Active and resting stages of zooplankton and its seasonal evolution in a hypersaline temporary pond of the Mediterranean coast (the "Vecchia Salina", SE Italy)

Abstract

The species composition of zooplankton and its variability were studied with an integrated water-sediment analysis for a period of two years in a hypersaline temporary pond in SE Italy. The basin was affected by extended drought during summer, and even during the wet period the sodium chloride salinity was never below 42.5‰. The zooplankton showed the presence of seasonal species (mainly Anostraca), together with not seasonal, but opportunistic ones (mainly Rotifera, and Ciliophora) with a shorter life cycle. Rotifers (mainly Hexarthra fennica, and Proales similis), and ciliates (Fabrea salina) attained 99-100% of total planktonic organisms in certain periods. Resting stages were extracted from the upper 6 cm of 14 sediment cores collected during a dry period (August 1998). A total of 24 resting morphotypes (cysts) were listed-more than double the number of the active organisms (11) found in the plankton over the two years. The seasonal succession of species was different in the two years studied. This fact, together with the richness of the cyst bank of the sediment, indicates that in each period the water column shows only a portion of the biodiversity, which the sediment contains unexpressed as resting stages. The study of cyst distribution (both horizontal and vertical) in sediments provided complementary suggestions to understand the space-time distribution of the plankton organisms. Laboratory tests showed that hatching of different cysts generally occurred in a wide range of salinity conditions, and was not synchronous. This allowed us to assume that even the cyst hatching rate could be an adaptation to highly variable extreme environments.