Of the 17 open-sea sub-basins, good status (concentrations below the threshold value) for dissolved inorganic phosphorus (DIP) was only achieved in the Bothnian Bay. The Kattegat, Great Belt and Kiel Bay were near to their respective threshold values (ER < 1.2). The Bay of Mecklenburg as well as Gdansk Basin showed ER values < 1.5 and The Sound 1.52, but the remaining open sea sub-basins are generally more distant from their threshold values, with many areas expressing ER values close to or greater than 2. Moreover, it appears that the Gdansk Basin, Arkona Basin, Bornholm Basin, and the Gulf of Finland show slight increasing trends in DIP within this assessment period (Results figure 2). It is likely that the changes between 2013 and 2016 relate to the natural phenomenon of the major Baltic inflow. This might have stirred up some legacy phosphorus from deeper waters and the sediments below, redistributing it to surface waters. The Kattegat, Great Belt, The Sound, Kiel Bay, and Bay of Mecklenburg in turn appear to have received a clear pulse of DIP in 2013, and almost returned to pre-inflow levels from 2014 (Results figure 2).
Results figure 1. Status evaluation of the DIP indicator, presented as eutrophication ratio (ER). ER shows the present concentration in relation to the threshold value, increasing along with increasing eutrophication. The threshold value is ER ≤ 1.00 (achieve good status).
Results figure 2. Winter DIP concentrations (dark blue line; average for 2011-2016) and threshold levels as agreed by HELCOM HOD 39-2012 (red dashed line). It should be noted that the results for Bornholm Basin strongly depend on stations in the open-sea area of Pomeranian Bay, which is influenced by the Odra plume.
Results table 1. Threshold values, present concentration (as average 2011-2016), eutrophication ratio (ER) and status of DIP 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 value has not been reached (fail – i.e not good status).
Of all coastal waters that were assessed, good eutrophication status is only found in some areas of the Swedish and Polish coastal waters (Results figure 1). Winter DIP concentrations were only assessed in the coastal and transitional waters of Latvia, Poland and Sweden (Results table2). In the Polish coastal waters 39% of the area (km2) was in good status and 61% failed to achieve good status. In Swedish coastal waters 48% of the area was in good status, 7% was not assessed and 45% failed to achieve good status. In the coastal waters of Latvia 100% of the areas failed to achieve good status.
Results table 2. Results for national coastal DIP indicators, as reported by countries. The table includes information on the assessment unit (CODE, defined in the HELCOM Monitoring and Assessment Strategy Annex 4), assessment period (start year and end year), average concentration during assessment period, threshold values, units, and Eutrophication Ratio (ER). The ER is coloured red or green to denote if the status evaluation has been failed or achieved, respectively. For Sweden - indicates only status provided and not raw result value, and note countries use different units in their reproting. In Swedish waters only the water bodies that were assessed are shown. ER is a quantitative value for the level of eutrophication, calculated as the ratio between the threshold value and the present concentration and forms the basis for the status assessment – when ER > 1, threshold value has not been achieved and status is failed (not good).
The long-term trends are provided as additional information and do not influence the status assessment.
For some basins, measurements reach back to the 1960s. After the marked increase of dissolved inorganic phosphate (DIP) that started in the 1960s and 1970s, concentrations generally remained at a high level, though strong fluctuations occur as a result of various internal processes and generally no clear trends can be seen in most assessment regions (Results figure 3). However the increase in DIP concentrations during recent years, particularly visible in the Bothnian Sea, Gulf of Riga, Gdansk Basin, Northern Baltic Proper and Åland Sea is interesting. Major Baltic Inflows (MBIs) are of great importance in this respect. After the MBIs of 1975/76, 1983 and 1993 lower phosphate concentrations in the subsequent years were measured, whereas a comparable decrease after the MBI of 2003 was not be observed, indicating that the vertical transport through the permanent halocline is not sufficiently understood. The historicity of the inflow events and the seasons of MBIs as well as the intensity and depth of vertical mixing must be considered (Nausch et al. 2008; Reissmann et al. 2009). The process is additionally complicated since mixing of oxygenated waters into sulfidic waters causes formation of manganese and iron phosphate precipitates (Dellwig et al., 2010) that cause transport of large amounts of phosphate to the sediments (Relevance figure 1). So it appears that the recent inflow with a short period of re-oxygenation mainly caused entrainment of phosphate into the surface waters, likely by increased turbulence and replacement of deeper waters. Based on the current data, intermittent removal by precipitation appeared quantitatively not so important.
Result figure 3. Temporal development of winter dissolved inorganic phosphorus (DIP) concentrations in the open-sea assessment units from 1960s to 2016. Dashed lines show the five-year moving averages and error bars the standard deviation. Green lines denotes the indicator threshold. Significance of trends was assessed with Mann-Kendall non-parametric tests for period from 1990-2016. Significant (p<0.05) deteriorating trends are indicated with orange data points. No significant improving trends were detected.
The confidence of the indicator status evaluation in open sea areas, based on the spatial and temporal coverage of data as well as the accuracy of the protocol for setting threshold values, was high in the Kattegat, Arkona and Bornholm Basin, low in The Sound, Gdansk Basin, Western Gotland Basin, Gulf of Riga, Åland Sea and The Quark, and moderate in all remaining sub-basins. The low confidence was caused by a lack of monitoring data.
Results figure 4. Indicator confidence, determined combining information on data availability and the accuracy of the protocol for setting threshold values. Low indicator confidence calls for an increase in monitoring.
The indicator confidence was estimated through confidence scoring of the threshold value (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.