Introduction

Phytoplankton are free-floating uni-cells and colonies that grow photoautotrophically in aquatic environments. Phytoplankton play a key role in the primary production and global nutrient cycles of the Earth (Daniel 2001) by making up the main producers in any given water body (Biddanda and Benner 1997). It colonizes the upper part of the water column, down to the limit of penetration of light. The structure and abundance of the phytoplankton populations are mainly controlled by inorganic nutrients such as nitrogen, phosphorus, and silica (Daniel 2001) and mainly available nitrogen as nitrate, nitrite and ammonia, phosphorus as soluble orthophosphate (USEPA 2000) and silicone as silicate forms.

Phytoplankton communities are sensitive to changes in their environment and therefore phytoplankton total biomass and many phytoplankton species are used as indicators of water quality (Reynolds et al. 2002; Brettum and Andersen 2005).

Wholly, the use of living organisms to determine the presence, amounts, changes in and effects of physical, chemical, and biotic factors in the environment are termed biological monitoring (Baker 1976). Therefore, the water quality has to be monitored by measuring the phytoplankton content in order to come up with preventive measures such as aeration to prevent fish kills during the decay of the phytoplankton biomass, and the sustainable use of drinking water supply and recreational activities (Imhoff and Alberrecht 1975) and other uses of the water bodies. This work was aimed to determine the impacts of physicochemical parameters on the phytoplankton compositions and abundances in Selameko manmade reservoir, Debre Tabor, South Gondar, Ethiopia.

Materials and methods

Description of the study area

This study was conducted at Selameko manmade reservoir, south-west of Debre tabor town, Ethiopia. This manmade reservoir is found 2513 meter above sea level (masl), specifically located at 38°05′E and 11°53′24″N (Fig. 1). The reservoir was constructed in 2007 with a total of 11.6 hectares (ha) with 20 m depth to irrigate nearly 63 ha. The catchments of the reservoir are extended from 2513 to 2726 masl with 879.25 ha of total area.

Fig. 1
figure 1

Map and location of the study area (Selameko Manmade Reservoir)

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Hydrology and climate

Based on the National Metrology Agency of Bahir Dar Branch Office, the mean annual temperature of the study area is 16.23 °C (ranges from 9.2 to 23.26 °C) (from 1997 to 2009 years) and its mean annual rainfall is 1371.2 mm (ranges from 1096.7 to 1645.7 mm). The climate of reservoir is characterized roughly by four seasons: (1) A main-rainy season (MRS) with heavy rains during July–September, (2) a post-rainy season (PORS) between October and November, (3) a dry season (DS) between December and April and (4) a pre-rainy season (PRS) from May to June (Tamiru 2006; Ayalew et al. 2007).

Measurements of physicochemical parameters

Samples of physicochemical parameters were collected from August 2009 to May 2010 in the four seasons, i.e., main rainy/MRS/, post-rainy/PORS/, dry/DS/, and pre-rainy/PRS/seasons. The water samples were taken only from littoral (SI) and open water zone (SII) two times from each for each parameter (Fig. 1).

Water temperature and pH, dissolved oxygen (DO), total dissolved substances (TDSs), and water transparency were determined in situ. Water temperature and pH were measured with coupled pH/TDS/CON Meter (Model Tochpro II); DO was determined by portable oxygen analyzer (JPB-607); TDSs were measured by cond/TDS meter (Model CE 470 Cond. Meter 01189); water depth was measured by standardized meter (the calibrated meter that had metal tip part was dipped into the reservoirs water until it touched the bottom of the reservoir three times at the central and the average recorded depth was taken) and transparency was measured by standardized Secchi disk. The physico-chemical parameter measurements were taken after the probes dipping down from the surface water to 50 cm down in the reservoir water. Major nutrients, nitrate (NO3-N), phosphate (PO4-P), and silicate (SiO2), were measured in situ immediately by using a portable water analyzer kit (Wagtech international, Palintest transmittance display photometer 5000, Palintest Ltd., and UK) (Palintest Ltd 1989). The collected water samples from the two sites were first filtrate by Whatman GF/C, 0.6–0.7 µm pore size membrane filter to avoid unnecessary large-sized particulate materials (debris) that cause further nutrient release before nutrient analysis was made.

