Secchi depths were measured from Pohjanpitäjänlahti bay in May 2021 from four stations along a coastal salinity gradient with a Secchi disc 20 cm in diameter, except at station 1, where Secchi depth was measured with a white canister of roughly equal size due to the strong river flow. 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.