Climate change impacts and vulnerability in Europe 2016

Climate change impacts on environmental systems
4.1.6 Ocean oxygen content
Key messages
• Dissolved oxygen in sea water affects the metabolism of species. Therefore, reductions in oxygen content (i.e. hypoxic or
anoxic areas) can lead to changes in the distribution of species, including so called 'dead zones'.
• Globally, oxygen-depleted areas have expanded very rapidly in recent decades. The number of 'dead zones' has roughly
doubled every decade since the 1960s and has increased from about 20 in the 1950s to about 400 in the 2000s.
• Oxygen-depleted zones in the Baltic Sea have increased more than 10-fold, from 5 000 to 60 000 km 2 , since 1900, with
most of the increase happening after 1950. The Baltic Sea now has the largest dead zone in the world. Oxygen depletion
has also been observed in other European seas in recent decades.
• The primary cause of oxygen depletion is nutrient input from agricultural fertilisers, causing eutrophication. The effects
of eutrophication are exacerbated by climate change, in particular increases in sea temperature and in water-column
stratification.
Relevance
Accelerated nutrient flow into the sea (mostly from
agricultural fertilisers) in combination with warming
water temperatures can lead to large phytoplankton
blooms and subsequent increases in primary
production (a process called eutrophication). When
these organisms sink to the sea floor, oxygen is utilised
in their decomposition. If mixing within the water
column cannot supply enough oxygen to the sea floor,
this can lead to oxygen reduction (hypoxia) to levels
that severely limit biological activity and ultimately to
complete oxygen depletion (anoxia).
Most organisms, including marine organisms, require
oxygen for their metabolism. Therefore, lower oxygen
concentrations in seawater affect the physiology,
composition and abundance of species. Rising
water temperatures will have knock-on effects on a
number of different chemical processes in the marine
environment. For example, as the temperature rises,
oxygen becomes less soluble in water, resulting in
lower oxygen concentrations (Keeling et al., 2010); at
the same time, the oxygen demand for metabolism
increases (Pörtner, 2010). Insufficient oxygen supply
to organisms will eventually have knock-on effects
on productivity, species interactions and community
composition at the ecosystem level.
Oxygen depletion can occur episodically (less than
once per year), periodically (several times per year
for short periods) and seasonally (each summer),
and eventually it can become persistent. The Baltic
Sea has the largest dead zone in the world, which
includes large areas of persistent oxygen depletion.
Oxygen-depleted areas are an example of how one
type of anthropogenic pressure, nutrient input causing
eutrophication, is exacerbated by climate change (here
increasing temperature) through multiple different
linkages with biology, from cellular levels to community
and ecosystem levels. For example, land‐based
nutrient enrichment can lead to a redistribution
in the vertical distribution of primary production.
Such increased nutrient input can increase primary
production in the surface layer, where the oxygen
produced can be exchanged with the atmosphere.
The organic material produced will sink through the
pycnocline (i.e. the ocean layer with a stable density
gradient, which hinders vertical transport) using
oxygen as it decomposes. At the same time, when
primary production occurs below the pycnocline, the
oxygen produced will stay in the bottom layer. As
climate change influences stratification parameters
(see Section 4.1.4), and consequently the depth of
the pycnocline, it could influence light availability
for primary production in the deeper layer, possibly
decreasing oxygen production. In this case, the
interaction between climate change and eutrophication
can have impacts on biodiversity, plankton
communities and oxygen conditions (Lyngsgaard et al.,
2014). Oxygen depletion may also interact with other
anthropogenic stressors in affecting marine ecosystems
and fisheries, such as overfishing or the introduction of
invasive species (Diaz and Rosenberg, 2008).
Past trends
Dissolved oxygen in marine ecosystems has changed
drastically in a very short period compared with other
variables of the marine environment. While hypoxic
zones occur naturally in some regions, increased
nutrient loads from agricultural fertilisers have
caused oxygen-depleted areas or even anoxic areas
(so‐called 'dead zones') to expand globally since the
Climate change, impacts and vulnerability in Europe 2016 | An indicator-based report
119