Nitrogen and phosphorus loads reached peaks around 1980 at about 3.1 and 5.8 times the preindustrial loads, respectively. These high nutrient inputs, which increased strongly after 1950 (Figure 4), caused eutrophication of the Baltic Sea. Nitrogen loads have dropped by a quarter and phosphorous loads have halved since 1980, mostly due to better sewage treatment. Current nitrogen loads are comparable to the inputs in the 1970s, while phosphorus is approximately at levels seen in the 1950s. Eutrophication remains one of the major pressures on the Baltic ecosystem, having both direct and indirect impact.
Nutrient loads still exceed the goals of the HELCOM Baltic Sea Action Plan in the central Baltic Sea and the Gulf of Finland; further load reduction has been agreed by HELCOM.
Due to the high nutrient input, water column nutrient concentrations increased from the beginning of the 20th century, by 1.7 times for nitrogen and by 2.6 times for phosphorus. During the 1990s, increases in nutrient concentrations halted in most areas (Figure 5). With the exception of the eastern Gotland Basin, nitrogen concentrations declined. Phosphorus concentrations, however, remained at high levels or even increased when compared with 2007–2011. Phosphorus is retained in the Baltic Sea sediments, resulting in a longer residence time than nitrogen and long-lasting effects of past phosphorus loading. With nutrient concentrations remaining at high levels, the intensity of summer phytoplankton blooms did not change after 1990.
Figure 5: Total nitrogen (top) and phosphorus (bottom) concentrations in the eastern Gotland Basin and the Gulf of Finland. Horizontal green lines indicate Baltic Sea Action Plan targets.
Figure 4: Waterborne and total nutrient inputs to the Baltic Sea. The horizontal green line indicates the maximum allowable input defined in the Baltic Sea Action Plan.