THE EFFECTS OF SOLAR-POWERED MIXERS ON THE THERMAL AND OXYGEN STRATIFICATION IN TWO SOUTHEASTERN US RESERVOIRS

Harmful algal blooms (HABs) have been increasing over the last 50 years due to anthropogenic nutrient enrichment and increasing temperatures. HABs can impact lake ecosystem function, leading to ecological, economic, and societal impacts. One HAB control method is the use of solar-powered mixers, which decrease the stability of reservoirs thermal stratification by increasing turnover intensity. We aim to determine the impact of mixers on the timing of thermal stratification and thermocline and oxycline depth by comparing two similar warm monomictic reservoirs in central Georgia – Lake Varner (LV), which utilizes mixers, and Hard Labor Creek Reservoir (HLCR), which does not. From Jan. 2020 – Dec. 2021, temperature data loggers were deployed at 1-meter increments along a 10-meter-deep array in each reservoir. Dissolved oxygen (DO) measurements were also recorded monthly.

The presence of mixers did not appear to impact the onset of stratification in the spring, as both reservoirs had near identical dates for the development of a thermocline and each reservoir’s thermocline first appeared at the same depth in both years. However, in the autumn of both years, the thermocline eroded a month earlier in LV, where mixers are present, compared to HLCR. As the slope of the thermal gradient across the metalimnion decreases prior to overturn in the fall, the thermocline’s ability to act as a barrier to mixing reduces. While the depth of the solar mixer intakes is unchanged, the removal of the thermal barrier may allow water from deeper in the water column to begin moving upward towards the mixer intakes. When colder deep water is pumped to the lake’s surface, the turnover process is accelerated.

Both reservoirs had similar oxygen profiles during stratification, but in LV, DO levels increased more quickly in autumn compared to HLCR during both years. The earlier disappearance of the thermocline allows the oxygen to begin mixing earlier, which allows for oxygen levels to increase earlier. However, it is also possible that the biome of LV, affected by both the older age of the reservoir and the presence of the mixers, may impact rates of photosynthesis and, in turn, DO. These findings suggest the impact of epilimnetic mixers on full lake thermal and oxygen processes are minimal, with only minor changes observed during fall turnover.