This 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.
HELCOM common monitoring of the phytoplankton community, the methods for sampling, sample analysis and calculation of carbon biomass are described in general terms in the HELCOM Monitoring Manual (http://www.helcom.fi/action-areas/monitoring-and-assessment/manuals-and-guidelines/phytoplankton-guidelines).
For time-series calculations, it is important to have as regular datasets as possible. At least monthly sampling during the growth period is needed to design reference growth curves. If sampling dates or numbers of samples are very irregularly distributed, monthly means have to be calculated before further analysis. If historical datasets are not available, time-series data should be collected during at least 10 years to include natural interannual variability. The data must represent the upper mixed layer. FerryBox data can be additionally used assuming that that the sampling depth (usually 4−5 m) represents the upper surface layer as the ship creates turbulence when moving.
Current monitoring is not formalised for this indicator. The indicator is currently being tested using data collated for the described test assessment units. Sufficiently frequent sampling is seldom available through monitoring programs (see also Heiskanen et al., 2016). Moreover, the open sea monitoring activities of many countries have been reduced during the last years. This is in some areas (Gulf of Finland, Northern Baltic Proper) compensated by increasing activities of sampling by FerryBox systems. A more detailed scheme of stations and sampling times of recent monitoring activities can be provided.
This test indicator is operational as:
The interval of sampling should be regular and the frequency at least once a month during the growth period. In some areas, especially offshore, it can be supported also by FerryBox sampling. For time being, microscopic analysis is part of Ferrybox sampling only in Estonian monitoring program. The time-scale for data sets should be at least 10 years to create type- or site-specific reference growth curves. Some recommendations for spatial resolution have been given recently (Jaanus et al., 2017) and this will be an important consideration when defining the appropriate scale of assessment units monitored.