Impacts of the sediment-laden Elwha River plume on light attenuation in the water column and kelp habitat

Abstract

River dam removals are a form of sediment supply to the sub-tidal areas near river mouths. Suspended sediment in the sub-tidal water column can attenuate light, decreasing the amount of light available to seabed photosynthesizers, but not much is known about the effects of sediment plume light attenuation on kelp growth. During a recent dam removal project on the Elwha River, kelp densities in the sub-tidal region declined. In this study we evaluate trends in kelp growth since dam removal on the Elwha River began, using dive surveys from 2010 to 2013, and relate these trends to long-term river turbidity and light intensity data obtained during a 3-day study period in April 2014. The results suggest that the river plume attenuates light and has been at least partially responsible for decreases in kelp densities, but that this is not the sole determining factor in kelp growth. Another factor hypothesized to influence kelp growth is a difference in substrate size on either side of the river mouth.