Spatial patterns of oxygen states affect lake food web structure through the response of cold-water fish.

Surface water warming and bottom water oxygen depletion, accelerated by global warming, limit the distribution of cold-water fish in the pelagic areas of temperate lakes. While surface warming is well known to cause drastic changes to food web structure by limiting the distribution of top predators, the effect of hypoxia on cold-water fish distribution and food webs are lacking because hypoxic conditions are often spatially heterogeneous. We investigated how the spatial development of dissolved oxygen conditions is associated with the distribution of cold-water fish (kokanee and rainbow trout) and their prey resources. To address this, we compared the spatial distribution of cold-water fish and their prey resources between two pelagic sites during summer in a small temperate lake characterized by spatially heterogeneous dissolved oxygen conditions associated with its unique topography. We found that eDNA copies of kokanee were concentrated in areas with relatively favorable dissolved oxygen concentrations during the summer, when their vertical distribution was strongly constrained by low oxygen concentrations and high temperatures. These findings suggest that spatial heterogeneity in dissolved oxygen conditions can alter the pelagic distribution of cold-water fish during summer. Furthermore, changes in the distribution of cold-water fish potentially contributing to spatial variation in predation pressure, influencing the spatial distribution of zooplankton. These patterns are consistent with the possibility that spatial variation in fish distribution contributed to differences in predation pressure.