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No. 76: Intercalibration of chlorophyll a between Denmark, Norway and Sweden

Carstensen, J. 2016. Intercalibration of chlorophyll a between Denmark, Norway and Sweden. Western Baltic (BC6), Kattegat (NEA8b) and Skagerrak (NEA8a, NEA9 and NEA10). Aarhus University, DCE – Danish Centre for Environment and Energy, 38 pp. Technical Report from DCE – Danish Centre for Environment and Energy No. 76. http://dce2.au.dk/pub/TR76.pdf

Summary

In this report reference conditions and class boundaries for high-good and good-moderate boundaries of phytoplankton biomass have been intercalibrated between Denmark, Norway and Sweden. Phytoplankton biomass was assessed using chlorophyll a, which is a measure of the photosynthetic active biomass in phytoplankton cells and provides a proxy measure of the phytoplankton biomass. The intercalibration approach followed the EC guidelines and was carried out separately for five common intercalibration types: BC6, NEA 8a, NEA 8b, NEA 9 and NEA 10. BC6 is located within the Baltic Geographical Intercalibration Group and the four other types are located in the North East Atlantic Geographical Intercalibration Group. The three countries use the same data acquisition for chlorophyll a, but assessment methods (metrics) differ.

A combined dataset with data from the three national monitoring programmes in Denmark, Norway and Sweden was established as the basis for the intercalibration. The three different national chlorophyll a metrics have been calculated for all water bodies using this combined dataset, allowing a direct comparison of the different assessment methods. The Danish metric was selected as the common metric, and both the Norwegian and Swedish metrics correlated strongly with the Danish metric. The chlorophyll a metrics were also strongly correlated to the total nitrogen concentration and no adjustments caused by potential biogeographical differences were needed.

Existing reference conditions and class boundaries for the three countries were translated to the common metric scale for each of the five types using the established relationships between national chlorophyll a metrics. In BC6 and NEA 8b differences in reference conditions and class boundaries between Denmark and Sweden were small, when these were compared with the common metric scale. In NEA 8a, 9 and 10, differences between Norway and Sweden were somewhat larger. Intercalibration of reference conditions and class boundaries was mainly done by averaging national boundaries translated to the common scale, but in NEA 8a, 9 and 10 some of the values were set to appropriate values, obeying the natural gradient from land towards the open sea and reference conditions and targets for good environmental status set by OSPAR for the open Skagerrak. Intercalibrated values for the common chlorophyll a metric were translated back to the national metric using established relationships. The final outcome of the intercalibration exercise was a set of intercalibrated chlorophyll a values for the five shared types in the Baltic GIG and NEA GIG, which will be submitted by the agencies in Denmark, Norway and Sweden to the European Commission decision on intercalibration.