Subtidal kelp, over 20 species in total, are abundant in the Puget Sound but no data exists on how they are impacted by the multitude of overwater structures found there. There are currently over 9,000 overwater structures creating 9 km2 of shade in the Sound. This research quantifies various overwater structures’ impacts on the productivity and distribution of subtidal kelp beds. Three sets of floating docks ranging in size from 35 to 66 m2, located at Deception Pass and Camano Island State Parks, were sampled twice during the summer of 2017. Georeferenced underwater video surveys were conducted by paddling a floating platform with a depth adjustable camera/laser array to record the presence of subtidal kelp, which was primarily sugar kelp (Saccharina latissimi), although all species were surveyed. Parallel transects out to eight meters from each dock and in each control, spaced two meters apart, were followed using an on-board GPS. The transects and underwater video footage were correlated, using their time stamps, to create one-meter grid cells which were encoded and mapped to show presence of subtidal kelp. Wet biomass and morphometric measurements were taken from kelp collected from thirty samples along the transects at each site. Data was also collected for related environmental conditions at each site, including light extinction, depth and substrate. Light extinction coefficients were calculated using an array of eleven photosynthetically active radiation sensors deployed at various depths and distances from each dock, as well as within each paired control site. Substrate samples taken with a Peterson dredge, along each transect, were analyzed for organic content and particle size distributions. Proportional coverage and densities of subtidal kelp were statistically compared for significant differences between the docks and their paired control sites and were correlated with related environmental conditions using nonparametric tests
