Inter-annual variability in Delaware Bay brackish marsh vegetation, USA

Abstract

Reports of sudden marsh browning, or even dieback, suggest that the many heretofore “healthy” coastal marshes have reached some tipping point with respect to sea level rise, necessitating better and more widespread monitoring. In this paper, we examine spatial and temporal variations in marsh vegetation cover, substrate wetness, and sediment exposure for mesohaline to oligohaline marshes in Delaware Bay over a 15-year period (1993–2008) using three spectral indices (the Normalized Difference Vegetation Index, the Normalized Difference Water Index, and the Normalized Difference Soil Index) based on Landsat Thematic Mapper and Enhanced Thematic Mapper + imagery. In general, degrading marsh areas show low percentages of vegetation cover compared to bare marsh substrate, and substrate wetness tends to be high. But this characterization is not consistent from one year to the next, and in marshes that are in incipient stages of degradation, apparent vegetation health can improve substantially for a few years. Detailed transect data collected from July to September in an area of Bombay Hook National Wildlife Refuge, where little marsh loss was evident, document considerable variability in vegetation dynamics. The marshes along the transect kept pace with the major transgressive pulse of the 1990s, but as the rate of sea level rise decreased after 2000, vegetation vigor fell, especially in 2004, the year after Hurricane Isabel. The years of maximum vegetation cover, 2003 and 2005, coincided with short-term, sea level high stands and/or very wet and cooler summers. We theorize that after keeping up with the dramatic rise in sea level during the 1990s, marsh surface elevations in these microtidal systems are now too high to allow adequate flushing of sulfides and low dissolved oxygen waters except for high precipitation events or short-term sea level rises. If this situation were to continue, it could affect the “health” of marshes that otherwise were accommodating high rates of sea level rise well.