The core indicator 'Oxygen debt' is applied in the Bornholm Basin, Gdansk Basin, Western Gotland Basin, Eastern Gotland Basin, Northern Baltic Proper and Gulf of Finland. Values below the threshold values have not been achieved in any of these sub-basins, indicating not good status in all areas where an indicator evaluation has been made (Result figure 1 and Results table 1) and levels have remained relatively constant during the current assessment period (Results figure 2). Oxygen debt is largest in the Baltic Proper, corresponding to hypoxic conditions starting just below the halocline. An oxygen debt threshold value exist for the Åland Sea, Bothnian Sea and Bothnian Bay, but currently the oxygen debt algorithm has not been adjusted and optimized to provide reliable indicator results for those assessment units.
Result and confidence figure 1. Status of the 'Oxygen debt' indicator below the halocline, presented as eutrophication ratio (ER). ER shows the present concentration in relation to the threshold value, increasing along with increasing eutrophication. Good status is achieved when ER ≤ 1.00.
Results figure 2. Average oxygen debt below the halocline (black line; average for years 2011-2016) and threshold values.
Results table 1. Threshold values, present concentration (as average 2011-2016), eutrophication ratio (ER) and status of oxygen debt in the open-sea basins. ER is a quantitative value for the level of eutrophication, calculated as the ratio between the threshold value and the present concentration – when ER > 1, threshold values has not been reached.
Oxygen debt below the halocline has increased in all basins since the early 1900's. The increase has been strongest in the Baltic Proper (Results figure 3). The Bornholm Basin experiences larger inter-annual variability because of larger variations in the oxygen concentrations, mainly as a result of natural water flows or processes.
Results figure 3. Temporal development in the core indicator 'Oxygen debt' in the Baltic Proper (containing Eastern Gotland basin, Gdansk basin, Western Gotland basin, Northern Baltic Proper and Gulf of Finland), showing the volume specific oxygen debt below the halocline based on the data and sub-basin division delineation of HELCOM (2017). The dashed line shows the five-year moving average. The significance of the trend was tested for the period 1990-2012 by the Mann-Kendall non parametric test. Orange color indicates significant (p<0.05) deteriorating trend: an increasing trend in oxygen debt signifies deteriorating oxygen conditions.
The confidence of the indicator status evaluation is based on the spatial and temporal coverage of data as well as the accuracy of the threshold-setting protocol. High confidence was attained in all the sub-basins where the indicator was applicable (Results figure 4).
Results figure 4. Indicator confidence, determined by combining information on data availability and the accuracy of the threshold-setting protocol. Low indicator confidence calls for increase in monitoring.
The indicator confidence was estimated through confidence scoring of the threshold (ET-Score) and the indicator data (ES-Score). The ET-Score was rated based on the uncertainty of the threshold value setting procedure. The ES-Score is based on the number as well as spatial and temporal coverage of the observations for the assessment period 2011-2016. To estimate the overall indicator confidence, the ET- and ES-Scores were combined. See
Andersen et al. 2010 and Fleming-Lehtinen et al. 2015 for further details.