| Summary |
An observational and experimental study was conducted within a mainland marsh of the Virginia Coast Reserve on the Delmarva Peninsula in Virginia. This study was designed to evaluate the effects of sea-level induced high marsh subsidence and ecosystem state change on the existing high marsh plant community dominated by Spartina patens and Distichlis spicata. Five sites were chosen based on their apparent stages of progression along a proposed conceptual model of ecosystem state change. The sites were characterized by: (1) a solid turf with very little microtopographic relief, (2) prolonged flooding and development of a hummock and hollow terrain, and (3) an eroding tributary to the major tidal creek within the marsh. Water levels and interstitial water salinity were measured over a period of two years within the five sites chosen for study. Experimental flooding of bordered plots within these sites was conducted in the second year of the study. End-of-year aboveground biomass and aboveground plant adenine nucleotide concentrations and adenylate energy charge (AEC) ratios were measured to assess the effects of state change and experimental flooding on plant community composition and stress levels on individual species. There was a shift in community structure from S. patens dominance to D. spicata dominance within areas experiencing state change. While salinity levels differed significantly between sites, recorded values were well within known tolerances of both species, leaving differences in flooding patterns as the most likely cause for community change. Measurements of adenine nucleotides and AEC ratios gave results opposite from expected, with higher levels recorded in areas undergoing state change. When examined with biomass data, the increased levels of adenine nucleotides and AEC within hummocked areas of the marsh may indicate a stress response caused by plant metabolic acceleration and subsequent increased glucose consumption due to flooding-induced fermentative processes. Experimental flooding produced no measurable effects on plant biomass, and variable results in plant adenylates. Only S. patens exhibited a stress response due to flooding, with lowered adenosine triphosphate and total adenine nucleotide concentrations within flooded plots in the non-hummocked sites. While prolonged periods of inundation are stressing both high marsh co-dominants, D. spicata and S. patens, short-term effects of tidal flooding as a stressor are more evident for S. patens. Results support the observed shift in species dominance found along the sequence of state change from organic high marsh to mineral low marsh. |
