The Subtropical Estuarine Complex of Paranagua Bay, Brazil
133
The mean precipitation during the rainy season is more than three times
higher than that of the dry season.
10.2.1 Geomorphologic Processes
Physiographic structures of Paranagmi Bay are geologically ephemeral
and depend on interactions between the upland freshwater drainage
system and tidal marine conditions. The bay can be divided into a mouth
zone, an estuary funnel, and a meander zone (Lessa et al. 1998; Fig. 10.1).
The mouth zone corresponds to the lower part of the estuary, which is surrounded by beach ridge plains. Islands in the mouth zone (i.e., Cotinga
Island) emerged from shallow estuarine areas (Angulo 1992). The main
channel has a depth of more than 10m (max. 33m) and the bottom sediments are characterized by well-sorted fine to very fine sand, with fines
(grain size <0.062 mm) increasing from sea (0 %) towards land ( 40 %).
The estuary funnel represents the widest cross-section in the estuary with
depths (maxima 15m near Paranagua harbor) varying between 4 and 10m
and decreasing upstream. Most of the bottom in the funnel zone is characterized by muddy sediments (<50% sand; Bigarella et al. 1978). In the
upper funnel zone non-vegetated elongated low-tide sandbars represent a
bay-head delta typical of tide-dominated environments. The upper 5 km of
the estuary with shallow channels ( <4 m) and strong fluvial influence
represent the meander zone. The bottom sediments are characterized by
poorly sorted fine to medium sand ( <40% fines), though grain sizes above
2 mm are common. Several low-vegetated non-tidal islands occur near the
mouth of small rivers in the lower meander zone. A larger influx of fluvial
sediments into the Paranagua Bay estuary during the last decades may
have caused an expansion of tidal sandbars and the fluvial sand deposits at
the head of the estuary (Fig. 10.2).
10.2.2 Physical Characteristics
The Paranagua Bay estuarine complex has been classified as a partially
mixed estuary (type B), with lateral heterogeneity (Knoppers et al. 1987), a
mean depth of 5.4m, a total water volume of 1,410 9 m 3 and a residence time
of 3.49 days (FUNPAR 1997). As circulation patterns and stratification vary
between seasons, mean salinity and water temperature in summer and in
winter are 12-29 and 23-30°C and 20-34 and 18-25°C, respectively. The
structure and function of the estuarine system is influenced by different
gradients. A salinity-energy gradient from freshwater to marine conditions
133
The mean precipitation during the rainy season is more than three times
higher than that of the dry season.
10.2.1 Geomorphologic Processes
Physiographic structures of Paranagmi Bay are geologically ephemeral
and depend on interactions between the upland freshwater drainage
system and tidal marine conditions. The bay can be divided into a mouth
zone, an estuary funnel, and a meander zone (Lessa et al. 1998; Fig. 10.1).
The mouth zone corresponds to the lower part of the estuary, which is surrounded by beach ridge plains. Islands in the mouth zone (i.e., Cotinga
Island) emerged from shallow estuarine areas (Angulo 1992). The main
channel has a depth of more than 10m (max. 33m) and the bottom sediments are characterized by well-sorted fine to very fine sand, with fines
(grain size <0.062 mm) increasing from sea (0 %) towards land ( 40 %).
The estuary funnel represents the widest cross-section in the estuary with
depths (maxima 15m near Paranagua harbor) varying between 4 and 10m
and decreasing upstream. Most of the bottom in the funnel zone is characterized by muddy sediments (<50% sand; Bigarella et al. 1978). In the
upper funnel zone non-vegetated elongated low-tide sandbars represent a
bay-head delta typical of tide-dominated environments. The upper 5 km of
the estuary with shallow channels ( <4 m) and strong fluvial influence
represent the meander zone. The bottom sediments are characterized by
poorly sorted fine to medium sand ( <40% fines), though grain sizes above
2 mm are common. Several low-vegetated non-tidal islands occur near the
mouth of small rivers in the lower meander zone. A larger influx of fluvial
sediments into the Paranagua Bay estuary during the last decades may
have caused an expansion of tidal sandbars and the fluvial sand deposits at
the head of the estuary (Fig. 10.2).
10.2.2 Physical Characteristics
The Paranagua Bay estuarine complex has been classified as a partially
mixed estuary (type B), with lateral heterogeneity (Knoppers et al. 1987), a
mean depth of 5.4m, a total water volume of 1,410 9 m 3 and a residence time
of 3.49 days (FUNPAR 1997). As circulation patterns and stratification vary
between seasons, mean salinity and water temperature in summer and in
winter are 12-29 and 23-30°C and 20-34 and 18-25°C, respectively. The
structure and function of the estuarine system is influenced by different
gradients. A salinity-energy gradient from freshwater to marine conditions