g., Eby and Crowder, 2002). However, documenting these effects on fish growth, survival, and significant, find more long-term population-level responses has proven difficult. Bottom hypoxia in many north temperate systems, such as Lake Erie, persists for a short time period (days to months; Rucinski et al., 2010), making hypoxia effects on fish difficult to distinguish from other seasonal processes. In addition, while nutrient additions can exacerbate hypoxia, they can also increase system productivity and increase prey production through bottom-up processes. Such positive effects can be particularly strong if bottom hypoxia forces prey organisms higher in the
water column where many zooplankton taxa have higher growth rates because of higher temperature, light, and phytoplankton abundance (e.g., Goto et al., 2012). While definitive in situ ecological impacts have been hard to quantify, laboratory studies have demonstrated the potential for some Lake Erie fish and zooplankton to be negatively affected by direct exposure to low DO concentrations.
For example, while the relatively tolerant yellow perch (Perca flavescens) Smoothened inhibitor can survive at low DO concentrations, both consumption and growth rates decline under hypoxia ( Roberts et al., 2011). Further, hypoxia may lead to reduced prey production because some zooplankton prey species experience poor survival under hypoxia (e.g., Daphnia mendotae; Goto et al., 2012). In contrast, other zooplankton taxa seem to be able to survive prolonged hypoxia (see Vanderploeg et al., 2009a), but may use the hypoxic zone as a refuge from predation. Additionally, the growth and survival rates of some preferred benthic prey (e.g., Chironomidae) are largely unaffected by low DO conditions ( Armitage et al., 1995). Potential in situ impacts of hypoxia on mobile fish species in Lake Erie appear to be indirect and vary among species. For example, hypoxia-intolerant rainbow smelt 3-oxoacyl-(acyl-carrier-protein) reductase (Osmerus mordax) entirely avoid hypoxic waters in CB by migrating horizontally or moving up into
a thin layer of the water column just above the hypoxic zone ( Pothoven et al., 2012 and Vanderploeg et al., 2009b). By contrast, while some yellow perch move horizontally away from the CB hypoxic region, many remain in this region, but move higher in the water column, and undertake short feeding forays into the hypoxic zone ( Roberts et al., 2009 and Roberts et al., 2012). Owing to these taxon-specific responses, hypoxia may reduce the overlap between predator and prey or facilitate predator foraging success, as both prey and predator are squeezed into the same area of the water column. In Lake Erie, the diets of emerald shiner, a warm-water epilimnetic zooplanktivore, seemed unaffected by hypoxia ( Pothoven et al., 2009) and their foraging rates may even be increased as zooplankton are forced into the epilimnion.