The Baltic proper eutrophication - a runaway system that passed the tipping point at the end of the 1950s

The magnitude of the annual biological production in the Baltic proper is determined by the phosphorus (P) concentration C in the surface layer in winter. C is proportional to the total P supply TPS to the water column. TPS has three components; the land-based supply LPS; the ocean supply OPS; and the internal supply IPS from anoxic bottoms. The OPS is minor. The land-based P source, LPS, culminated in the 1980s and at present it has about the same value as in the early 1950s. Despite this, C still increases, and the present time C is at least 3 times higher than C in the 1950s. This runaway evolution of the Baltic proper P content demonstrates that the evolution of C cannot be explained only by the evolution of the external sources LPS and OPS. The runaway behaviour suggests that there is a positive feedback between the state C and the supply TPS. It is shown that the internal P-supply IPS provides such a positive feedback via its dependence on the area of anoxic bottoms Aanox, because IPS is proportional to Aanox and Aanox is proportional to C so that IPS is proportional to C. The internal supply IPS thus increases with C if there are anoxic bottoms. Anoxic bottoms start to occur when C passes the threshold value Ct which occurs when TPS passes the threshold value TPSt. This happened in the Baltic proper at the end of the 1950s. A time-dependent P model describes the evolution of C in the Baltic proper from 1950 to the present quite well.