Ecosystem services in fresh, brackish and marine marshes

Overview C. Craft (IU), in collaboration with S. Pennings (UH) and S. Joye (UGA), is examining how ecosystem services vary among tidal fresh, brackish and marine marshes as part of a separately funded grant from EPA. They used field and laboratory measurements, geographic information systems, and simulation modeling to investigate the potential effects of accelerated sea-level rise on tidal marsh area and delivery of ecosystem services along the Georgia coast. Model simulations using the Intergovernmental Panel on Climate Change (IPCC) mean and maximum estimates of sea-level rise for the year 2100 suggest that salt marshes will decline in area by 20% and 45%, respectively. The area of tidal freshwater marshes will increase by 2% under the IPCC mean scenario, but will decline by 39% under the maximum scenario. Delivery of ecosystem services associated with productivity (macrophyte biomass) and waste treatment (nitrogen accumulation in soil, potential denitrification) will also decline. These findings suggest that tidal marshes at the lower and upper salinity ranges, and their attendant delivery of ecosystem services, will be most affected by accelerated sealevel rise, unless geomorphic conditions (ie gradual increase in elevation) enable tidal freshwater marshes to migrate inland, or vertical accretion of salt marshes to increase, to compensate for accelerated sea-level rise. As part of this work, S. Pennings and H. Guo (Ph.D. student, UH) evaluated the factors creating plant community structure along the estuarine salinity gradient. They sampled plant biomass and diversity at replicate fresh, brackish and marine tidal marshes on the Satilla, Altamaha and Ogeechee Rivers. A total of 109 plant species were found, but 11 species dominated the plant communities. Site-level species richness decreased across the salinity gradient (Figure 9) and did not differ among rivers (Satilla=11.3, Altamaha=13.0, Ogeechee= 12.2). Plot-level richness showed a similar pattern. Vegetation was tallest at fresh sites dominated by Zizaniopsis and shortest at salt marsh sites dominated by Spartina. Standing biomass was greatest at brackish sites and tended to decrease away from the creekbank in both brackish and fresh sites, whereas in the salt marsh sites biomass was greatest in the midmarsh zones dominated by Juncus roemerianus. Standing stocks of C, N and P were estimated based on total biomass, the relative abundance of different plant species, and the elemental composition of each plant species. Total carbon stocks paralleled patterns of biomass in that they were greatest at the brackish sites and lowest at the salt marsh sites. Nitrogen stocks decreased across sites as salinity increased and were greatest in the creekbank zone. Phosphorus stocks did not differ between fresh and brackish sites, but were lower at salty sites.