Mixing regimes in the equatorial crater lakes of western Uganda

In order to understand the ecological functioning of lake ecosystems, little is more crucial than knowledge of their mixing regime. However in tropical regions, this information is scarce for all except the largest lakes, and often inferred from occasional observation of physical water-column properties. In this study, a total of 276 water-column profiles of temperature, conductivity and dissolved oxygen collected between 2000 and 2017 in 60 volcanic crater lakes in equatorial western Uganda are used to classify these lakes according to mixing regime. Most study lakes are small (on average 29 ha), but together they cover a spectacularly large gradient in maximum depth (Z(max)) between 1.4 m and >220 m, and consequently they display a wide range of mixing regimes. We identified four lakes as continuous warm polymictic, 12 as discontinuous warm polymictic, and 11 other lakes mix completely once or twice each year. We classify these 11 lakes as equatorial dimictic, because they experience deep mixing during both dry seasons separated by long periods of stable stratification, while the season of complete mixing may differ between years and among neighbouring lakes. The 33 remaining lakes do not usually mix completely and are thus meromictic. Relative depth (Z(r)) is the main factor controlling the mixing regime of these lakes. The presence of a shallow permanent chemocline reduces mixing depth, and the smallest meromictic lakes (< 3 ha) tend to occasionally mix deeper than predicted by Z(r). Based on these variables, predictions can be made about the mixing dynamics of Ugandan crater lakes not covered by our dataset. Previous suggestions that the mixing depth of tropical crater lakes is strongly influenced by the wind shelter provided by a high crater rim or by water-column transparency could not be confirmed.