CHAPTER 2 • The Maracaibo System: A Physical Profile
31
at 25 m, all authors that have studied the nutrients content in the Maracaibo system
have reported similar high concentrations of phosphorus and similar increments with
depth. Redfield and Doe (1964) recorded values of about 1-4 mg-atoms 1-1 of inorganic
phosphorous in the epilimnium, and more than 7 mg-atoms 1-1 in the hypolimnium.
Variations throughout the year are considerable. In a study of phosphate in the saline
cone, Parra Pardi (1979) found the lowest values, 1.0 mg-atom 1-1 for the surface and
4.0 mg-atoms 1-1 for the bottom, in September, and the highest, 1.5 mg-atom 1-1 for the
surface and 15.0 mg-atoms 1-1 for the bottom, in October. Battelle Memorial Institute
(1974) reported a peak value for the bottom at 31 m of 151 mg-atoms 1-1 (4.9 mg 1-1) in
January 1973, when the surface water had 2.4 mg-atoms 1-1 (0.05 mg 1-1). The maximum
value registered during 1982-1984, at 1 m below the surface, from six different locations throughout the lake, was 10.32 mg-atoms 1-1 (Escam 1991).
Since the content of phosphorus in the Gulf of Venezuela at the surface is only
0.2-0.4 mg-atoms 1-1, and 1.0 mg-atom 1-1 at the bottom (Redfield 1955), the phosphate
in the lake must come necessarily from terrestrial sources. In this regard, Hobson (1979)
found that in the rivers of the southwestern part of the Lake, phosphate concentrations in the "white water" of the Santa Ana River ranged from 0.93-2.17 mg-atoms 1-1, and
averaged 1.5 mg-atom 1-1 and the "black water" of the Concepci6n River (with a high concentration of lignin), ranged from 0.93-2.48 mg-atoms 1-" and averaged 1.5 mg-atom 1-1;
in the Lake itself the concentration ranged from 1.86-6.20 mg-atoms 1-1, and averaged
5.27 mg-atoms 1-1. One of the sources of phosphate are the mines located near the headwaters of the Catatumbo River and the recently discovered phosphate rocks which
cover a large extension in the cordillera south and south-west of the lake (Parra Pardi
1986). On the other hand the increment in phosphorous with depth is attributed to
the sinking of organisms from the surface (Redfield et al. 1955) and the slow outward
circulation of the cone-shaped hypolimnium. L6pez-Hermindez et al. (1980) have suggested that the retention of phosphorus by adsorption in the sediments of the lake
contribute to the control of phosphorus in the water.
The main sources of nitrogen compounds in the lake are the sewage and industrial
discharges in the vicinity of the city of Maracaibo. This nutrient-rich water is incorporated into the saline wedge that penetrates into the lake through the Strait. The ammonia in the lake follows the same pattern of distribution of the phosphorous once it
is incorporated to the cone-shaped epilimnium (Battelle Memorial Institute 1974). In
a typical distribution the ammonia does not exceed 6 mg-atoms 1-1 up to a depth of
10 m throughout the lake, but increases rapidly to reach more than 180 mg-atoms 1-1
at the base of the cone (Parra Pardi 1979).
Parra Pardi (1979) found the following values for the N total/P relationship: tributary rivers 1-5; sewage 4.5-7.5; industrial discharges 2-10. The mean value obtained for
the epilimnium was less than 5. The mean value in the hypolimnium has been estimated in 5, with occasional mean values of over 15. When Ntotall P < 5, the nitrogen is
considered as the limiting factor: this can be considered the condition for most of the
lake waters. The nitrogen balance in the lake has been established by Delft Hydraulics
and Haskoning (1991) as described in Table 2.2.
