Limnological studies on a shallow reservoir in western Venezuela (Tulé Reservoir)

Limnologica 31 (2001) 139-145 http://www.urbanfischer.de/j ournals/limno

LIMNOLOGICA © by Urban & Fischer Verlag

Departamento de Biologfa, Facultad Experimental de Ciencias, Universidad del Zulia, Maracaibo, Venezuela

Limnological Studies on a Shallow Reservoir in Western Venezuela (Tul6 Reservoir) RITAPAEZ, GILIARUIZ, ROMULOMARQUEZ,Luz MARINASOTO,MARYNESMONTIEL& CARLOSLOPEZ With 6 Figures and 2 Tables Key words: Limnology, neotropics, reservoir, nutrients, Venezuela, zooplankton, phytoplankton primary productivity

Abstract Limnological features of Tul6 Reservoir, a large and shallow reservoir in western Venezuela were studied from May 1992 to December 1993. This reservoir is polymictic, with a low water transparency. Levels of nitrogen were high and phytoplankton productivity was low. The phytoplankton productivity values may be explained by the low concentrations of orthophosphate during the larger part of the sampling period and the frequent mixing and resuspension of sediments. Heterotrophic bacteria were between 1651.67 and 4365.00 Colony-forming Units/ml. Species composition of the crustacean zooplankton community was similar to other neotropical eutrophic reservoirs.

Introduction Limnological characterization is a fundamental tool for designing of the conservation and management of lakes and reservoirs, However, except for Brazilian reservoirs (TUNDISI et al. 1993), the knowledge about neotropical reservoirs is generally very limited. In Venezuela, the building of artificial reservoirs has rapidly increased in recent decades (CRESSA et al. 1993), but our knowledge on these waterbodies has progressed very little since the preliminary data of LEWIS & WEIBEZAHN(1976). The number of Venezuelan reservoirs from which limnological informations are available is very low (SORIANO& CRESSA 1989; CRESSA & SENIOR 1990; GONZALEz et al. 1991; SOTO et al. 1994; INFANTE82; 1NFANTE 1994; VEGAS-VILARRUB~A 1995; SOTO & LOPEZ 1996; GONZALEZ & ORTAZ 1998; MOaALES & LOPEZ 1998; MARIN et al. 1999), representing less than 5 % of the total number of artificial reservoirs in the country (CmcssA et al. 1993). On the other hand, most available data are for large and deep reservoirs.

In this paper, we describe the limnological features of a large but shallow reservoir (Tul6 Reservoir) and their spatial and seasonal variations.

Study Site Tul6 Reservoir is a large (56.6 km 2) and shallow waterbody (mean depth 7.2 m and maximun depth 9.7 m), located at 3 4 m above sea level in western Venezuela (10°53'N 72°08'W) (Fig. 1). Construction was completed in 1971 and the impoundment is used to supply water to the main cities of the region for irrigation and for flood control. The main tributaries are the Cachirf River, Colorado Stream and the discharge channel from Socuy Reservoir. The bedrock of the drainage area is heterogeneous. The surrounding vegetation is represented by tropical semideciduous forest and zones with intense agricultural activities (SMITH 1985). The drainage area has a bimodal pattern of precipitation with maximum peaks in April-May and OctoberNovember (EsPINOZA 1992). Available limnological informations are restricted to phytoplankton species composition (YACUBSON 1980), a checklist of rotifer species (D~Az & CASTELLANO 1988), the microbiological quality of the waters (Rulz & MONTIEL DE MORALES 1996) and epizoans on crustacean zooplankton (LOPEZ et al. 1998).

