The Baltic Sea a century ago — a reconstruction from model simulations, verified by observations

“Pre-industrial” trophic conditions in the Baltic Sea were simulated with SANBALTS (Simple As Necessary BAltic Long-Term large Scale) model. External nutrient inputs to the major basins of the Baltic Sea a century ago were reconstructed from various literature and data sources. The reconstructed input of total nitrogen was less than a half and that of total phosphorus was about a third of their contemporary values. The simulated “pre-industrial” conditions are validated by comparison to actual historical data on the water transparency, oxygen concentration, primary production, and net sediment accumulation. The “pre-industrial” trophic state could have been more phosphorus limited than today because simulated basin-wide annual averages of dissolved inorganic phosphorus concentrations of 0.06–0.3 µM P are about 40–80% of their present day values, while dissolved inorganic nitrogen concentrations of 2–4 µM N are almost the same as today or even slightly higher.