Quantifying interior coastal marsh degradation at two North Carolina marshes

Abstract

Marshes along the coast of North Carolina are currently at risk due to ongoing land loss, and as they are highly productive waterways, understanding the processes driving land loss is critical. By focusing on two marshes adjacent to waterways — Roanoke Marsh, south of Manns Harbor, and Mackay Island National Wildlife Refuge — we created a dataset of land loss rates from 1983 to 2016, both within the marsh interior (due to expanding ponds) and on the outer edge of the marsh (coastal retreat). We investigated the hypothesized primary driver behind the interior pond expansion (wind-driven waves in the pond interior) and the coastal edge retreat of the marsh (wind-driven waves within Currituck Sound). The total land area lost over the 34- year study period was 1.49 km2 and 2.32 km2 on Roanoke Marsh and Mackay Island, respectively. The percentage of total land lost due to internal pond expansion was 60% in Roanoke Marsh and 44% in Mackay Island. Internal pond expansion is at least of equal importance to outer coastal retreat for net land loss in these coastal marshes. The local wind has a dominant direction from the north-northwest with more energetic winds during the winter. However, the wind directions and direction of pond expansion do not appear to be correlated. This may be because the winds are bimodal and drive expansion in multiple directions. In addition, there is subsidence in this portion of North Carolina that may be an additional factor contributing to the pond area expansion.