Effects of salinity and water level on coastal marshes: an experimental test of disturbance as a catalyst for vegetation change

Abstract

Wetlands in many coastal areas are threatened by sea level rise. Higher sea level will increase the frequency and duration of inundation in coastal areas and, in some systems, result in higher salinity as well. While the effects of salinity and water level on coastal marsh vegetation have been widely investigated, the role of disturbance in causing shifts in vegetation due to changes in salinity or water level has received little attention. We examined interactions of disturbance (clipping of aboveground vegetation) with salinity and water level treatments in a factorial arrangement using oligohaline marsh mesocosms. The mesocosms all contained soil and vegetation from two adjoining plant communities, one dominated by Spartina patens (Ait.) Muhl. and the other by Sagittaria lancifolia L. We assessed vegetation responses quarterly for 1 year and related responses to soil redox potential ( E h), sulfide concentration, salinity, and pH. We found that salinity and water level had significant effects on species richness only following disturbance and that similar patterns occurred for aboveground biomass of dominant and subdominant species. Following disturbance, aboveground biomass of Sagittaria lancifolia was significantly reduced by salinity but not by flooding (due to a capacity for seed germination under flooded conditions and rapid vegetative growth following disturbance), while the more salt-tolerant Spartina patens was eliminated by flooding but not affected by salinity. Disturbance led to almost complete eradication of vegetation under flooded, saline conditions. In the absence of disturbance, Sagittaria lancifolia biomass increased while Spartina patens was not affected in response to flooding, and neither species was significantly affected by salinity. Responses of vegetation corresponded with lower E h in flooded and saline treatments and higher sulfide concentration in saline treatments. Our results suggest that disturbance is an important component of vegetation change in response to rising sea level, catalyzing rapid shifts in vegetation structure or accelerating wetland loss.