Summary

In 2004 an intercalibration of plankton monitoring was conducted as part of the quality assurance of the Danish Nation-wide Aquatic Monitoring and Assessment Program. The intercalibration focused on calculation of abundance and carbon biomass of phyto-, micro- and mesozooplankton, with most participants representing private consulting companies and regional authorities involved in the Danish monitoring program. A few participants were from Swedish and Norwegian institutions.

Main results from the phytoplankton intercalibration were:

• The calculated total carbon biomass showed good agreement between participants with a coefficient of variance (CV) of 12% and very high similarity for replicate and triplicate samples counted by the same participant (CVs of 4% and 1%, respectively).

• Carbon biomass of individual species calculated by the different participants varied substantially more than the total biomass.

• Diversity (Shannon’s index) calculated from abundance (cells l-1) varied dramatically while diversity calculated from carbon biomass of individual species showed good agreement between participants.

• Major differences were found in the number of specimens and taxa calculated by the different participants. Combined with the very similar total biomass obtained by the participants, the additional effort put into quantifying the rarer species will not have a major impact on the estimate of the total biomass or the diversity based on biomass. Abundance of the rare species may, however, be important in the monitoring of e.g. toxic species.

• Several synonyms were used for a number of species, and there is a need for adjustment of the taxonomy to ensure comparable data.

• Cell volumes of the quantified species obtained either from measurements of specimens in the samples or by the use of constant species specific volumes varied between the participants.

Main results from the micro- and mesozooplankton intercalibration were:

• To describe interactions of the plankton it is essential to separate the heterotrophic dinoflagellates from the autotrophic ones. For the small species this requires the use of epifluorescence microscopy. However, most of the biomass of the community of heterotrophic dinoflagellate arises from large species that are identifiable in the inverted microscope. Therefore it will be valuable to put extra effort into identification and quantification of the most common large species like Gyrodinium spirale og G. dominans.

• Size rather than taxonomical position determines the ecological role of the ciliates. Thus it will be profitable to put effort into counting a larger number of individuals divided into well-defined size classes rather than attempting a correct taxonomical identification.

The results of the intercalibration illustrate the need for recurring intercalibrations to ensure that monitoring data are comparable.

Full report in pdf-format (814 kB).