Seasonal variation in phytoplankton Seasonal variations in ... - Unn

Lakes & Reservoirs: Research and Management 2006 11: 63–72

Blackwell Publishing Asia Seasonal variation in phytoplankton

Seasonal variations in phytoplankton populations in Ogelube Lake, a small natural West African Lake

Original Article

Nkechinyere O. Nweze Department of Botany, University of Nigeria, Nsukka, Nigeria

Abstract Water samples were collected on a monthly basis at three locations along the north–south axis of the lake at 1 m depth intervals for 1 year. A sedimentation technique was used for microscopic examination of the samples. The monthly mean and seasonal phytoplankton densities were calculated. The original data were square-root transformed, and analysis of variance performed with SPSS version 10.0. The significant treatment effects were determined using Fisher’s least significance difference at the 5% probability level. The mean phytoplankton population per millilitre was very high for the entire sampling period. There was a phytoplankton bloom formation in the early rainy season, with the mean total phytoplankton population density for the rainy season (April–October) being significantly higher than that of for the dry season (November–March). A population increase was observed during the late part of the dry season to the early part of the rainy season. During the rainy season, Chlorophyceae (mostly desmids) were most abundant, followed by Cyanobacteria, Bacillariophyceae, Euglenophyceae, Dinophyceae, Cryptophyceae, Chrysophyceae and Xanthophyceae, in decreasing order of abundance. This order changed slightly in the dry season, when there was relative abundance of Bacillariophyceae over Cyanophyceae, and Dinophyceae over Euglenophyceae. The low population of Euglenophyceae indicates that the organic pollution is still low, with the predominance of desmids, indicating an oligotrophic lake condition.

Key words desmids, Ogelube lake, oligotrophy, phytoplankton, seasonal variation.

INTRODUCTION Ogelube Lake is one of the four small natural lakes in the Opi area of Enugu State, Nigeria. This area is the focus of recent limnological and ecological investigations because of the ecological implications of a new highway being constructed in this area (Biswas 1984; Evurunobi 1984; Hare 1986; Biswas & Nweze 1990). The construction work has created some ecological disturbances, including intense human interference through excavation of river sand by building contractors, and erosion from the road under construction. The erosion problem has resulted in suspension of the road construction, submergence of some farmland and partial silting of one of the lakes. The lake is in the derived savanna region, located between 6° 44′ 50′′ to 6° 44′ 58′′N and 7° 29′ 37′′ to 7° 29′ 41′′E, at an elevation of ≈ 244 m a. s. l. (Nweze 2003). The area has two seasons – the rainy season (April–October) and

Corresponding author. Email: [email protected] Accepted for publication 13 April 2006.

Doi: 10.1111/j.1440-1770.2006.00292.x

the dry season (November–March), with the cold, dry harmattan period dominated by the north-east trade winds that carry dust from the Sahara Desert from December to January. Hare (1986) estimated the dimensions of the lake at high and low water levels as 2.0 and 1.3 ha, respectively; mean depth (z) of 2.17 m and 1.27 m, respectively; and maximum depth (z max.) of 3.95 m and 2.40 m, respectively (Fig. 1). The lake has no permanent inflowing or outflowing streams, but there is an overflow channel at its southern end. The sources of water inputs are seasonal, usually being direct precipitation in the form of rainfall, inflows from run-off from surrounding hills and areas, and possibly water replacement from a water table below the lake, which accounts for the permanence of the water. There is a dense forest consisting of oil palm trees, ferns and other trees on the northern, north-western and eastern sides of the lake. Trees such as Lophira lanceolata Van Tiegh. ex Keay., Parkia clappertoniana Keay. and Elaies guineensis Jacq.; herbs such as Aspilia africana (Pers.) C.D. Adams and Tridax procumbens Linn.; and grasses © 2006 Blackwell Publishing Asia Pty Ltd

