AbstractFour Cuban reservoirs (Higuanojo, Tuinicu, Lebrije and Zaza) in the Sancti Spiritus province were studied in 1988–1989. The study was based on monthly samples of physical, chemical and biological components of the pelagic ecosystem.The reservoirs were warm‐monomictic with more or less complete water mixing during the dry season and stable stratification during the wet season when inflow of water was highest. The mixing event was very important in regulating the biology and chemistry of the reservoirs. During the stratified period the water was anoxic below 10 m depth and reduced compounds, especially ammonium, accumulated. In the reservoirs with less stable stratification, ammonium was occasionally oxidized and the nitrogen lost to the atmosphere through denitrification. Phytoplankton biomass varied considerably among reservoirs and was not only correlated to total‐P, but also to light, Zooplankton biomass and composition was related to availability of good quality food and to the intensity of fish pred