Measurement of phytoplankton community

Phytoplankton

Integrated phytoplankton samples were collected two times for each from littoral (SI) and open water zone (SII) using Van Dorn water sampler. The collected samples were concentrated in 100 mL using 55 µm (mesh opening) phytoplankton net and preserved with Lugol’s solutions. Again, 100 mL was allowed to settle in graduate cylinder overnight and the supernatant was siphoned off till 10 mL remained. Of this concentrated sample, 1 mL was used in a Sedgwick-Rafter Cell, of which 100 microscopic field were counted for major species according to Wetzel and Likens (2000). Identification and counting were made under Olympus (CH-2) compound microscope (200X) based on a key guideline of Yamaguchi and Gould (2007) and Blomqvist and Olsen (1981), and abundance of each species of phytoplankton was calculated based on Lind (1979). Chlorophyll-a (Chl-a) was calculated based on monochromatic methods (Lorenzen 1967 in Wetzel and Likens 2000).

Data analyses

Analysis of variance (ANOVA) was used to test significant differences between those like spatial and temporal variations of physico-chemical and phytoplankton (P < 0.05). Tukey (Honestly Significantly Differently Test) test was used to determine significance in mean catches and estimates. Stepwise regression was used to test the relationship between the physico-chemical parameters and biological variables; the observed (r) values were then compared to the table values at P < 0.05 level of significance, whereas abundance of phytoplankton was analyzed using percentage. Two-way ANOVA was applied for both physicochemical and phytoplankton. In general, data were calculated and organized using appropriate statistical software, such as the SPSS (2007) version 16 and SAS (2003).

Results and discussion

Physicochemical measurements (in seasons and sites)

Most of the physicochemical parameters are very suitable for phytoplankton growth. The ANOVA result showed that there were highly significant differences in temperature, pH, DO, WTD, TDS, NO3-N, PO4-P, SiO2 and chl-a among seasons and between sites (Table 1). The Tukey test also showed similar scenario (Table 1). Specifically, temperature of the reservoir water ranged from 18.7 °C (PORS, SI) to 24.2 °C (DS, SI and SII, PRS, SII). This registered temperature is good for planktons but not optimum for the growth of fish as suggested by Korai et al. (2008) which is between 22 and 31 °C. pH of the reservoir extended from 7.01 (SII) to 8.01 (SI). These pH values are good for aquatic life including fish and fall (Oso and Fagbuaro, 2008) within the EPA Redbook recommended range for fresh water (6.5–9.0) (Schmirz 1996) and recommended by others (Chapman 1996; Goldman and Horne 1983). DO was ranged from 5.0 (DS) to 6.15 mg/L (MRS). The obtained DO concentration satisfied the minimum recommended standard (>5 ppm) set by EPA Redbook and others (USEPA 2008; Yajurvedi 2008) and good for fishing and planktons. The transparency depth was ranged from 32 (MRS, SI) to 97 cm (PORS, SII) and is eutrophic (Horne and Goldman 1994). The lowest was due to the presence of high suspended matter (sands, silts) that increased water turbidity in the MRS (Meesukko et al. 2007; Rafique et al. 2002), and the presence of large numbers phytoplankton that favored by high nutrient loads in rain season (Mustapha and Omotosho 2005). Similarly, the TDSs of the reservoir were between 67.1 (in PORS, SI, SII) and 137.2 ppm (in MRS, SII). This registered value is very conducive for the growth of aquatic organisms including fish (Mohamed et al. 2009) (Table 1).

