Following the concept of Broad Belt Transects (BBTs) proposed by the MARBEFES project a summer sampling campaign (27/28-08-2024) from the plume zone of the Curonian Lagoon to the open Baltic Sea, from 11 stations, covering the transect of about 130 km, was performed to investigate a distribution and functional diversity gradient of the microplankton: phytoplankton and microzooplankton. Integrated microplankton samples were taken from a depth of 1-10 m and fixed with Lugol solution based on HELCOM recommendations. An advanced imaging flow cytometer - FlowCam (FlowCam® Cyano, Yokogawa Fluid Imaging Technologies, USA) - was used to identify and count phytoplankton and microzooplankton species/groups (2-300 µm). Different morphological traits (such as width, length, thickness, abundance, biovolume) calculated by FlowCam and ecological traits (such as motility, nutrition, etc.) were used for this assessment