Environmental Engineering Reference
In-Depth Information
with other factors (rising CO
2
, altered ocean circulation), and the potential
synergistic or antagonistic effects of multiple stressors must be considered.
Satellite observations indicate a strong negative relationship, at inter-
annual time scales, between marine primary productivity and surface warm-
ing in the tropics and subtropics, most likely due to reduced nutrient supply
from increased vertical stratification (Behrenfeld et al., 2006). Satellite data
also indicate that the very lowest productivity regions in subtropical gyres
expanded in area over the past decade (Polovina et al., 2008), although
these trends may be due to interannual variability (Henson et al., 2010).
Numerical models project declining low-latitude marine primary produc-
tion in response to 21st century climate warming (Sarmiento et al., 2004;
Steinacher et al., 2010) (Figure 5.19). Warmer, more nutrient-poor condi-
tions in the subtropics could enhance biological nitrogen fixation (Boyd
and Doney, 2002), an effect that may be amplified by higher surface water
CO
2
levels (Hutchins et al., 2009). The situation is less clear in temperate
and polar waters, although there is a tendency in most models for increased
production due to warming, reduced vertical mixing, and reduced sea-ice
cover. For example, the rapid warming and sea-ice retreat along the West
Antarctic Peninsula has lead to a poleward shift in the region of strong sea-
sonal primary production that has impacts for higher trophic levels including
seabirds (Montes-Hugo et al., 2009). In most open-ocean regions, however,
the climate signal in primary production and other ecosystem properties may
be difficult to distinguish from natural variability for many decades (Boyd et
al., 2008; Henson et al., 2010). Changes in atmospheric nutrient deposition
(nitrogen and iron) linked to fossil-fuel combustion and agriculture also can
alter marine productivity but mostly on regional scales near industrial and
agricultural sources (Duce et al., 2008; Krishnamurthy et al., 2009).
Subsurface oxygen levels likely will decline due to warmer waters (low-
er oxygen solubility) and altered ocean circulation, leading to an enlarge-
ment of open-ocean oxygen minimum zones and stronger coastal oxygen
depletion in some regions (Keeling et al., 2010; Rabalais et al., 2010). Low
subsurface O
2
, termed hypoxia, occurs naturally in open-ocean and coastal
environments from a combination of weak ventilation and/or strong organic
matter degradation. Dissolved O
2
gas is essential for aerobic respiration, and
low O
2
levels negatively affect the physiology of higher animals leading to
so-called “dead-zones” where many macro-fauna are absent. Coastal hypox-
ia can lead to marine habitat degradation and, in extreme cases, extensive
fish and invertebrate mortality (Levin et al., 2009; Rabalais et al., 2010). Ex-
panded open-ocean oxygen minimum zones would increase denitrification
and may contribute to increased oceanic production of the greenhouse gas
