To build a forecasting tool for the state of eutrophication in the Archipelago Sea, we fitted a generalized additive mixed model to the marine biological data, collected over the years 2011-2017 by an automated monitoring buoy at the Seili station. The resulting "Seili-index" can be used to predict chlorophyll content of the sea water a number of days ahead using the temperature forecast as a covariate. An array of test predictions on the 2017 data set shows that the index is adept at predicting the amount of chlorophyll especially at the greater depths.