454
15 Vertical Structure of Ocean Waters and Biological Productivity
15.2 Upwelling and Biological Production
15.2.1 General Overview
Upwelling regions around the globe make up about 0.1% of the ocean's surface area, however, these areas provide more than 40% of all the commercial
fish captured globally. In general, there are a few types of upwelling: coastal
upwelling, upwelling driven by river runoff, upwelling due to tidal mixing and
upwelling induced by large-scale circulations. In this section, we will concentrate on coastal upwelling. Other types of upwelling and their corresponding
biological consequences will be summarized briefly.
As was shown in Sect. 13.2.4, the movement of freshwater towards the sea
creates a shear which generates the progressive mixing of fresh and salt water in
the estuary. Due to the entrainment mechanism, the freshwater flow carries out
to the sea several times its own volume of salt water from below the pycnocline.
Bottom water inflowing to the estuary is nutrient-rich through its contact with
the decomposing organisms on the sea bottom. These nutrients are vertically
transported into the surface waters. The phytoplankton take up nutrients,
zooplankton eat phytoplankton which are in turn eaten by fish. This regular
food web induces further sinking of organic matter which enriches the water
moving up the estuary near the sea floor (Mann, 1991).
When tidal currents are very strong, they can break down the stratification
in the water column and cause the complete mixing of the freshwater with
the underlying salt water. Nutrient-rich water from the bottom is distributed
throughout the water column and provides the basis for the phytoplankton
growth. However, due to high turbidity, the penetration of light is limited and
phytoplankton production may be much less than in a stratified estuary (see
Sect. 13.2.4 and Bowman et al., 1986).
In Sect. 7.7.4 we have described two other examples of upwelling driven by
large-scale circulation, namely polar upwelling and equatorial upwelling. Much
of the high aquatic productivity around the Antarctica results from upwelling
at the Antarctic Divergence. As much as 75% of the primary production close
to the Antarctic continent is accomplished by microplankton (20 /lm-2 mm),
and other primary food sources include detritus and bacteria. A bio-optical
algorithm based upon monthly climatological phytoplankton pigment concentration from the coastal zone colour scanner show that there is a large variation
of annual primary production over the Southern Ocean (all area south of 50 0 S)
from 630 TgC/year in the South Indian Ocean up to 1222 TgC/year in the Ross
Sea, which accounts for nearly 28% of the annual production of the Southern
Ocean (Arrigo et al., 1998).
The zone of maximum primary production advances poleward with the progression of the summer and the retreat of sea ice. Although the phosphate
(P04) and nitrate (N03) concentrations persist in these waters due to the
widespread upwelling, there are several factors limiting the primary production. The most important are the low angle of light incidence, strong winds
15 Vertical Structure of Ocean Waters and Biological Productivity
15.2 Upwelling and Biological Production
15.2.1 General Overview
Upwelling regions around the globe make up about 0.1% of the ocean's surface area, however, these areas provide more than 40% of all the commercial
fish captured globally. In general, there are a few types of upwelling: coastal
upwelling, upwelling driven by river runoff, upwelling due to tidal mixing and
upwelling induced by large-scale circulations. In this section, we will concentrate on coastal upwelling. Other types of upwelling and their corresponding
biological consequences will be summarized briefly.
As was shown in Sect. 13.2.4, the movement of freshwater towards the sea
creates a shear which generates the progressive mixing of fresh and salt water in
the estuary. Due to the entrainment mechanism, the freshwater flow carries out
to the sea several times its own volume of salt water from below the pycnocline.
Bottom water inflowing to the estuary is nutrient-rich through its contact with
the decomposing organisms on the sea bottom. These nutrients are vertically
transported into the surface waters. The phytoplankton take up nutrients,
zooplankton eat phytoplankton which are in turn eaten by fish. This regular
food web induces further sinking of organic matter which enriches the water
moving up the estuary near the sea floor (Mann, 1991).
When tidal currents are very strong, they can break down the stratification
in the water column and cause the complete mixing of the freshwater with
the underlying salt water. Nutrient-rich water from the bottom is distributed
throughout the water column and provides the basis for the phytoplankton
growth. However, due to high turbidity, the penetration of light is limited and
phytoplankton production may be much less than in a stratified estuary (see
Sect. 13.2.4 and Bowman et al., 1986).
In Sect. 7.7.4 we have described two other examples of upwelling driven by
large-scale circulation, namely polar upwelling and equatorial upwelling. Much
of the high aquatic productivity around the Antarctica results from upwelling
at the Antarctic Divergence. As much as 75% of the primary production close
to the Antarctic continent is accomplished by microplankton (20 /lm-2 mm),
and other primary food sources include detritus and bacteria. A bio-optical
algorithm based upon monthly climatological phytoplankton pigment concentration from the coastal zone colour scanner show that there is a large variation
of annual primary production over the Southern Ocean (all area south of 50 0 S)
from 630 TgC/year in the South Indian Ocean up to 1222 TgC/year in the Ross
Sea, which accounts for nearly 28% of the annual production of the Southern
Ocean (Arrigo et al., 1998).
The zone of maximum primary production advances poleward with the progression of the summer and the retreat of sea ice. Although the phosphate
(P04) and nitrate (N03) concentrations persist in these waters due to the
widespread upwelling, there are several factors limiting the primary production. The most important are the low angle of light incidence, strong winds