Effects of multiple stresses hydropower, acid deposition and climate change on water chemistry and salmon populations in the River Otra, Norway

The Otra River in Southern Norway is impacted by acid deposition, hydropower development (around 40% of the river has been modified for electricity production) and, increasingly, by climate change. The Otra supports populations of both land-locked and migratory (anadromous, moving between rivers and the sea) salmon which have been severely impacted by acidification in the latter part of the 20th century.

Environmental policy and conservation initiatives have prompted a reduction in acid deposition into the Otra since the 1980s, which has caused a partial recovery of both populations of salmon. However, in order to predict and manage the long-term health and status of the Otra’s salmon populations, it is important to consider acidification as part of a multiple stress combination affecting the river alongside hydropower and climate change.

A MARS team led by Raoul-Marie Couture used a set of linked process-oriented models to provide estimates of future water discharge and chemistry and their effects on fish populations in the Otra River. The models were run to 2100 using two Representative Concentration Pathway climate scenarios: RCP4.5, in which global carbon emissions peak at 2040 then decline; and RCP8.5 in which global carbon emissions continue to rise through the 21st century.

The projected changes in climate produced only small – but ecologically positive – changes in the water chemistry of the Otra River. Run-off was predicted to increase by around 30%, largely during winter (as a result of increased precipitation and snowmelt), which, when coupled with projected decreases in acid deposition through the 21st century reduces the possibility of acidification. And, linked to this, the likelihood of river water pH dropping below levels where fish are significantly stressed (5.8 for parr and 6.2 for smolt) is reduced.

The study suggests that future climate change may cause slight improvements the water chemistry conditions for salmon populations in the Otra River through the 21st century. However – as the authors acknowledge – this result addresses only one aspect of climate change and not others such as increased water temperatures, river basin vegetation growth and soil mineralisation. Future river run-off levels will be affected by the continuation of hydropower projects on the river.

As such, the multiple stressor combination of acidification, hydropower and climate change on the Otra River are fundamentally linked, and whilst this study sheds new light on their interactions, their complex, interconnected nature provides ongoing challenges for environmental modelling and management.