AUTHOR=Stigebrandt Anders , Andersson Ambjörn 

TITLE=The Eutrophication of the Baltic Sea has been Boosted and Perpetuated by a Major Internal Phosphorus Source

JOURNAL=Frontiers in Marine Science

VOLUME=7

YEAR=2020

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2020.572994

DOI=10.3389/fmars.2020.572994

ISSN=2296-7745

ABSTRACT=<p>The phosphorus (P) concentration <italic>c1</italic> in the surface layer of the Baltic proper in winter depends on the land-based P source <italic>LPS</italic>, and the ocean P source <italic>OPS</italic>, which are known. It also depends on the internal P source <italic>IPS</italic> from anoxic bottoms and the sum of internal and external P sinks <italic>TPsink</italic>, which are estimated in this paper. <italic>IPS</italic> is parameterized as <italic>fs</italic>·<italic>Aanox</italic>, where <italic>fs</italic> is the specific annual mass flux of P from anoxic sediments and <italic>Aanox</italic> is the area of anoxic bottoms, and <italic>TPsink</italic> is parameterized as <italic>c1</italic>·<italic>TRVF</italic>, where <italic>TRVF</italic> is the total removal volume flux. We use a time-dependent P budget model, and 47 years of observational data, and the method of least squares to determine the best estimates of the unknown parameters <italic>fs</italic> and <italic>TRVF</italic>. The result is <italic>TRVF</italic> = 3,000 km<sup>3</sup> year<sup>−1</sup> and <italic>fs</italic> = 1.22 tons P km<sup>−2</sup> year<sup>−1</sup>. With these parameter values, the model gives a quite good description of the observed evolution of <italic>c1</italic>. The observed runaway evolution of <italic>c1</italic>, with increasing <italic>c1</italic> since the 1980s although the land-based supply <italic>LPS</italic> has been halved, is well-described by the model. It is concluded that the internal P source <italic>IPS</italic> provides a positive feedback mechanism that has boosted and perpetuated the eutrophication of the Baltic proper and that <italic>IPS</italic> is the major driver of the Baltic Sea eutrophication since the late 1990s. It is suggested that measures to eliminate <italic>IPS</italic> should be included in the management strategy to reduce the eutrophication of the Baltic proper.</p>