Methods Samples were taken at monthly or at longer intervals in two sampling periods. During the first period, a total of 9 samplings were performed from May 1992 to April 1993. Vertical series of samples were 0075-9511/01/31/02-139 $ 15.00/0

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Samp,ing Channelfrom Stations S ° c u y t ~

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i 10° O0'N

Colorado ~fU v LaProvincia utre,,~

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Fig. 1. Location of Tul6 Reservoir and sampling stations.

taken each 2 m from surface to bottom using a Van Dorn sampler (21) only at Station A, which is located near the dam (Fig. 1). At this station, we made the following measurements: temperature was measured with a thermometer inside the sampler; dissolved oxygen was determined by Winkler method; pH and conductivity were measured with a portable sensor equipment and water transparency with a white Secchi disk, 20 cm in diameter. Measurements of alkalinity, dissolved silica, total phosphorus, total nitrogen and ammonium were performed according to standard methods (MACKERETHet al. 1979; A.RH.A. 1992; GOLTERMANet al. 1971). Orthophosphate, nitrite and nitrate were determined by ion chromatography. Phytoplankton productivity was measured by determining the changes in oxygen concentration in dark and clear bottles, incubated "in situ', during 4 hours throughout water column. Crustacean zooplankton was collected with a conical plankton net (65 jam mesh aperture) towed through the water column (filtered volume about 0.5 m3). Organisms were fixed with acidified Lugol's solution and preserved with formaldehyde solution (4%). Subsamples were obtained using a Folsom subsampler, and counting was made in Petri dishes. Benthos samples were collected with an Ekrnan dredge of 625 cm 2 are a, samples were sieved through a 0.125 mm mesh and were preserved with formaldehyde solution. During the second period of sampling, a total of 7 samplings was performed from June to December 1993. Only one surface sample was collected at each sampling date at all stations (A-E) (Fig. 1). Samples for heterotrophic bacteria were collected in a sterile bottle and were counted by the Pour Plate Method (A.RH.A. 1992). At each of the stations, temperature, pH, water transparency, conductivity, dissolved oxygen, nitrite, nitrate, and total phosphorus were measured according to the methods previously described. 140

Limnologica 31 (2001) 2

Results and Discussion Temperature, transparency and dissolved oxygen Water temperature ranged between 28 and 33 °C, with a mean value of 29.6 °C, throughout the column of water at Station A. S o m e mixing periods alternating with periods of thermal stratification (May, October 1992 and February 1993) were observed (Fig. 2). M a x i m u m difference between the surface and the bottom was 3 °C in October 1992. Tul6 Reservoir may be considered as a warm polymictic waterbody. This thermal pattern is c o m m o n in shallow neotropical reservoirs (TUNDISI et al. 1993; HENRY 1995). In Tul6 Reservoir frequent mixing events are related to the strong winds and absence of hills or mountains around the reservoir. Water transparency was between 0.40 and 0.96 m, with a mean value of 0.51 m at Station A. At this station, lower values occurred during mixing periods (Fig. 2). In the remaining stations (B-E), transparency was between 0.18 and 0.81 m and lower mean values were found near tributaries (Table 1). Transparency appears to depend primarily of resuspension of sediment from the bottom, and contribution from the catchment of suspended organic and inorganic particles, particularly during the rainy season. Dissolved oxygen fluctuated between 4.4 and 9.0 mg/1, with a mean value of 6.5 mg/1 in water column at Station A. Changes in vertical distribution (Fig. 3) were related to ther-

0g

0 1 2

E

3

V

r-

4

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a

Fig. 2. Temporal changes of transparency (Secchi disk) and vertical profiles of temperature (°C) at Station A.

5

M

J

J

A

O

J

F

1992

M

A

1993

01 2-

E

V

e(I)

1:1

34-

1 M

Fig. 3. Temporal variations of profiles of dissolved oxygen (rag/l) at Station A.

mal stratification events. On the surface, values of dissolved oxygen were comparable between stations (Table l). Calculated values of saturation for the water column at Station A mostly show a subsaturation with values between 57.9-119.7%, the mean value was 88.6%. On the bottom, oversaturation was found only during the mixing periods. Two factors can explain the oxygen deficit on the bottom during the thermal stratification periods: high rates of decomposition of organic matter and low contributions of oxygen from photosynthetic activities, as a consequence of low transparency.

