Taxonomic composition and abundance of phytoplankton in the middle reach of the karstic Zrmanja Estuary (Croatia)

The taxonomic composition and abundance of phytoplankton were analyzed in relation to thermohaline and hydrological conditions in the middle reach of the karstic river Zrmanja Estuary, eastern Adriatic coast, Croatia, in the period December 1981 to November 1982, and June 1998 to June 1999. General trophic interrelations were characterized by the spring-summer development of marine phytoplankton (mainly diatoms) and autumn- winter development of small herbivores (copepods, nauplii, copepodites). Two maxima of naked ciliates indirectly indicated the regeneration of alochtonous organic matter in winter, and autochtonous organic matter in late summer. In general, dominant phytoplankton species provided seasonal recurrent assemblages. Freshwater phytoplankton above the halocline is less abundant in the lower than in the upper reach of the estuary. Instead of freshwater phytoplankton, the brackish diatom Thalassiosira (a short-chained, small- celled species) was abundant above the halocline during the winter-spring period