Effects of climate change on temperature and oxygen stratification of mid-latitude reservoirs

River impoundments (i.e., reservoirs) are the dominant lake type in mid-latitude regions but have received comparatively little attention in recent assessments of climate change effects on inland waters. We analyzed temperature and oxygen profiles collected since the 1970s from >100 reservoirs within the Commonwealth of Virginia. We found that unlike naturally formed lakes in northern regions, which have experienced surface warming and bottom cooling, reservoirs show warming trends in both surface and bottom layers. Their response may be indicative of the combined effects of changes in heat exchange at the lake surface and warming of river inflows. Surface temperatures increased more rapidly among deep, near-dam stations (0.56 +/- 0.15 degrees C decade(-1)) relative to shallow stations in the upper reservoir (0.35 +/- 0.24 degrees C decade(-1)), whereas bottom temperatures increased faster among shallow stations (0.50 +/- 0.14 degrees C decade(-1)) relative to deep stations (0.15 +/- 0.06 degrees C decade(-1)). Among deep stations, faster surface warming and slower bottom warming resulted in strengthening temperature gradients and greater oxygen depletion in bottom waters. At shallow stations, faster warming at the bottom relative to the surface resulted in a weakening of temperature gradients and increasing oxygen at the bottom. Weakening of thermal gradients among weakly stratified mid-latitude lakes may compress the range of depths over which stratification occurs. Latitudinal differences in the strength of stratification may be an important factor determining the sensitivity of lakes to climate warming.