The effects of Sesarma reticulatum (L.) herbivory and sea level rise on creek expansion in Cape Cod salt marshes

Abstract

High densities of herbivorous purple marsh crabs (Sesarma reticulatum) have caused major vegetation loss in salt marshes across Cape Cod (Massachusetts, USA). As creekbanks are a preferred habitat for this species, much of the damage is concentrated along these edges, resulting in an elevated potential for creek widening through erosion and bank slumping. The presence and/or spatial distribution of S. reticulatum is variable across this region with impacts ranging from minor to severe, and this provides an opportunity to distinguish between background vs. S. reticulatum-enhanced widening. To accomplish this, GIS tools were used to delineate portions of tidal creek systems in 18 marshes, half of which have experienced substantial creekbank vegetation losses from crab-driven herbivory. Delineations were based on georeferenced aerial photography between 2005 and 2021. In addition, creek widths at discrete locations in the delineated systems were measured in each photo year using GIS tools to estimate rates of bank retreat. A smaller number of narrow <2.0 m mosquito ditches were analyzed in the same way to further stratify the data by initial width. Between 2010 and 2021, all tidal conduits ranging from the larger, natural creeks to mosquito ditches widened at a rate that was significantly higher in marshes with no Sesarma damage. While all marshes are under threat from SLR, creek-widening in those experiencing intense crab herbivory may have a greatly shortened lifespan.