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

Abstract

The phosphorus (P) concentration c1 in the surface layer of the Baltic proper in winter depends on the land-based P source LPS, and the ocean P source OPS, which are known. It also depends on the internal P source IPS from anoxic bottoms and the sum of internal and external P sinks TPsink, which are estimated in this paper. IPS is parameterized as fs·Aanox, where fs is the specific annual mass flux of P from anoxic sediments and Aanox is the area of anoxic bottoms, and TPsink is parameterized as c1·TRVF, where TRVF 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 fs and TRVF. The result is TRVF = 3,000 km3 year−1 and fs = 1.22 tons P km−2 year−1. With these parameter values, the model gives a quite good description of the observed evolution of c1. The observed runaway evolution of c1, with increasing c1 since the 1980s although the land-based supply LPS has been halved, is well-described by the model. It is concluded that the internal P source IPS provides a positive feedback mechanism that has boosted and perpetuated the eutrophication of the Baltic proper and that IPS is the major driver of the Baltic Sea eutrophication since the late 1990s. It is suggested that measures to eliminate IPS should be included in the management strategy to reduce the eutrophication of the Baltic proper.

Global climate change and the Baltic Sea ecosystem: direct and indirect effects on species, communities and ecosystem functioning

“…The nutrient load into the sea will probably increase, especially in the northern Baltic Sea where precipitation is probably increasing the most (Lessin et al, 2014;Huttunen et al, 2015;Christensen et al, 2022) but also in the southern Baltic Sea (M. . It has also been projected that the total phosphorus loading (from terrestrial areas of Finland) will increase relatively more than that of nitrogen (Huttunen et al, 2015) and, together with the internal loading of phosphorus from sediments (Lessin et al, 2014;Stigebrandt et al, 2014;Stigebrandt and Anderson, 2020), phosphorus availability to primary producers may increase. If the N : P ratio of the surface layer will decline, the spring bloom will decline and more excess phosphate will be available for the summer cyanobacteria communities after the spring bloom (Lessin et al, 2014).…”

“…Several modelling studies project an increase in total phytoplankton concentration (chlorophyll, in mg m −3 ), until the end of the century, with the increase manifested especially in summer (Meier et al, 2012a, b;Lessin et al, 2014;Skogen et al, 2014;Ryabchenko et al, 2016). As hypoxia and associated internal loading of phosphorus will probably be enforced by global warming (Meier et al, 2019b;Tomczak et al, 2021), it has even been suggested that this "vicious circle of eutrophication" (Vahtera et al, 2007), will prevent the success of nutrient abatement measures unless internal loading of phosphorus is reduced (Gustafsson et al, 2012;Stigebrandt and Anderson, 2020).…”

Investigating Hypoxic and Euxinic Area Changes Based on Various Datasets From the Baltic Sea

“…Instead, in all three Gotland sub-basins, the size of the euxinic area was linked to the residual bottom temperature (explained variance: 0.6 2 = 36%, see Figure 6). Due to a greater decomposition of organic matter and due to a lower oxygen solubility in warmer bottom water (e.g., Stigebrandt and Andersson, 2020), the euxinic area increased with the bottom temperature, even on time scales that were larger than those of the direct influence of individual saltwater inflows. The correlations between the hypoxic area and the physical drivers of basins closer to the North Sea connection (Arkona and Bornholm basins, Slupsk Channel) were relatively weak, although for the Bornholm Basin the stronger correlations with physical factors could be attributed to its longer water residence time compared to the Arkona Basin (e.g., Lass et al, 2005;Meier, 2007).…”

Remediating Agricultural Legacy Nutrient Loads in the Baltic Sea Region

“…The annual exchange rate for 2013 (1 SEK = 0.1156 €) is used for conversion. Extent according to [72] (0.092 Mt P) dates back to 2005; while a more recent study by the same research group estimates 0.060 Mt P [97]. No repetition is considered, as oxygenation is assumed to only be necessary for 10-15 years [98].…”