Results and Confidence

This pre-core indicator and its threshold values are yet to be commonly agreed in HELCOM.
The indictor is included as a test indicator for the purposes of the 'State of the Baltic Sea' report 2018, and the results are to be considered as intermediate.

Good status was not reached in any of the assessment units where the indicator was applied. The sub-basins causing greatest concern regarding status during the assessment period 2011-2015 were the Bothnian Sea, Northern Baltic Proper and Gulf of Riga, where the eutrophication ration (ER) was ≥1.50 (Results table 1). ER values were also above the threshold value in the other assessed areas, and the assessment units closest to achieving good status were the Arkona Sea, Bornholm Basin, Eastern Gotland Basin and Western Gotland Basin (Results figure 1 and Results Table 1).


Results figure 1. Status of cyanobacterial surface accumulations, presented as eutrophication ratio (ER). ER shows the present concentration in relation to the threshold value, increasing along with increasing eutrophication. The threshold value has been achieved when ER ≤ 1.00.

Results table 1. Information on threshold values, present concentration and status (good status /not good status) of the Cyanobacterial Bloom Index indicator. The indicator values are based on two parameters, Cyanobacterial Surface Accumulations and Cyanobacteria biomass (See Thresholds and Status evaluation table 1 and Assessment protocol).


Long-term trends

Blooms of nitrogen-fixing cyanobacteria are a natural phenomenon in the Baltic Sea (Bianchi et al. 2000, Poutanen & Nikkilä 2001, Westman et al. 2003), and have been observed in phytoplankton sampling in the early 1900s (Hällfors et al. 2012). The blooms became extensive during the 20th century, and have occurred commonly in the Baltic Proper and the Gulf of Finland since the 1960s (Finni et al. 2001, Poutanen & Nikkilä 2001).

The longest satellite based time series on algae accumulations from the Baltic Sea is presented by Kahru and Elmgren (2014). According to this satellite-based investigation, cyanobacterial blooms have increased in the Baltic Sea since the late 1970s (Result figure 2). Also, the total area covered by the blooms has increased during this time. However, the development has not been linear as the period with the lowest amount of blooms occurred in the late 1980s – early 1990s.


Result figure 2. Mean fraction of cyanobacteria accumulations (FCA%) and total accumulated area affected (TA) in the entire Baltic Sea between 1979 and 2013 based on satellite detection (from Kahru and Elmgren, 2014).


The general trend in indicator status for the sub-basins indicates a worsening trend (i.e. decreasing Cyanobacterial Bloom Index values) for the Northern Baltic Proper (Results figure 3A) and Bothnian Sea (Results figure 3B). For the Gulf of Finland and Eastern Gotland Basin, recent years (circa 2005 onwards) show slightly better status than during the earlier time period of 1998-2004. However, observations before 1996 also generally show on better status than the years following this time point.


Figure 3A)


Figure 3B)


Results figure 3 (A and B). Temporal development of 'Cyanobacterial bloom index' in the open-sea assessment units: A) In the Eastern Gotland Basin, the Northern Baltic Proper and the Gulf of Finland in 1990-2014, and B) in the Arkona Basin, the Bornholm Basin, the Gulf of Riga and the Bothnian Sea in 2003-2015. Dashed lines show the five-year moving averages. Green lines in B denote the indicator threshold. Thresholds are not included in A due to differences in sub-basin division (sub-basin division given in Kahru & Elmgren 2014). Significance of trends was assessed with Mann-Kendall non-parametric tests for the whole data sets. Significant (p<0.05) deteriorating trends are indicated with orange data points, and no significant improving trends were found in the current data set.


Confidence of the indicator status evaluation

The confidence of the status estimate was not assessed in absence of methodology to define status confidence (ES_SCORE). Further development of this indicator is required and is underway.