Executive Summary
As population pressure increases along the Georgia coast, coastal managers require more and better data regarding coastal resources and human impacts to these resources to carry out their mission, particularly in the expansive and productive salt marshes. Understanding the impact of private recreational docks on saltmarsh ecosystems is considered by many to be a critical need, given that these structures shade the marsh and that their numbers are increasing rapidly with little understanding of their cumulative effects.
Until recently, no systematic study had been carried out examining this issue in the southeastern US, with the exception of one local study in SC (Sanger and Holland 2002). To address this data need, the Georgia Coastal Zone Management Program supported a study of dock proliferation and shading impacts on Wilmington Island, GA in Chatham County (Alexander and Robinson, 2004). The results of that study document a 90% increase in total dock area and a 73% increase in number of docks from 1970 to 2000. Approximately half of the total dock area in 2000 was constructed above, and thus overshadowing, the ubiquitous Spartina alterniflora saltmarsh vegetation. The shading effect created on average a 56% decrease in vegetation stem density beneath docks when compared to areas adjacent to docks. This stem density reduction represents a potentially important and previously unquantified term in the carbon budget of the marsh, which provides food and critical habitat for many commercially important species.
The present study quantifies the stem density, biomass and carbon produced in the Spartina marsh at sites shaded by private recreational docks and at sites adjacent to those under-dock sites. Plots were sampled by clipping all the grass in 0.25 m2 quadrats and separating the vegetation into dead and living groups. The living group was further sorted into short and tall subgroups. All vegetation was dried and weighed to determine the biomass within each group. An average of 87% of the biomass in each plot was contained within the living, tall stem group. Similar to the previous study, stem density was reduced under docks an average of 50%. Living aboveground biomass was 154 – 825 g/m2 and total aboveground biomass was 249 – 1226 g/m2 , similar to other studies from the southeastern US and the average carbon production was 167 gC/m2. Detailed height measurements demonstrated that tall vegetation was taller beneath the dock than in control sites, probably as a result of etoliation in response to shading. These detailed height data were used to calculate the average weight per stem and weight per linear centimeter, allowing us to address the issue of plant robustness at dock versus control sites. Lower stem densities of tall plants were associated with higher average biomass per stem. Although tall vegetation under the dock was taller and contained more mass per stem than those in the control sites, the decrease in stem density was significant enough to offset this increase in individual stem mass.
The 50% stem density reduction results in a consequent reduction between 21-37% of biomass and carbon produced per meter square under a dock structure. Applying the present results to the Alexander and Robinson (2004) study of Wilmington Island, the organic carbon reductions predict that the unrealized organic carbon contribution to the saltmarsh ecosystem under the average modern dock (123 m 2 ) would be between 4.3-7.6 x 103 gC per dock per year. By applying this 21-37% decrease to all docks on Wilmington Island to assess impact on an island-wide basis, dock footprints over vegetation are presently reducing the available organic carbon between 0.84-1.5 x 106 gC/y. Using State-wide data for dock numbers and sizes, these reductions suggest that private recreational docks are reducing organic carbon input between 1.0-1.7 x 107 gC/y. The Kneib (2003) trophic model shows that the 21-37% decrease in biomass equates to reductions of 0.5-0.9 g dw nekton/m2 in total annual primary nekton production. For migrant nekton (i.e., penaeid shrimp and finfish), which make up 33% of the total nekton, primary nekton reductions are between 1.8-3.2 x 104 g ww nekton around Wilmington Island and between 0.6-1.2 x 106 g ww State-wide. Carrying this analysis further to determine the potential loss of harvestable-size, migrant nekton awaits species-specific biological data. This assessment of dock shading impacts should be refined by further research and points to the importance of assessing cumulative impacts of human activity in the coastal zone as these impacts are concentrated in critical nursery areas for commercial species.
