Deoxygenation of the Baltic Sea during the last century

Deoxygenation is a global problem in coastal and open regions of the ocean, and has led to expanding areas of oxygen minimum zones and coastal hypoxia. The recent expansion of hypoxia in coastal ecosystems has been primarily attributed to global warming and enhanced nutrient input from land and atmosphere. The largest anthropogenically induced hypoxic area in the world is the Baltic Sea, where the relative importance of physical forcing versus eutrophication is still debated. We have analyzed water column oxygen and salinity profiles to reconstruct oxygen and stratification conditions over the last 115 y and compare the influence of both climate and anthropogenic forcing on hypoxia. We report a 10-fold increase of hypoxia in the Baltic Sea and show that this is primarily linked to increased inputs of nutrients from land, although increased respiration from higher temperatures during the last two decades has contributed to worsening oxygen conditions. Although shifts in climate and physical circulation are important factors modulating the extent of hypoxia, further nutrient reductions in the Baltic Sea will be necessary to reduce the ecosystems impacts of deoxygenation.Dead zones are hypoxic (low-oxygen) areas unable to support most marine life, and over the past 50 y they have spread rapidly in the open ocean (1) as well as in coastal ecosystems (2). Global warming is thought to be a major driver for these changes (3), although biogeochemical factors have also been recognized, especially in coastal marine ecosystems (4, 5). In the Baltic Sea, the present spread of hypoxia is the combined result of climate changes influencing deepwater oxygenation (6) and increased eutrophication (7, 8), resulting in a hypoxic area ranging between 12,000 and 70,000 km² with an average of 49,000 km² over the time period 1961–2000 (7). Here, we separate the effects of the two factors on oxygen conditions.Physical factors are an important consideration in whether an ecosystem will experience hypoxia. The Baltic Sea is naturally prone to hypoxia due to a restricted water exchange with the ocean and a long residence time above 30 y (9, 10). Saltier, denser water from the North Atlantic flows over a series of shallow sills in the Danish Straits to ventilate waters below the permanent halocline and are governed by meteorological-induced variations in sea levels (11), displaying variations at decadal scales (12, 13). The dense saltwater inflows bring new supplies of oxygen to bottom waters, but at the same time enhance stratification, creating larger bottom areas that experience hypoxia (14). In particular, the ventilation of the deeper waters is attributed to events of larger inflows of high-saline water (>17), termed Major Baltic Inflows (MBIs), that have been less frequent in the last three decades (6).Climate warming decreases oxygen solubility due to higher water temperature, increases stratification, and enhances respiration processes (15). Climate warming is likely to be accompanied by increased precipitation and inflows of freshwater and nutrients to coastal waters in many areas of the globe. Increasing nutrient inputs from land stimulates primary production and export of organic material to the deep waters, thereby disrupting the subtle natural balance between oxygen supply from physical processes and oxygen demand from consumption of organic material. However, the importance of decreasing oxygenation versus increasing nutrient inputs for explaining the recent spread of hypoxia is not known (6, 7).Water column measurements of dissolved oxygen concentrations began around 1900 with more regularly spaced measurements commencing in the 1960s (Fig. S1), allowing a more consistent assessment of the spatial extent of hypoxia (7, 14). The sparse temporal and spatial resolution of oxygen data before 1960 allowed only assessing hypoxia at specific locations (16) or specific years (17). To our knowledge, our study is the first to report basin-wide trends of stratification and oxygen conditions from 1898 to present, and here we will focus on the two basins that have perennial hypoxia—the Bornholm Basin and the Gotland Basin (Fig. S2). These two basins are connected via a channel with a sill depth of 60 m.