Environmental Engineering Reference
In-Depth Information
alterations in nutrient supply and production are predominantly negative,
due to reduced vertical mixing. In high latitude regions the resultant
increased stability of the water column may, however, have a positive effect
on production in spite of reduced nutrient supply because phytoplankton
will no longer be mixed down to depths greater than their compensation
depth (the depth at which respiration loss exceeds photosynthetic gain)
(Behrenfeld et al. 2006).
Sarmiento et al. (2005) have performed a comparative study on primary
production using empirical models for a set of seven biomes (marginal sea
ice; subpolar; subtropical seasonally stratifi ed; subtropical permanently
stratifi ed; low latitude upwelling; tropical upwelling; tropical downwelling),
which are further subdivided into a total of 33 biogeographical provinces
resembling those of Longhurst (1998). A small global increase in marine
chlorophyll and primary production is predicted (< 10%) for 2050 and
2090, compared with the pre-industrial control scenario, but with quite big
regional differences. Decreases in the North Pacifi c and the area adjacent
to the Antarctic continent are slightly more pronounced than offset by
increases in the North Atlantic and the open Southern Ocean. The most
robust part of the outcome is the change in biome areas, with reductions
in the marginal sea-ice biome and increases in the permanently stratifi ed
subtropical gyre biome.
Other critical factor in determining the change in primary production
is temperature sensitivity of primary production for a given chlorophyll
level. This in itself determines whether primary production increases or
decreases at low latitudes, and whether there would be no change or quite
large increases in primary production at high latitudes (Brander 2010).
Satellite observations of ocean chlorophyll indicate that global ocean
annual primary production has declined by more than 6% since the early
1980s (Gregg et al. 2003). Global blended chlorophyll seasonal climatologies
were used as inputs to the Vertically Generalized Production Model or
VGPM (Behrenfeld and Falkowski 1997) to compute seasonal ocean primary
production. Nearly 70% of the global decline occurred in the high latitudes.
In the northern high latitudes, these reductions in primary production
corresponded with increases in sea surface temperature and decreases
in atmospheric iron deposition to the oceans, e.g., the processes involve
both direct and indirect effects on nutrient supply. In the Antarctic, the
reductions were accompanied by increased wind stress. It must be noted
that these declines in primary production at high latitude have been offset
by increases at low latitudes, and that three of the four low latitude basins
exhibited decadal increases in annual primary production.
