GA Tech Ocean Science and Engineering Presents Dr. James Morris, Baruch Institute and Dept. of Biol. Sciences, University of South Carolina

The objectives of this research have been to learn how salt marsh vegetation responds to sea- level rise (SLR) and when rates of sediment accretion will not be sustainable. This talk will focus on long-term data collected in marshes at North Inlet, SC since 1984. 

This work has led to development of a marsh equilibrium theory now widely used in models of sediment accretion. Marsh plants (Spartina alterniflora) regulate marsh elevation according to the rate of sea-level rise and flooding frequency. There is a range of flooding tolerance and an optimum relative elevation for plant growth. 

The equilibrium elevation is a function of the rate of SLR, suspended sediment concentration, and productivity. This is key to understanding how marshes keep pace with sea level. The rate of SLR at North Inlet now exceeds the accretion rate, and marshes there will drown in coming decades. Applying this model to other estuaries, we predict that marsh survival is proportional to tide range. 

Marsh plants also orchestrate the nutrient cycles that support their productivity, and at North Inlet these cycles show a high degree of order and self-determination. The inventories of N and P in porewater are small fractions of what is required to support primary production. 

Primary production is N-limited and this is consistent with the N:P ratio declining from 14 during early spring to 5 in late summer. There are losses, especially of N, from drainage, denitrification, and export of organic production. These have to be compensated by gains from N fixation, most likely from coupled sulfate reduction. 

There is also a coupling between sulfide, iron, and phosphorous that appears to conserve P during the winter and generate soluble P during the active growing season. These couplings coordinate the biogeochemical cycles, and this extends to the tidal creeks that are dominated by benthic sources of nutrients.