Fresh, K.L., B. Williams, D. Pentilla. 1995. Overwater Structures and Impacts on Eelgrass in Puget Sound,Washington. Puget Sound Research '95 Proceedings.

STUDY OBJECTIVES: Impact Assessment. Determine if small single-family residence docks cause a decline in eelgrass densities under and adjacent to piers and assess if the use of gratings mitigate the impacts.

METHODS: Empirical: eelgrass density was measured under and adjacent to a selection of seven "single- family" dock sites in Puget Sound from Roche Harbor to South Hood Canal. Baseline measurements and one year of post-project monitoring were also taken for five sites to assess effectiveness of mitigating impacts with grating.

OBSERVED DENSITY DECLINES: Six out of the seven sites selected to measure declines without mitigating construction methods demonstrated measurable declines and/or absence of eelgrass growing under the docks. The one site that did not show measurable impact appeared to have a mitigating aspect to its structure as the dock moved up and down and side to side with tidal fluctuations.

Four of the five structures evaluated for alternative mitigating construction methods demonstrated eelgrass densities decline when compared to undisturbed reference areas. At the fifth structure site, eelgrass density increased but increased substantially less than it increased in the reference area.

COMMENTS: Preliminary results support the hypothesis that shading is the major cause of eelgrass density loss. This is demonstrated by significant reductions in density under docks and diminished reductions under docks with grating used to mitigate shading impacts. Structure length, height over the bottom, design, orientation, and local environmental conditions (i.e. current patterns) may also play a role in the nature and extent of impact.