2.2 Evidence for ocean deoxygenation and its patterns: Atlantic Ocean Olof Linden 2.2 Evidence for ocean deoxygenation and its patterns: Atlantic Ocean Olof Linden World Maritime University, Malmö, Sweden 2.2 SECTION Summary • No other chemical parameter is of such importance to all higher marine life as dissolved oxygen. From available data it is obvious that oxygen-limited areas have expanded throughout most of the Atlantic during the last 50 to 100 years and are clearly related to human activities. • Oxygen limited waters, hypoxic and even anoxic conditions are now found in many coastal areas in the Atlantic Ocean including in connected seas like the Mediterranean, the Black Sea, the North Sea and the Baltic Sea, and the Gulf of Mexico. • Sediment studies show that anoxic conditions have been present in deep waters in the past, long before anthropogenic activities started to have an influence on the marine environment. However, the present observed deoxygenation in coastal and offshore areas is clearly linked to human activities such as input of nutrients and organic matter, and increasing temperatures as a result of climate change. • The expanding oxygen minimum zones affect the entire Atlantic ocean ecosystem, its biodiversity and productivity. • In addition to many coastal waters, oxygen limited waters are also found at mid-water depths (typically 300- 1000 m) in the Atlantic Ocean basins, particularly the Equatorial and South Atlantic. The oxygen concentrations in these areas have decreased during the last 60 years, partly due to ocean warming, and partly as a result of decreased mixing and ventilation. • Continued spreading of deoxygenated areas in the Atlantic, as well as the rest of the world ocean, must be considered a most serious environmental problem with enormous potential impacts on biodiversity and human societies. Ocean deoxygenation: Everyone’s problem 37 2.2 Evidence for ocean deoxygenation and its patterns: Atlantic Ocean Atlantic Ocean deoxygenation Potential consequences Oxygen limited areas have expanded over most of the • If the conditions continue to deteriorate a number of Atlantic Ocean during the last 50 to 100 years. human ocean and coastal activities are in jeopardy including fishing, aquaculture and coastal tourism. Human activities are the reasons for the expanding • If the release of nutrients, oxygen depleting organic oxygen limited areas. substances and greenhouse gases are not dealt with, the deoxygenated areas will expand as well as extend into deeper parts of the Atlantic Ocean. Consequences can now more often be noted in many • Coastal areas will lose fisheries, tourism and may even coastal areas and marginal seas where the smell of be uninhabitable to marine life. anoxic water is a regular phenomenon. In addition to large parts of the coastal waters of the • If the deoxygenation of entire ocean basins continue Atlantic, over the last 60 years large ocean basins human life will be seriously threatened, not only in such as the Equatorial and southern Atlantic are being coastal areas but everywhere on the planet. affected by decreasing oxygen levels. 2.2.1 Introduction regions. The oxygen-minimum zone typically occurs at depths between 300 and 1200 m which is the zone The Atlantic Ocean, including marginal seas like the where the permanent thermocline is found. A number Mediterranean, Gulf of Guinea, Black Sea, North Sea, of other physical processes also influence the levels Baltic Sea, and Gulf of Mexico, is the second largest of oxygen at a particular depth in the ocean, such as body of water on the planet. The surface area of about circulation, and down- and up-welling. The Atlantic is 106 million km2 with an average depth is about 3600 m, characterized by massive down-welling in the North and a maximum depth of 8500 m. The Atlantic Ocean Atlantic and Norwegian Sea. Here North Atlantic Deep is the drainage basin for more land area than any other Water (NADW) is formed which feeds the upwelling in ocean basin. Seven of the world’s ten largest rivers many other regions of the world ocean. There is also empty into the Atlantic Ocean. The Atlantic Ocean basin significant although less pronounced down-welling in is the only part of the world ocean with significant water the South Atlantic near Antarctica. exchange with the Arctic Ocean. Upwelling areas are found in several parts of the Atlantic There are more and longer data series of physical/ Ocean (Figure 2.2.1). At these sites nutrient rich deep chemical and biological parameters from the Atlantic water is brought up to the surface through a combination (particularly the North Atlantic) than from any other part of wind and Ekman drift and the Coriolis force. Ekman of the world ocean. Such data series of measurements drift is the process where winds generate currents and in some cases stretch as far back as 150 years. due to such currents upwelling occurs, a process called the Coriolis effect. This deep water may, however, be In the Atlantic Ocean, as in most of the