This dataset consists of analysed phytoplankton data from the years 2001-2017 inclusive collected from the SmartBuoy moorings at Celtic Deep, Dowsing, Gabbard/West Gabbard/WestGabbard2, Warp and Liverpool Bay using WMS and EDU automated water samplers. SmartBuoys consist of a 4 meter tall stainless steel frame with a fibreglass toroid for buoyancy. Instruments are attached at a nominal 1 meter depth to the frame. The water samplers collect into pre-spiked bags containing acidified Lugols Iodine on a pre-determined cycle over the course of a deployment. Following recovery these samples were analysed for a full community of phytoplankton species using the Utermohl method on high-power inverted microscopes. Phytoplankton cell counts are abundance values reported in Cells per Litre. These deployments form part of the National Marine Monitoring Programme (NMMP) funded by the UK Government's Department for Environment, Food and Rural Affairs (Defra). SmartBuoy phytoplankton data is primarily used for eutrophication monitoring and feeds into OSPAR and MSFD assessments. Buoys have also been deployed as part of short term (up to 3 years) research contracts. Phytoplankton samples have been collected using SmartBuoys since April 2001 at a variety of locations in the North Sea, Irish Sea and Celtic Sea. Blanks in the dataset constitute information which is unavailable and should be treated as missing.