Distribution of phytoplankton along the thermohaline gradient in the north-eastern Adriatic channel; winter aspect

The distribution of phytoplankton and its relation to the hydrographic features in the north-eastern Adriatic was investigated in February 2008. The area of interest included a thermohaline gradient in the channel situated between the coast and the islands lying parallel to the coast. The gradient is controlled by the influx of oligotrophic karstic riverine water at the south-eastern end, submarine springs in the middle part, and warmer offshore waters at the north-western end of the channel. The change of temperature and salinity in the estuarine transition zone was accompanied by abundant diatoms and dinoflagellates below the halocline, with dominant chain-forming diatoms (Chaetoceros, Bacteriastrum) in abundances reaching 5 ×105 cells dm-3. The impact of coastal submarine springs detected by infrared remote sensing resulted in the growth of cyanobacteria in the nitrogen-depleted surface waters. The greater contribution of picoplankton, as well as of nanoplanktonic coccolithophorids and cryptophytes, in the outer channel system indicated their preference for oligotrophic conditions. Flow cytometric counts of nanophytoplankton were 10-30 times greater than inverted microscope counts. Cyanobacteria were about five times more abundant than picoeukaryotes. The study demonstrates how different techniques (remote sensing and in situ investigations) can be useful in understanding the biological and hydrographic set-up in the specific oligotrophic eastern Adriatic coastal environment.