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

The study of diel vertical movements of zooplankton in a small meromictic k arstic lake (lake La Cruz), presenting highly stratified waters, was perfor med using two different methodologies: (i) samples were taken along the ver tical profile and were compared at different hours in a diel cycle; (ii) so me plankton traps were located at different depths, covering different peri ods of time, to catch organisms going upwards and downwards. The main subje ct of this study has been the vertical movements affecting rotifers since t hey were dominant in the zooplankton of this lake, but we have also include d the results obtained for other zooplankton groups. The results indicate a n almost general movement in epilimnetic waters corresponding to the 'norma l' pattern of migration (ascent at dusk and descent at dawn); in metalimnet ic and hypolimnetic waters, vertical movements were greatly reduced and wer e more independent of the diel cycle. The results also confirm that the den se populations at these levels of the oxic-anoxic boundary are made up of a ctive animals. Among the causes accounting for our results, the cycles of a ctivity (mainly feeding) connected with the diel cycle could be important i n explaining vertical movements of filter-feeder rotifers. In larger zoopla nkters, visual predation might be the main reason for such behavior.