The primary production of the system is high (Battelle Memorial Institute 1974;
Rodriguez and Conde 1989), but its distribution is asymmetrical in the lake, with the
eastern side considerable more productive. This is particularly true for a segment of
coastal waters between 9° 40'N and 10 0 30'N where the largest concentration of oil pro-
31
at 25 m, all authors that have studied the nutrients content in the Maracaibo system
have reported similar high concentrations of phosphorus and similar increments with
depth. Redfield and Doe (1964) recorded values of about 1-4 mg-atoms 1-1 of inorganic
phosphorous in the epilimnium, and more than 7 mg-atoms 1-1 in the hypolimnium.
Variations throughout the year are considerable. In a study of phosphate in the saline
cone, Parra Pardi (1979) found the lowest values, 1.0 mg-atom 1-1 for the surface and
4.0 mg-atoms 1-1 for the bottom, in September, and the highest, 1.5 mg-atom 1-1 for the
surface and 15.0 mg-atoms 1-1 for the bottom, in October. Battelle Memorial Institute
(1974) reported a peak value for the bottom at 31 m of 151 mg-atoms 1-1 (4.9 mg 1-1) in
January 1973, when the surface water had 2.4 mg-atoms 1-1 (0.05 mg 1-1). The maximum
value registered during 1982-1984, at 1 m below the surface, from six different locations throughout the lake, was 10.32 mg-atoms 1-1 (Escam 1991).
Since the content of phosphorus in the Gulf of Venezuela at the surface is only
0.2-0.4 mg-atoms 1-1, and 1.0 mg-atom 1-1 at the bottom (Redfield 1955), the phosphate
in the lake must come necessarily from terrestrial sources. In this regard, Hobson (1979)
found that in the rivers of the southwestern part of the Lake, phosphate concentrations in the "white water" of the Santa Ana River ranged from 0.93-2.17 mg-atoms 1-1, and
averaged 1.5 mg-atom 1-1 and the "black water" of the Concepci6n River (with a high concentration of lignin), ranged from 0.93-2.48 mg-atoms 1-" and averaged 1.5 mg-atom 1-1;
in the Lake itself the concentration ranged from 1.86-6.20 mg-atoms 1-1, and averaged
5.27 mg-atoms 1-1. One of the sources of phosphate are the mines located near the headwaters of the Catatumbo River and the recently discovered phosphate rocks which
cover a large extension in the cordillera south and south-west of the lake (Parra Pardi
1986). On the other hand the increment in phosphorous with depth is attributed to
the sinking of organisms from the surface (Redfield et al. 1955) and the slow outward
circulation of the cone-shaped hypolimnium. L6pez-Hermindez et al. (1980) have suggested that the retention of phosphorus by adsorption in the sediments of the lake
contribute to the control of phosphorus in the water.
The main sources of nitrogen compounds in the lake are the sewage and industrial
discharges in the vicinity of the city of Maracaibo. This nutrient-rich water is incorporated into the saline wedge that penetrates into the lake through the Strait. The ammonia in the lake follows the same pattern of distribution of the phosphorous once it
is incorporated to the cone-shaped epilimnium (Battelle Memorial Institute 1974). In
a typical distribution the ammonia does not exceed 6 mg-atoms 1-1 up to a depth of
10 m throughout the lake, but increases rapidly to reach more than 180 mg-atoms 1-1
at the base of the cone (Parra Pardi 1979).
Parra Pardi (1979) found the following values for the N total/P relationship: tributary rivers 1-5; sewage 4.5-7.5; industrial discharges 2-10. The mean value obtained for
the epilimnium was less than 5. The mean value in the hypolimnium has been estimated in 5, with occasional mean values of over 15. When Ntotall P < 5, the nitrogen is
considered as the limiting factor: this can be considered the condition for most of the
lake waters. The nitrogen balance in the lake has been established by Delft Hydraulics
and Haskoning (1991) as described in Table 2.2.
The primary production of the system is high (Battelle Memorial Institute 1974;
Rodriguez and Conde 1989), but its distribution is asymmetrical in the lake, with the
eastern side considerable more productive. This is particularly true for a segment of
coastal waters between 9° 40'N and 10 0 30'N where the largest concentration of oil pro-