Risk assessment for salmon from water quality changes following timber harvesting.

Area of Science:

• Environmental Science
• Ecotoxicology
• Aquatic Ecology

Background:

• Land use, like timber harvesting, can alter surface water quality.
• Previously, minor water quality changes below standards were considered harmless.
• Recent concerns for threatened salmon populations necessitate reevaluating environmental impacts.

Purpose of the Study:

• Develop a risk assessment for salmon exposed to water quality changes after timber harvesting.
• Evaluate the combined toxicity and exposure risks of nitrate-nitrogen, chloride, and suspended sediment.
• Assess the probable risk to coho salmon (Oncorhynchus kisutch) from nitrate-nitrogen exposure.

Main Methods:

• Literature review and Alsea Watershed Study data were used to develop toxicity relationships.
• Toxicity was assessed using LC50 (lethal concentration for 50% of population) for nitrate-nitrogen.
• Risk was predicted based on nitrate-nitrogen concentrations and suspended sediment exposure.

Main Results:

• A reference catchment with higher nitrate-nitrogen showed low risk due to low suspended sediment.
• A harvested catchment with lower nitrate-nitrogen exhibited higher risk due to increased suspended sediment transport.
• The risk assessment model demonstrated sensitivity to suspended sediment concentrations.

Conclusions:

• Suspended sediment transport significantly influences the risk of nitrate-nitrogen exposure to salmon.
• Current risk assessments may underestimate impacts if solely focused on chemical concentrations.
• Further research is needed to refine risk assessments for salmonid populations affected by land use changes.