Abstract
Dissolved oxygen in the sea is essential for marine fauna and biogeochemical processes. Decline in the sea water oxygen concentration is considered to be an effect of eutrophication, also exacerbated by climate change. The Baltic Sea is one of the most eutrophic seas in the world and is located in northern Europe. It is a vulnerable, brackish, semi-enclosed sea, suffering from high pressures from human activity. This leads to increased hypoxic and anoxic areas, which can be used as a measure of the environmental state. In the present study the extent of anoxic (O2 < 0 ml l–1) and hypoxic (O2 < 2 ml l–1) areas were estimated for the autumns in 1960–2019 using vertical profiles of observed oxygen concentrations in the Baltic proper and four sub-areas of the Baltic proper: the Bornholm Basin, the western, northern and eastern Gotland basins. From vertical profiles of observed salinity, the annual average of the halocline depths in the four sub-basins were estimated. The results imply regime shifts toward increased anoxic area extents in the Gotland basins around the turn of the 20th century. In autumn 2018, the extent of anoxic bottom areas in the Baltic Sea was record high since the start of the data series. During the later part of the studied period the depths of the halocline coincide with the depth of the hypoxia in the Gotland basins. This implies that in these basins a worst-case scenario for the extent of hypoxic areas seems to be reached.
Highlights
Deoxygenation has become a worldwide marine environment issue (Breitburg et al, 2018) and more than 400 dead zones have been identified since the mid-20th century (Rabalais et al, 2007; Diaz and Rosenberg, 2008; Murphy et al, 2011; Strokal and Kroeze, 2013; Su et al, 2017)
We focus on (1) the Baltic proper, here defined by the sub-basins Arkona Basin, Bornholm Basin, eastern, western and northern Gotland basins, Gdansk Bay, Gulf of Riga, and Gulf of Finland as in Hansson et al (2019), as well as (2) the four sub-basins Bornholm Basin (BB), and eastern (EGB), western (WGB), and northern (NGB) Gotland basins in which the shallowest areas closest to land are not included (Figure 1)
The hypoxic area extent in 2018 that covered about 33% (84,600 km2) of the Baltic proper bottom area was only seen once before, in autumn 2007 (Figure 2A, Hansson et al, 2019)
Summary
Deoxygenation has become a worldwide marine environment issue (Breitburg et al, 2018) and more than 400 dead zones have been identified since the mid-20th century (Rabalais et al, 2007; Diaz and Rosenberg, 2008; Murphy et al, 2011; Strokal and Kroeze, 2013; Su et al, 2017). The spread of hypoxia (low oxygen) and anoxia (no oxygen) areas in coastal waters is mainly due to eutrophication caused by nutrient enrichment from intensified agriculture and waste-water discharges since the mid-20 century (Rabalais et al, 2010). Eutrophication and expansion of hypoxic areas will likely exacerbate in a warmer future climate (Altieri and Gedan, 2015; Breitburg et al, 2018). Increased discharge of freshwater and nutrient loads to coastal seas may enhance primary production and sedimentation of organic matter, which may amplify mineralization and oxygen consumption in deeper layers. Climate change may affect the hypoxic/anoxic areas by modifying inflow dynamics and/or vertical mixing processes for deep, isolated, water masses and by that the transport of oxygen to these layers. The bottom water oxygen conditions affect the nutrient exchange in the sediment-water interface (Sundby et al, 1992; Viktorsson et al, 2013; Bonaglia et al, 2014) and a deficiency may lead to a “vicious circle” due to increased phosphorus release and nitrogen removal (Vahtera et al, 2007)
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