Long-term data show effects of atmospheric temperature anomaly and reservoir size on water temperature, thermal structure, and dissolved oxygen

Predicting changes in reservoir conditions from climatic warming is hindered by the paucity of long-term data on temperature and thermal and oxygen structure replicated across a range of reservoir sizes. The present study characterizes seasonal patterns in temperature, thermal structure, and dissolved oxygen availability in reservoirs, and evaluates how critical periods for aquatic organisms (i.e., periods of maximum temperature and minimum oxygen) for these features are affected by atmospheric temperature anomalies at different time lags. Temperature and dissolved oxygen were measured from May through October at 1 m intervals from surface to bottom for 10 reservoirs sampled between 14 and 21 years during 1995–2016. For most temperature and oxygen metrics July was the period of thermal maxima and oxygen minima across reservoirs, exceptions were thermocline depth, which was static from May through October, and bottom water temperature which peaked in August. Surface water temperature of reservoirs increased 0.6 °C for every 1.0 °C increase in atmospheric temperature in July independent of reservoir area. Although the percentage of water that was oxic in July decreased with increasing July air temperatures, it increased with warming air temperatures from February to April. These long-term data derived patterns highlight the importance of reservoir size and lag periods in building a framework for predicting climate-induced changes in the thermal and oxygen environments of reservoirs, which have important implications for water quality and ecosystem processes and the associated dynamics of reservoir flora and fauna.