64

N. O. Nweze

such as Digitaria gayana (Kunth), Staff ex A. Chev., Eragrostis aegyptica (Willd.) Delile, Laudetia arundinacea (Hochst. ex A. Rich.) Steud., and Pennisetum polystachion (Linn.) Schult, are some of the plants growing luxuriantly around the lake. The south-western side of the lake is farmed occasionally, while local children hunting rodents burn the eastern side during most dry seasons. Floating aquatic flowering plants were absent in the open water at

the sampling points, but Nymphaea micrantha Guill. and Per. (water lily) grows abundantly at the lake’s northern end. As reported by Hoque and Ezepue (1977), the soil around the lake is enriched with unconsolidated quartz arsenites and friable sand that is typically white in colour, but is sometimes stained reddish brown by iron. It is overlain by red earth composed of laterite, formed by the weathering and ferruginization of the rock. The lake bed is a mixture of clayey soil, decaying vegetation and probably igneous rock below, which prevents loss of water by seepage. The lake shoreline consists mostly of decaying vegetation and mud, although sand was observed on the south-west shore. There is little information on the small natural lakes of this nature in Nigeria. There is a tendency to overlook them, focusing instead on reservoirs. Some aspects of Ogelube Lake have been published. Hare and Carter (1984) discussed the diel and season fluctuations of the physicochemical parameters, whereas Hare (1986) reported on the general limnology of the lake with a zoological bias, and Biswas and Nweze (1990) focused on the checklist of phytoplankton. This study on Ogelube Lake presents the seasonal variations in the phytoplankton species and classes during the rainy season, when the lake is rarely visited by the indigenes, and during the dry season, when it is used for bathing, washing and in situ fermentation of cassava (Manihot esculenta Cranz) tubers. The area has experienced intense anthropogenic activities, including intense farming, pastoralists and fishing activities over the last 20 years, necessitating the publication of this information to provide a database for comparison with the current lake environment. Thus, it provides some information for monitoring the effects of human activities on a small lake of this type. Moreover, it adds to the pool of information on the algal flora of Nigerian inland waters.

MATERIALS AND METHODS

Fig. 1.

Bathymetric map of Ogelube Lake, Opi, Nigeria (A, B,

C = sampling sites) (After Hare 1986).

© 2006 Blackwell Publishing Asia Pty Ltd

Sampling trips were made from May 1981 to May 1982 to the sampling site at monthly (28 days) intervals, usually on Saturdays, except when weather conditions made the roads inaccessible. Eight trips were made during the rainy season, and five trips during the dry season. Water samples were collected from three mid-lake locations along the north–south axis of the lake, about mid-day at the surface, 1 m depth and prebottom depth, using an Irwin sampler (Welch 1952). Fluctuations in environmental factors such as depth, water temperature, colour, transparency, pH and iron were measured with standard methods for this purpose (APHA, 1976).

Seasonal variation in phytoplankton

Three 25 mL subsamples were removed from each depth for phytoplankton studies, and preserved with Lugol’s iodine (Prescott, 1964). The sedimentation technique was used for microscopy (Lund et al. 1958) on 1 mL or 5 mL aliquots (depending on the concentration of the samples), with all examinations done with a Wild Heerburg, M40 inverted microscope under × 600 magnification. The general algal identification was carried out using the techniques of Prescott (1962, 1964), Drouet (1966), Fritsch (1965), Thomasson (1957, 1960, 1965), Thompson (1966), Patrick (1966), Belcher and Swale (1976, 1979), and Subberaju and Suxena (1979). Desmids were identified, primarily using the techniques of West and West (1904 –11) and West and Carter (1923). Chlorococcales were identified, using the techniques of Prescott (1962) and Hindak (1977), and Cyanophyceae/Cyanobacteria using primarily the techniques of Desikachary (1959) and Prescott (1962, 1964). The monthly mean algal population densities for the lake were calculated from mean location values. The population densities (individuals per millilitre) of phytoplankton for the rainy season were calculated by dividing the total number of individuals encountered during the rainy season by 72 (3 depths × 3 locations × 8 trips), whereas those of the dry season were divided by 45 (3 depths × 3 locations × 5 trips). Simple percentages of the seasonal species composition were calculated. The total numbers of individuals encountered at the various depths (surface, 1 m and prebottom) and locations (A, B and C) sampled for the rainy and dry seasons were calculated, with the data being square-root transformed in order to equalize variations in the data. Thereafter, an analysis of variance (ANOVA) was performed with SPSS version 10.0. The data were analysed as factorial in completely randomized design. This model was necessary to estimate the main effects of seasons, classes of algae/organisms and the interactions between the two main effects. Significant treatment effects were determined by Fisher’s least significance difference (F-LSD) at a 5% probability level. Values were retransformed back to the original figures for the presentation of the study results. The computerized Pearson correlation analyses were carried out between the phytoplankton and the environmental factors. All tests of significance were at the 95% confidence limit at 12 d.f.

