456
15 Vertical Structure of Ocean Waters and Biological Productivity
euphotic zone: primary production ~ (nutrients)
sediment
benthic ~
trophic level
pelagic
upwelling
trophic
level
Fig. 15.1: Schematic representation of nutrient cycling in coastal upwelling system
(adapted from Glantz, 1996)
(1985) reported results of comprehensive studies on primary production in an
upwelling area at 15°S on the coast of Peru. They found that the rate of nitrate
uptake in the shallow, as well as the deep, euphotic zone is controlled by light
in the nutrient-rich, freshly upwelled waters. Once phytoplankton have been
upwelled, it is rarely mixed below the euphotic zone because of the relatively
strong stratification. The surface mixed layer in the area is rather shallow and
only occasionally deeper than 20 m. The spatial distribution of the primary
production in the surface waters is shown in Fig. 15.2. The pattern clearly
demonstrates the areas affected by coastal upwelling as well as areas affected
by the coastal band of equatorial upwelling.
Measurement by MacIsaac et al. (1985) showed that there is a consistent
pattern of physiological transformation starting at the upwelling centre. In this
sense, the coastal upwelling ecosystem has quasi time-space stability. However,
the scale of this stability is rather small, when compare with oceanic scales,
being of the order of 10 to 100 km and 1 to 10 days. Apart from this mesoscale
variability, the magnitude of biological production varies interannualy.
There is experimental evidence that biological productivity decreases during
EI Nino events. The coastal upwelling continues but the water entrained is
warmer and poorer in nutrients. The thermocline is progressively depressed
toward and below the depth of entrainment (40 to 80 m) and smaller quantities of nutrients are transported to the surface. This in turn causes primary
production of organic material to decrease proportionally. Also, the amount
of light available to a phytoplankton population for the synthesis of organic
15 Vertical Structure of Ocean Waters and Biological Productivity
euphotic zone: primary production ~ (nutrients)
sediment
benthic ~
trophic level
pelagic
upwelling
trophic
level
Fig. 15.1: Schematic representation of nutrient cycling in coastal upwelling system
(adapted from Glantz, 1996)
(1985) reported results of comprehensive studies on primary production in an
upwelling area at 15°S on the coast of Peru. They found that the rate of nitrate
uptake in the shallow, as well as the deep, euphotic zone is controlled by light
in the nutrient-rich, freshly upwelled waters. Once phytoplankton have been
upwelled, it is rarely mixed below the euphotic zone because of the relatively
strong stratification. The surface mixed layer in the area is rather shallow and
only occasionally deeper than 20 m. The spatial distribution of the primary
production in the surface waters is shown in Fig. 15.2. The pattern clearly
demonstrates the areas affected by coastal upwelling as well as areas affected
by the coastal band of equatorial upwelling.
Measurement by MacIsaac et al. (1985) showed that there is a consistent
pattern of physiological transformation starting at the upwelling centre. In this
sense, the coastal upwelling ecosystem has quasi time-space stability. However,
the scale of this stability is rather small, when compare with oceanic scales,
being of the order of 10 to 100 km and 1 to 10 days. Apart from this mesoscale
variability, the magnitude of biological production varies interannualy.
There is experimental evidence that biological productivity decreases during
EI Nino events. The coastal upwelling continues but the water entrained is
warmer and poorer in nutrients. The thermocline is progressively depressed
toward and below the depth of entrainment (40 to 80 m) and smaller quantities of nutrients are transported to the surface. This in turn causes primary
production of organic material to decrease proportionally. Also, the amount
of light available to a phytoplankton population for the synthesis of organic