European Ocean Biodiversity Information System

In June 2013 and April 2014, sampling was carried out at four sites selected as representatives of different environmental issues and anthropogenic impacts Samples (5 L) were concentrated with a 10-μm mesh, reduced to 250 mL and immediately fixed with buffered formalin (4 % final concentration). Subsamples (10–25 cm3) were examined in a settling chamber using a Leica DMI 3000B inverted microscope equipped with phase contrast and bright-field illumination at ×200 magnification, according to the method of Utermöhl (1958). The entire surface of the chamber was examined. Among the microzooplankton communities, five main groups were considered and distinguished as ciliates (naked and tintinnids), heterotrophic dinoflagellates, other protozoa and micrometazoans. The phototrophic ciliate Mesodinium rubrum was treated together with the aloricate heterotrophic ciliates. Empty loricae of tintinnids were not differentiated from filled ones because tintinnid protoplasts are attached to the lorica by a fragile strand, which detaches with ease during collection and fixation of the samples. For each taxon, biovolumes were estimated by measuring the linear dimension of each organism with eyepiece scale and equating shapes to standard geometric figures. The obtained biovolumes were converted to carbon content by applying the following conversion factors and formulae according to Verity and Langdon (1984) for tintinnids, Putt and Stoecker (1989) for naked ciliates, Edler (1979) for athecate and thecate dinoflagellates and Beers and Stewart (1970) for other protozoa.