Wednesday, September 1, 2010

Landforms of the Chesapeake: Eroding Coastal Wetlands

Severely eroded tidal wetland in Southern Maryland
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Tidal wetlands are very important to the Chesapeake Bay. They act as a last line of natural filters for land-based runoff and sediment, and provide an extreme diversity of fish, wildlife, and invertebrates in the zone between low tide and high tide. However, these wetlands are being lost (converted to unvegetated mud flats) at an alarming rate due to...factors unknown! So let's back up a minute.
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As the sea level has gradually risen since the end of the last ice age (about 10,000 years ago), areas that were once marginal land were submerged. Dry land was saturated and in large areas of the Bay, turned into coastal wetlands. These areas are very dynamic - they change with tides, and with erosion and deposition from storm events. Beginning in the early 1800s, Americans filled large areas of this marginal, waterfront land to create the type of land-harbor interface we all see in the ports of New York, Boston, Chicago, Baltimore, Charleston, etc. Basically, so you could pull a horse up to a boat that drafts 15 feet of water. Hundreds of thousands of acres of wetlands were lost this way on the east coast of the United States. This direct loss of coastal wetlands probably peaked between 1880 and 1970, and has been decreasing rapidly since then, as tightening regulations and enforcement make it increasingly unprofitable to fill in coastal wetlands. Yet, wetlands continue to be lost every year. How?
Moderately eroding tidal wetland in Southern Maryland
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That's where it gets complicated. Known factors include:

1) the subsidence of coastal marshes - basically sinking under their own weight, 2) increased boat traffic, average boat speed, and wake size/force, and 3) Accelerated volumes and speed of runoff from waterfront farms and developments

Additional possible factors include:

1) the loss of SAV beds that used to attenuate stormwater flow, wave energy, and boat wakes. This loss has been ongoing, but was accelerated hugely by the impacts from Hurricane Agnes in 1972 2) continuing and/or accelerated sea level rise

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The interesting thing is that even biologists can't agree on the problem or the solution. The National Marine Fisheries Service, on the one hand, funds the restoration of these coastal wetlands in the Chesapeake Bay through what's called the Living Shoreline Initiative. Seems like a good (although costly) approach, right? On the other hand, they have a regulatory biologist on staff who thinks that unvegetated mud flats - one of the only increasing habitats (besides concrete) in the Chesapeake Bay - is more valuable to fish than coastal wetlands. And of course, anyone who has either done fish research or spent one day of their life fishing can tell you that saltwater fish prefer structure. This biologist's actual argument is that "when you look in the water on a mud flat, you can see fish - but you can't see fish in the marsh." This brilliant fisheries biologist is interested in curtailing or stopping the restoration of coastal wetlands because he can't see fish in the marsh. I guess it's not occurred to him that the fish use structure in the marsh to avoid detection.
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In Maryland, we've made slight progress on restoring lost coastal wetlands by having it turned into law. When a landowner wants to install rip rap or bulkhead (both of which have low shallow water habitat value), they are now required to justify why a "living shoreline" wetland was not feasible on their property. It's a small step - and I'll take it. Look - just for a moment - at my picture of a healthy coastal wetland in South Carolina, and contrast it to the two pictures above.
Stable tidal wetland on Folly Island, South Carolina

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