Increased sediment load during a large-scale dam removal changes nearshore subtidal communities

Area of Science:

• Coastal Ecology
• Marine Biology
• Environmental Science

Background:

• The Elwha River dam removal released over 10 million tonnes of sediment into the coastal marine ecosystem.
• This massive sediment influx significantly altered the nearshore subtidal environment.

Purpose of the Study:

• To assess the biological responses of the nearshore subtidal community to sediment deposition and suspended sediment.
• To understand the impacts of dam removal-induced sedimentation on marine life.

Main Methods:

• Utilized time series data including bathymetry, substrate grain size, and turbidity.
• Conducted scuba dive surveys and towed video observations before and during dam removal.
• Analyzed changes in macroalgae, invertebrate, and fish populations.

Main Results:

• Macroalgae cover significantly decreased in areas with persistent sediment deposition.
• Most invertebrate and fish densities declined with deposition, but some taxa like bivalves, flatfish, and sand lance increased in specific sediment conditions.
• Areas without deposition showed varied responses, with some species benefiting from increased particulate matter or reduced competition.

Conclusions:

• Sedimentation from dam removal profoundly impacted coastal benthic communities, with varied species-specific responses.
• Early signs of macroalgal recovery suggest potential for long-term ecosystem resilience.
• Findings offer critical insights for future dam removal projects and understanding sedimentation effects.