The fall and rise of kelp abundance and recruitment timing as related to changes in suspended sediment and light availability during and following Elwha dam removal

Large amounts of sediment entered the Strait of Juan de Fuca during a staged, three year removal of two dams on the Elwha River, elevating suspended sediment over a large area and creating persistent deposits near the river mouth. Previously abundant kelp and other macroalgae were sensitive to the sediment influx, rapidly declining to low levels as assessed by annual summer scuba and towed video surveys. A negative correlation between percent cover of macroalgae and (remotely sensed) near-surface suspended sediment concentration suggested that reduced light contributed to the kelp decline. In the three years since the end of dam removal, kelp rebounded as suspended sediment decreased, and kelp have almost completely recovered in areas of former abundance that lack sediment deposits. Changes in suspended sediment during and after dam removal also affected kelp recruitment timing. We sampled monthly during the growing season (February-July) two and three years after the end of dam removal to establish kelp recruitment timing associated with kelp recovery. No kelp were visible in February, small kelp appeared in March, and maximum kelp coverage was reached in June. By comparison, recruitment of visible kelp was delayed until August during dam removal. Kelp responsiveness has spurred research into how suspended sediment affects light availability and how light availability controls kelp recruitment and abundance. We will report light levels associated recruitment timing during the kelp rebound, and hindcast to light levels associated with delayed recruitment and reduced abundance during the dam removal period. Kelp are an important component of temperate coastal ecosystems. Understanding effects of increased sediment and reduced light on nearshore habitats is relevant to managing human activities that alter sediment inputs