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, and the results are to be considered as intermediate.
The evaluation is based on phytoplankton data of the spring period (March-May) from the upper mixed layer (Results table 1). Precondition for a valid calculation is a check whether the spring bloom is sufficiently represented in the data. The biomass of diatoms or dinoflagellates has to exceed a threshold of 1000 µg/L at least once. If this is not achieved, the calculation of an "alternative" Dia/Dino index, based on silicate consumption, may be applied, as described by Wasmund et al. (2017). As seen from Results table 1, the bloom was always sufficiently represented during the assessment period from 2011 to 2016.
Results table 1. Maximum and mean biomass of diatoms and dinoflagellates in the years of the assessment period.
The good status is defined at a Dia/Dino index > 0.5. It was failed in 2012, 2013, 2015, 2016, and in the total average over the assessment period (Results table 2.)
Results table 2. Dia/Dino index in the Eastern Gotland Basin, separated for the years of the assessment period. Number of data points per season are given as "n".
Temporal trends provide additional information on the spread and variability of the Dia/Dino index. The original data for each year from 1981 to 2016 is shown in Results figure 1 (red line). It reveals a high variability. The spread over the entire potential range from 0 to 1 indicates the high sensitivity of this indicator. If the spring bloom was missed by the routine sampling, i.e. biomass of diatoms or dinoflagellates was smaller than 1000 µg/L, the use of the standard Dia/Dino index was not allowed. In that case, the alternative Dia/Dino index (green line) may be tried. It has to be noted, that its threshold for the good status is higher (in this case at 0.84). Also in years of extreme outliers that failed the good status, the alternative Dia/Dino index may be checked. This applies, for example, to the year 1984, when the standard Dia/Dino index indicated a bad status whereas the status may be still good according to the alternative Dia/Dino index. The alternative Dia/Dino index should only be used in exceptional and justified cases.
Results figure 1. Annual values of the standard Dia/Dino index and the alternative Dia/Dino index from 1981 to 2016. Dashed lines indicate the thresholds for the good status. GES = Good status.
For trend analyses, the curves should be smoothed, for example by using the 3-year moving average as shown in Results figure 2. By this processing, the strong decline of diatom blooms, as discovered by Wasmund et al. (1998), becomes obvious. After the very bad status in the 1990s concerning the diatom blooms (cf. also Klais et al. 2011), the system recovered under high fluctuations. On average, it still did not reach the good status.
Results figure 2. 3-year moving average of the standard Dia/Dino index and the alternative Dia/Dino index from 1981 to 2016. Dashed lines indicate the thresholds for the good status. GES = Good status.
The indicator confidence depends on the data basis. The Dia/Dino index is based on data of the dominating phytoplankton groups, diatoms and dinoflagellates, that are more robust than those of rare phytoplankton groups. Moreover, these groups are easily to identify even without specific expert knowledge. This makes the original phytoplankton data robust.
As seen from Results table 1, the bloom was always sufficiently represented during the assessment period from 2011 to 2016. The number of data points in each year is sufficient (see Results table 2). The standard Dia/Dino index can additionally be checked by an alternative method based on silicate data. As both methods result in the same evaluation for each year and show the same trends during the assessment period, the confidence of the assessment is considered to be high.