Influence of the Hyporheic Zone on Supersaturated Gas Exposure to Incubating Chum Salmon

Abstract

AbstractHydroelectric dam operation causes total dissolved gas (TDG) to be seasonally elevated in the lower Columbia River, as surface water concentrations approach 120%. Federally protected chum salmon Oncorhynchus keta embryos incubating in nearby spawning areas could be affected if depth‐compensated TDG concentrations within the hyporheic zone exceed 103%. The objective of this study was to determine whether TDG of the hyporheic zone in two chum salmon spawning areas—one in a side channel near Ives Island, Washington, and another on the main‐stem Columbia River near Multnomah Falls, Oregon—was affected by the elevated TDG of the surface water. Depth‐compensated hyporheic TDG did not exceed 103% at the Multnomah Falls site. However, in the Ives Island area, chum salmon redds were exposed to TDG greater than 103% for more than 300 h. In response to river depth fluctuations, TDG varied significantly in the Ives Island area, suggesting increased interaction between the hyporheic zone and surface water at that site. We conclude from this study that the interaction between surface water and the hyporheic zone affects the concentration of TDG within the hyporheic zone directly via physical mixing and indirectly by altering water chemistry and thus dissolved gas solubility. Consideration of these interactions is important when estimating TDG exposure within egg pocket environments and will enable resource managers to optimize recovery strategies.