Population structure and abundance of zooplankton along the Krka river estuary in spring 2006

Abstract

           The zooplankton assemblage was investigated in April 2006 at 9 stations along, and in front of, the highly stratified salt-wedge Krka river estuary, using 53- and 125-μm mesh Nansen nets. Tintinnids, copepods and their developmental stages and mollusc larvae dominated the zooplankton community, with 28 tintinnid and 41 copepod taxa recorded in the investigated area. Both tintinnid and copepod communities showed a general decrease in abundance and increase in species number from the head to the mouth of estuary, with a clear dominance of estuarine-neritic species. The most abundant tintinnids were Tintinnopsis campanula and Favella ehrenbergii. Small cyclopoid copepod Oithona nana dominated in the upper reaches, presumably feeding opportunistically on abundant organic matter accumulated at the permanent halocline. The contribution of calanoids intensified towards marine stations, except in the eutrophicated Šibenik harbour area in the middle reaches, where cyclopoid Oithona nana again proliferated. An assessment of the sampling performances of the 53- and 125-μm nets and data on zooplankton population structure indicated that the 125-μm mesh size plankton net in combination with Niskin bottles would give the optimal insight into the ecology of all zooplankton components in future investigations of this and similar highly stratified estuaries.