Responses of Coastal Wetlands to Rising Sea-Level Revisited: The Importance of Organic Production

Abstract

Abstract A network of 15 Surface Elevation Tables (SET) at North Inlet estuary, SC, have been monitored on annual or monthly time scales beginning from 1990 to 1996. The initial elevations spanned a range from suboptimal to superoptimal relative to the vertical growth range of the dominant vegetation, Spartina alterniflora. Of 98 time series, 20 have elevation gains equal to or exceeding the local rate of sea-level rise (SLR, 0.34 cm/yr). The elevation gain in North Inlet is dominated by organic production and, we hypothesize, is proportional to net ecosystem production. The rate of elevation change was 0.47 cm/yr in plots experimentally fertilized for 10 years with N&P compared to nearby control plots that have gained 0.1 cm/yr in 26 yr. The excess gains and losses of elevation in fertilized plots are accounted for by changes in belowground biomass and turnover. This is supported by bioassay experiments in marsh organs where in 3 years the belowground biomass of fertilized S. alterniflora plants increased by 1,772 g m-2 yr-1, which is equivalent to 2.1 cm/yr. Root biomass was greater in the fertilized treatment than in controls, but in both treatments, roots plateaued at about 973 g m-2 and 613 g m-2, respectively. Growth of belowground biomass was dominated by rhizomes, which continued to grow at a rate of 1,227 g m-2 yr-1 in the fertilized treatment after 3 years. Wetlands like North Inlet could be classified as autonomous because they depend on in situ organic production to maintain elevation. Autonomous wetlands are more vulnerable to SLR because their elevation gains are limited by net ecosystem production whereas minerotrophic wetlands are limited ultimately only by the mineral sediment supply.