Environmental Engineering Reference
In-Depth Information
in regional weather patterns, acidifi cation of oceans, variations in nutrient
loads or alteration in ocean circulation (Brierley and Kingsford 2009). All
these changes and others that may be occurring, affect biological processes
taking place in the ocean at all levels, from the molecular to the ecosystemic
one (Drinkwater et al. 2010). There is broad consensus that contemporary
global climate change is a reality, and that much of the ongoing change is
a direct result of human activity (IPCC 2007a). In particular, burning fossil
fuels, making cement and changing land use have driven atmospheric
carbon dioxide concentrations up from a pre-industrial value of about
280 ppm to 385 ppm in 2008 (Meure et al. 2006) (Fig. 1). Annual increases
are now exceeding 2 ppm, an emission trend that exceeds the worst case
scenario discussed at the Intergovernmental Panel on Climate Change
(IPCC 2007b). There is a direct link between global temperature and CO
2
concentration (IPCC 2007a). The increased heating in the lower atmosphere/
Earth's surface (radiative forcing) resulting from the “greenhouse” effect
caused by increasing atmospheric CO
2
, methane and other gases (at a
value of about 3 W.m
-2
, following IPCC 2007c) is unprecedented in at least
the last 22,000 years (Joos and Spahni 2008) and has already had direct
physical consequences for the marine environment and organisms living
there. These include increases in mean global sea surface temperature by
0.13ºC per decade since 1979, and ocean interior temperature by >0.1ºC
since 1961, increasing wind velocity and storm frequency, changes in ocean
circulation, vertical structure and nutrient loads (IPCC 2007c), as well as
rising sea level by more than 15 cm in the last century (Rahmstorf 2007)
(Fig. 1), and presently by a mean of about 3.3 mm per year. Because the
oceanic and atmospheric gas concentrations tend towards equilibrium,
increasing CO
2
pressure drives more CO
2
into the ocean, where it dissolves
forming carbonic acid (H
2
CO
3
) and thus increases ocean acidity; ocean pH
has dropped by 0.1 (a 30% increase in H
+
ion concentration) in the last 200
years (The Royal Society 2005) (Fig. 1).
Marine ecosystems clearly respond to changes in ocean variability
and climate over a wide range of spatial and temporal scales (Mann and
Lazier 1996, Southward et al. 2005, Drinkwater et al. 2010). The processes
through which the physical environment affects the factors controlling
primary production have long been known (Sverdrup 1953). These include
infl uence on upper layer nutrient levels through mixing or upwelling, light
levels through the effects of cloudiness or sea-ice coverage, and stratifi cation
through changes in mixing or heat and salt fl uxes (Lavoie et al. 2009). For
example, the relationship between mixing and production of phytoplankton
in the North Atlantic depends upon the ratio of Sverdrup's critical depth
in spring to the mixed-layer depth at the end of the winter (Dutkiewicz et
al. 2001). Where this ratio is near 1, as in the subtropical gyre, increased
mixing reduces stratifi cation which tends to increase primary production
