Environmental Engineering Reference
In-Depth Information
Open-ocean deoxygenation has been observed in the thermocline of
the North Pacific and tropical oceans over decadal periods, perhaps due
to natural climate variability (Mecking et al., 2008). Models project long-
term reductions of 1-7% in the global oxygen inventory and expansions of
open-ocean oxygen minimum zone over the 21st century (Frölicher et al.,
2009; Keeling et al., 2010). The duration, intensity, and extent of coastal
hypoxia has also been increasing substantially over the last half-century,
but primarily due to elevated fertilizer run-off and atmospheric nitrogen
deposition that contribute to coastal eutrophication, enhanced organic mat-
ter production, and export and subsurface decomposition that consumes
O
2
. Climate change could accelerate coastal hypoxia via surface warming
and regional increases in precipitation and river runoff that increase wa-
ter-column vertical stratification; on the other hand, more intense tropical
storms could disrupt stratification and increase O
2
ventilation (Rabalais et
al., 2010). Expanding coastal hypoxia is also induced in some regions by
reorganization in ocean-atmosphere physics. Off the Oregon-Washington
coast, increased wind-driven upwelling is linked to the first appearance of
hypoxia, and even anoxia, on the inner-shelf after five decades of hypoxia-
free observations (Chan et al., 2008). Further south in the California Current
System, the depth of hypoxic surface has shoaled along the coast by up to
90 m (Bograd et al., 2008). The same physical phenomenon, along with the
penetration of fossil-fuel CO
2
into off-shore source waters, are introducing
waters corrosive to aragonite (
W
< 1) onto the continental shelf (Feely et al.,
2008). There is conflicting evidence on how coastal upwelling may respond
to climate change, and impacts may vary regionally (Bakun et al., 2010).
Laboratory and mesocosm experiments indicate that many marine or-
ganisms are sensitive to elevated CO
2
and ocean acidification, with both
positive and negative physiological responses (Fabry et al., 2008; Doney et
al., 2009a,b; NRC, 2010). The projected rates of change in global ocean pH
and
W
over the next century are a factor of 30-100 times faster than temporal
changes in the recent geological past, and the perturbations will last many
centuries to millennia. Although there are spatial and temporal variations in
surface seawater pH and saturation state, projected future surface water pH
values for the open-ocean are below the range experienced by contempo-
rary populations, and the ability of marine organisms to acclimate or adapt
to the magnitude and rate of change is unknown.
The largest identified negative impacts are on shell and skeleton growth
by calcifying species including corals, coralline algae, and mollusks. Cor-
als utilize the aragonite mineral form of calcium carbonate, and the rate
of coral calcification declines with falling aragonite saturation state even
when waters remain supersaturated, and corals appear to need saturation
