Importance of water-level fluctuation on population dynamics of cladocerans in a hypertrophic reservoir (Lake Arancio, south-west Sicily, Italy)

During a period of three years (1990-1991 and 1993), we studied the population dynamics of planktonic cladocerans in a hypertrophic reservoir. Weekly sampling revealed that the five most common species followed a trend which reflects the peculiar hydrological characteristics of the reservoir and their key position in the pelagic food web. In particular, 1991 was characterized by a strong water inflow which probably interfered with the reproductive activities of the dominant fish population (Rutilus rubilio) and reduced the concentration of inedible planktonic algae allowing the development of small Chlorococcales. This event was associated with higher population densities of Daphnia hyalina compared to the other years of the survey. In spring 1991, an extended clear-water phase was observed and Secchi disk depth increased to 6 m, whereas in the other years it did not surpass 1 m. In addition, the D. hyalina population persisted throughout the summer in 1991, whereas it started to decline at the end of June in the other years. This development of D. hyalina probably influenced the population dynamics of the other cladoceran species in the reservoir, and in particular reduced the summer growth of Diaphanosoma lacustris and delayed the occurrence of Bosmina longirostris. Stomach analysis indicated that D. hyalina is the preferred food item of juvenile (less than two months old) R. rubilio. Overall, the hydrology of the reservoir was observed to interact with the trophic processes in the pelagic environment of the ecosystem in at least two different ways: via bottom-up processes, influencing phytoplankton dynamics, and via top-down processes, regulating the predation efficiency of the planktivores. ©1997 Kluwer Academic Publishers.