J

7.

J

A

1992

0

J

F

M

A

1993

Conductivity, alkalinity, pH and dissolved silica Conductivity was between 40.9 and 191.5 pS/cm, with a mean value of 150.3 ~S/cm for water column at Station A (Table 1). During the thermal stratification periods, lower values were observed on the bottom. Alkalinity ranged between 36 and 94 rag/1 for bicarbonates, and 0 to 16 mg/1 for carbonates in Station A, while the pH values were between 6.8 and 9.6, with a mean value of 7.8 at the same station. Lower values were found on the bottom, Limnologica 31 (2001) 2

141

Table 1. Mean values of some limnological features of Tu16 Reservoir. Maximum and minimum values in parenthesis. (In station A, mean values were calculated from samples collected throughout the column of water between May 1992 and April 1993. In the remaining stations, mean values were calculated from surface samples collected between June and December 1993.) Parameter

A*

B**

C**

D**

E**

Depth (m)

6.8 (6-7.2)

2.1 (0.6-4)

1.9 (0.5-3.5)

3.3 (2-5.2)

4.0 (2.5-5.1)

Transparency (m)

0.51 (0.40-0.96)

0.33 (0.21-0.47)

0.42 (0.14-0.52)

0.62 (0.45-0.61)

0.67 (0.46-0.81)

Dissolved oxygen (mg/1)

6.5 (4.4-9.4)

6.9 (3-7.6)

6.5 (4.4-9.7)

6.7 (6-7.2)

6.3 (6.1-7.2)

Conductivity (pS/cm)

150.3 (40.9-191.5)

.

.

.

.

Total alkalinity (mg CaCO3/1)

70 (36-94)

.

.

.

.

pH

7.8 (6.8-9.8)

8.1 (7.7-9.8)

8.2 (7.8-8.9)

8.4 (7.7-8.4)

8.3 (7.8-8.9)

N = number of samples. *Depth and transparency (N = 9). Mean values of remaining features were calculated from vertical samples (N = 44). ** N -- 7. Table 2. Mean values of nutrients and heterotrophic bacteria. Maximum and minimum values in parenthesis. (In station A, excepting heterotrophic bacteria, mean values were calculated from samples collected throughout the column of water between May 1992 and April 1993. In remaining stations, mean values were calculated from surface samples collected between June and December 1993.) Parameter

A*

B**

Dissolved silica (rag/l)

5.2 (3.3-10.6)

.

Nitrite (pg/1)

35.7 (0-300)

30.8 (8-60)

Nitrate (~ag/1)

45.0 (2-154)

28.2 (2-50)

Ammonium (pg/1)

280 (43-1030)

.

.

.

.

Total nitrogen (,ug/1)

670 (0-1680)

.

.

.

.

Orthophosphate (gg/1)

27 (4.0-150)

108 (10-308)

Total phosphorus (pg/1)

120 (30-381)

.

Heterotrophic bacteria 2870.42 (Colony-forming units/ml) (998-9100)

C** .

D**

E**

39.8 (14-72)

30.0 (12-40)

32.7 (9-66)

21.5 (4-49)

33.1 (2-61)

18.1 (2-38)

8 (1-20)

9 (1-20)

1676.67 (1100-2890)

1651.67 (567-7464)

.

.

338 (10-609) .

4365.00 (810-8289)

. 3688.75 (678-8387)

.

N = number fo samples. * Mean values of heterotrophic bacteria were caIculated from surface samples (N = 7). In remaining features (N = 44). ** N = 7.

during the thermal stratification periods. On the surface samples, pH was about 8.1 and 8.4 (Table 1). Dissolved silica was present in the range of 3.3-10.6 mg/1, with a mean value of 5.2 mg/1 at Station A (Table 2).

