Refuge availability and sequence of predators determine the seasonal succession of crustacean zooplankton in a clay-turbid lake

The contribution of predators and abiotic factors to the regulation of the biomass and seasonal succession of crustacean zooplankton was studied in Lake Rehtijarvi (southern Finland). Field data in combination with bioenergetics modeling indicated that invertebrate planktivory by Chaoborus depressed cladoceran populations during early summer. In particular, bosminids that generally form the spring biomass peak of cladocerans in stratified temperate lakes did not appear in the samples until July. In July, predation pressure by chaoborids was relaxed due to their emergence period and cladoceran population growth appeared to be limited by predation by planktivorous fish. The effect of fish predation was amplified by reduced refuge availability for cladocerans. The concentration of dissolved oxygen below the epilimnion was depleted, forcing cladocerans to move upward to less turbid and thus more dangerous water layers. The effect of size selective predation by fish resulted in reduced mean size of cladocerans during the period when refuge thickness (thickness of the water layer with oxygen concentration < 1 mg l(-1) and water turbidity > 30 NTU) was lowest. The results confirmed that in clay-turbid lakes, invertebrate predators could be the main regulators of herbivorous zooplankton even when cyprinid fish are abundant.