Effects of Bridge Shading on Estuarine Marsh Benthic Invertebrate Community Structure and Function

ABSTRACT / The effect of bridge shading on estuarine marsh food webs was assessed by comparing benthic invertebrate communities beneath seven highway bridges with marshes outside of bridge-affected areas (reference marshes). We used light attenuation and height–width ratio (HW ratio), which takes into account the two main bridge characteristics that deter-mine the degree of shading, to quantify the impact of shad-ing on invertebrate communities. Low bridges, with HW ratio 0.7 and light attenuation greater than 85–90%, had benthic invertebrate densities and diversity that were signifi-cantly 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) a