Aarhus Universitets segl

No. 128: Intercalibration of phytoplankton studies in marine areas 2018

Jakobsen, H.H. 2019. Interkalibrering af fytoplanktonundersøgelser i marine områder 2018. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 28 s. - Teknisk rapport nr. 128


In support of the Danish Nationwide Monitoring and Assessment Program for the Aquatic and Terrestrial Environments (NOVANA), a phytoplankton inter-calibration was executed in the summer of 2018 focusing on species identification and enumeration, species diversity index calculations and calculation of phytoplankton carbon biomass. Inter-calibration is a part of the national quality assurance procedure where all consultants and data providers have to participate regularly. Phytoplankton inter-calibration is a part of the framework agreement between DCE/Aarhus University and the Danish Environmental Protection Agency. Four participants from Denmark participated in the intercalibration (appendix 5.1). Data were reported to the data portal ‘STOQ’ operated by the Danish Nature Agency, and from where data used in this report were drawn. Significant results of the phytoplankton inter-calibration are:

Based on the number of cells counted and the volume measurements of the dominant cells, all four consultants provided data accordingly to the guidelines stated in the technical instructions. However, one exception was that one of the consultants apparently only measured the cell sizes in one of the analyzed triplicate samples.

The species composition was overwhelmingly dominated by diatoms whereas other species (dinoflagellates and cryptophytes) made a small fraction.

A total of 27 species were identified in the sample, of which 11 species (key species) were identified by all consultants. The key species biomass accounted for > 95% of the total biomass. The average phytoplankton biomass at class level varied accordingly across all participants and showed a slight statistical difference between the participants for diatoms (P = 0.03) and dinoflagellates (P = 0.01), but not for cryptophytes.

Similarity analysis based on  presence/absence of species gives an overall picture of the "uniformity (similarities), of the sample at the level as a whole community composition. There was a similarity > 80 % between three of the participating four consultants. There is no general accepted level of on satisfactory performance, but based on studies in the primary literature, 80% is concluded satisfactory (Culverhouse et al. 2003).

The sample size affects the number of species observed in a given sample. This can be corrected for, so-called "rare faction" correction. After this correction, the mutual difference between the consultants' species count was reduced from 40% to 17%, which points to differences in the counting strategies.

Compared to previous inter-calibrations held in 2004 and 2012, were the variation in measurements of cell sizes significantly by the consultants' significantly reduced. In addition, the variation between consultants' biomass determination has decreased from »40 to 11% since 2012.

The inter-calibration was concluded with a concluding meeting at Aarhus University, Department of Bioscience (Roskilde). At the meeting, data from the present report were presented and discussed.