Salt Marsh Primary Production and Its Responses to Relative Sea Level and Nutrients in Estuaries at Plum Island, Massachusetts, and North Inlet, South Carolina, USA

Feedbacks between flooding and plant growth that help to stabilize marshes against rising sea level are being investigated in estuaries at Plum Island, Massachusetts, and North Inlet, South Carolina. Net annual primary production of the marsh grass Spartina alterniflora has been quite variable through the years, and correlates positively with sea level during the growing season at both sites. The elevation of the marsh surface relative to mean high water determines the duration of flooding, or hydroperiod, that in turn affects plant growth. The effect of flooding was tested experimentally using an in situ bioassay to simulate growth at different relative elevations. At North Inlet, we found a parabolic response to relative elevation, with clear evidence of minimum and maximum vertical limits and an optimal elevation for growth. The Plum Island bioassay provided evidence of the super-optimal side of the growth curve. In both marshes, the responses of S. alterniflora to rising sea level, at their current elevations, are consistent with the bioassay results. This growth curve is important because it defines suboptimal elevations that are unstable for marshes and super-optimal elevations that are stable. Instability results when an increase in sea level decreases primary production, leading to declines in mineral sedimentation and sediment organic matter accretion. Conversely, stability results when rising sea level stimulates primary production, leading to increased sedimentation and organic matter accretion. There also has been interannual variability in the maximum standing biomass (a proxy for productivity) of another marsh grass, Spartina patens, but no significant correlation has been found with sea level, possibly due to methodological limitations. Finally, both Spartina species responded positively to nitrogen and have remained highly productive for 13 years of fertilization at Plum Island and 30 years at North Inlet.