Deadly Threat No. 3: Ocean deoxygenation

Principally, seawater’s oxygen concentration is determined by both the diffusion of atmospheric oxygen across the air-sea interface and the consumption through microbial respiration (Falkowski et al., 2011). The resultant oxygen concentration naturally varies with time and space, but in no way accounts for the decline in oxygen content that we are seeing today. Referred to as ocean deoxygenation, it is the third ‘deadly’ threat that the ocean faces and comes in two forms.

The first form of oxygen depletion, occurring in the central North Pacific Ocean and tropical oceans, relates to the impacts of global warming and regional freshening. In both of these open basins, observations over the past 50 years have shown falling oxygen levels (Keeling et al., 2010). Behind this decline are a number of factors connected to climate change (Bijma et al., 2013), such as:

1. The decreased solubility of oxygen in a warming ocean.
2. The increase in respiration rates with rising ocean temperatures.
3. The reduced ventilation in high-latitude areas linked to increased ocean stratification. The latter inhibits the exchange of oxygen between surface and subsurface waters, ultimately reducing the supply of oxygen to the ocean’s interior (Keeling et al., 2010).

Currently, though, it remains uncertain as to whether this deoxygenation in the open ocean is a long-term trend related to climate change (the factors listed above), the outcome of natural cyclical processes, or a combination of both (Bijma et al., 2013).

Elsewhere, on the western coast of North America, anoxic (lacking oxygen) waters have been found on the inner shelf, adjacent to the upwelling zone. These continental margin upwelling systems are naturally prone to deoxygenation, due both to the low oxygen source waters and the high flux of nutrients that promote high phytoplankton biomass that then settles to the seabed where it is decomposed by respiring bacteria. However, for the western coast of North America, it was discovered that the upwelling of oxygen depleted waters was due to an increase in the frequency and strength of upwelling inducing winds; events symptomatic of climate change (Chan et al., 2008).

The second form of oxygen depletion refers to the rapid rise in coastal hypoxia from anthropogenic eutrophication. This results from nutrients found in agricultural fertilisers and sewage pollution that find their way into the ocean through urban runoff, causing blooms of algae and plankton (Bijma et al.,2013). Eventually these die and fall to the seabed, where they are decomposed by oxygen consuming bacteria. This lowers the oxygen content of the water, depleting the supply available for marine life. It is for this reason that these hypoxic areas have become known as marine dead zones – there is simply not enough dissolved oxygen to support marine life. Since the introduction of industrially produced nitrogen fertiliser in the late 1940s, oxygen depleted zones have spread, but with a lag of 10 years between their use and the incidence of hypoxia. Indeed, over the past 50 years, the number of dead zones has approximately doubled each decade (Diaz and Rosenberg, 2008). This correlates to human’s continued settlement in coastal regions.

Figure 1 shows the location of the ocean's dead zones. Those areas most affected include the Baltic Sea, which harbours seven of the world’s 10 largest marine dead zones, and also the northern Gulf of Mexico (National Geographic, 2013).   Inland seas and estuaries are also extremely prone to coastal hypoxia; some examples include the Kattegat, the Black Sea and Chesapeake Bay (IPSO, 2013).

Figure 1. Map of aquatic dead zones around the world (NASA, 2010). Click here to see the high-resolution version.

However, it is important to note that the research into ocean deoxygenation is still in its infancy. Prior to 1960, it is extremely rare to find accurate measurements of oceanic oxygen concentrations, making it hard to accurately discern patterns (Falkowski et al., 2011). Large gaps in data also make it difficult to make reliable predictions for the future. Keeling et al. have predicted that by 2100, the ocean’s oxygen content will have declined by between 1 and 7% - notice the uncertainty (2010). This will undoubtedly have profound consequences on the ocean’s biology, but also its biogeochemical cycling. This post has ended up rather long though, so I shall save these for next time!