Pacific Coastal Biome
413
with filaments and arcuate features of chlorophyll that reach far offshore; these appear
to be not unlike the Heida eddies discussed earlier but nevertheless, as off Alaska, coastal
chlorophyll enhancement does propagate far seaward to the west off Chile.
But to return to the canonical upwelling system of this province: the biological seasons
are dominated by processes that occur off Peru, where austral spring-summer upwelling
is the principal time-dependent forcing. The biological evolution of a parcel of upwelled
water, which requires about 10 days to complete, has been well worked out (see MacIsaac
et al., 1985; Harrison et al., 1981) in the Humboldt Current, in a location (Cape NazcaCape Santa Ana) having consistent upwelling for several weeks. In the actual upwelling
zone, < 7 km from the coast, nitrate remains relatively constant at about 20 M in the
upwelling water, whereas silicate supply varies according to its source. The evolution
of phytoplankton growth passes progressively through several phases: a small inoculum
of phytoplankton cells is upwelled from 50–60 m depth, the origin of the seed stock
being cells sinking from farther seaward; these cells are physiologically conditioned to
low light levels and therefore have low rates of nutrient uptake and growth. In addition,
the upwelled water itself may require conditioning by exposure to light to permit active
growth of algae. If this occurs, it consists of progressive modification of trace metal
chemistry, exchanging available copper and manganese ions.
Progressively, the cells entrained in the upwelled water shift up to increased physiological rates so that as stratification is induced by solar heating in the upwelled parcel,
the entrained cells are adapted to high irradiance and high nutrient levels. The now fastgrowing cells, held in a very shallow (< 10–15 m) Ekman layer, reduce ambient nitrate
levels quite rapidly and there is a massive increase in standing stock of cells. Rates of
primary production are maximal at 0–10 m, whereas two depths of high concentration
of chlorophyll may occur—the first near the depth of maximum production rate and the
other, a DCM, near the bottom of the thermocline, although this may merely represent
unconsumed, sinking cells. Subsequently, in response to nutrient depletion in the Ekman
layer, the cells respond by limiting some of their cellular processes, sequentially slowing
nutrient uptake, photosynthesis, and storage of carbohydrates and lipids. Because most
of the growth can be attributed to diatoms, the limiting nutrient may be silicate rather
than nitrate. In the maximum growth phase, a typical upwelling plume has a phytoplankton species assemblage dominated by no more than 10 species of diatoms (Detonula,
Chaetoceros, Hemiaulis, Rhizosolenia, and Thalassiosira) together with dinoflagellates and
< 10-mm flagellates. The total cell number of diatoms is usually about the same as the
total of the other two groups.
Herbivorous mesozooplankton consume only a small fraction (10% is a typical finding)
of the daily primary production. Three copepods dominate the biomass of mesozooplankton offshore (Calanus chilensis, Centropages brachiatus, and Eucalanus inermis), of
which only the last continues to perform diel migrations while it is entrained into an
upwelling feature. These species aggregate close to the surface under such conditions and
their layer depth coincides with the lower part of the upper chlorophyll layer—perhaps
near the depth of maximum production rate (Herman, 1984).
At midshelf depths (120 m and beyond), during the upwelling season off Peru, it was
found that these large copepods had different feeding strategies and that the differences
were strengthened during periods of low food availability: Calanus feeds continuously as
a herbivore, but avoids the surface layers by day; Centropages is an omnivore, feeding only
at night when offshore; and Eucalanus feeds intensively by day on POM in the anoxic
layer, but near-surface at night (Boyd et al., 1980). Inshore, near the foci of upwelling
cells, the zooplankton is dominated by small copepods, just as it is off Alaska: species
of Paracalanus, Oithona, and Acartia dominate this fauna within the cores of upwelling
cells, their abundance responding negatively to SST (Escribano and Hidalgo, 2000).
413
with filaments and arcuate features of chlorophyll that reach far offshore; these appear
to be not unlike the Heida eddies discussed earlier but nevertheless, as off Alaska, coastal
chlorophyll enhancement does propagate far seaward to the west off Chile.
But to return to the canonical upwelling system of this province: the biological seasons
are dominated by processes that occur off Peru, where austral spring-summer upwelling
is the principal time-dependent forcing. The biological evolution of a parcel of upwelled
water, which requires about 10 days to complete, has been well worked out (see MacIsaac
et al., 1985; Harrison et al., 1981) in the Humboldt Current, in a location (Cape NazcaCape Santa Ana) having consistent upwelling for several weeks. In the actual upwelling
zone, < 7 km from the coast, nitrate remains relatively constant at about 20 M in the
upwelling water, whereas silicate supply varies according to its source. The evolution
of phytoplankton growth passes progressively through several phases: a small inoculum
of phytoplankton cells is upwelled from 50–60 m depth, the origin of the seed stock
being cells sinking from farther seaward; these cells are physiologically conditioned to
low light levels and therefore have low rates of nutrient uptake and growth. In addition,
the upwelled water itself may require conditioning by exposure to light to permit active
growth of algae. If this occurs, it consists of progressive modification of trace metal
chemistry, exchanging available copper and manganese ions.
Progressively, the cells entrained in the upwelled water shift up to increased physiological rates so that as stratification is induced by solar heating in the upwelled parcel,
the entrained cells are adapted to high irradiance and high nutrient levels. The now fastgrowing cells, held in a very shallow (< 10–15 m) Ekman layer, reduce ambient nitrate
levels quite rapidly and there is a massive increase in standing stock of cells. Rates of
primary production are maximal at 0–10 m, whereas two depths of high concentration
of chlorophyll may occur—the first near the depth of maximum production rate and the
other, a DCM, near the bottom of the thermocline, although this may merely represent
unconsumed, sinking cells. Subsequently, in response to nutrient depletion in the Ekman
layer, the cells respond by limiting some of their cellular processes, sequentially slowing
nutrient uptake, photosynthesis, and storage of carbohydrates and lipids. Because most
of the growth can be attributed to diatoms, the limiting nutrient may be silicate rather
than nitrate. In the maximum growth phase, a typical upwelling plume has a phytoplankton species assemblage dominated by no more than 10 species of diatoms (Detonula,
Chaetoceros, Hemiaulis, Rhizosolenia, and Thalassiosira) together with dinoflagellates and
< 10-mm flagellates. The total cell number of diatoms is usually about the same as the
total of the other two groups.
Herbivorous mesozooplankton consume only a small fraction (10% is a typical finding)
of the daily primary production. Three copepods dominate the biomass of mesozooplankton offshore (Calanus chilensis, Centropages brachiatus, and Eucalanus inermis), of
which only the last continues to perform diel migrations while it is entrained into an
upwelling feature. These species aggregate close to the surface under such conditions and
their layer depth coincides with the lower part of the upper chlorophyll layer—perhaps
near the depth of maximum production rate (Herman, 1984).
At midshelf depths (120 m and beyond), during the upwelling season off Peru, it was
found that these large copepods had different feeding strategies and that the differences
were strengthened during periods of low food availability: Calanus feeds continuously as
a herbivore, but avoids the surface layers by day; Centropages is an omnivore, feeding only
at night when offshore; and Eucalanus feeds intensively by day on POM in the anoxic
layer, but near-surface at night (Boyd et al., 1980). Inshore, near the foci of upwelling
cells, the zooplankton is dominated by small copepods, just as it is off Alaska: species
of Paracalanus, Oithona, and Acartia dominate this fauna within the cores of upwelling
cells, their abundance responding negatively to SST (Escribano and Hidalgo, 2000).