RESULTS The percentage compositions of the various classes of algae for the entire sampling period are presented in Table 1. The results illustrated the abundance of Chlorophyceae, followed by Cyanobacteria, Bacillariophyceae, Dinophyceae, Euglenophyceae, Cryptophyceae, Chrysophyceae and

65

Xanthophyceae in decreasing order. Subsequent investigations from 1985 –1986 by Nweze (2003) confirmed this trend. Inspection of the mean population during the rainy season showed that Chlorophyceae was most abundant, followed by Cyanobacteria, Bacillariophyceae, Euglenophyceae, Dinophyceae, Cryptophyceae, Chrysophyceae and Xanthophyceae, in order of abundance. This order of abundance was somewhat altered during the dry season, when there was a relative abundance of Bacillariophyceae over Cyanobacteria, and Dinophyceae over Euglenophyceae. Combined analysis of the various phytoplankton classes during the rainy and dry seasons is presented in Table 2. The main effect of classes showed that at P = 0.05 (F-LSD = 96.739), there were significant dif ferences in the populations of Chlorophyceae and all other classes. However, Cyanobacteria, Bacillariophyceae, Dinophyceae, Table 1.

Composition of various classes of algae in Ogelube Lake Composition (%) Rainy

Dry

Mean

Classes

season

season

annual value

Chlorophyceae

91.0

46.36

89.04

6.24

11.27

6.46

Bacillariophyceae

1.51

26.72

2.28

Dinophyceae

0.33

8.91

0.50

Euglenophyceae

0.42

3.45

0.46

Cryptophyceae

0.25

2.55

0.30

Chrysophyceae

0.01

0.55

0.03

Xanthophyceae

0.005

0.18

Cyanobacteria

Total

Table 2.

100

0.01

100

100

Combined analysis of phytoplankton classes during the

rainy and dry seasons Seasons Main effect Classes

Rainy

Dry

of classes

Chlorophyceae

22 434.91

212.15

11 323.53

Cyanobacteria

1334.86

106.04

720.45

Bacillariophyceae

973.42

506.90

740.16

Dinophyceae

119.42

51.50

85.46

Euglenophyceae

132.41

24.52

78.46

Cryptophyceae

162.12

44.38

103.25

Chrysophyceae

17.19

1.63

9.14

Xanthophyceae

0.97

2.39

1.68

3146.91

118.67

1632.77

Main effect of season

Fisher’s least significance difference (0.05) comparing: season = 26.707; classes = 96.739; season by classes = 193.963.



1.00 8.65* 1.26 *, significant at 95%; NA, not applicable; NS, not significant.

5.29* 0.34 0.44 Organism × depth × season

0.25NS

0.08NS NA 0.72

NS NS NS NS NS

20.30* NA

1.85NS 0.714NS 2.10NS 8.58* 0.730NS 7.80* 0.260NS 0.422NS

17.84* 236.69* Organism × season

Depth × season

0.117NS 11.18* 16.63* 13.21* 21.20*

4.86*

3.42* NA

82.97* 23.64*

1.54NS 2.22*

74.26* 31.36*

9.41* 32.64*

19.73* 94.98*

10.69* NS

432.70*

0.577 0.45NS

1561.68* Season

Organism × depth

4.35*

34.29* NA

1.37NS 1.26NS

10.63* 66.16*

6.11* 10.47*

360.99* 428.10*

33.75* 11.65*

27.46* 71.63*

0.044NS 0.44NS

255.14*

Depth

Organism

Chrysophyceae Cryptophyceae Euglenophyceae Dinophyceae Desmidiales

Chlorococcales

Cyanobacteria

Bacillariophyceae

F-values Results of the analysis of variance of observed algal groups Table 3.

