Tidal Marsh Nutrient Allocation and Plant Diversity Along a Salinity Gradient.

Increased salinity driven by sea-level rise is expected to change tidal marsh plant and nutrient distributions. Spartina alterniflora, considered a poor interspecies competitor, expanded its range in the lower Chesapeake Bay into an interior, previously freshwater area largely co-dominated by Peltandra virginica and Zizania aquatica. We explored the unexpected appearance of S. alterniflora through measures of plant diversity, soil and vegetative nutrient content, and phosphorus pools in each of four marshes along a salinity gradient [freshwater, fresh+ (transitional), oligohaline, mesohaline]. Percent cover was estimated in June and September 2011 to capture intra-seasonal changes. Soil material and plant tissues were collected monthly June - October 2011 and analyzed for carbon (C), nitrogen (N) and phosphorus (P) content. A Bray-Curtis ordination showed the mesohaline marsh composition (% cover) clearly different from the other marshes, with the oligohaline site separating from the cluster of lower salinity marshes. Co-dominants at the freshwater end member of the gradient shifted to a clear dominant species at the oligohaline site, suggesting potential competitors with higher salt tolerance would need to outperform only a single species. The two species abundant at all but the mesohaline marsh shifted C and N content and ratios throughout the growing season, but there was no difference along the gradient. Soil C and N were significantly higher (p<0.05) at the freshwater marsh relative to other marshes. Phosphorus appears to be affected by a sediment shadow in the low salinity marshes, with content highest at the mesohaline marsh for most of the dates sampled. N:P suggests probable P limitation in all but the mesohaline marsh. Freely available inorganic P increased at the mesohaline marsh, although small in magnitude. Organically bound P was also highest at the mesohaline site, which may reflect plant detritus from stressed plants and deserves further investigation.