The phytoplankton composition and spatial distribution in the north-eastern Adriatic Channel in autumn 2008

Abstract

           The Velebit and Pag Channels form a 40 km long and 4-14 km wide system running parallel to the northeastern Adriatic coast. The aim of this study was to investigate phytoplankton abundance, composition and distribution along the system in the autumn period. Sampling was carried out at 8 stations in November 2008. The spatial distribution of physico-chemical parameters indicated the existence of a pronounced horizontal thermohaline and trophic gradient. From the inner part of the Velebit Channel, which is an extension of the karstic Zrmanja River estuary, to the outer end of the Pag Channel, which communicates with the open sea, the temperature and salinity ranged from 11.0 to 16.0 °C and from 23.4 to 38.2, respectively. Nutrient concentrations ranged from 0.001 to 0.1 µmol L^-1 for orthophosphates, from 0.85 to 7.0 µmol      L^-1 for total inorganic nitrogen and from 0.3 to 15.4 µmol L^-1 for silicates. Total abundance of phytoplankton decreased from 5.5 x 105 cells L^-1 to 8.5 x 104 cells L^-1 along the trophic gradient. Diatoms dominated the phytoplankton with maximum abundances from 7.3 x 104 cells L^-1 (85% of total phytoplankton) at the most oligotrophic stations up to 9.6 x 105 cells L^-1 (96% of total phytoplankton) at the nutrient enriched stations. Statistical analysis on the basis of environmental and biological parameters clustered the stations in three main groups. The first group represented the southern Velebit Channel with higher nutrient levels and phytoplankton dominated by Thalassionema nitzschioides. The second group included stations from the middle Velebit Channel with significantly lower levels of nutrients, and phytoplankton mostly dominated by Bacteriastrum sp. with a contribution by various diatom species. The third group represented the stations from the Pag Channel, and associated with the exclusive domination of Bacteriastrum sp. and a greater contribution of coccolithophorids. This paper also gives the first checklist of species present in the extremely oligotrophic area of the Velebit and Pag Channels. 148 taxa were identified: 91 diatoms, 47 dinoflagellates and 10 other flagellate morphotypes. Detailed electron microscopy analysis revealed three potentially toxic diatoms of Pseudo-nitzschia genus: P. calliantha, P. pseudodelicatissima and P. manii. The results of this work represent a valuable contribution to the knowledge of phytoplankton species composition and distribution relative to prevailing physico-chemical conditions in this poorly investigated transition zone.