Unexplained die off of salt marsh plants, particularly along creek edges and the low tide line, has become an increasing issue along the New England coast since the 1990s. Along marsh creek banks and harbor edges, salt marsh plants (particularly salt marsh cordgrass or Spartina alterniflora) began disappearing, leaving behind large swaths of exposed soil, filled with tunnels. The disappearance of grasses, both above and below ground, leaves marsh soils exposed and vulnerable to erosion (see the sidebar on marsh soils at the bottom of the post!) Given climate shifts and projected changes in sea level, salt marshes are already at risk and soil erosion from dieback makes these ecosystems even more threatened!
The Nantucket Conservation Foundation first began to document salt marsh dieback on Nantucket around 2010 (approximately 20 years after it became common on the mainland) and the dieback is still fairly minor although increasing in key marshes on island. The full picture of the cause of salt marsh dieback is still evolving but the direct cause appears to be increased feeding by the native purple marsh crab (Sesarma reticulatum). This crab is found in most New England marshes and feeds primarily on salt marsh cordgrass stems and roots while burrowing through marsh soil. Click here for an amazing GoPro video by my friend Marc Hensel of crabs feasting on Spartina! The reason why this native crab is suddenly increasing in numbers or increasing grazing is unknown but research is currently underway in many labs across New England. One hypothesis is that a decline in native predators such as striped bass, blue crabs, dogfish and cod may be allowing crab populations to increase; another is that changes in harbor water quality may be altering food availability for the crabs.
In 2015, our primary goal at NCF was to begin to take baseline data on the extent of salt marsh dieback on our properties and start simple estimates of crab populations in various salt marshes. Over June, July and August we spent many days walking through salt marshes and setting crab traps in areas of Polpis and Madaket Harbors.
Extensive areas of dieback were documented in Medouie Creek in Polpis Harbor and the adjacent Pocomo Meadows (just outside the Polpis Harbor entrance). Conversely, on the opposite end of the island, very little dieback was observed or documented in Eel Point marsh in Madaket Harbor.
In addition to documenting dieback, we set out passive crab traps in Medouie Creek, Pocomo Meadows and Eel Point Marsh. Traps are empty tennis ball cans, sunk intil the can rim is even with the soil surface - crabs will meander into this cans (fall in!) and can not escape out the slick sides. Checking traps in the morning (after 24 hours) helps guarantee the crabs survive to live another day!
Medouie Creek and Pocomo Meadows were dominated by purple marsh crabs (~78-90% of trapped crabs) with only a few other crab species mixed in - the native fiddler crab and the non-native Asian shore crab and Green crabs. Eel Point, on the other hand, had a lot fewer crabs and was primarily dominated by the native fiddler crab (~60-70%) and only 12% of crabs trapped were the purple marsh crab.
Survey results from 2015 show higher numbers of purple marsh crabs in the marshes that currently have salt marsh dieback. Marshes without dieback seem to have a higher diversity of different crab species. The question still remains, why are there more purple crabs? Decrease in predators? Some of that work may be answered by Graduate student Marc Hensel - check out his blog Marsh Life for more info on his research. For NCF, in the 2016 field season, we will be focusing our research efforts on the direct impacts of crabs on Spartina and will be caging salt marsh cordgrass to protect it from crab predation and seeing if that halts or reverses the impacts of salt marsh dieback. All of this will only help us understand the ecological processes of salt marsh dieback, not necessarily prevent or reverse it. Although, on Cape Cod and other marshes in New England, where this dieback has been in place longer, it may be cyclical with some marshes beginning to regrow as the crab populations rebalance. Our responsibility in the meantime then is to help protect our sensitive marsh soils from erosion and impacts until the ecological system of the purple marsh crab is rebalanced! Stay tuned for more information as this research progresses.
And if you are on Nantucket near a salt marsh and see what looks like salt marsh dieback - send me an email and let me know, we will add it to our database!
Marsh Soils Sidebar!
Soils can be either mineral based (sand, silt or clay) or organic based. Living on Nantucket - we are used to sandy soils!! Marsh soils are very different and primarily are organic-based with some overwashed sand occasionally mixed in. These marsh soils are typically made up of undecomposed organic material, otherwise known as peat. When soils are waterlogged particularly for a day or two, oxygen is often not present, making it a difficult and slow process for decomposing microorganisms to eat up and break down dead plant materials. Instead, when plants natural die in the fall, they accumulate on the soil surface and slowly turn into peat, becoming the soil seen in our salt marshes. As vegetation dies off, not to return, and crabs burrow through this peat, oxygen increases in the soil and it begins to decompose to organic soil. Bare marsh soils can start to break down, becoming unstable and subject to increased erosion from tides and ice. Increased marsh erosion means less salt marsh which can have negative impacts to the ecosystems of both the salt marsh and the harbor. Peat soils also accumulate and build up at very slow rates but can be quickly lost through erosion.