188
H. Mianzan et al.
(Framiiian and Brown 1996). At the deeper and open Uruguayan coast, the
proximity of the drainage channel causes a stratified and dynamic regime,
leading to a highly variable front. In contrast, the front is relatively homogeneous in the semi-enclosed and shallow Samboromb6n Bay, which is
distant from the main drainage channels (Lasta 1995; Guerrero et al.1997a).
Beyond the Barra del Indio shoal, the estuary discharges waters through the
Oriental Channel and the gently sloping Maritime Channel (Fig. 13.1),
though highly stratified salt wedge conditions may occur in one channel,
while the other is not or only partially stratified coupled with the two way
discharging pattern (Guerrero et al. 1997a). Further offshore, the partially
stratified regime is characterized by an increasingly thicker (7 -8 m) upper
layer. The boundary between the partially stratified regime and the open
ocean defines the surface salinity front (Fig.l3.2).
The budget between outflowing estuarine surface layer waters
(82,000 m 3 s- 1 ) and inflowing shelf waters (60,000 m 3 s- 1 ) accounts for weak
estuarine circulation and a residence time of 46.6 days (Guerrero, unpubl.
data). The dynamics of upper layer water, discharging over the continental
shelf, are mainly driven by wind stress (Guerrero et al. 1997a). During fall
and winter the diluted surface waters from the Rio de la Plata flow in an
NNE direction along the Uruguayan coast in response to the Coriolis force,
extending as far north as 28°S (Piola et al. 1999). Dominant onshore winds
force surface waters in a southward direction along the Argentine coast in
spring and summer. Continental shelf water, which intrudes upriver below
the diluted upper layer, remains unaltered under most wind conditions,
leading to an arrested position of the salt wedge regime over a long-time
scale. A disruption of water column stratification and mixing of the salt
wedge occurs only after several hours of strong onshore winds ( > 11 m s- 1 ),
though the salt wedge at the head of the estuary becomes reestablished
within 48-72 h. The dynamics of the Rio de la Plata estuary is controlled by
tide- and wind-driven waves and continental runoff but modified by topography and Coriolis force. The equilibrium between these forces is highly
variable, depending largely on the intensity of wind stress and freshwater
discharge (Guerrero et al. 1997a).
13.3 Biotic Components
Our treatment of floral and faunal assemblages is focussed at the key biotic components of the salt wedge regime and adjacent areas (Figure 2).
Broad descriptions on the whole system may be found in Cousseau ( 1985);
Boschi (1988); CARP (1989); and Wells and Daborn (1997).
H. Mianzan et al.
(Framiiian and Brown 1996). At the deeper and open Uruguayan coast, the
proximity of the drainage channel causes a stratified and dynamic regime,
leading to a highly variable front. In contrast, the front is relatively homogeneous in the semi-enclosed and shallow Samboromb6n Bay, which is
distant from the main drainage channels (Lasta 1995; Guerrero et al.1997a).
Beyond the Barra del Indio shoal, the estuary discharges waters through the
Oriental Channel and the gently sloping Maritime Channel (Fig. 13.1),
though highly stratified salt wedge conditions may occur in one channel,
while the other is not or only partially stratified coupled with the two way
discharging pattern (Guerrero et al. 1997a). Further offshore, the partially
stratified regime is characterized by an increasingly thicker (7 -8 m) upper
layer. The boundary between the partially stratified regime and the open
ocean defines the surface salinity front (Fig.l3.2).
The budget between outflowing estuarine surface layer waters
(82,000 m 3 s- 1 ) and inflowing shelf waters (60,000 m 3 s- 1 ) accounts for weak
estuarine circulation and a residence time of 46.6 days (Guerrero, unpubl.
data). The dynamics of upper layer water, discharging over the continental
shelf, are mainly driven by wind stress (Guerrero et al. 1997a). During fall
and winter the diluted surface waters from the Rio de la Plata flow in an
NNE direction along the Uruguayan coast in response to the Coriolis force,
extending as far north as 28°S (Piola et al. 1999). Dominant onshore winds
force surface waters in a southward direction along the Argentine coast in
spring and summer. Continental shelf water, which intrudes upriver below
the diluted upper layer, remains unaltered under most wind conditions,
leading to an arrested position of the salt wedge regime over a long-time
scale. A disruption of water column stratification and mixing of the salt
wedge occurs only after several hours of strong onshore winds ( > 11 m s- 1 ),
though the salt wedge at the head of the estuary becomes reestablished
within 48-72 h. The dynamics of the Rio de la Plata estuary is controlled by
tide- and wind-driven waves and continental runoff but modified by topography and Coriolis force. The equilibrium between these forces is highly
variable, depending largely on the intensity of wind stress and freshwater
discharge (Guerrero et al. 1997a).
13.3 Biotic Components
Our treatment of floral and faunal assemblages is focussed at the key biotic components of the salt wedge regime and adjacent areas (Figure 2).
Broad descriptions on the whole system may be found in Cousseau ( 1985);
Boschi (1988); CARP (1989); and Wells and Daborn (1997).