86
Osmar O. Moller Jr .. Patrice Castaing
The Patos Lagoon is the only connection with the South Atlantic Ocean for a
hydrographical basin that covers approximately 200 000 km 2 (Fig. 5.1b). The river discharge is characterized by strong floods during late winter/early spring season and
low to moderate runoff during the rest of the year (Fig. 5.2). Large interannual variations are common (Paim and Moller Jr. 1986). The discharge of the Guaiba River, the
most important tributary, may vary from 41-22000 m3 S-l (Rochefort 1958) with an
annual mean around 1000 m3 S-l (Bordas et al. 1984). The Jacui-Taquari River system
contributes with 85% of this value. Cam aqua River is important only during flood
periods when it may attain 5 000 m3 S-l (Herz 1977). No data at all exists for San Gon<;alo
channel (Fig. 5.1C), which connects Patos and Mirim Lagoons.
The tides are mixed and mainly diurnal reflecting the proximity of an amphidromic
point for the M2 tidal component (Vassie 1982). The mean range within the inlet is
0.32 m (Herz 1977). Friction provokes a liner attenuation as the tidal wave progresses
landward (Moller Jr. 1996).
Two different geomorphological units can be distinguished along the lagoon: the
lower (southern) and inner (northern) areas. They are naturally separated by the sandy
banks located in the area around Ponta da Feitoria (Fig. 5.1C) where depths are around
1 m (Delaney 1965). The inner lagoon covers 90% of the total surface, has a mean depth
of 5 m and is characterized by cuspate spits that had their origin associated with the
wind wave field (Toldo Jr. 1994). The southern portion of the lagoon, also considered
as the estuarine region (Closs 1962; Closs and Madeira 1968; Castello 1985), is dominated by very shallow zones (mean depth is 1.7 m) that are cut by natural and man
made channels. The cross sectional area decreases exponentially from Ponta da Feitoria
(7 x 10 4 m2) towards the mouth (10 4 m2) (Moller Jr. 1996). The estuary region is marked
by the presence of several embayments.
The inner limit attained by salt water excursion is around Ponta da Feitoria.
Figure 5.2 presents an overview of surface salinity distribution in the estuarine area
based on data collected in different seasons: summer (Fig. 5.2a), autumn (Fig. 5.2b,c),
and spring (Fig. 5.2d), among those reported by Castello (1976a-d; 1977a,b; 1978a,b).
The strongest part of the longitudinal salinity gradient is always observed north of
the entrance channel. Lateral salinity gradients are observed in the area where the cross
sectional area increases. Mean annual salinity of the estuarine area is 13 psu (Castello
1985), with instantaneous values ranging from zero to 34 psu. Vertical salinity distribution varies from salt wedge type to a well mixed structure (Calliari et al. 1980; Moller
Jr. et al. 1991).
5.3
Data Sampling and Treatment
5.3.1
Longitudinal Surveys
The longitudinal distribution of salinity and temperature were obtained in 1988 during a series of 12 cruises carried out along the entire lagoon as part of a multidisciplinary program named the Patos Lagoon Project. The sampling strategy consisted of
18 stations (Fig. 5.1a) located along the main estuarine channel and the deeper longitudinal axis of the lagoon and the Guaiba River. Salinity and temperature were measured with
Osmar O. Moller Jr .. Patrice Castaing
The Patos Lagoon is the only connection with the South Atlantic Ocean for a
hydrographical basin that covers approximately 200 000 km 2 (Fig. 5.1b). The river discharge is characterized by strong floods during late winter/early spring season and
low to moderate runoff during the rest of the year (Fig. 5.2). Large interannual variations are common (Paim and Moller Jr. 1986). The discharge of the Guaiba River, the
most important tributary, may vary from 41-22000 m3 S-l (Rochefort 1958) with an
annual mean around 1000 m3 S-l (Bordas et al. 1984). The Jacui-Taquari River system
contributes with 85% of this value. Cam aqua River is important only during flood
periods when it may attain 5 000 m3 S-l (Herz 1977). No data at all exists for San Gon<;alo
channel (Fig. 5.1C), which connects Patos and Mirim Lagoons.
The tides are mixed and mainly diurnal reflecting the proximity of an amphidromic
point for the M2 tidal component (Vassie 1982). The mean range within the inlet is
0.32 m (Herz 1977). Friction provokes a liner attenuation as the tidal wave progresses
landward (Moller Jr. 1996).
Two different geomorphological units can be distinguished along the lagoon: the
lower (southern) and inner (northern) areas. They are naturally separated by the sandy
banks located in the area around Ponta da Feitoria (Fig. 5.1C) where depths are around
1 m (Delaney 1965). The inner lagoon covers 90% of the total surface, has a mean depth
of 5 m and is characterized by cuspate spits that had their origin associated with the
wind wave field (Toldo Jr. 1994). The southern portion of the lagoon, also considered
as the estuarine region (Closs 1962; Closs and Madeira 1968; Castello 1985), is dominated by very shallow zones (mean depth is 1.7 m) that are cut by natural and man
made channels. The cross sectional area decreases exponentially from Ponta da Feitoria
(7 x 10 4 m2) towards the mouth (10 4 m2) (Moller Jr. 1996). The estuary region is marked
by the presence of several embayments.
The inner limit attained by salt water excursion is around Ponta da Feitoria.
Figure 5.2 presents an overview of surface salinity distribution in the estuarine area
based on data collected in different seasons: summer (Fig. 5.2a), autumn (Fig. 5.2b,c),
and spring (Fig. 5.2d), among those reported by Castello (1976a-d; 1977a,b; 1978a,b).
The strongest part of the longitudinal salinity gradient is always observed north of
the entrance channel. Lateral salinity gradients are observed in the area where the cross
sectional area increases. Mean annual salinity of the estuarine area is 13 psu (Castello
1985), with instantaneous values ranging from zero to 34 psu. Vertical salinity distribution varies from salt wedge type to a well mixed structure (Calliari et al. 1980; Moller
Jr. et al. 1991).
5.3
Data Sampling and Treatment
5.3.1
Longitudinal Surveys
The longitudinal distribution of salinity and temperature were obtained in 1988 during a series of 12 cruises carried out along the entire lagoon as part of a multidisciplinary program named the Patos Lagoon Project. The sampling strategy consisted of
18 stations (Fig. 5.1a) located along the main estuarine channel and the deeper longitudinal axis of the lagoon and the Guaiba River. Salinity and temperature were measured with