Euglenophyceae, Cryptophyceae, Chrysophyceae and Xanthophyceae showed no significant differences among themselves. Moreover, season-by-class interactions indicated that Chlorophyceae, Cyanobacteria and Bacillariophyceae were significantly higher in the rainy season than in the dry season at P = 0.05 (F-LSD = 193.96). The seasonal effect was highly significant, with a total phytoplankton population density value of 3146.91 for the rainy season value, compared to the dry season value of 118.67 at P = 0.05 (F-LSD = 26.707). Results of the ANOVA of the various groups investigated are presented in Table 3. The F-values showed that all groups were significant for season at P = 0.05. Furthermore, taxal composition (organisms) varied with the seasons, except for Chrysophyceae, where it was not applicable because only one genus was observed. Organism-by-depth interaction was significant for Cyanobacteria, Bacillariophyceae, Dinophyceae, Euglenophyceae and Xanthophyceae (F = 10.692; 32.642; 9.405; 2.216 and 3.419, respectively), whereas Chlorophyceae (Desmidiales and Chlorococcales) and Cryptophyceae were not significant. The effects of organism, depth and season of sampling and their first order and second order of interaction were significant for Cyanobacteria and Dinophyceae at P < 0.05 (F = 7.803 and 8.583, respectively). Analyses of the major taxa of Chlorophyceae encountered are presented in Table 4. The main effect of seasons for the two orders of Chlorophyceae encountered (Desmidiales and Chlorococcales) at P = 0.05 showed that Desmidiales were higher than Chlorococcales in the rainy season, with values of 21 396.55 and 1038.19, respectively. During the dry season, Chlorococcales were higher than Desmidiales, with values of 193.69 and 18.46, respectively. For Desmidiales, the seasonal effect was highly significant, with a rainy season value of 23 396.85, compared to a dry season value of 18.46 at P < 0.05, and F-LSD = 9.868. There were significant differences for all organisms at P < 0.05, with Staurastrum, Cosmarium and Spondylosium species being prominent. Season-by-organism interaction was highly significant at P < 0.05, F-LSD 51.38 for Closterium, Cosmarium, Spondylosium, Staurastrum and Staurodesmus, whereas the other desmids were not significant. The main ef fect of season for Chlorococcales, at P = 0.05 and F-LSD = 1.15, was significant. The season-by-organism interaction for the group and the main effect of organism at P < 0.05 also were significant for all genera encountered (F-LSD = 11.5 and 5.279, respectively). Scenedesmus, Ankistrodesmus Crucigenia and Chlorella were prominent in the rainy season, but Chlorella decreased in prominence during the dry season, while Crucigenia was dominant during the dry season. Analyses of the various taxa of Cyanobacteria, Bacillariophyceae and Dinophyceae are shown in Table 5. The main

N. O. Nweze Xanthophyceae

66

Seasonal variation in phytoplankton Table 4.

67

Total populations of various Chlorophyceae taxa observed during rainy and dry seasons Total population Main effect Rainy season (%)

Dry season (%)

of organism

(i) Desmidiales Closterium 1698.34 (1.32) 7.78 (7.02) Cosmarium 22 036.01 (17.16) 21.56 (19.46) Spondylosium 40 772.80 (31.76) 27.53 (24.85) Staurastrum 63 465.20 (49.43) 45.08 (40.70) Staurodesmus 399.5 (0.31) 8.04 (7.26) Others 9.33 (0) 0.78 (0.70) Main effect of season 21 396.85 18.46 F-LSD (0.05) comparing: season = 9.868; organism = 25.409; season-by-organism interaction = 51.34

853.06 11 028.79 20 400.17 31 755.14 203.77 5.06 10 707.66

(ii) Chlorococcales Ankistrodesmus 2001.09 (27.54) 237.31 (17.5) Chlorella 839.11 (11.55) 0.20 (0.05) Crucigenia 931.46 (12.82) 694.24 (51.20) Protococcus 517.62 (7.12) 1.02 (0.075) Scenedesmus 2388.36 (32.86) 375.59 (27.70) Tetradron 337.84 (4.65) 12.43 (0.92) Others 251.86 (3.47) 35.06 (2.59) Main effect of season 1038.19 193.69 F-LSD (0.05) comparing: season = 1.15; organism = 5.279; season-by-organism interaction = 11.75

1119.2 419.65 812.85 259.32 1381.98 175.14 143.46 615.94

Figures in parentheses are percentage composition; F-LSD, Fisher’s least significance difference. Table 5.

