Das Projekt "Phytoplankton ring test 2007 - Identification, counting and biomass determination of synthetic and natural phytoplankton samples" wird vom Umweltbundesamt gefördert und von Umweltbundesamt durchgeführt. In 2007, within the HELCOM Monitoring Programme of the Baltic Sea and as part of the quality assurance of the German Marine Monitoring Programme a common ring test on phytoplankton was carried out. 25 employees from 7 HELCOM member states took part in this inter-laboratory comparison. Three different typs of samples were analysed: a synthetic sample (series 1) consisting of different types of microparticles and two samples (series 2 and 3) consisting of phytoplankton from the western part of the Baltic Sea (Kieler Bucht). For series 1 a good comparability of counting and measuring results within a laboratory (repetitions) became apparent. A very good comparability of results between laboratories became recognizable as well, due to the simple geometric bodies of the microparticies. The results of series 1 show that the greater the number of microparticles and the smaller the size of microparticles was, the greater were the counting and measurement errors. Overall, it can be concluded that almost all participants were able to identify, count and measure most of the taxa of series 2 with adequate certainty. Participants attracting attention due to their z deep u score in series 1 did not necessarily in series 2 or vice versa. The results of series 1 and series 2 indicate that the counting strategies 'stripes' and 'fields' could cause higher mean values with a higher variability than the counting strategy 'whole chamber'. Because this ring test did not specifically concentrate on methodologically issues, the observed effects are not statistically ensured. Therefore, especially the suitability of the counting strategies 'stripes' and 'fields' for the determination of abundancies should be reviewed in detail in particularly prepared ring tests or laboratory comparisons. The technical equipment of the participants obviously was suitable for the routine monitoring. A direct dependence of counting and measurement results from microscopic equipment was not found. For species determination of many taxa, however, additional methods like special staining methods or scanning electron microscopy would be required. Because these methods arc time and cost intensive it is often difficult to find a suitable compromise. Nevertheless, especially methodological aspects of the phytoplankton analysis like the mentioned influence of the counting strategy on the abundance results should be further examined to better assess analytical errors. Assuming that the ring test samples compared to routine samples were most carefully examined and that participants invested more time than usual, it has to be expected that the detected variations in routine analysis are much higher. Participants reported many single proofs with respect to the comparability of the taxa composition results of series 3. Only a few taxa had been identified by all participants on species level. usw.