Phytolankton have been monitored in Sweden since 1983. The national monitoring program was initiated by the Swedish Environmental Protection Agency and is now financed by the Swedish Agency for Marine and Water Management. Monitoring is performed by Stockholm University, Swedish Meteorological and Hydrological Institute, Gothenburg University and Umeå University. Data are stored in the Swedish Ocean Archive database (SHARK), by the Swedish Meteorological and Hydrological Institute. Data are collected and analyzed according to the HELCOM COMBINE Manual - Part C Annex C6 Monitoring of phytoplankton species composition, abundance and biomass (https://www.helcom.fi/wp-content/uploads/2019/08/Guidelines-for-monitoring-phytoplankton-species-composition-abundance-and-biomass.pdf) or similar methods. "In short, analysis of phytoplankton species composition, abundance and biomass is carried out for the following purposes • to describe temporal trends in phytoplankton species composition, phytoplankton abundance, biomass as well as the intensity and occurrence of blooms • to describe the spatial distribution of phytoplankton species • to identify key phytoplankton species (e.g. dominating, harmful, potential non-indigenous and/or invasive species, as well as indicator species) • to provide basic data for complex ecosystem analyses, food web studies, modelling as well as political and social requirements such as indicators in the frame of the Marine Strategy Framework Directive of the European Union (MSFD; European Union 2008) and the EU Water Framework Directive (WFD; European Union 2000)” (HELCOM COMBINE Manual https://www.helcom.fi/wp-content/uploads/2019/08/Guidelines-for-monitoring-phytoplankton-species-composition-abundance-and-biomass.pdf) Information about the program and the methods are also available in Swedish at https://www.havochvatten.se/vagledning-foreskrifter-och-lagar/vagledningar/ovriga-vagledningar/undersokningstyper-for-miljoovervakning/undersokningstyper/vaxtplankton.html Data are collected and analyzed according to the HELCOM COMBINE Manual - Part C Annex C6 Monitoring of phytoplankton species composition, abundance and biomass (https://www.helcom.fi/wp-content/uploads/2019/08/Guidelines-for-monitoring-phytoplankton-species-composition-abundance-and-biomass.pdf) or similar methods.