Total populations of various taxa of Cyanobacteria Bacillariophyceae and Dinophyceae observed during the rainy and dry seasons Total population Main effect

Organism

Rainy season (%)

Dry season (%)

of organism

(i) Cyanobacteria Chroococcus 271.16 (4.06) 19.00 (3.58) Rhabdogloea 832.63 (12.48) 391.78 (73.92) Merismopedia 5053.29 (75.71) 75.50 (14.25) Microcystis 448.86 (6.73) 29.64 (5.59) Others 68.37 (1.02) 14.07 (2.65) Main effect of season 1334.86 106.04 F-LSD (0.05) comparing: season = 9.868; organism = 25.408; season-by-organism interaction = 51.34

145.08 612.21 2564.40 239.35 41.22 720.45

(ii) Bacillariophyceae Aulacoseira 1906.57 (97.93) 987.91 (97.45) Others 40.26 (2.07) 25.90 (3.26) Main effect of season 973.42 506.90 F-LSD (0.05) comparing: season = 4.581; organism = 4.581; season-by-organism interaction = 9.67

1447.24 33.08 740.16

(iii) Dinophyceae Peridinium 449.87 (94.18) 191.49 (92.96) Gymnodinium 26.57 (5.56) 14.41 (7.0) Glenodinium 0.35 (0.07) 0.00 (0) Cystodinium 0.87 (0.18) 0.09 (0.04) Main effect of season 119.42 51.50 F-LSD (0.05) comparing: season = − 0.15; organism = − 0.414; season-by-organism interaction = 0.899

320.68 20.49 0.18 0.48 85.46

Figures in parentheses are percentage composition; F-SLD, Fisher’s least significance difference.


68

N. O. Nweze

effects of organism and season-by-organism interaction for Cyanobacteria were significant at P < 0.05 (F-LSD = 25.408 and 51.34, respectively) for all organisms encountered – Chroococcus, Rhabdogloea, Merismopedia and Microcystis, except for others (minor organisms) that were not significant for season and organism interaction. Cyanobacteria of importance were Merismopedia, Microcystis and Rhabdogloea, in decreasing order, during the rainy season (75.7%, 12.48% and 6.73%, respectively), while the order was altered during the dry season (14.25%, 5.59%, and 73.92%, respectively). Other important genera were Chroococcus and Microcystis (Table 5-i). The main effect of season, organism and season-by-organism interactions at P < 0.05, and FLSD = 4.581, 4.581 and 9.67, respectively, was significant for Bacillariophyceae. The only diatom of significance was Aulacoseira, which dominated the group during both seasons. Its population was higher in the rainy season than during the dry season (1906.57 and 987.91 individuals, respectively; Table 5-ii). The main effect of season, organism and season-by-organism interactions at P < 0.05, and F-LSD = 0.15, 0414 and 0.899, respectively, was significant for Dinophyceae. Peridinium was the abundant genus during both the rainy and the dry seasons (94.18% and 92.96%, respectively). Gymnodinium was very low, whereas Glenodinium and Cystodinium were rare (Table 5-iii). The analyses of the various Euglenophyceae and Cryptophyceae taxa are presented in Table 6. The main effect of season, organism and season-by-organism interactions for Euglenophyceae was significant at P < 0.05, and FLSD = 0.46, 1.317 and 3.133, respectively. Trachelomonas had the highest mean population density during both the Table 6.

rainy and the dry seasons (73.71% and 89.24%, respectively), followed by Euglena. Euglena was rare during the dry season (1.48%). Other genera such as Phacus and Lepocinclis were scarce (Table 6-i). The main effects of season, organism and season-byorganism interactions for Cryptophyceae at P < 0.05, FLSD = 3.217, 3.217 and 6.936, respectively, was significant. Chroomonas was more abundant than Cryptomonas during both seasons (Table 6-ii). Dinobryon was the only Chrysophyceae observed during this study, and was scarce (Table 7-i). The main effect of season was significant at P < 0.05 and F-LSD = 0.286. For the Xanthophyceae, the main effect of organism and season-by-organism interactions was significant at P < 0.05 and F-LSD = −0.41, respectively. Centritractus was more abundant than Ophiocytium and Botryococcus (Table 7-ii). Monthly fluctuations in the population density of the total phytoplankton, Chlorophyceae, Cyanobacteria and Bacillariophyceae are shown in Figure 2. The total phytoplankton was highest in May 1981 (71 812 individuals mL−1), and lowest in November 1981 (237 individuals mL−1). There was a steady decrease in the population density from July to November, a slight increase in December, and a decrease in January. An increase followed from February to May (Fig. 2a). The population of Chlorophyceae followed the same trend as the total phytoplankton, being highest in May and lowest in November (Fig. 2b). The population of Cyanobacteria was highest in August, and lowest in November (Fig. 2c). They were a major component of the total phytoplankton population in September (34.99%), October (35.62%) and January (34.1%; data not presented in the figure). Bacillariophyceae had

