30
Gilberto Rodriguez
The salinity of the lake at any time may be obtained by numerical integration according to the relation
C(t) == C(t=O) + L.:1C
This model was tested using the values of Carter's (1955) hydraulic balance and other.
The salinities obtained closely correspond with the measured salinities, notwithstanding the quality of the data and the simplifications of the model.
2.7
Chemistry and Fertility of the Water
2.7.1
Dissolved Oxygen
The dissolved oxygen was carefully sampled by Parra Pardi (1977) in five sections
throughout the Strait, during a study of the self-purification of the estuary. Very small
variations were observed in the tridimensional pattern thus obtained, with differences
rarely above 1.5 mIl-I. Vertically the water is frequently super-saturated on the surface due to local algal blooms, but the oxygen content drops regularly as low as 40%
saturation at 10 m, and 30% at 14 m, although on no occasion an anoxic condition was
observed. These values fluctuate in 24 hour cycles by as much as 1 mIl-I, but these fluctuations are not clearly correlated with the tidal records. Along the channel, the oxygen content decreases as a consequence of the discharge of sewage and industrial
wastes from populated areas, but more normal values were found 18 km north of these
areas.
The oxygen content in Tablazo Bay varies widely with the local conditions, but no
anoxic areas has been observed. In mangrove areas of this zone Rodriguez (1973) recorded 98.2% saturation.
Anoxic conditions were reported by Gessner (1953), Redfield et al. (1955), Redfield and Doe (1964), and Parra Pardi (1979) at the bottom of the lake, corresponding with the more saline water of the cone-shaped hypolimnium, when the water
is completely stratified (Fig. 2.3c). The epilimnium display only moderate variations
in the oxygen content. The mean of 6 different locations throughout the lake at
1 m depth, for the years 1982-1984, was 5.14 ml 1-1 (96% saturation), with extreme values of 3.5 and 6.3 ml 1-1 (Escam 1991). The lowest values over the cone were observed
in December, when the access of saline water is restricted, the base of the cone is reduced and the apex approaches the surface. When the process is completed, the volume of the hypolimnium is considerable smaller, and in some years the water is completely mixed, as was observed by Redfield and Doe (1964) for November 1955 and December 1956.
2.7.2
Nutrients
From the time that Gessner (1953) found in his analysis of the waters of the lake up to
1.3 mg-atoms 1-1 (40 mg m-3) of phosphate in the surface and 7.1 mg-atoms 1-1 (220 mg m-3)
Gilberto Rodriguez
The salinity of the lake at any time may be obtained by numerical integration according to the relation
C(t) == C(t=O) + L.:1C
This model was tested using the values of Carter's (1955) hydraulic balance and other.
The salinities obtained closely correspond with the measured salinities, notwithstanding the quality of the data and the simplifications of the model.
2.7
Chemistry and Fertility of the Water
2.7.1
Dissolved Oxygen
The dissolved oxygen was carefully sampled by Parra Pardi (1977) in five sections
throughout the Strait, during a study of the self-purification of the estuary. Very small
variations were observed in the tridimensional pattern thus obtained, with differences
rarely above 1.5 mIl-I. Vertically the water is frequently super-saturated on the surface due to local algal blooms, but the oxygen content drops regularly as low as 40%
saturation at 10 m, and 30% at 14 m, although on no occasion an anoxic condition was
observed. These values fluctuate in 24 hour cycles by as much as 1 mIl-I, but these fluctuations are not clearly correlated with the tidal records. Along the channel, the oxygen content decreases as a consequence of the discharge of sewage and industrial
wastes from populated areas, but more normal values were found 18 km north of these
areas.
The oxygen content in Tablazo Bay varies widely with the local conditions, but no
anoxic areas has been observed. In mangrove areas of this zone Rodriguez (1973) recorded 98.2% saturation.
Anoxic conditions were reported by Gessner (1953), Redfield et al. (1955), Redfield and Doe (1964), and Parra Pardi (1979) at the bottom of the lake, corresponding with the more saline water of the cone-shaped hypolimnium, when the water
is completely stratified (Fig. 2.3c). The epilimnium display only moderate variations
in the oxygen content. The mean of 6 different locations throughout the lake at
1 m depth, for the years 1982-1984, was 5.14 ml 1-1 (96% saturation), with extreme values of 3.5 and 6.3 ml 1-1 (Escam 1991). The lowest values over the cone were observed
in December, when the access of saline water is restricted, the base of the cone is reduced and the apex approaches the surface. When the process is completed, the volume of the hypolimnium is considerable smaller, and in some years the water is completely mixed, as was observed by Redfield and Doe (1964) for November 1955 and December 1956.
2.7.2
Nutrients
From the time that Gessner (1953) found in his analysis of the waters of the lake up to
1.3 mg-atoms 1-1 (40 mg m-3) of phosphate in the surface and 7.1 mg-atoms 1-1 (220 mg m-3)