Long-term study of the effects of river inflow on oxygen stratification and its consequences for selected abiotic and biotic factors in a deep reservoir

One of the effects of climate change is an increase in extreme weather events, leading to more frequent droughts and floods, which alter river flow patterns and affect the distribution of dissolved oxygen (DO) in downstream reservoirs. Dissolved oxygen is crucial for biogeochemical cycles and aquatic communities. We investigated how fluctuations in river inflow and water exchange affect oxygen stratification in a submountain dam reservoir. We also assessed these effects on pH, changes in nutrients, chlorophyll a, and zooplankton density and composition. We analysed long-term (1994-2017) monitoring data from the deep Dobczyce Reservoir (southern Poland). We grouped the years in terms of DO concentrations in the metalimnion and hypolimnion. The results showed that the duration of oxygen stratification depends on the magnitude and timing of high inflows. Years with low summer inflows and limited mixing resulted in prolonged stratification lasting 3-4 months, with hypoxic conditions covering up to 83 % of the meta- and hypolimnion. Years with floods in May or September shortened stratification. In years with large summer inflows, oxygen stratification was strongly disturbed or not developed. In addition, years with high summer inflows showed higher concentrations of nutrients and chlorophyll a and greater zooplankton density dominated by opportunistic r-strategists (Polyarthra vulgaris and Bosmina longirostris). Water exchange rates were not accurate indicators of oxygen conditions. Thus, river flow variability plays a crucial role in shaping oxygen distribution, water quality and food web structure in deep submountain reservoirs.