Effects of Shading from Bridges on Estuarine Wetlands

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 human populations in coastal areas increase, so does the need for an improved and more extensive infrastructure including bridging across estuaries and marshes when building roads. Very little research has been conducted to quantify the effects of shading on marsh function and productivity, and to determine the amount of mitigation that should be required to offset these effects. The objectives of this research were to determine the effects of bridge height and width on marsh productivity and function by directly assessing light attenuation under bridges, determining the effects of shading on the dominant salt marsh species, and comparing 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. 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 attenuation less than 250 umol mˉ²sˉ¹ under the bridges. At a HW ratio of 0.68 bridge effects were still detected, although it was 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. Regression analyis showed a clear correlation between secondary productivtiy and bridge HW ratio, (r²=.95). Low bridges, with HW ratios of <0.7 and light attenuation <260 mol mˉ²sˉ¹ (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 organisms/m²) of densities measured in adjacent reference marshes (119,329-173,351 organisms/m²). Likewise, there were fewer taxa under low bridges (5.8/11.35 cm² core) as compared to the reference marshes (9.0/11.35 cm² core). Density 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 and may not require compensatory mitigation.

  • Supplemental Notes:
    • This research was funded by the U.S. Department of Transportation, University Transportation Centers Program.
  • Corporate Authors:

    North Carolina State University, Raleigh

    Department of Soil Science, Campus Box 7619
    Raleigh, NC  United States  27695-7919

    North Carolina State University, Raleigh

    Center for Transportation and the Environment, Centennial Campus, Box 8601
    Raleigh, NC  United States  27695-8601

    North Carolina Department of Transportation

    P.O. Box 25201, 1 South Wilmington Street
    Raleigh, NC  United States  27611

    Research and Special Programs Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Authors:
    • Broome, S W
    • Craft, C B
    • Struck, S D
    • SanClements, M
  • Publication Date: 2005-6


  • English

Media Info

  • Media Type: Print
  • Edition: Final Report
  • Features: Figures; References; Tables;
  • Pagination: 66p

Subject/Index Terms

Filing Info

  • Accession Number: 01051549
  • Record Type: Publication
  • Report/Paper Numbers: FHWA/NC/2003-07
  • Created Date: May 22 2007 7:07PM