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Expansion of Pannes
Citation:
Watson, E. Expansion of Pannes. IN: Sound Update, Long Island Sound Study, Stony Brook University, Stony Brook, NY, 4, (2015).
Impact/Purpose:
The Long Island Sound Study is publishing a special issue in their journal, Sound Update, on salt marsh loss for Long Island. This special issue will be released with a State of New York funded trends analysis for Long Island coastal wetlands that shows significant loss rates (~25%) between the 1970s and early 2000s despite strong laws preventing loss to coastal development. The LISS has asked seven scientists to discuss factors responsible for wetland loss. This paragraph discusses panne expansion, or the formation of shallow depressions on marsh landscapes, which several studies report as responsible for most marsh fragmentation that is occuring in the U.S. Northeast.
Description:
For the Long Island, New Jersey, and southern New England region, one facet of marsh drowning as a result of accelerated sea level rise is the expansion of salt marsh ponds and pannes. Over the past century, marsh ponds and pannes have formed and expanded in areas of poor drainage, behind blocked mosquito ditches, and in the centerpoint of grid-ditched marsh islands. The main processes thought be responsible include accelerated sea level rise and the abandonment of mosquito ditch maintenance. Where tidal flooding is increasing rapidly, locations closer to tidal channels (<~30 m) are able to respond more robustly. Accumulation of sediment is focused along tidal creeks and better drainage allows marsh plant growth to build marsh peat. Conversely, for the marsh interior, both sediment accumulation and plant productivity are limited, which contributes to the formation of shallow depressions and marsh drowning. The shift from mosquito ditch maintenance to open water marsh management is also implicated in pond and panne formation and expansion. Where channels become blocked, or shift from facilitating to preventing drainage, areas of stagnant water develop, and plants become stunted or die. Pond and panne expansion are concentrated in areas with low tidal range (>1m). Where tidal range is greater, channel incision leads to the cyclic formation and draining of such features with creek incision (Wilson et al. 2014). While published studies indicate that interior ponding and rapid expansion are the largest contributor to contemporary marsh loss in the region (Kearney et al. 1988; Hartig et al. 2002; Smith 2009), interior ponds are also common features and an ecologically valuable component of marsh landscapes (Adamowicz and Roman 2005), making it important to develop a distinction between stable ponds and those contributing to long-term and directional marsh vegetation loss.
