This dataset contains earth observation data summarized per 20k grid in the BAltic Sea used for Chlorophyll-a indicator in the HELCOM Third Holistic Assessment of the Ecosystem Health of the Baltic Sea (HOLAS 3).

This core indicator evaluates the average chlorophyll-a concentration in surface water (0 – 10 m) during summer (June – September) during the assessment period 2016-2021. The measured average chlorophyll-a is a combined estimate of three types of data: 1) in-situ measurements from oceanographic monitoring data (surface water 0-10 m) and 2) earth observation data. 3) Ferrybox data. More detailed description on methodology can be found at HELCOM Core indicator report at https://indicators.helcom.fi/indicator/chlorophyll/ This dataset displays the result of the indicator in HELCOM Assessment Scale 4b (Division of the Baltic Sea into 19 open sea sub-basins and division of coastal areas to WFD water types or water bodies). The open sea areas have been assessed based on HELCOM Monitoring data, earth observation data and ferrybox data. Results for coastal areas are based on national WFD results reported by HELCOM EG EUTRO. Attribute information: "HELCOM_ID" = Code of the HELCOM scale 4b assessment unit "Country" = Country/ Opensea "level_2" = name of the subbasin "level_4" = Name of the HELCOM scale 4 assessment unit "Area (km2)" = Area of the HELCOM scale 4 assessment unit "Indicator" = Name of indicator “Season” = Season of the indicator (Summer = June-September) “Period” = Assessment period (20162021 = 2016 – 2021) Indicator value (ES) = Indicator value calculated for the assessment unit "Standard deviation (STD)" = Standard deviation of data used for calculating ES "Number of samples " = Number of samples by assessment unit "Theshold value (ET)" = Commonly agreed thershold value "Ratio between ES and ET (ER)" = Eutrophication ratio: Ratio between ES and ET (in case of indicator with positive response to eutrophication) "ES_SCORE” = Confidence based on the data used for calculating ES "ET_SCORE” = Confidence of target-setting procedure "I_SCORE” = Confidence (%) = average of ES-Score and ET-Score "Indicator Weight" = Indicator Weight in integrated eutrophication assessment “Status” = Status of the indicator (“Good”, “Not good”, “Not assessed” or "Not applicable") “Confidence” = Confidence of result based on I-SCORE (“High”, “Moderate” or “Low”) "AULEVEL" = Assessment unit level used for the indicator

This dataset contains ferrybox data reported by HELCOM Contracting Parties and used for Chlorophyll-a indicator in the HELCOM Third Holistic Assessment of the Ecosystem Health of the Baltic Sea (HOLAS 3). The Ferrybox data consists of on-board measurements taken on commercial ship routes and collated by Finnish Environment Institute SYKE. Data has been reported to ICES using the ICES Oceanographic data format for PUMP data type. https://www.ices.dk/data/data-portals/Pages/ocean-format.aspx https://www.ices.dk/data/Documents/ocean/ICES-Oceanography-Data-Submission-Format.zip Attribute description (ODV format): Cruise: Cruise label to identify the given cruise Station: Station label to identity the given station yyyy-mm-ddThh:mm:ss.sss: Date and time of the given station (year, month, day, hour, minute and seconds) in UTC Latitude [degrees north]: Latitude in decimal degrees in WGS84 Longitude [degrees south]: Longitude in decimal degrees in WGS84 Bot. Depth [m]: Bottom Depth in metres Depth [m]: Depth [m] of the given sample QV:ODV:Depth [m]: Quality Flag of the given Quality Flag Scheme and variable Temperature [degC]: Temperature in degree Celsius of the given sample QV:ODV:Temperature [degC]: Quality Flag of the given Quality Flag Scheme and variable Practical Salinity [dmnless]: Practical Salinity of the given sample QV:ODV:Practical Salinity [dmnless]: Quality Flag of the given Quality Flag Scheme and variable Dissolved Oxygen [ml/l]: Dissolved Oxygen in ml/l of the given sample QV:ODV:Dissolved Oxygen [ml/l]: Quality Flag of the given Quality Flag Scheme and variable

Springtime Chl-a concentration is here used as a proxy for productive surface waters. In the Baltic Sea Impact Index (BSII), areas with high springtime phytoplankton production will be given higher importance, as they are considered important areas for the Baltic Sea food web. In the current map, mean of springtime maximum weekly values (weeks 12-22, years 2003-2011) Chl-a concentration of the surface waters has been used, derived from satellite data (MERIS). Years 2003-2011 have been used, as there is no MERIS data available for years 2012-2016. The data for eastern Baltic Sea is provided by the Finnish Environment Institute (~300m resolution). Outside this high resolution data, MERIS-data downloaded from JRC-database has been used (~4 km resolution, to calculate average of maximum monthly values for April or May for 2003-2011). Both datasets were converted to 1 km x 1 km grid cells.