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Population pressure along the Georgia coast has greatly increased the number of docks that extend across the marsh, impacting marsh ecosystems. To understand the patterns and impacts of dock proliferation, aerial photography and field data were used within a geographic information system (GIS) to quantify the number of docks and the area of docks affecting marsh. Digitising the perimeter of docks, we created maps showing the footprint of docks from 1970 to 2000 on Wilmington Island, GA. These maps document a 90% increase in total dock area (24,048 m2 in 1970 to 45,679 m2 in 2000) and a 73% increase in number of docks during this period (174 docks in 1970 to 301 docks in 2000). Indicators of shading impacts by these docks to the Spartina alterniflora marsh (e.g., variation in stem density and plant height) were quantified during the summer of 2003. We established 56 transects beneath 52 docks, sampling beneath and 5m adjacent to the docks with a 0.1 m2 quadrat. Average stem density was 56% lower beneath docks than adjacent to docks and paired data were statistically different. Plant height was not significantly different beneath or adjacent to docks. Although existing dock structures only shade 0.5% of the marsh surrounding Wilmington Island, approximately 1% of the marsh may eventually be shaded assuming current parcel configurations (i.e., no subdividing will occur) and maximum estimates of cumulative impact suggest that 4-6% of the marsh could be shaded under current Georgia law. Although presently unquantified, this shading and stem density reduction may be important for the carbon budget of the marsh, which provides critical habitat for many commercially important species. Additional information needs identified during this study include: an assessment of the potential for and significance of enhanced Spartina wrack accumulation around dock structures; a determination of the impact to benthic habitats by floating docks that sit on the bottom at low tide; and a comparative analysis of coastal county dock statistics with the baseline and trend data in this study.
Author: Alexander, CR and Robinson, MH
Year: 2004
Document Source: Database
Avalability: Digital
Interaction: Shading
Confidence: Good
Quantified: Quantified
Interaction significance: Stem density of Spartina alterniflora was reduced 56% directly beneath docks when compared with adjacent areas. For management purposes however, we need to quantify how these reductions in stem density are related to the carbon budget of the saltmarsh system. Currently, the impacts docks are having on the carbon budget are unknown.
Purpose of research: University based research report
Context: Study in the US.
Mitigation/Best practice: Not a focus of the study
Download the report: Impacts of Recreational Docks on the Saltmarshes of Georgia.pdf (1,774.80 KB)
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