Subtidal Invertebrate and Fish Responses to Increased Sediment Load During Elwha Dam Removal

Abstract

Increased sediment delivery to the coastal ocean can affect marine organisms through burial, scour, and turbidity. Staged removal of two dams from the Elwha River during 2011-2014 increased sediment loads to the Strait of Juan de Fuca compared to background levels. We measured the timing, magnitude, and spatial extent of sediment inputs, and concurrently conducted scuba surveys of benthic macroinvertebrates and fish to assess their responses and investigate the role played by physical changes in shaping community change. Sand or mud deposition \u3e10 cm occurred over large areas around the river mouth starting in 2012 and increasing in 2013, burying several of our sampling sites. The spatial extent of the sediment-laden river plume was greater than the region of persistent sediment deposition. Most invertebrate taxa were negatively impacted by sediment deposition but unaffected by increased turbidity in areas lacking deposition. In contrast, the abundance of feather duster tubeworms (Sabellidae) and clams increased in response to increased turbidity, or alternatively to reduced vegetation stemming from increased turbidity, in areas lacking deposition. Sand lance and flatfish abundance increased in response to sediment deposition but greenlings, gunnels and ratfish decreased. Turbidity increases did not affect fish. The amount of sediment eroding from the former reservoirs is now substantially less than during dam removal, but hydrodynamic processes are reworking recent deposits in the Strait. Our ongoing surveys will assess trajectories of physical and biological response as the system recovers towards a natural sediment regime