266
Chapter 9: The Atlantic Ocean
sharp turbidity front, visible in NOAA-AVHRR and in chlorophyll imagery. This migrates
seasonally: upstream and westward in summer (minimum discharge) and downstream
and eastward in winter (maximum discharge).
Perhaps because it is not one of the world’s major upwelling sites, the physical
mechanism that forces upwelling episodes at Cape Frio is better known than the ecological consequences, although the physiological parameters of phytoplankton involved
in upwelling blooms here have been observed to respond appropriately to light, nutrients, and temperature (Gonzalez-Rodriguez et al., 1992; Gonzalez-Rodriguez, 1994). The
upwelling of SACW is episodic, and the biological response is rapid, though the upwelled
water does require conditioning by superficial heating and stratification before primary
production rate increases significantly.
The anticipated physiological conditioning of algal cells has been followed in this
upwelling, showing that the light-saturation curve responds to conditions of upwelling
and downwelling with changing values. A typical upwelling event lasts from 5 to 10 days,
during which successive reductions in wind speed induce surface heating so that the rate
of primary production starts to increase by about day 5 to introduce the “production
phase” of the event. During this period, the mixed layer (typically 10 m deep, 15–20
C
at the surface, and 15–20 mol NO 3 ) accumulates only about 5–6 mg chl m
−3 from a
production rate of about 10 mg C m
−3 hr
−1 . Subsequently, as the system returns to a
downwelling state, chlorophyll biomass rapidly falls by an order of magnitude, though
productivity remains at about 2 or 3 mg C m
−3 hr
−1 for some time. This suggests that
the upwelling bloom is rapidly and heavily grazed, a suggestion for which there is some
experimental support. There is also some indication that the magnitude of the production
cycle may respond to the initial inoculum of cells in the upwelling parcel of water rather
than to its nutrient load. The newly upwelled water contains a sparse population of
benthic diatoms that are rapidly replaced by an intense bloom of neritic and opportunistic
diatoms.
Farther poleward, at 25–30
S (or from Santos to Cape St. Marta), recent studies during
austral summer over and beyond the shelf have confirmed the oligotrophic nature of
the tropical water (0–200 m) carried by the Brazil Current, and the ecological effects of
intrusions of SACW across the shelf (Metzler et al., 1997). These surveys of the Brazilian
COROAS expeditions found highly variable rates of nitrate uptake by phytoplankton in
this hydrographically dynamic region. Differences between inshore and oceanic profiles
of production rate and chlorophyll biomass, and the concomitant physiological constants, were predictable; over the shelf, <32% of the nitrogen utilized by autotrophic
cells was obtained from NO 3 . At coastal stations and midshelf at 30
S, the DCM was
found by Odebrecht and Djurfeldt (1989) to be dominated by larger autotrophic cells
(>20 m) while the surface layer was populated by the smaller pico and nano fractions;
chlorophyll maxima in the DCM were due to the presence of large centric diatoms (Coscinodiscus, Thalassiosira) having high cellular chlorophyll content. Here, the DCM flora
is sustained by nutrients supplied by bottom-driven turbulence during both upwelling
and downwelling conditions.
There appears to be a clear distinction between zooplankton populations that are
characteristic of SACW irruptions onto and across the shelf and the shelf plankton proper,
which is dominated by neritic species; several accounts speak of Calanoides carinatus and
Ctenocalanus vanus as indicators of SACW water as far south as Rio de Janeiro. At Cabo
Frio, Valentin (1984) describes two characteristics species groups associated with various
nearshore neritic conditions: (i) herbivores—Ctenocalanus vanus, Calanoides carinatus,
Penilia avirostris, and (ii) omnivores, predators—Siphonophora, Chaetognatha, Corycaeus
amazonicus, Eucalanus pileatus, Clausocalanus acuicornis. An intermediate offshore neritic
zone is characterized by a great abundance of appendicularians, whereas the oceanic
Chapter 9: The Atlantic Ocean
sharp turbidity front, visible in NOAA-AVHRR and in chlorophyll imagery. This migrates
seasonally: upstream and westward in summer (minimum discharge) and downstream
and eastward in winter (maximum discharge).
