The effects of dock shading on the seagrass Halodule wrightii in Perdido Bay, Alabama

Aboveground and belowground biomass, density, blade length, and chlorophyll content of seagrass growing directly under docks were compared with adjace nt unshaded sites. The amount of light reduction due to dock shading was me asured using quantum spherical irradiance meters. Seagrasses were present u nder docks; they were shaded such that light levels were 19% and 16% of sur face irradiance, at shallow and deep sites, respectively. Shoot density was 40-47% lower in shaded plots than in unshaded plots; total biomass was 30- 33% lower in shaded plots than in unshaded plots. In contrast, blade length and chlorophyll content were increased at shaded plots relative to unshade d plots. Seagrasses were not found under docks at light levels less than 14 % of surface irradiance. These results are in agreement with published in s itu compensation irradiance estimates of 15-18% for Halodule wrightii in Te xas coastal waters. Shading effects were most apparent between the hours of 1000 and 1500; therefore, the light received during the early morning and late afternoon hours may be critical for plants growing under docks. The no rth-south orientation of the docks in this study is likely an important fac tor contributing to the continued survival and growth of the seagrasses.