A breathless ocean

Ordinarily, ocean surface waters have an oxygen concentration of 5-8 ml l^-1. However, as I discussed last time, climate change is altering the ocean’s oxygen content, causing concentrations in some areas to plummet. In regions considered to be under ‘extreme hypoxia’, dissolved oxygen content is less than 2 ml l^-1; a substantial decline from the norm. It is, therefore, hardly surprising that this has drastic consequences for marine life (Bijma et al., 2013).

With oxygen as the principal constraint on growth, declining oxygen levels affect the functioning and growth of many marine organisms (Zimmer, 2010). Many species exhibit stress-related behaviour and for those most vulnerable, such as crabs and starfish (bottom-dwellers), extreme hypoxic conditions can cause widespread mortality (Gewin, 2010).

As deoxygenation has increased, the depth of oxygen minimum zones has shoaled. This has compressed habitats for marine organisms that have a high metabolic rate and oxygen demand. As a consequence, encounter rates between predators and prey have been altered and many species have been forced to migrate in search of oxygenated waters. This has meant we have seen large-scale shifts in the distribution of species (Stramma et al., 2011). However, fishermen in certain regions of the world have learnt to take advantage of this behaviour. Unfortunately for fish, this has meant that even if they manage to swim away and escape the hypoxia, the narrowed water column they can then live in makes them much easier to catch and increasingly vulnerable (Gewin, 2010). Alongside habitat compression, extreme hypoxia also results in a loss of fauna and together, these seriously impact ecosystem energetics and function. This is primarily because microbes decompose the organisms that die, instead of fish predators, and this diverts energy flows away from the higher trophic levels (Diaz and Rosenberg, 2008).

Sustained hypoxic conditions can also affect global biogeochemical cycles. As oxygen concentrations decline, a change in bacteria occurs - from those that require oxygen in order to thrive, to bacteria for whom oxygen is toxic. However, these new bacteria participate in denitrification, which reduces the concentration of nitrate in the ocean and produces nitrous oxide, thereby limiting ocean productivity (CLAMER, 2011). As nitrous oxide is a potent greenhouse gas, ocean deoxygenation could further amplify global warming (Zimmer, 2010).

However, not all species suffer under extreme hypoxic conditions. Humboldt squid are one such example; tolerant of low-oxygen concentrations they feast on the remains of bottom dwellers that have died due to oxygen depletion (Gewin, 2010). Similarly, jellyfish also tolerate lower oxygen concentrations and, consequently, can thrive in hypoxic areas. This is partially because they are able to store reserves of oxygen in their jelly.

Humboldt squid

Overall though, as Diaz and Rosenberg state, ‘there is no other variable of such ecological importance to coastal marine ecosystems that has changed so drastically over such a short time as dissolved oxygen’ (2008: 929).  Ocean deoxygenation is a major global environmental problem today and one that has detrimental consequences for marine life and ecosystems.