Interactive physical and biotic factors control dissolved oxygen in salmon spawning streams in coastal Alaska

Abstract

Streamwater dissolved oxygen (DO) concentrations are driven by interacting physical and biotic parameters. Future DO depletion events in small, coastal salmon streams are therefore likely to be driven by changes in hydrology in addition to atmospheric warming. We measured DO, temperature, discharge and spawning salmon abundance in upstream (reference reach) and downstream salmon bearing reaches of four streams in southeast Alaska to determine how multiple physical and biotic factors interact to control streamwater DO. Stream temperature ranged from 5.1 to 15.8 °C and fell within the optimum range that is considered favorable for salmon physiology. Concentrations of DO ranged from 2.8 to 12.3 mg/L, with concentrations significantly lower (p < 0.01) in the downstream compared to upstream sites when spawning salmon were present. These findings likely indicate that spawning salmon can substantially alter ecosystem respiration and thus DO regimes in stream ecosystems. Furthermore, DO concentrations in lower Peterson Creek were especially low (< 4.0 mg/L) in early August when stream temperature exceeded 14 °C, discharge was low and spawning salmon were abundant. These results illustrate that the impacts of enhanced ecosystem respiration due to high densities of spawning salmon, elevated stream temperature and reduced aeration stemming from low streamflow are likely additive in terms of reducing DO. Furthermore, it is highly likely that stray salmon released from local hatcheries augmented spawner densities in our study streams. This suggests that the straying of hatchery salmon into natural stock salmon streams may contribute to streamwater DO depletion via enhanced stream ecosystem respiration.