Perhaps because it is not one of the world’s major upwelling sites, the physical
mechanism that forces upwelling episodes at Cape Frio is better known than the ecological consequences, although the physiological parameters of phytoplankton involved
in upwelling blooms here have been observed to respond appropriately to light, nutrients, and temperature (Gonzalez-Rodriguez et al., 1992; Gonzalez-Rodriguez, 1994). The
upwelling of SACW is episodic, and the biological response is rapid, though the upwelled
water does require conditioning by superficial heating and stratification before primary
production rate increases significantly.
The anticipated physiological conditioning of algal cells has been followed in this
upwelling, showing that the light-saturation curve responds to conditions of upwelling
and downwelling with changing values. A typical upwelling event lasts from 5 to 10 days,
during which successive reductions in wind speed induce surface heating so that the rate
of primary production starts to increase by about day 5 to introduce the “production
phase” of the event. During this period, the mixed layer (typically 10 m deep, 15–20
C
at the surface, and 15–20 mol NO 3 ) accumulates only about 5–6 mg chl m
−3 from a
production rate of about 10 mg C m
−3 hr
−1 . Subsequently, as the system returns to a
downwelling state, chlorophyll biomass rapidly falls by an order of magnitude, though
productivity remains at about 2 or 3 mg C m
−3 hr
−1 for some time. This suggests that
the upwelling bloom is rapidly and heavily grazed, a suggestion for which there is some
experimental support. There is also some indication that the magnitude of the production
cycle may respond to the initial inoculum of cells in the upwelling parcel of water rather
than to its nutrient load. The newly upwelled water contains a sparse population of
benthic diatoms that are rapidly replaced by an intense bloom of neritic and opportunistic
diatoms.
Farther poleward, at 25–30
S (or from Santos to Cape St. Marta), recent studies during
austral summer over and beyond the shelf have confirmed the oligotrophic nature of
the tropical water (0–200 m) carried by the Brazil Current, and the ecological effects of
intrusions of SACW across the shelf (Metzler et al., 1997). These surveys of the Brazilian
COROAS expeditions found highly variable rates of nitrate uptake by phytoplankton in
this hydrographically dynamic region. Differences between inshore and oceanic profiles
of production rate and chlorophyll biomass, and the concomitant physiological constants, were predictable; over the shelf, <32% of the nitrogen utilized by autotrophic
cells was obtained from NO 3 . At coastal stations and midshelf at 30
S, the DCM was
found by Odebrecht and Djurfeldt (1989) to be dominated by larger autotrophic cells
(>20 m) while the surface layer was populated by the smaller pico and nano fractions;
chlorophyll maxima in the DCM were due to the presence of large centric diatoms (Coscinodiscus, Thalassiosira) having high cellular chlorophyll content. Here, the DCM flora
is sustained by nutrients supplied by bottom-driven turbulence during both upwelling
and downwelling conditions.
There appears to be a clear distinction between zooplankton populations that are
characteristic of SACW irruptions onto and across the shelf and the shelf plankton proper,
which is dominated by neritic species; several accounts speak of Calanoides carinatus and
Ctenocalanus vanus as indicators of SACW water as far south as Rio de Janeiro. At Cabo
Frio, Valentin (1984) describes two characteristics species groups associated with various
nearshore neritic conditions: (i) herbivores—Ctenocalanus vanus, Calanoides carinatus,
Penilia avirostris, and (ii) omnivores, predators—Siphonophora, Chaetognatha, Corycaeus
amazonicus, Eucalanus pileatus, Clausocalanus acuicornis. An intermediate offshore neritic
zone is characterized by a great abundance of appendicularians, whereas the oceanic