrounding sea level rise.

To give the marsh a bit of an elevation boost and keep out crabs, scientists at Cape Cod National Seashore have experimented with adding sand above the peat to very small sections of the marsh. Vegetation growing in plots of sand thrived at the new elevations and were devoid of crab burrows and grazing. This method could potentially be used on larger areas of the marsh.

 

Here, the crab-exclusion mesh has been laid down beside surviving vegetation in the marsh. The marsh vegetation can then spread horizontally underground and then grow up through the mesh, where it can thrive safe from purple marsh crab grazing.

Photo/Stephen Smith, NPS

 

A non-sand-augmented plot (left) beside a sand-augmented plot (right). The plot with sand contains vegetation and no burrows, while the plot without it has been extensively burrowed in and stripped of vegetation.

Photo/Stephen Smith, NPS

 

Relevant Scientific Publications

• Bertness, M. D., & Silliman, B. R. (2008). Consumer control of salt marshes driven by human disturbance. Conservation Biology, 22(3), 618-623.
• Bertness, M. D., Holdredge, C., & Altieri, A. H. (2009). Substrate mediates consumer control of salt marsh cordgrass on Cape Cod, New England. Ecology, 90(8), 2108-2117.
• Bertness, M. D., Brisson, C. P., Bevil, M. C., & Crotty, S. M. (2014). Herbivory drives the spread of salt marsh die-off. PloS one, 9(3), e92916.
• Holdredge, C., Bertness, M. D., & Altieri, A. H. (2009). Role of crab herbivory in die‐off of New England salt marshes. Conservation Biology, 23(3), 672-679.
• Smith, S. M. (2009). Multi-decadal changes in salt marshes of Cape Cod, Massachusetts: a photographic analysis of vegetation loss, species shifts, and geomorphic change. Northeast Nat, 16, 183-208.
• Smith, S. M., & Tyrrell, M. C. (2012). Effects of mud fiddler crabs (Uca pugnax) on the recruitment of halophyte seedlings in salt marsh dieback areas of Cape Cod (Massachusetts, USA). Ecological Research, 27(1), 233-237.
• Smith, S. M., Medeiros, K. C., & Tyrrell, M. C. (2012). Hydrology, herbivory, and the decline of Spartina patens (Aiton) Muhl. in outer Cape Cod salt marshes (Massachusetts, USA). Journal of Coastal Research, 28(3), 602-612.
• Smith, S. M. Substrate Properties Affect the Recovery of Spartina alterniflora from Drought and Herbivory. In: Carlos Busso (ed), From seed germination to young plants: ecology, growth, and environmental influences, Chapter 6. Nova Publishers.
• Smith, S. M., Tyrrell, M. C., & Congretel, M. (2013). Palatability of salt marsh forbs and grasses to the purple marsh crab (Sesarma reticulatum) and the potential for re-vegetation of herbivory-induced salt marsh dieback areas in cape cod (Massachusetts, USA). Wetlands ecology and management, 21, 263-275.
• Smith, S. M. (2015). Vegetation change in salt marshes of Cape Cod National Seashore (Massachusetts, USA) between 1984 and 2013. Wetlands, 35(1), 127-136.
• Smith, S. M., & Lee, K. D. (2015). The influence of prolonged flooding on the growth of Spartina alterniflora in Cape Cod (Massachusetts, USA). Aquatic Botany, 127, 53-56.
• Smith, S. M. (2015). Does loss of salt marsh vegetation caused by a native grapsid crab improve habitat suitability for the Atlantic mud fiddler (Uca pugnax)?. Journal of Crustacean Biology, 35(5), 616-621.
• Smith, S. M., & Green, C. W. (2015). Sediment suspension and elevation loss triggered by Atlantic mud fiddler crab (Uca pugnax) bioturbation in salt marsh dieback areas of southern New England. Journal of Coastal Research, 31(1), 88-94.
• Smith, S. (2024). The effects of Sesarma reticulatum (L.) herbivory and sea level rise on creek expansion in Cape Cod salt marshes. Continental Shelf Research, 272, 105146.