Knowledge and implications of global change in the oceans for biology, ecology, and ecosystem services - Ocean Sustainability in the 21st Century - page 131

decreasing the ocean's CO 2 uptake efficiency. Another potentially serious consequence of
global warming is a decrease in the dissolved O 2 content of the world's oceans (Keeling
et al., 2010 ). Oxygen is directly linked to carbon via photosynthesis and respiration, so
the distribution of O 2 in the ocean is a diagnostic of the rate at which organic matter is
produced, redistributed, and decomposed in the ocean. The loss of dissolved O 2 (deoxy-
genation) is predicted, not just because O 2 is less soluble in warmer water but also be-
causeglobalwarmingmayincreaseupperoceanstratification,therebyreducingtheO 2 sup-
ply to the ocean interior (Sarmiento et al., 1998 ; Bopp et al., 2001 ; Keeling and Garcia,
2002 ) . Systematic deoxygenation and the extension of oxygen minimum zones are increas-
ing throughout the world oceans and can have widespread consequences in the biogeo-
chemical cycling of carbon, nitrogen, and many other important elements (P, Fe, Mn, etc.).
Oxygen is also fundamental for all aerobic life, including organisms living in the dark
ocean interior. Most organisms are not very sensitive to O 2 levels as long as concentrations
are high enough or they are able to survive for a short period of their life without oxygen.
However, once O 2 drops below a certain threshold, the organism suffers from a variety of
stresses, leading ultimately to death if the concentrations stay too low for too long (hypoxic
conditions). Thresholds for hypoxia vary greatly between marine taxa, with fish and crus-
taceans tending to be the most sensitive (Vaquer-Sunyer and Duarte, 2008 ). Anoxia is rare
in the water column of the modern ocean, while deep hypersaline anoxic basins (DHABs)
have been discovered in the eastern Mediterranean Sea, and in the Red Sea (Danovaro et
al., 2010 ) . Suboxic and hypoxic conditions occur in all oceans and these oxygen minimum
zones (OMZs) will expand in the future as documented by declines in O 2 concentrations
over the past 50 years in the subpolar North Pacific and in tropical OMZs (Whitney et al.,
2007 ; .Stramma et al., 2008 ; Levin, 2010 ) . At very low O 2 concentrations (< 5 μmol kg −1 ),
major changes in biogeochemical cycling occur and nitrate becomes important in respira-
tion. Once nitrate is exhausted, the biogeochemistry then tends to be dominated by sulfate-
reducing microbes, which convert sulfate to sulfide.