Phosphorus and nitrogen Orthophosphate was detected in May, July and August, 1992 and January, 1993, when the concentration was between 142

Limnologica 31 (2001) 2

4.0 and 150 ~g/l throughout the water column at Station A. The mean value was 27 pg/1 (Table 2). On surface samples from other stations, mean values were found between 9 and 338 ~tg/1, higher values were found near to tributaries at stations B and C (Table 2). Total phosphorus was between 30 and 381 pg/1, with a mean value of 120 pg/1 in Station A (Table 2). Nitrate concentration fluctuated between 2 and 154 pg/1, with a mean value of 45 pg/1 throughout the water column at

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J

J

l O

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1993

N

D

Fig. 4. Temporal fluctuations of heterotrophic bacteria (Colonyforming units/ml) in sampled stations.

350'

[ ] Gross Productivity [ ] Net Productivity

300' el 250

E 20o 150

E 100 50"

M

J

J

A

0

g

J

1992

M A

1993

Fig. 5. Temporal variations of phytoplankton productivity (mg C/m3h) at Station A.

Station A (Table 2). Maximum values were found in the period of thermal stratification. In surface samples, mean values were in the range of 18.1-33.1 ~g/1 (Table 2). Nitrite was between undetected values and 300 pg/1, with a mean value of 35.7 ~ag/1in Station A. In the other stations mean values were in the range of 30.0-39.8 pg/1 (Table 2). Ammonium was between 43 and 1030 ~g/1, with a mean value of 280 lag/1 in Station A (Table 2). At the same station, total nitrogen was between 3.0 and 1680 gg/1, with a mean value of 670 ~tg/1 (Table 2). These levels of nitrogen nutrients are higher in comparison to the values reported from other neotropical reservoirs (CRESSA& SENIOR 1990; TUND~S~et al. 1993) and show primarily the effects of inflows of nutrient-rich hypolimnetic waters from Socuy Reservoir into Tul6 Reservoir through the discharge channel (SoTO et al. 1994) and the effects of agricultural activities in the catchment area. In lakes and ponds, ratio N:P has been used sucessfully to establish limiting nutrient. VOLLENWEIDER(1968) proposed a ratio of 9:1 (N:TP) for phytoplankton. Waterbodies with a ratio higher than 9:1 are potentially limited for phosphorus. When ratio is lower than 9:1, the waterbody is limited for nitrogen. In Tul6 Reservoir, mean ratio N:P is 12.8:1, according to values suggested by VOLLENWEIDER(1968), phosphorus can be the limiting nutrient in this system. Similar situations have been reported for other neotropical reservoirs (GIANESELLAGALVAO 1985; CRESSA& SENIORS 1990; TUNDISIet al. 1993; MARIN et al. 1999). However, some problems can emerge, when models relating directly phytoplankton productivity with nutrients availability are applied to reservoirs. According to LrND et al. (1993), such abiotic factors as turbidity and short water-residence time can be of greater importance for algal production in reservoirs than the availabiliy of nutrients. Particularly, in shallow and polymictic reservoirs, where usually the mixing depth exceeds the depth of the euphotic zone, low light availability limits phytoplankton production ( K I M M E L & GROEGER 1984; KIMMELet al. 1990).

100

Heterotrophic bacteria L. 80 I1)

ma

L

Q. 60 ..Q

E 40 z 20

Mean values of heterotrophic bacteria were found to be between 1651.67 and 4365.00 Colony-forming units/ml. The highest values were found in stations B and C, which are the nearest to tributaries (Table 2). Maximum values were observed during the rainy season (Fig. 4). This pattern of spatial and temporal changes also shows the effects of the contributions of organic matter from the tributaries and run-off waters.

Phytoplankton productivity M

J

J 1992

A

O

J

F

M

A

1993

Fig. 6. Temporal fluctuations of crustacean zooplankton at Station A.

