IMIS

Samples were fixed with Ca(HCO3)2-buffered formaldehyde (0.8% final concentration) and cell abundance and species composition of phytoplankton were estimated according to Utermöhl’s method (1958), using an inverted microscope equipped with phase contrast, at 200-320-400x final magnifications. Since the beginning of the time-series, species identification capability has changed over time, due to instrumental improvement (optical microscopy magnification increase) and to improved expertise of people in charge of identification. Besides, increased taxonomic knowledge and improved species distribution information has ruled out the presence of some taxa previously described or has lead to change in name. Therefore, taxonomic level of determination may have become lower: e.g. from species to genus. This may have led to apparent disappearance or, conversely, new appearance of taxa over the time series. A taxonomic quality control check is currently being carried out. Furthermore, since 1986, there have been modifications in phytoplankton cell counting procedures, which were motivated by decrease in overall cell abundance. Until mid 2002, the same method was adopted for all taxa: cells were counted in defined random fields or along transepts with length equal to the diameter of the sedimentation chamber and width equal to the field of the eyepiece (Hasle, 1978). In mid 2002, the method of enumeration was adapted to fit the abundance of the various groups. Abundant cells were counted in defined random fields, rarer species were counted on the whole chamber (Fonda Umani et al., 2007). The method adopted up to 2002 might have led to over-estimate of less abundant species, while no modifications in counting method over the whole time series have involved abundant taxa (e.g. Undet. Flagellates, Cryptophyceae undet, Skeletonema spp., Chaetoceros spp.) Due to scattered metadata information during the earlier part of the time series (1986 – 2002), a more detailed backward Quality Control is not possible. In the North Adriatic, a general decrease in phytoplankton abundance has been described in the same period by several other institutes working in the area, which increases the confidence in the whole observed data pattern. References Fonda Umani S., Del Negro P. Larato C., De Vittor C., Cabrini M., Celio M., Falconi C., Tamberlich F. and Azam F. 2007 Major interannual variations in microbial dynamics in the gulf of Trieste (Northern Adriatic Sea) and their ecosystems implications. Aquatic Microbial Ecology 46: 163-175. Hasle G.R. 1978 Using the inverted microscope. In: Phytoplankton Manual (ed. Sournia A.) UNESCO, Paris: 191-196.