Status evaluation is measured in relation to scientifically based and commonly agreed sub-basin specific threshold values, which defines the concentration that should not be exceeded (Thresholds and Status evaluation figure 1).
Thresholds and Status evaluation figure 1. Schematic representation of the threshold value applied in the 'oxygen debt' core indicator, the threshold values are assessment unit specific (see Thresholds and Status evaluation table 1).
These indicator threshold values were based on the results obtained in the TARGREV project (HELCOM 2013a), also taking advantage of the work carried out during the EUTRO PRO process (HELCOM 2009) and national work for EU WFD. The final threshold values were set through an expert evaluation process done by the intersessional activity on development of core eutrophication indicators (HELCOM CORE EUTRO) and the threshold values were adopted by the HELCOM Heads of Delegations 39/2012 (Thresholds and Status evaluation table 1). The threshold values for oxygen debt in Åland Sea, Bothnian Sea and the Bothnian Bay were however endorsed at a later stage by the HELCOM State and Conservation 5E-2017 but the algorithm for calculating the indicator remains to be adjusted and optimized for these areas.
The threshold values for oxygen debt were defined from the 95th percentiles during the pre-eutrophied period before 1940, detected through change-point analysis for all assessment units.
Thresholds and Status evaluation table 1. Assessment unit specific threshold values for the core indicator 'oxygen debt'.