Surface water temperature is increasing in reservoirs worldwide, including in subtropical regions. This warming is linked to the increased frequency and duration of droughts, which has consequences for oxygen availability and metabolism. We used a distributed network of stationary loggers (n = 5-7), synoptic surveys using a sensor-equipped boat, and surface water temperature derived from the Landsat 8 satellite to characterize ecosystem-level changes in dissolved oxygen and temperature in a eutrophic, polymictic Texas reservoir. The 3-year study period included a prolonged drought that dropped the reservoir to about half of its capacity and included several weeks with no surface water inflow, followed by a high precipitation event that filled the reservoir to capacity. Sites had dissolved oxygen saturation ranging from 4.8 to 195% during the drought compared to 78.4 to 139% following a flood event. Measurements for the same month across 3 years show that patterns observed extend beyond seasonal cycles with the range of temperature during drought being 11°C compared to 5.4°C at drought onset and 3.9°C after the drought. This dry-wet sequence was associated with a multi-year La Niña followed by El Niño. Reductions in allochthonous inputs and shallower water levels were observed to produce high phytoplankton productivity which made diel swings in oxygen concentrations more variable. Our case study highlights how reduced inflows into warming water bodies with high nutrient levels are likely to impact dissolved oxygen availability. This research is a step towards understanding reservoir ecosystem dynamics and spatial heterogeneity under variable hydroclimatic conditions.