Total population of various taxa of Euglenophyceae, and Cryptophyceae observed during the rainy and dry seasons Main effect

Organism

Rainy season

Dry season

of organism

101.63 (19.19)

1.45 (1.48)

51.53

7.44 (1.40)

2.38 (2.43)

4.91

(i) Euglenophyceae Euglena Lepocinclis Phacus

30.19 (5.70)

6.73 (6.86)

18.46

Trachelomonas

390.37 (73.71)

87.54 (89.24)

238.96


132.41

24.52

78.46

F-LSD (0.05) comparing: season = 0.46; organism = 1.317; season-by-organism interaction = 3.133 (ii) Cryptophyceae Chroomonas

210.71 (64.99)

74.80 (84.28)

Cryptomonas

113.52 (35.01)

13.95 (15.72)


162.12

44.38

F-LSD (0.05) comparing: season = 3.217; organism = 3.217; season-by-organism interaction = 6.936 Figures in parentheses are percentage composition; F-LSD, Fisher’s least significance difference.


142.76 63.74 103.25


69

two population peaks, that in July being less than that in April. Their density was low between August to November, with the lowest density occurring in October. There was an abrupt increase in December, a decrease in early January, and finally an increase to a peak level in April (Fig. 2d). Monthly variations in the populations of Euglenophyceae, Dinophyceae, Cryptophyceae, Chrysophyceae and Xanthophyceae are presented in Figure 3. Euglenophyceae occurred mostly between May to November. Its population was highest in June, and lowest in April (Fig. 3a). It formed a significant component of the phytoplankton in November, being next in abundance to Chlorophyceae (data not

Table 7.

shown). The Dinophyceae population was consistently low throughout the study period. Its peak occurred in April, and was lowest in September and October (Fig. 3b). Cryptophyceae were observed mostly from June to August (Fig. 3c), Chrysophyceae in July and November, and Xanthophyceae in December (Fig. 3d,e). The three groups formed an insignificant part of the total phytoplankton. Total phytoplankton, total Chlorophyceae and total Cyanophyceae exhibited a significant negative correlation with transparency (r = – 0.7418, −0.7275 and −0.7324, respectively). Bacillariophyceae had a highly significant negative correlation with depth and colour (r = –0.7156 and 0.6924, respectively). The total Euglenophyceae

Total population of various taxa of Chrysophyceae and Xanthophyceae observed during the rainy and dry seasons Total population Main effect

Organism

Rainy season

Dry season

of organism

(i) Chrysophyceae Dinobryon

17.19

1.63

9.41


17.19

1.63

9.41

F-LSD (0.05) comparing: season = − 0.286; organism = not applicable season-by-organism interaction = not applicable (ii) Xanthophyceae Centritractus

1.47 (50.52)

0.45 (6.27)

0.96

Ophiocytium

1.24 (42.61)

6.73 (93.73)

3.99

Botryococcus

0.20 (6.87)

0.00

0.10


0.97

2.39

1.68

F-LSD (0.05) comparing: season = 2.62; organism = − 0.41; season-by-organism interaction = − 0.41 Figures in parentheses are percentage composition; F-SLD, Fisher’s least significance difference.

Fig. 2.

Monthly variations in populations

(individuals/ml) of Total Phytoplankton (a), Chlorophyceae (b), Cyanophyceae (c) and Bacillariophyceae (d) in Ogelube lake.


70

N. O. Nweze

exhibited a significant positive correlation with rainfall (r = +0.5703).

DISCUSSION Cyanobacteria are known to be more abundant than the other classes of algae in most shallow, tropical lakes (Egborge 1973; Howard-Williams & Ganf 1981). This was not the case, however, for Ogelube Lake, for which Chlorophyceae, and particularly desmids, were most abundant. This finding indicates an oligotrophic lake condition, conforming to the observations by Prescott (1962), Hutchinson (1967), Beadle (1974), Reid and Wood (1976) and Round (1977) that the abundance of desmids is common in oligotrophic, soft-water lakes that contain low calcium concentrations. An earlier study by Hare (1986) also revealed a low calcium content. The absence of water snails, coupled with high desmid flora recorded by Biswas and Nweze (1990), is all suggestive of an oligotrophic water condition. A high number of different species also confirms the suggestion that the lake exhibits an oligotrophic condition, as previously noted by Beadle (1974). The increased phytoplankton observed from February to May suggests that it is the period of maximum growth or multiplication of cells. Most previous investigations on tropical waters have reported higher phytoplankton populations occur in dry, rather than in rainy seasons (Egborge 1979a). In contrast, the results of this study indicated the highest phytoplankton population during the

