Diel vertical movements of zooplankton in lake La Cruz (Cuenca, Spain)

The study of diel vertical movements of zooplankton in a small meromictic karstic lake (lake La Cruz), presenting highly stratified waters, was performed using two different methodologies: (i) samples were taken along the vertical profile and were compared at different hours in a diel cycle; (ii) some plankton traps were located at different depths, covering different periods of time, to catch organisms going upwards and downwards. The main subject of this study has been the vertical movements affecting rotifers since they were dominant in the zooplankton of this lake, but we have also included the results obtained for other zooplankton groups. The results indicate an almost general movement in epilimnetic waters corresponding to the 'normal' pattern of migration (ascent at dusk and descent at dawn); in metalimnetic and hypolimnetic waters, vertical movements were greatly reduced and were more independent of the diel cycle. The results also confirm that the dense populations at these levels of the oxic-anoxic boundary are made up of active animals. Among the causes accounting for our results, the cycles of activity (mainly feeding) connected with the diel cycle could be important in explaining vertical movements of filter-feeder rotifers. In larger zooplankters, visual predation might be the main reason for such behavior.