15.2 Upwelling and Biological Production
455
and large waves (see, for example, Massel, 1996a). Thrbulence induced by
winds and waves drags phytoplankton down to depths where light intensity is
too low to permit photosynthesis (Pinet, 1992). A full description of the food
web in the Southern Ocean is beyond the scope of this book and the reader
should consult the numerous biological books and papers on this subject (for
example, Burton, 1977; Gray and Christiansen, 1985; Laws, 1985).
Equatorial upwelling is a result of the complex interaction of the westwardflowing North and South Equatorial currents and the eastward-flowing Equatorial Countercurrent and Equatorial Undercurrent. Corio lis deflection induces
divergence of the equatorial water and an upwelling of cold water from depth.
Despite seasonal variations in the nutrient content of the surface water, primary
production remains remarkably stable throughout the year and food chains in
the photic zone are long, from phytoplankton to large predators such as marlin
and shark.
An oceanographic survey of the eastern tropical Pacific Ocean in AugustNovember 1990, confirmed a productive high-chlorophyll surface layer in two
upwelling regions, namely the equatorial divergence (east of the Galapagos) and
the countercurrent divergence, more than 1000 km west of the Costa Rica Dome
(Fiedler et al., 1991). The observed rates of 14C phytoplankton productivity
were of the order of 400-800 mg C/m 2 /day in equatorial water between the
Galapagos and longitude 1400W. However, the productivity is limited by N0 3
availability. Excess N03 persists in the euphotic zone and is advected to weak
upwelling regions, where it can persist for longer than 200 days.
15.2.2 Physical Factors and Primary Production in Coastal Upwelling Ecosystems
Coastal upwelling regions have been well studied, partly due to important fish
stocks which exist in these areas. The factors controlling high biological production have been extensively reviewed by Boje and Tomczak (1978), Richards
(1981) and Mann and Lazier (1996) and the basic physical mechanisms involved
in coastal upwelling were described in Chap. 7. The typical nutrient cycling in
a coastal upwelling system is schematically shown in Fig. 15.1. However, there
is a great variability of the physical processes occurring in the major upwelling
areas, namely the Oregon and California coasts, west coasts of Southern Africa
and Peru, Spanish Sahara, and Somalia coasts.
To illustrate the linkage between physical and biological processes in an upwelling ecosystem, we will concentrate on the Peruvian upwelling system at
15°S, and Chilean upwelling (the Gulf of Aranco). The first system was studied
intensively in 1976--1977 during the JOINT II experiments (Mann and Lazier,
1996). Due to a relatively narrow shelf (20 km) with a steep slope at the
shelf break, and constant and relatively weak winds, offshore transport occurs
mainly in the top 20 m and the wind-induced mixing does not penetrate deeply.
The poleward countercurrent, flowing beneath the equatorward coastal current
is situated at intermediate depth over the continental shelf. MacIsaac et ai.
455
and large waves (see, for example, Massel, 1996a). Thrbulence induced by
winds and waves drags phytoplankton down to depths where light intensity is
too low to permit photosynthesis (Pinet, 1992). A full description of the food
web in the Southern Ocean is beyond the scope of this book and the reader
should consult the numerous biological books and papers on this subject (for
example, Burton, 1977; Gray and Christiansen, 1985; Laws, 1985).
Equatorial upwelling is a result of the complex interaction of the westwardflowing North and South Equatorial currents and the eastward-flowing Equatorial Countercurrent and Equatorial Undercurrent. Corio lis deflection induces
divergence of the equatorial water and an upwelling of cold water from depth.
Despite seasonal variations in the nutrient content of the surface water, primary
production remains remarkably stable throughout the year and food chains in
the photic zone are long, from phytoplankton to large predators such as marlin
and shark.
An oceanographic survey of the eastern tropical Pacific Ocean in AugustNovember 1990, confirmed a productive high-chlorophyll surface layer in two
upwelling regions, namely the equatorial divergence (east of the Galapagos) and
the countercurrent divergence, more than 1000 km west of the Costa Rica Dome
(Fiedler et al., 1991). The observed rates of 14C phytoplankton productivity
were of the order of 400-800 mg C/m 2 /day in equatorial water between the
Galapagos and longitude 1400W. However, the productivity is limited by N0 3
availability. Excess N03 persists in the euphotic zone and is advected to weak
upwelling regions, where it can persist for longer than 200 days.
15.2.2 Physical Factors and Primary Production in Coastal Upwelling Ecosystems
Coastal upwelling regions have been well studied, partly due to important fish
stocks which exist in these areas. The factors controlling high biological production have been extensively reviewed by Boje and Tomczak (1978), Richards
(1981) and Mann and Lazier (1996) and the basic physical mechanisms involved
in coastal upwelling were described in Chap. 7. The typical nutrient cycling in
a coastal upwelling system is schematically shown in Fig. 15.1. However, there
is a great variability of the physical processes occurring in the major upwelling
areas, namely the Oregon and California coasts, west coasts of Southern Africa
and Peru, Spanish Sahara, and Somalia coasts.
To illustrate the linkage between physical and biological processes in an upwelling ecosystem, we will concentrate on the Peruvian upwelling system at
15°S, and Chilean upwelling (the Gulf of Aranco). The first system was studied
intensively in 1976--1977 during the JOINT II experiments (Mann and Lazier,
1996). Due to a relatively narrow shelf (20 km) with a steep slope at the
shelf break, and constant and relatively weak winds, offshore transport occurs
mainly in the top 20 m and the wind-induced mixing does not penetrate deeply.
The poleward countercurrent, flowing beneath the equatorward coastal current
is situated at intermediate depth over the continental shelf. MacIsaac et ai.