The open-sea core indicators are updated using data reported by Contracting Parties to the HELCOM COMBINE database hosted by ICES, using the algorithms developed for the eutrophication assessment work flow. The values are achieved using indicators specifications shown in Assessment protocol table 1 (see HELCOM Eutrophication assessment manual).
Assessment protocol table 1. Specifications of the core indicator Secchi depth.
The indicators within the criteria were weighted according to their relevance for eutrophication in each sub-basin. The weight was evenly distributed within the criterion, unless there was a justification to do otherwise. For Secchi depth, the weight was assigned according to the available information on the light absorption by colored dissolved organic matter (CDOM) and the relationship between CDOM absorption and chlorophyll a concentration in the sub-basin (Assessment protocol table 2), respectively. The weight was distributed equally (no weight assigned) for most sub-basins but in the Gulf of Finland and especially in the Gulf of Bothnia Secchi depth received a reduced weight due to increased absorption of light by CDOM. This makes Secchi depth a less reliable indicator of eutrophication, and therefore it received a smaller weight in those basins.
Assessment protocol table 2. Secchi depth and chlorophyll a have been weighted according to available information on CDOM absorption of light and the relationship between CDOM light absorption and chlorophyll a (chl-a) concentration in the sub-basin.
The core indicator is applicable in the 17 open sea assessment units (from one nautical mile from the baseline seawards).
The assessment units are defined in the HELCOM Monitoring and Assessment Strategy Annex 4.