ACTIVE AND RESTING STAGES OF ZOOPLANKTON AND ITS SEASONAL EVOLUTION IN A HYPERSALINE TEMPORARY POND OF THE MEDITERRANEAN COAST (THE “VECCHIA SALINA”, TORRE COLIMENA, SE ITALY).

The species composition of the zooplankton and its variability were studied in an integrated water-sediment analysis for a period of two years in a hypersaline temporary pond in the SE Italy. 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, and Ciliates attained 99-100 % of total plankton 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 active organisms (11) found in the plankton over the two years studied. The seasonal succession of species was different in the two years. This fact, together with the richness of the cyst bank in the sediments, 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. Laboratorary tests showed that hatching of different cysts generally occurred in a wide range of salinity conditions, and it was not synchronous. This allowed us to assume that even the cyst hatching rate could be an adaptation to highly variable extreme environments.