world ocean, oxygen limited or hypoxic and such waters are found the oxygen concentrations in the water column are often around 100 to 300m depth and sometimes even generally characterized as slightly supersaturated in at the surface. Pronounced upwelling areas are found the surface water, more-or-less undersaturated at for example off the south-west coast of Africa where the intermediate depth, and with higher levels of oxygen, Benguela Current brings pulses of hypoxic deepwater to although still undersaturated, in deeper waters. This the surface along the coast of South Africa, Namibia and difference in the oxygen concentration is a reflection Angola (Arntz et al., 2006; Mohrholz et al., 2008; Monteiro of the production and uptake of oxygen through et al., 2006, 2008; Prochazka et al., 2005) (Figure 2.2.1). photosynthesis and air-sea gas exchange near the The Canary Current is a wind-driven surface current surface, and the consumption through respiration and which is part of the North Atlantic Gyre. Along the north- degradation processes below. Deeper waters tend west African coast off Mauretania and West Sahara to have higher oxygen levels because of supply of the Canary Current, due to the Ekman drift, produces well-oxygenated bottom waters and low degradation a pronounced upwelling which brings oxygen-depleted rates. These well-oxygenated bottom waters originate but nutrient-rich deep water to the surface (Gyory et al., from down-welling/thermohaline circulation in polar 2007; Hempel, 1982). Furthermore, seasonal upwelling 38 Ocean deoxygenation: Everyone’s problem 2.2 Evidence for ocean deoxygenation and its patterns: Atlantic Ocean SECTION 2.2 SECTION Figure 2.2.1 The Atlantic Ocean with areas of permanent or seasonal upwelling of deep water (dark blue areas). Such areas are common in the basins along the western coasts of the Europe and Africa but occurs also along large parts of the coastline in Europe and North America as well as in the Baltic and Black Seas. Major surface currents are also illustrated (orange arrows). areas related to the West African monsoon are found in often leads to increased concentrations of nutrients the central equatorial Atlantic Ocean, typically occurring and organic matter (e.g. Seitzinger et al., 2010; Sinha from May through September (Christian & Murtugudde, et al., 2017). This leads to eutrophication which results 2003). The area affected by this upwelling may be as in increased decomposition and decreased oxygen large as 2 million km2 (Subramaniam et al., 2013). concentrations (hypoxia) which may further lead to anoxia (no oxygen). There are numerous reports of 2.2.2 Trends and impacts such conditions along much of the world’s coastlines (see for example reviews by Diaz & Rosenberg (2008) 2.2.2.1 Coastal areas and Rabalais et al. (2014), as well as the assessments carried out by the World Resources Institute (2010)). Under natural conditions shallow waters on the In the Atlantic Ocean region eutrophic conditions are continental shelf generally tend to have higher nowadays more or less permanent in parts of the Gulf concentrations of oxygen because of higher primary of Guinea, Abidjan Lagoon, along the Atlantic coast of productivity and being better ventilated. However, a Spain and Portugal, English Channel, Wadden Sea, number of anthropogenic activities affect the levels of Kattegat and Skagerrak, up to 40% of the Baltic proper, oxygen in coastal oceanic waters. Coastal waters are Bay of Gdansk, Gulf of Riga, the Black Sea including affected by forestry, agriculture and urbanization which Azov Sea and Sea of Maramara. Ocean deoxygenation: Everyone’s problem 39 2.2 Evidence for ocean deoxygenation and its patterns: Atlantic Ocean Figure 2.2.2 Map showing the distribution of 415 hypoxic and eutrophic coastal systems worldwide. Most of these areas are found in the Atlantic. From WRI (2010). A contributing factor to decreasing oxygen conditions in many coastal areas, particularly near major concentrations is the increasing ocean water rivers or in enclosed or semi-enclosed sea areas. In the temperature. Global warming results in increasing Atlantic Ocean depressed oxygen levels as a result of water temperatures particularly in surface waters, increased outflow of organic matter and nutrients have which also leads to decreasing dissolved oxygen been observed along most of the coastal areas. In some concentrations as warmer water holds less oxygen. In cases, the deoxygenation is temporary and affects the Baltic Sea where the relative importance of physical relatively small areas. However, there is an increasing forcing versus eutrophication is debated, analysis of number of large coastal areas that are more-or-less oxygen and salinity profiles collected over the last 115 permanently affected by hypoxia or anoxia. Recovery years show that increasing temperatures over the last from prolonged periods of anoxia takes many years two decades has contributed to worsening oxygen (Diaz & Rosenberg, 2008; Pearson & Rosenberg, 1978).