Table 1 Physico-chemical parameters of Selameko reservoir from 2009 to 2010 respect to seasons and locations
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The concentration of NO3-N was between 0.1 and 2 mg/L, for PO4-P was between 0.08 and 0.83 mg/L, and for silicate was between 0.09 and 22.5 mg/L. All nutrients, the highest values were recorded in MRS due to surface water inflow that brought nutrients from the surrounding agricultural areas (Meesukko et al. 2007; Stanley et al. 2003) and the leachates of municipal wastes from waste disposal sites and sanitary landfills (Bennett 1998). Additionally, PO4-P concentration was due to washing and bathing with phosphate-based detergents and soaps (Davies et al. 2009), and silicate was due to the presence of high rate of rock and soil weathering in the water body (Little 2004). All these high concentrations of nutrients abnormally increased plankton abundances and composition in rainy time than other seasons. Generally, the obtained nitrate concentration is tolerable by fish and other aquatic organism and satisfies surface water quality standards (<5 mg/L) (PCD 1997 in Chattopadhyay and Banerjee 2007) and fulfills the minimum level of nitrate in lake to be productive (Yajurvedi 2008). The registered silicate concentration is good for plankton growth and satisfies freshwater ranges from 1 to 30 mg/L (Wetzel 2001 in Meesukko et al. 2007; Chapman 1996). However, the obtained concentration of PO4-P is greater than other standards (0.005–0.020 mg/L PO4-P, Chapman 1996; 0.01–0.03 ppm phosphorus, Yajurvedi 2008) and an indication of the presence of pollution.

Phytoplankton compositions and abundances (in season and sites)

A total of 36 genera from seven taxonomic families and three groups were recorded during the study period.

The ANOVA result showed that there were highly significant differences in zygnemaphyceae, bacillariophyceae, chlorophyceae, cryptophyceae, cyanophyceae, euglenenophyceae, and dinophyceae among seasons and between sites (Tables 2 and 3). According to stepwise regression, total number of phytoplankton had positive relationship with physicochemical parameters (R2 = 0.99, P < 0.001, N = 16). Grand total of phytoplanktons = +2.6 × e + 6 NO3-N, +4.5 × e + 6 PO4-P, +1.8 × e + 5 SiO2, +1 × e + 5 chl-a.

Table 2 Seasonal and temporal abundance (ind/L) and distribution of phytoplankton in Selameko manmade reservoir during the four seasons in two locations, South Gondar, Debre Tabor
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Table 3 The mean differences of (Tukey Test) biological parameters (phytoplankton) of Selameko manmade reservoir, South Gondar (Debre Tabor) from 2009 to 2010
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From the total numbers of phytoplankton, the MRS was higher than the other seasons and PORS was the least abundance. The highest record was due to the presence of excess nutrients that come from human sources such as waste and agricultural runoffs (Osondu 2008) mainly attributed to the increase in nitrates, phosphates, and silicates (Kobbia et al. 1991) (Table 1). These alternative nutrients became food sources of various phytoplankton types. The lowest abundance in PORS was probably due to the presence of small amounts of nutrients like phosphates and nitrates. From all seasons, Bacillariophyceae was the most abundance family due to majorly the presence the high concentration and most favorite food alternative, silicate (Radwan 2005; Little 2004) and to some extant nitrate (Khenari 2007). Among Bacillariophyceae, Melosira and in Cyanophyceae, Microcystis were some of the dominant genera and were indicators of the presence of pollution (Heiskary and Markus 2001) and the alkaline nature of the reservoir water (Onyema 2007). Totally, very large numbers of phytoplankton were recorded. The total numbers of phytoplankton had positive correlation with NO3-N, PO4-P, and SiO2 as well as chl-a. When the concentrations of nutrients increased, the phytoplankton abundances increased and vices verse (Onyema 2007). Being these, very large concentrations of chl-a (39.29 µg/L) were recorded in MRS than any other seasons (Meesukko et al. 2007; Radwan 2005). Based on this registered chl-a concentration, reservoir water is productive (eutrophic) because its value is between 10 and 40 µg/L (ADEQ 2008), and 3.0–78.0 µg/L (Wetzel 1983).

Conclusions

Based on the present observation, Selameko manmade reservoir is rich in species diversity and composition and the nutrient status is high enough to support the plankton community. However, the high concentration of PO4-P indicates the presence of strong anthropogenic pressure. To solve such big pressure, both basin and reservoir management are recommended to solve such acute problems.