[CT Birds] New Perspective on Tidal Marsh Change

Parrot sisserou at charter.net
Wed Nov 30 20:18:37 EST 2016

Greetings, here are some new observations from the long-term studies at Barn Island as supported by some of the ecological literature (difficult to find studies of the unditched marsh).

Long-term studies of marsh change at Barn Island WMA suggest that our understanding of salt marshes is skewed by the extensive grid ditching for mosquito control in the early part of the last century - much of which was done in the 1930’s under the WPA but CT had a control program dating back to at least the 1910’s.  Last year I was challenged again to reconstruct the pre-ditching marsh landscape and discovered that the natural marsh has a levee and basin topography.  The levees are elevated high marsh adjacent to the creeks and the levees are wide and continuous.  The levees support Spartina patens - the grassy meadow habitat of saltmarsh sparrow and other marsh birds.  Between the levees and the upland are the basins (aka pannes or depressions) which support stunted S. alterniflora pannes and salt pannes (no vegetation).

Grid ditching overdrained the marsh and allowed for S. patens to expand into the basins - the colonial farmers knew this for they constructed ditched into the basins result in more patens for haying.  At Barn Island there is a farm cart path from the upland to bayside levees in order to harvest the patens on the natural levees.  Most of us know the ditched marsh to have extensive patens meadows and we know this in part to maintenance of ditches by Mosquito Control over the decades.  This is not a stable condition.  Dr. Frank Egler in 1947 found the marshes at Barn Island to be dominated by patens - only 15 years after ditching.  By 1964, the marshes were reverting to the wetter basin vegetation and patens was declining.  By 1974 Barn Island was not the wetland complex Egler described in 1947.

Aerial photo analysis reveals that the levees, as much as 55 meters wide, began to shrink following ditching and the patens of the levees were replaced by another high marsh grasses - Juncus gerardii or black grass (actually a rush).  These changes point to a lowering of high tide as the ditches are flooding the basins first and before the basins can fill (requiring an enormous amount of water) - the tide changes and the levees cease to flood.  

Many tidal wetland scientists attribute the increased wetness of the marsh to sea level rise but there is almost no study the role of ditching and now aggrading ditches upon this landscape.  At Barn Island there is a remote area (Sassafras Marsh) which has reverted to the levee and basin topography - the levees are growing (one is 30 meters wide and supporting pure patens) and the second one - still growing is but 15 meters wide.  It appears that the marshes are reverting to the natural landscape.  Connecticut College owns a natural marsh on the Thames River and it is tracking sea level rise but the ditched marshes are not (one cannot dispute this statement but it does not mean that sea level is the primary driver in the marshes becoming wetter and not keeping pace with sea level rise if they are trying to reestablish and equilibrium state).  The largest unditched marsh in the northeast is Barnstable Marsh on Cape Cod.  The unditched portions have been stable for decades and I presume the levees are the primary nesting area for the saltmarsh sparrow but it does not appear that this natural reference marsh is subject to avian studies or even basic ecological studies that would help us to understand the levee and basin topography and processes.

If marshes are reverting to the levee and basin topography, how can we accelerate levee building to create more sparrow habitat.  Prior to ditching, the extensive creek network at Great Meadows in Stratford formed nearly continuous levees near the mouth of Lewis Gut - probably due to the velocity of the water at the mouth of the Gut putting massive amounts of sediment into the downstream creek network.  At Barn Island, a secondary tidal creek passed through impoundment 3 and provided tidal flow to the marsh called impoundment 4.  Dike construction in 1947 eliminated that tidal flow but today the installation of a second culvert on the east side of this former impoundment might help to accelerate levee growth along the banks of this creek.  

A problem in some marshes - as the levees on the ditch margins - the wetland between these margins can become too wet and lose all their vegetation except for the levees.  Managers are concerned that if these levees breach - the soft sediments in the pools may wash away creating deep water.  Managers are trying various experiments to prevent signficant loss of wetland vegetation.  Some of this salt panne and pool habitat are beneficial to the wading marsh birds.  So how do we promote levee growth and maximize vegetation in the basins?  The Barn Island marshes were maintenance ditched in 1979 but in the case of the Sassafras Marsh only the eastern levee was reditched.  Today the basin is fully vegetated, no pannes or pools.  Can selective ditching be a tool to maximize marsh vegetation?  LeMay found on the Plum Island, MA marshes that the basin vegetation of the natural marsh is 10 cm higher than the compable habitat of the ditched marsh.  Wilson recently found at the same location that as the ditches aggrade, tidal creeks grow and if they incise a levee and drain a pool, tall S. alterniflora colonizes the bottom of the pool and the sedimentation rate is twice that of sea level rise.

For more information on the natural marsh, ditced marsh and reverting marsh read their modules:  http://www.sound.uconn.edu/lissm/barn_island/about.html
There is a fourth module to read called Marsh Migration.  The long-term studies at Barn Island allow us to describe the vegetation cycles associated with marsh migration, the cycles repeat themselves every 20 years.  The amount of new marsh formed in a 20 year period is one the order of 1 to 2 meters and often this is adjacent to forest, which I am told is not ideally suited for sparrows.  In some places at Barn Island there is a wide belt of patens near the upland but in many places it is stunted S. alterniflora.  The low hills of the coast will prevent formation of significant areas of new marsh.  

Ron Rozsa, Ashford
Plant Ecologist

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