Effects of Shading by Bridges on Estuarine Wetlands

Abstract

Salt and brackish water marshes are integral parts of the coastal ecosystem, performing important nutrient cycling and hydrologic functions as well as providing habitat and breeding grounds for many coastal species. Despite their many benefits estuarine marshes have faced continued degradation in the past, which has led to a no net loss policy of these areas. As coastal populations increase so does the need for an improved and more extensive infrastructure including bridging estuarine marshes when building roads. Presently, mitigation is required to alleviate the effects of shading from bridges spanning marshes; however, mitigation is expensive and increases construction costs. Little research has been conducted to quantify the effects of shading by bridges on marsh function and productivity. These effects must be quantified to understand the effect of bridge height and width on the underlying marsh. Should the effects of shading be minimal then mitigation requirements may be reduced, in turn reducing construction costs. The objectives of this project were to determine the effects of bridge height and width on marsh productivity and function, directly assess light attenuation under bridges, and compare the effects of shading on the dominant salt marsh species. Seven different bridges representing various height and width combinations were selected for study. Measurements were taken at random both under and beyond the influence of these bridges. Photosynthetically available light was measured. Plant samples were clipped, dried and weighed to determine aboveground biomass, average stem height, number of stems, number of flowers and basal area. Soil cores were taken to a depth of 30cm and C and N were determined.

Both bridge height and width heavily influence the degree to which shading by bridges effects the underlying vegetation. All plant variables measured show a strong bridge effect at HW ratios less than 0.5 and light attenuation less than 250 umol m-2s-1 under the influence of the bridge. At a HW ratio of 0.68 a bridge effect was still detected although it was greatly diminished. Thirty-two comparisons were made between areas under and outside the influence of bridges having HW ratios greater than 0.7, only four significant differences were detected. Regression analysis shows a clear correlation between marsh productivity and bridge HW ratio, (r2=.95). Data indicates that shading by bridges having HW ratios above 0.7 do not adversely impact the productivity or function of the underlying marsh. The best design is a bridge greater than 9 m over the underlying marsh that is narrow enough to keep the HW ratio greater than 0.7.