Photosynthetically active radiation (PAR) was measured underwater at various depths from Pohjanpitäjänlahti bay in May 2021 from four stations along a coastal salinity gradient with an upward-facing light meter. PAR underwater was measured by lowering the light sensor to the desired depth. Light intensity at the surface was measured simultaneously with an upward-facing light logger at the boat to provide reference values for light intensity. The measurements were done 8 times from a small boat. The data was collected to a thesis funded by Walter and Andreé de Nottbeck Foundation.