Fig. 3.

rainy season (April–October) rather than during the dry season (November–March). This finding was correlated with early rains, which caused large quantities of nutrients to enter the lake from storm-generated run-off from the surrounding cultivated/seasonally burned land, which might have induced the phytoplankton growth that culminated in the peak value observed in May. The significant negative relationships between total phytoplankton, the major classes Chlorophyceae and Cyanophyceae, and water transparency support the observations of Biswas (1969) and Egborge (1979a) that phytoplankton abundance reduces transparency. As there was no overflow during this period, the lake acted as a nutrient sink, thereby causing increased biological activity. Thus, there was a mild increase in the lake’s eutrophication status at this period, as indicated by the high incidence of Euglenophyceae (80 individuals mL−1) that Egborge (1979a) reported as indicative of eutrophic waters. Whereas Euglenophyceae was higher at the beginning of the rainy periods (hence the positive correlation), the Dinophyceae population was low, with the converse situation occurring during the dry season. The observed phytoplankton population decrease with the progression of the rainy season might be attributable to the reduced water transparency, and to the diminished nutrient content, cloud cover and dilutional effects of rain (Evurunobi 1984), also consistent with the observations of Ruttner (1968), Biswas (1969) and Egborge (1979b). The December increase in phytoplankton population could be a

Monthly variations in populations (individuals/ml) of Euglenophyceae (a), Dinophyceae (b), Cryptophyceae (c), Chrysophyceae (d)

and Xanthophyceae (e) in Ogelube lake.



result of nutrient recycling because of harmattan winds, causing overturn of the water column (Carrick et al. 1993). The relative abundance of Bacillariophyceae during the dry season (December–February) indicated that the water was fairly clean during this period, given that this group of phytoplankton is usually associated with clean waters (American Public Health Association 1976). There was a variation in the composition of the various phytoplankton classes. Only the Chrysophyceae was sparsely represented. The F-values for the ANOVA showed that the Chlorophyceae groups (Desmidiales and Chlorococcales) and Cryptophyceae were uniformly distributed with depth. Although Cyanobacteria, Bacillariophyceae, Euglenophyceae and Euglenophyceae increased with depth, Dinophyceae decreased with depth, whereas Xanthophyceae were greater at the 1 m depth. There were significant seasonal variations in the observed phytoplankton populations, with most groups significantly higher in the rainy season than in the dry season. The exception was Chlorococcales, as shown by the F-LSD values at P = 0.05 for seasons.

CONCLUSION The results showed that the rainy season phytoplankton population was larger than that of the dry season, and that the period of the actual population increase was during the onset of the rains (April–May). Increased rains and cloud cover led to a steady decline in the phytoplankton population until the dry season (November–March). Chlorophyceae was the dominant class, with desmids also being significant, followed by Cyanobacteria. The abundance of desmids indicates an oligotrophic lake condition. The classes Bacillariophyceae, Dinophyceae, Euglenophyceae, Cryptophyceae, Chrysophyceae, Chrysophyceae and Xanthophyceae were low. The important phytoplankton taxa in the lake were Staurastrum, Cosmarium, Spondylosium, Closterium, Staurastrum, Crucigenia, Scenedesmus, Ankistrodesmus, Chlorella, Protococcus, Tetraedron (Chlorophyta); Chroococcus, Rhabdogloea, Merismopedia, Microcystis (Cyanophyta); Aulacoseira (Bacillariophyta); Peridinium (Dinophyta); Euglena, Trachelomonas (Euglenophyta); and Cryptomonas and Chroomonas (Cryptophyta). Villagers use the lake for bathing, washing and in situ fermentation of cassava during the dry season. These study results indicated that the water was best for such purposes during the dry season, when the pollution indicators (Euglenophyceae) were low, thereby suggesting organic pollution and phytoplankton density also were low. The lake is sacred to the indigenous people in the basin that worship in shrines around the lake, and that do not fish in it. It appears that the construction of the new road

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did not induce pollution because the lake is located some distance (120 m) from the road and a sandy river (a tributary of the Uhere River), thereby not being readily accessible to the impacts from the road. Long-term study would be necessary to establish any adverse effects of intensifying farming around the lake, and from the impacts related to increasing access to tourists for picnics.

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