Disturbance in Georgia salt marshes: variation across space and time

We documented the frequency and effect on live biomass of five different types of disturbance over 14 years in creekbank and mid-marsh zones of eight salt marshes dominated by Spartina alterniflora in Georgia, USA. Wrack (floating debris) and creekbank slumping were the most common disturbances at the creekbank, and snails were the most common disturbance agent in the mid-marsh. Disturbance frequency varied among sites due to differences in plot elevation and landscape position. Wrack disturbance at the creekbank was positively correlated with plot elevation, and both initial slumping and terminal slumping of creekbank plots were negatively correlated with plot elevation. Wrack disturbance at the creekbank and snail disturbance in the mid-marsh were also most common at barrier island vs. interior marshes. Disturbance varied up to 14-fold among years. Wrack disturbance at the creekbank was negatively correlated with river discharge and sea level, and initial slumping of creekbank plots was also negatively correlated with sea level. The different disturbance types varied in their effects on end-of-year standing plant biomass. At the creekbank, wrack disturbance reduced biomass in affected plots by similar to 46%, but slumping did not affect biomass until the plot was totally lost. In the mid-marsh, slumping and wrack were not important disturbances, but snail disturbance reduced biomass in affected plots by similar to 70%. In addition, abiotic conditions (river discharge, maximum monthly temperature, sea level, and precipitation) strongly affected year-to-year variation in biomass. Across the entire landscape, fewer than a quarter of the plots on average were disturbed, and disturbance reduced overall standing biomass by similar to 18% in the creekbank zone and similar to 3% in the mid-marsh zone. Our results indicate that wrack has fairly strong effects on end-of-year biomass at the creekbank. Overall, however, variation in abiotic conditions among years had stronger effects on end-of-year standing biomass in both marsh zones than did disturbance.