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 analysis required for the indicator evaluation is that the biomass of planktonic diatoms is divided by the biomass of autotrophic (+ mixotrophic) dinoflagellates. In order to let this indicator range from 0 to 1, the ratio is calculated as follows:
The following conditions have to be fulfilled for the analysis to be valid:
Note 1: For practical reasons, a representative sample from the upper mixed layer irrespective of the sampling depth should be sufficient. Only in spring, the upper mixed layer is rather deep and comprises the whole euphotic (trophogenic) layer. Deep chlorophyll maxima, frequently formed by dinoflagellates, seem to rarely occur in spring. The influence of day-time is low, and thus the time of day need not be considered in the sampling guidelines.
Note 2: Diatoms are always considered as autotrophic, but dinoflagellates may also be mixotrophic or heterotrophic. The mode of nutrition is difficult to identify. Pigmented dinoflagellates are considered as autotrophs. Even the chloroplasts are sometimes hard to recognize. The bloom-forming dinoflagellates of the spring (Peridiniella catenata, Biecheleria baltica, Gymnodinium corollarium, Scrippsiella hangoei; cf. Klais et al. 2013) are autotrophs. A minor error in a few doubtful dinoflagellates will not affect the index.
Note 3: The biomass in the numerator and denominator has to be given in the same units. Wet weight or carbon units can be used, but as carbon data are frequently lacking in older data, wet weight is preferred. If carbon units are used the Dia/Dino index is skewed. As large diatoms have a big vacuole that contains only little organic carbon, the Dia/Dino index will be lower in comparison with that based on wet weight. However, especially in spring, when small diatoms dominate, the deviation is not as large as in other seasons and may be acceptable.
Note 4: If sampling dates or numbers of samples are very irregularly distributed during the spring months, monthly means have to be calculated before seasonal means are calculated from the monthly means.
Note 5: The Dia/Dino index reflects the conditions during the spring bloom because this is the most prominent bloom in the annual cycle. The strongest effect of eutrophication (new nutrients) and global warming is expected in spring. Other arguments for using spring data are given in Note 3. Spring is defined as the period from March to May in the Baltic Proper.
Note 6: It has to be assured that the bloom was met. We suggest a biomass threshold of 1000 µg/L which has to be exceeded either by the diatoms or the dinoflagellates as a criterion. If this value is missed, the standard Dia/Dino index must not be calculated.
Note 7: Missing the diatom bloom may have two consequences: (1) The regular Dia/Dino index cannot be calculated because the threshold was missed (Note 6) or (2) the biomass threshold is just passed but the Dia/Dino index is unusually low nevertheless. In that case the diatom biomass can be calculated on the basis of silicate consumption as originally suggested by Wasmund et al. (2013). The resulting alternative Dia/Dino index is calculated as follows:
The alternative Dia/Dino index is normally higher than the standard Dia/Dino index because the silicate consumption estimates the maximal possible diatom biomass (cf. Results figure 1). Therefore, different good status values have to be derived.
For the time being, this assessment is made only for the Eastern Gotland Basin.