Light attenuation was measured in Pohjanpitäjänlahti bay in May 2021 at four stations along a coastal salinity gradient. Light attenuation was measured with Secchi disc and a light meter (400-700 nm). In addition, optically active substances, i.e., factors affecting light attenuation, were measured. These parameters included chlorophyll-a, particulate matter and colored dissolved organic matter (CDOM). In addition, salinity and temperature was measured at each station. The samples were collected 8 times with a small boat. The data was collected to a thesis funded by Walter and Andreé de Nottbeck Foundation.