• Penttila,D.and D.Doty.1990. Progress Report. Results of 1989 Eelgrass Shading Studies in Puget Sound. Washington Department of Fisheries, Marine Fish Habitat Investigations Division.
REGION: PNW Species: eelgrass
Page: Study Objectives, Methods & Findings | Conclusions
STUDY OBJECTIVES: Mitigation/Impact Assessment. Determine effects of direct shading on marine vegetation community, in particular beds of eelgrass.
METHODS: Empirical: quadrat, diver eelgrass surveys. Three Puget Sound study sites with littoral zone structures were selected on the basis of the presence of homogenous eelgrass, homogenous bottom topography and bottom topography unaffected by the construction or long term presence of the structure itself. The sites were surveyed for assessment of plant stature and counts per area during July-September 1989. A qualitatively different fourth site was also sampled at an intertidal eelgrass bed near an "oyster rack" culture structure.
FINDINGS: All fixed dock structures reduced eelgrass density to zero even when visual light attenuation did not approach full darkness with little or no evidence of impacts on the stature of the surviving plants. The oyster rack site also significantly reduced eelgrass density from 244.5 plants per m 2 to 10.6 plants per m 2 directly beneath racks. In contrast, the floating dock site with the chained-anchor moorage system that allowed a swing with wind and tidal currents showed no negative impacts on the density of the eelgrass in the structure's vicinity. This may be due to the flexible nature and movement of the dock and the resulting lack of shading cast continuously over any given bottom area.