The oxygen debt is defined as the "missing" oxygen relative to a full saturated water column, another term often used is the apparent oxygen utilization (AOU). By using the oxygen debt instead of the actual oxygen concentration the variations due to temperature controlled solubility is excluded.
It is preferable if oxygen data from the whole water profile is available but this is often not the case and especially not for older data. To overcome this problem the oxygen profile has instead been modelled with the information from salinity profiles, which typically has a higher vertical resolution than the oxygen in older data.
Generalized additive models (GAM) were applied to describe the temporal, seasonal and spatial variation in each basin. From these models, seasonal and spatial effects can be averaged over providing a time series mostly representing annual variations linked to nutrient inputs. The oxygen debt algorithm is done using the BALTSEM assessment units and the results are afterwards transferred to the HELCOM assessment units. For more details of the method see BSEP 133 and the R-scripts (https://github.com/ices-tools-prod/HEAT/tree/master/scripts/OxygenDebt).
The assessment units for the indicator are the open sea assessment units of the 17 Baltic Sea sub-basins.
The assessment units are defined in the HELCOM Monitoring and Assessment Strategy Annex 4.