The Convergence Ecosystem in the Southwest Atlantic
149
intervals). Precipitation is more frequent and higher in winter. Extreme
rainfall or drought periods are associated with El Nino-southern oscillation phenomena (Seeliger et al. 1997).
The tides are amplified by SE onshore winds which increase southward
(1m at 25°S, 8 m at 45°S). Amphidromic conditions exist at 30-32°S, 41 os,
and 47°S where oscillation periods of shelf water approach those of oceanic tides. Annual freshwater runoff (mean 20,000-25,000 m 3 s- 1 ) from the
Rio de la Plata drainage basin (3,170,000 km 2 ; Guerrero et al. 1997) varies
considerably between seasons. During maximum runoff in fall/winter,
offshore winds divide the outflowing water into a smaller, southward flowing branch and, owing to the Coriolis effect, into a larger branch along the
Uruguayan coast. The northern branch mixes with freshwater runoff from
the Patos Lagoon (annual mean 700-3,000 m 3 s- 1 , maximum in winter/
spring) over the southern Brazilian shelf (Moller et al. 1991). Owing to the
dilution of modified SACW and SAW, the coastal waters ( CW) below the
low salinity surface layer are defined as subtropical shelf waters (STSW)
and subantarctic shelf water (SASW) over the southern Brazilian and the
Argentine shelf, respectively. STSW and SASW are separated by an oceanographic front (subtropical shelf front) at approximately 32°S. The front is
an extension of the Brazil/Malvinas confluence over the shelf and runs in a
north-south direction between the 50-m isobath and the shelfbreak (Piola
et al. 2000).
11.3 Fertilization Processes
Continental runoff generates a low density and stable surface water layer,
the estuarine plume (Fig. 11.2A), which extends for about 50 km (Patos
Lagoon; Hartmann et al. 1986) and 240 km (Rio de la Plata; Guerrero et al.
1997) offshore. The freshwater outflow influences shelf eutrophication via
nutrient input and vertical stratification (sigma-t gradients up to 2.5 m- 1 ;
Brandhorst and Castello 1971; Lima 1992). As a consequence, short-term
and seasonal changes in freshwater volume and composition cause variability of chemical parameters over the shelf. Phosphate concentration
(0.4-1.0 J..lM) is generally in the same range as in SAW and SACW,
while average total suspended matter (> 15 mg 1- 1 ) and silicate (>20 J..lM)
are highest in low density coastal water ( Carreto et al. 1986; Niencheski and
Fillmann 1997). Owing to in- or nearshore phytoplankton uptake (Abreu et
al. 1995; Ciotti et al. 1995), outflowing freshwater tends to be poor in nitrate
( <3.0 J..lM). Low N:P ratios (approx. 5:1) indicate that phytoplankton production is regulated by nitrogen flow, though nearshore bottom turbulence
149
intervals). Precipitation is more frequent and higher in winter. Extreme
rainfall or drought periods are associated with El Nino-southern oscillation phenomena (Seeliger et al. 1997).
The tides are amplified by SE onshore winds which increase southward
(1m at 25°S, 8 m at 45°S). Amphidromic conditions exist at 30-32°S, 41 os,
and 47°S where oscillation periods of shelf water approach those of oceanic tides. Annual freshwater runoff (mean 20,000-25,000 m 3 s- 1 ) from the
Rio de la Plata drainage basin (3,170,000 km 2 ; Guerrero et al. 1997) varies
considerably between seasons. During maximum runoff in fall/winter,
offshore winds divide the outflowing water into a smaller, southward flowing branch and, owing to the Coriolis effect, into a larger branch along the
Uruguayan coast. The northern branch mixes with freshwater runoff from
the Patos Lagoon (annual mean 700-3,000 m 3 s- 1 , maximum in winter/
spring) over the southern Brazilian shelf (Moller et al. 1991). Owing to the
dilution of modified SACW and SAW, the coastal waters ( CW) below the
low salinity surface layer are defined as subtropical shelf waters (STSW)
and subantarctic shelf water (SASW) over the southern Brazilian and the
Argentine shelf, respectively. STSW and SASW are separated by an oceanographic front (subtropical shelf front) at approximately 32°S. The front is
an extension of the Brazil/Malvinas confluence over the shelf and runs in a
north-south direction between the 50-m isobath and the shelfbreak (Piola
et al. 2000).
11.3 Fertilization Processes
Continental runoff generates a low density and stable surface water layer,
the estuarine plume (Fig. 11.2A), which extends for about 50 km (Patos
Lagoon; Hartmann et al. 1986) and 240 km (Rio de la Plata; Guerrero et al.
1997) offshore. The freshwater outflow influences shelf eutrophication via
nutrient input and vertical stratification (sigma-t gradients up to 2.5 m- 1 ;
Brandhorst and Castello 1971; Lima 1992). As a consequence, short-term
and seasonal changes in freshwater volume and composition cause variability of chemical parameters over the shelf. Phosphate concentration
(0.4-1.0 J..lM) is generally in the same range as in SAW and SACW,
while average total suspended matter (> 15 mg 1- 1 ) and silicate (>20 J..lM)
are highest in low density coastal water ( Carreto et al. 1986; Niencheski and
Fillmann 1997). Owing to in- or nearshore phytoplankton uptake (Abreu et
al. 1995; Ciotti et al. 1995), outflowing freshwater tends to be poor in nitrate
( <3.0 J..lM). Low N:P ratios (approx. 5:1) indicate that phytoplankton production is regulated by nitrogen flow, though nearshore bottom turbulence