The Convergence Ecosystem in the Southwest Atlantic
151
southern Brazil coincided with large freshwater outflow in El Nino years
(Ciotti et al. 1995), interannual differences seem to be controlled by continental runoff.
Upwelling of nutrient-rich SACW in northern and of SAW in southern
areas between 23°S and 40°S is caused by cold cyclonic and warm anticyclonic vortices, owing to the wind regime, shelf and slope topography,
and current shear between the Brazil Current and contiguous waters.
In summer, dominance of NE winds causes divergence of coastal surface waters and the advection of deep SACW (10-20 °C, salinity 35-36)
over the shelf between 23°S and 31 °S (Fig. 11.2B,C; Ferreira da Silva et al.
1984; Matsuura 1986). SACW is the major source of dissolved nitrate
(>20 flM) and N:P ratios of approximately 16:1 favor primary production.
The size of production depends on the extension of SACW over the shelf
and whether it reaches the euphotic zone. Ascending waters tend to
reduce mixing depth in relation to euphotic depth, generating phytoplankton subsurface maxima and enhanced local primary productivity
(Brandini 1990; Aidar et al. 1993; Odebrecht and Djurfeldt 1996). During
strong NE winds (Fig. 11.2C), SACW enrichment reaches surface waters
(Hubold 1980). Except for vortex-driven upwelling of SACW, the moderately stratified thermohaline inhibits primary production, owing to dominance of oligotrophic TW (silicate ~5f1M, phosphate, nitrate, ammonium
~1f1M) at the surface (Brandini 1990; Niencheski and Fillmann 1997).
South of 38°S, fertilization at the shelf break front (up to 45 km wide)
between shelf waters and the western edge of the Malvinas Current
(Podesta 1997) sustains high phytoplankton biomass (>2 mg m- 3 ) during
summer (Carreto et al. 1995). The horizontal shift of this front between outer and inner shelf regions during summer and spring/fall,
respectively, depends on cyclical advance/reverse movements of the Malvinas Current.
In winter and spring, the shear between the Brazil Current and shelf
waters causes wave-like meanders and vortices and leads to enhanced subsurface phytoplankton biomass along the slope (24-31°S), which is associated with nutrient-rich SACW upwelling (Fig. 11.2D; Matsuura 1986;
Brandini 1990; Lima et al. 1996). Between 31°S and 35°S, the advection of
northward flowing SAW at the shelf break results in a horizontal thermohaline gradient and horizontal current shear leads to the development of
warm anticyclonic and cold cyclonic vortices, causing SAW upwelling
(Fig. 11.2E). Under these conditions, shelf and slope waters become weakly
stratified (Lima 1992). Primary production is enhanced when SAW (nitrate
and silicate -11.0 flM, ammonium 6.6f1M, phosphate 1.1f1M; Niencheski
and Fillmann 1997) reaches the euphotic zone, resulting in subsurface
chlorophyll-a nuclei and integrated chlorophyll-a concentrations above
151
southern Brazil coincided with large freshwater outflow in El Nino years
(Ciotti et al. 1995), interannual differences seem to be controlled by continental runoff.
Upwelling of nutrient-rich SACW in northern and of SAW in southern
areas between 23°S and 40°S is caused by cold cyclonic and warm anticyclonic vortices, owing to the wind regime, shelf and slope topography,
and current shear between the Brazil Current and contiguous waters.
In summer, dominance of NE winds causes divergence of coastal surface waters and the advection of deep SACW (10-20 °C, salinity 35-36)
over the shelf between 23°S and 31 °S (Fig. 11.2B,C; Ferreira da Silva et al.
1984; Matsuura 1986). SACW is the major source of dissolved nitrate
(>20 flM) and N:P ratios of approximately 16:1 favor primary production.
The size of production depends on the extension of SACW over the shelf
and whether it reaches the euphotic zone. Ascending waters tend to
reduce mixing depth in relation to euphotic depth, generating phytoplankton subsurface maxima and enhanced local primary productivity
(Brandini 1990; Aidar et al. 1993; Odebrecht and Djurfeldt 1996). During
strong NE winds (Fig. 11.2C), SACW enrichment reaches surface waters
(Hubold 1980). Except for vortex-driven upwelling of SACW, the moderately stratified thermohaline inhibits primary production, owing to dominance of oligotrophic TW (silicate ~5f1M, phosphate, nitrate, ammonium
~1f1M) at the surface (Brandini 1990; Niencheski and Fillmann 1997).
South of 38°S, fertilization at the shelf break front (up to 45 km wide)
between shelf waters and the western edge of the Malvinas Current
(Podesta 1997) sustains high phytoplankton biomass (>2 mg m- 3 ) during
summer (Carreto et al. 1995). The horizontal shift of this front between outer and inner shelf regions during summer and spring/fall,
respectively, depends on cyclical advance/reverse movements of the Malvinas Current.
In winter and spring, the shear between the Brazil Current and shelf
waters causes wave-like meanders and vortices and leads to enhanced subsurface phytoplankton biomass along the slope (24-31°S), which is associated with nutrient-rich SACW upwelling (Fig. 11.2D; Matsuura 1986;
Brandini 1990; Lima et al. 1996). Between 31°S and 35°S, the advection of
northward flowing SAW at the shelf break results in a horizontal thermohaline gradient and horizontal current shear leads to the development of
warm anticyclonic and cold cyclonic vortices, causing SAW upwelling
(Fig. 11.2E). Under these conditions, shelf and slope waters become weakly
stratified (Lima 1992). Primary production is enhanced when SAW (nitrate
and silicate -11.0 flM, ammonium 6.6f1M, phosphate 1.1f1M; Niencheski
and Fillmann 1997) reaches the euphotic zone, resulting in subsurface
chlorophyll-a nuclei and integrated chlorophyll-a concentrations above