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. As coastal populations increase, so does the need for an improved and more extensive infrastructure including bridging across estuaries and marshes. Mitigation is now required to alleviate the effects of shading from bridges spanning marshes. However, little research has been conducted to actually quantify the effects of bridge height and width on marsh productivity. The objectives of this research are to 1) determine the effects of bridge height and width on marsh productivity and function by directly assessing light attenuation under bridges, 2) to determine the effects of shading on the dominant salt-marsh plant species, and 3) to compare benthic invertebrate communities beneath seven highway bridges with marshes outside of bridge-affected areas (reference marshes).
Photosynthetically-available light was measured. Plant samples were clipped, dried, and weighed to determine above-ground biomass, average stem height, number of stems, number of flowers, and basal area. Soil cores were taken to a depth of 30 cm. To determine soil carbon, nitrogen, and below-ground biomass. Results indicate that both bridge height and width influenced the degree to which shading by bridges affected the underlying vegetation. All plant variables measured showed a strong bridge effect at height-to-width (HW) ratios less than 0.5 and light measurements less than 250 umol m -2s-1 under the bridges. At a HW ratio of 0.7, bridge effects were still detected, although they were greatly diminished. Of thirty-two comparisons between areas under and outside the influence of bridges having HW ratios greater than 0.7, only four significant differences were detected. Secondary production was also clearly correlated with the bridge HW ratio, (r 2=.95). Low bridges, with HW ratios of <0.7 and light attenuations of <260 mol m -2s-1 (photosynthetic photon flux units), had benthic invertebrate densities and diversity that were significantly lower than reference marshes. Density of benthic invertebrates at low bridges was 25-52% (29,685-72,920 organism/m 2) of densities measured in adjacent reference marshes (119.329-173,351 organism/m 2). Likewise, there were fewer taxa under low bridges (5.8/11.35 cm 2 core) as compared to the reference marshes (9.0/11.35 cm 2 core). Densities of numerically dominant taxa (oligochaetes, nematodes) as well as surface- and subsurface deposit feeders also were reduced by shading of low bridges. Decreased invertebrate density and diversity beneath low bridges was attributed to diminished above- and below-ground macrophyte biomass that presumably resulted in fewer food resources and available refuges from predators. 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.
