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.
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