ABSTRACT
This chapter discusses the role of the limiting nutrient. It examines the way in which the limiting nutrient differs between water basins, which has implications for nutrient reduction requirements as well. For the Baltic Sea, nitrogen fixation by cyanobacteria is an important process and contributes to about 15–20 per cent of the nitrogen inputs. The various sub-basins of the Baltic Sea have differences in water and nutrient residence times, load and internal biogeochemical processes that will alter the nitrogen:phosphorous ratios to a very high degree. Within the modelling framework, more nitrogen is incorporated into organic matter and subsequently lost via denitrification if the phosphorous levels are kept high. The chapter illustrates the effect on the net exports of nitrogen and phosphorous to the Baltic in relation to different levels of phosphorus reduction in the inputs to the Gulf of Riga.