The mean value of gross productivity throughout water column at Station A was 131.66 mg C/m 3 • h and net productivity was 38.15 mg C/m 3 • h. Higher values were found near the surface and decreased irregularly to the bottom. Seasonally, a Limnologica 31 (2001) 2

143

maximum peak in gross productivity was observed in May 1992 and two minor peaks in October, 1992 and March, 1993 (Fig. 5). The peak in May 1992 corresponded to the highest values of water transparency (Fig. 2). Productivity values of phytoplankton at Station A are lower with respect to other neotropical reservoirs (SoPdANO & CRESSA 1989; TUNDISI et al. 1993). This situation may be explained by the low concentrations of orthophosphate during the bigger part of the sampling period and the frequent mixing and resuspension of sediments. Also, mixing and resuspension of sediments can explain the significant difference between values of gross and net phytoplankton productivity in Tul6 Reservoir. In turbid and well mixed reservoirs, planktonic algae are exposed not only to rapidly fluctuating light but intermittently to aphotic conditions (KIMMELet al. 1990).

Zooplankton and benthos The crustacean zooplankton community at Station A was composed of one calanoid copepod (Notodiaptomus rnaracaibensis KIEFER), tWO cyclopoid copepods (Thermocyclops decipiens KIEFER, Mesocyclops brasilianus KIEFER) and four cladoceran species (Ceriodaphnia cornuta SARS; Bosmina tubicen BREHM; Moina sp.; Diaphanosoma spp.). Apart from Moina sp., which was extremely rare, all crustacean taxa were present throughout the sampling period. Nauplii and cyclopoid copepods were dominant taxa, except for the period March-April 1993, when B. tubicen showed maximum abundance and was the dominant taxon (Fig. 6). Dominance of cyclopoid copepods, especially T. decipiens, and small cladoceran species, is a typical feature of crustacean zooplankton communities in eutrophic reservoirs in South America (ARCIFA 1984; ARCIFA et al. 1992; L6PEZ & BELLO 1993). Abundance of crustacean zooplankton at station A fluctuated between 24.9 and 108.5 ind/1. Maximum values were found between May and July during the rainy season (Fig. 6). Range of body size in crustacean zooplankton was between 0.35-1.2 mm. According to ARCIFA(1984) and ARC~FAet al. (1992), this range of body size is small in comparison with values reported to temperate lakes. Two epizoan taxa, Synedra sp. (a pennate diatom) and Epistylis sp. (a ciliate protozoa), were found on exoskeleton of cyclopoid copepods and cladoceran species. Maximum prevalence of these epibionts on crustaceans was found in the early dry season (L6PEZ et al. 1998). The benthic community was very poor, only Chaoborus species larvae were collected at Station A. In contrast with Socuy Reservoir (LOPEZ & CRESSA 1996), which is connected with Tul6 Reservoir through the discharge channel, in Tul6 Reservoir Chaoborus species larvae showed low population densities and were found only in the benthos. Species composition and abundance of fish communitites are poorly studied. However the presence of planktivorous fishes such Astyanax spp. and Xenomelaniris spp. is known in this reservoir. 144

Limnologica 31 (2001) 2

Acknowledgement: We express our thanks to staff of Laboratorio de Qufmica Ambiental in Facultad Experimental de Ciencias of Universidad del Zulia, and to Instituto para la Conservaci6n de Lago de Maracaibo for some chemical analyses, and J.J. EWALDfor revision of English grammar. Special thanks to Consejo de Desarrollo Cienfffico y Human/stico de la Universidad del Zulia for financial support. Corrections and suggestions from anonymous referees improved greatly the manuscript.

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Received: February 16, 2000 Accepted: August 17, 2000

Correspondence address: Prof. CARLOSLOPEZ, Departamento de Biologia, Facultad Experimental de Ciencias, Universidad del Zulia, Apdo. 15299, Zona Postal 4005-A, Las Delicias, Maracaibo, Venezuela; e-mail: [email protected]

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