engleski

Epiphytic diatoms in the Zrmanja Estuary

The karstic river Zrmanja flows into the central Adriatic Sea (eastern coast, Croatia). Most of the year, considerable freshwater discharge and low tides (on average 20 cm) maintain a 20 km long, highly stratified estuary. The freshwater input varies between 38 and 456 m3 sec-1. The river flows through the sparsely inhabited Northern Dalmatian plateau, and due to very low nutrient concentrations it is one of the most pristine European rivers. We present a preliminary report on species composition of epiphytic diatoms in the surface layer of the upper reach of the estuary (station Z4, near Obrovac, 5 m deep). The research was performed during 1998/1999. The surface layer salinity ranges between 0 and 6. The surface temperature varied between 10 and 24oC. The halocline at station Z4 is below 2 m depth. Salinity in the bottom layer varied between 0 and 22. The relatively high pH (>8) value in the surface layer indicated the impact of alkaline karstic waters. Forty-four diatoms were determined in the surface layer of the estuary. Species with the maximum relative abundance (h=4) (Cocconeis pediculus EHR., Diatoma vulgaris BORY), and dominant species (Cocconeis pediculus EHR., C. placentula EHR., Rhoicosphaenia curvata KÜTZ., Synedra ulna (NITZ.) EHR., with an average relative abundance between 1.6 and 2.0 and frequency of finding between 80-100%) were determined. Diatoms were composed of species indifferent to salinity (Eunotia arcus EHR., Fragilaria capucina DESM., Gyrosigma acuminatum (KÜTZ.) R.), oligohalobic species (Cymbella affinis KÜTZ., C. minuta HILSE ex RABH., Pinnularia viridis (NITZ.) EHR.) and eurihaline species (Diploneis dydima (EHR.) EHR., Navicula gracilis EHR., Rhoicosphaenia curvata KÜTZ.). The low Wegel index of saprobity (1.79) indicated the low trophic state of the surface waters (I-II class, according to Pantle and Buck). Further investigations will give additional insight into the great biodiversity of diatoms in the brackish environment of the Zrmanja estuary.

epiphytic diatoms; estuary; brackish; Croatia

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