28
Gilberto Rodriguez
is responsible for 56.9% of the freshwater that enters the lake (Escam 1991). The salinity, which at the mouth of this river is 0.02 increases rapidly with the distance
(Gessner 1956):
Distance from the mouth (km) 0.0
Salinity (S)
0.33
2·4
4·8
7·2
0.62
1.03
1.33
However, the low salinity water is limited to a thin layer «1 m) of the effluent plume
which overrides lake water for a distance of 1 km. Below this layer the salinity increases
sharply to uniform values in the range of 2-3 (Hyne et al. 1979).
Towards the end of the dry season, brackish water penetrate through the Strait to
the deeper portions of the lake. This water contribute to the sharp vertical stratification of the lake which forms a cone-shaped hypolimnium located towards th~ center
of the lake, surrounded by lower density water. The water of the epilimnium moves
counter-clockwise around this limnetic cone like a large-scale vortex, with velocities up to 0.55 m S-I. This cyclonic circulation moves water by erosion from the hypolimnium to the epilimnium, where it becomes completely mixed. The volume of the
cone, which can reach 1,4 of the total volume of the lake is variable, expanding and contracting in a seasonal pattern, according to the variable inputs of brackish water from
the Strait or freshwater from the rivers. The saline cone is a permanent feature of
the lake and rarely it has been observed to disappear (Redfield and Doe 1964; Parra
Pardi 1983)
The history of salinities in the lake is particularly interesting in order to determine
whether the navigation channel which was dredged to 14 m in 1953, has produced a
salinization of the lake (Rodriguez 1984). The extant records, available since 1937, show
that since this year up to 1959 the mean salinity was 1.2 with extreme values of 0.7
and 2.0. Since 1959 and up to 1990 the mean salinity experienced a modest but consistent increase to a mean of 3.1 with extreme values of 2.3 and 4.2 (Parra Pardi 1986;
Escam 1991).
Superimposed to the long range increment in salinities, there are also fluctuations
in the yearly values, with peak salinities in the years 1939, 1964, 1974, 1979, and 1983.
Rainfall records for 35 years (1911-1946) in the city of Maracaibo (Rodriguez 1973),
evaluated through periodogram analysis, a method devised to detect rhythmicities
in temporal series, show that maxima of precipitation occur with a periodicity of
6 years. A similar result was found in the analysis of the discharge of the Limon River,
on the west coast of the lake. It is possible that the fluctuations of salinities in the lake
are the result of the periodicities of rainfall over the basin, distorted by the inertia in
the large body of water of the lake.
2.6.2
Salt Intrusion
In partially mixed estuaries, like the Maracaibo Estuary, the tidal movements produce
a cyclic oscillation and a periodic mixing which result in vertical and longitudinal
gradients of salinity. This intrusion, together with the tides, seems to be one of the
main factors in the silting of the navigation channel and for this reason it was studied
by several authors (Partheniades 1966; Partheniades et al. 1966; Harleman et al. 1966,
Gilberto Rodriguez
is responsible for 56.9% of the freshwater that enters the lake (Escam 1991). The salinity, which at the mouth of this river is 0.02 increases rapidly with the distance
(Gessner 1956):
Distance from the mouth (km) 0.0
Salinity (S)
0.33
2·4
4·8
7·2
0.62
1.03
1.33
However, the low salinity water is limited to a thin layer «1 m) of the effluent plume
which overrides lake water for a distance of 1 km. Below this layer the salinity increases
sharply to uniform values in the range of 2-3 (Hyne et al. 1979).
Towards the end of the dry season, brackish water penetrate through the Strait to
the deeper portions of the lake. This water contribute to the sharp vertical stratification of the lake which forms a cone-shaped hypolimnium located towards th~ center
of the lake, surrounded by lower density water. The water of the epilimnium moves
counter-clockwise around this limnetic cone like a large-scale vortex, with velocities up to 0.55 m S-I. This cyclonic circulation moves water by erosion from the hypolimnium to the epilimnium, where it becomes completely mixed. The volume of the
cone, which can reach 1,4 of the total volume of the lake is variable, expanding and contracting in a seasonal pattern, according to the variable inputs of brackish water from
the Strait or freshwater from the rivers. The saline cone is a permanent feature of
the lake and rarely it has been observed to disappear (Redfield and Doe 1964; Parra
Pardi 1983)
The history of salinities in the lake is particularly interesting in order to determine
whether the navigation channel which was dredged to 14 m in 1953, has produced a
salinization of the lake (Rodriguez 1984). The extant records, available since 1937, show
that since this year up to 1959 the mean salinity was 1.2 with extreme values of 0.7
and 2.0. Since 1959 and up to 1990 the mean salinity experienced a modest but consistent increase to a mean of 3.1 with extreme values of 2.3 and 4.2 (Parra Pardi 1986;
Escam 1991).
Superimposed to the long range increment in salinities, there are also fluctuations
in the yearly values, with peak salinities in the years 1939, 1964, 1974, 1979, and 1983.
Rainfall records for 35 years (1911-1946) in the city of Maracaibo (Rodriguez 1973),
evaluated through periodogram analysis, a method devised to detect rhythmicities
in temporal series, show that maxima of precipitation occur with a periodicity of
6 years. A similar result was found in the analysis of the discharge of the Limon River,
on the west coast of the lake. It is possible that the fluctuations of salinities in the lake
are the result of the periodicities of rainfall over the basin, distorted by the inertia in
the large body of water of the lake.
2.6.2
Salt Intrusion
In partially mixed estuaries, like the Maracaibo Estuary, the tidal movements produce
a cyclic oscillation and a periodic mixing which result in vertical and longitudinal
gradients of salinity. This intrusion, together with the tides, seems to be one of the
main factors in the silting of the navigation channel and for this reason it was studied
by several authors (Partheniades 1966; Partheniades et al. 1966; Harleman et al. 1966,