Environmental Engineering Reference
In-Depth Information
seawater temperature, and a lower oxygen exchange with underlying layers
due to enhanced stratifi cation (Keeling et al. 2010). Moreover, the emergence
of oxygen minimum zones in coastal environments (Diaz and Rosenberg
2008), tropical oceans (Bograd et al. 2008, Stramma et al. 2008), and the
North Pacifi c Ocean (Whitney et al. 2007, Pierce et al. 2012) have increased
in the last decades resulting in habitat compression and mass mortalities.
Dissolved oxygen concentration strongly constrains the development of
most fi sh species, which are recognized as especially vulnerable to hypoxic
conditions (Vaquer-Sunyer and Duarte 2008). Although much research is
being currently undertaken, some studies have already highlighted the
profound infl uence of decreasing oxygen on marine fi sh. For example,
hypoxic conditions impair predator avoidance capacity of mesopelagic
fi shes in the California Current, since fi sh are forced to move vertically
from deep water to better oxygenated-surface waters, where they are
highly vulnerable to visually-orientated predators (Koslow et al. 2011).
Other authors predicted a considerable habitat loss for benthic fi shes as
a consequence of the shoaling of the upper hypoxic zone in the Southern
California Bight (McClatchie et al. 2010). This phenomenon is likely to
affect most marine habitat worldwide, and is expected to be more severe
in shallow areas where the hypoxic conditions can easily be transferred to
the entire water column. In fact, the shoaling of hypoxic layers triggered a
substantial vertical habitat loss in the tropical Atlantic Ocean, which could
lead pelagic species to be highly vulnerable to surface fi shing gear and thus
could be potentially exposed to overfi shing (Prince et al. 2010, Stramma
et al. 2012).
Local weather variability.
Modifi cations in local weather as a result of climate
warming are refl ected in the increasing occurrence of episodic extreme
events, such temperature and precipitation extremes, storms and droughts
(Easterling et al. 2000). For example, the magnitude of El Niño-Southern
Oscillation (ENSO) has increased in the last decades, leading to more
extreme El Niño events (Dai et al. 1997). The productivity of the pelagic
habitat of the Tropical Pacifi c is tightly coupled to the El Niño-Southern
Oscillation, and the fi sh community is deeply modifi ed during strong El
Niño episodes (Lehodey et al. 2006). The intensifi cation of climate variability
exposes marine organisms to an increased environmental uncertainty, which
lead to unpredictable ecosystem responses.
The modifi cations of the Atlantic meridional overturning is expected to
impact the evolution of the global climate change (Bryden et al. 2005). Further
simulations predict that the slowing of the Atlantic Ocean circulation can
cause the collapse of plankton as a result of the reduced upwelling of deep,
nutrient-rich waters (Schmittner 2005), and the deleterious effect might
spread up in the trophic web causing severe reduction of fi sh stocks. The
