For PIE marshes, onward must be upward

Salt marshes and other coastal wetlands are on the front lines of sea level rise.  Marsh survival, and the persistence of the buffering capacity they provide between the oceans and the uplands, depends on their ability to increase their elevation at a rate that at least equals the rate of sea level rise.   For marshes that cannot gain elevation by migrating to higher ground due to natural or man-made barriers, elevation increase must be accomplished by accumulating sediment or organic matter, a process called “accretion”.  This is the situation for PIE marshes, which are facing rapid sea level rise.   Are PIE marshes keeping up?

Currently, most of PIE salt marsh area is “high marsh”, dominated by the marsh grass Spartina patens, or salt marsh hay.  High marsh exists at an elevation above mean higher high tides; that is, these marshes are only flooded during extreme high, or spring, tides.  “Low marsh” areas at PIE, which are flooded at least partially during every high tide, are found along the creek edges.  These marshes are dominated by smooth cordgrass, or Spartina alterniflora, which is adapted to tolerate the frequent flooding. 

PIE scientists have been measuring the rate of elevation change in these marshes using sediment elevation tables (SETs).   The location and elevation relative to established benchmarks (NAVD88) of each SET installation is determined using high-precision GPS equipment.   Each SET consists of a stable, horizontal arm or plate that is suspended above the marsh surface on a support pole that was driven into the ground to the point of “refusal”.  Thin rods are lowered through the plate to the marsh surface, and the distance from the plate to the surface is precisely measured.  Accretion rates are derived from our long term dataset of changes in these measurements.

We have found that accretion rates differ with elevation and with vegetation type.   At present, low elevation sites (~0.9 m NAVD88) dominated by S. alterniflora are accreting at rates ranging from about 6.5-10.5 mm/year.  High elevation sites (~1.35 M NAVD88) dominated by S. patens are accreting more slowly at about 2.5 - 5.5 mm/year.  This means that at the historical rate of sea level rise of 2.8 mm/yr, most of the marshes would be able to keep up.  However, at the current accelerated rate of 4 mm/year, most high marsh will not be able to accrete fast enough and will drown.  Under this scenario, the surviving marshes will transition to predominantly S. alterniflora low marsh, and will occupy a smaller area, sandwiched between wider and deeper creeks and upland geology.  Understanding the impact that this change in marsh geomorphology will have on marsh food webs, fish growth, and the ability of marshes to provide storm buffers for coastal communities is a current research focus for the PIE LTER.

for more information, see Wilson, 2014