Abiotic variables within the scope of this concern are water acidity, oxygen content, and temperature, and how forest service road (FSR) construction and logging practices affect salmon spawning habitat (Watts et al., 2005). These variables are largely influenced by the two other identified concerns, meaning resource managers must consider their interaction prior to developing a management plan. In this respect, process-based approaches are desirable, suggesting a modification to actions taken within the riparian area, for example, as means of resolving abiotic concerns.
Infilling of salmon spawning gravels with fine sediment deposition may also result in decreased oxygenation of water making contact with salmonid eggs. Siltation leads to increased impermeability of the stream bed and decreased hyporheic flow, preventing crucial egg-water gas exchange (Pike et al., 2010). Adult free-swimming salmonids require approximately 4 mg/L of dissolved oxygen in surrounding water, while salmon eggs are said to require higher concentrations (Hinch, 1998). Levels below 2 mg/L result in die-off at most salmonid life stages. One must note that water-available oxygen decreases with increasing water temperature and decomposition of organic matter (Hinch, 1998). Therefore, resource managers must consider decreased dissolved oxygen concentrations in the stream bed relative to free-water oxygen level, especially if fine particle deposition is observed (Hinch, 1998; Pike et al., 2010). Construction of FSRs through sensitive riparian environments are the primary cause of fine sediment release; disturbance of topsoil horizons allows for transportation of fine mineral layer components, transported by flows propagated by the smooth surface of the road.
Stream temperature is one of the most important factors regulating physical and biological processes in aquatic systems and responds to energy exchanges across the air–water interface, conductive and advective exchanges across the streambed, and groundwater inputs (Beschta et al., 1987; Evans et al., 1998). Timber harvesting within the riparian zone removes shading vegetation, increasing stream exposure and stream bed material to direct solar radiation, often resulting in increased maximum temperatures and diurnal fluctuations (e.g., Rishel et al. 1982; Johnson and Jones 2000). Furthermore, increased sediment load may result in heating of the water’s surface and cooling of the stream bed as light is intercepted at lower depths; siltation of the gravel bed may exacerbate this, reducing hyporheic flows and potentially affecting bed temperature. These temperature changes can influence aquatic ecosystems, often negatively, and are a significant concern in forest management (Hicks et al. 1991). Academic literature notes that the average summer maximum temperature in headwater streams has been shown to increase by 5.2°C following harvest; minimum temperatures increasing by 0.9°C, and diurnal fluctuations by 6.6°C (Anderson, 1973; Beschta et al., 1987; Hewlett and Fortson, 1982; Rishel et al., 1982; Brownlee et al., 1988; Macdonald et al., 1998; Johnson and Jones, 2000). Hinch (1998) notes an increase to 30°C in some clearcut streams―a lethal water temperature for salmonids. If temperatures are too high organism metabolism increases, limiting rate of growth; if stream temperature is too low the fish lack the required energy to perform necessary tasks. In terms of prevention, maintaining hyporheic flow patterns, conserving riparian canopy, and minimizing upstream effects is crucial in this regard (Pike et al., 2010; Hinch, 1998).
Stream acidity is rarely a concern, but may not decrease below a pH of 4 as this is lethal to most species of salmonid (Hinch, 1998). This value fluctuates with temperature, sediment load, flow, and stream inputs. In cut-off areas with low flows and significant pooling the pH may be observed to decrease below average stream measurements. An increase in organic matter and plant exudate input from bordering riparian areas contribute to this (Hinch, 1998). Furthermore, widespread drying of the riparian bank may impact stream pH, especially if water-level is low, as acid litter layers build up while vegetation takes up basic ions for growth (Kelly-Quinn et al., 2008). Furthermore, prescribed burns may result in acidification of streams within a close proximity and the disturbance of soils due to construction may cause oxidation of deposits high in sulfide or prompt the acceleration in decomposition of organic matter due to an increase in aeration and temperature (Hinch, 1998; Bayley et al., 1992).