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DEVELOPING INDICATORS OF SALT MARSH HEALTH
Citation:
Wigand, C, R A. McKinney, AND M Charpentier. DEVELOPING INDICATORS OF SALT MARSH HEALTH. Presented at Ecological Society of America, Snowbird, Utah, Aug. 6-10, 2000.
Description:
We relate plant zonation in salt marshes to key ecosystem services such as erosion control and wildlife habitat. Ten salt marshes in Narragansett Bay, with similar geological bedrock and sea exchange, were identified to examine plant zonation. Sub-watersheds adjacent to the salt marshes were characterized by land use practices and stream nutrient concentrations. We use a space-for- time approach: gradients of low to high % residential land use and low to high nitrate concentrations in the headwater streams were measured for the salt marshes. In the Fall, there was a significant correlation (p < 0.05; r = +0.84) between total dissolved nitrogen and % residential land use among the ten sites. Ribbed mussel stable nitrogen isotopic signatures reflected the nitrogen sources from the adjacent watersheds and were significantly correlated with % residential (p < 0.05; r = +0.68). Because human wastewater is known to enrich stable nitrogen isotope ratios in organisms, our results suggest that with increasing residential development, the salt marshes are receiving increasing nitrogen loads associated with human activities. Results from plant zonation transects in the salt marshes showed a significant correlation between the areal extent ofSpartina patens in the high marsh and the total number of plant zones (p < 0.05; r = +0.69). Furthermore, the transects revealed an inverse relationship between the areal extent of S. patens and % residential (p < 0.05; r = -0.66). We propose that the low marsh species, Spartina alterniflora displaces S. patens when salt marshes are enriched with nitrogen, and we support this hypothesis with a significant inverse correlation between the areal extent of S. patens and S. alterniflora. In addition, the stable nitrogen isotopic ratio of the aboveground tissue of S. alterniflora was directly correlated with % residential (p < 0.05; r = +0.73). Since the root structure of S. patens is better than S. alterniflora at promoting peat accumulation, we propose that salt marshes are more susceptible to erosional processes when enriched with nitrogen. Finally, we describe how the number of plant zones and areal extent ofS. patens are good indicators of salt marsh condition.