Introduction:

The aquatic environments are subjected to high temporal andspecial variability. This is due to the abundance and microalgal species composition as a result of interaction between physical,chemical and biological variables. The causes explained not only by the equilibrium approach (which permits the coexistence of specieslimited by different resources), but also by the non-equilibrium approach (which accepts the frequency of environmental variability,allowing species that share the same resources to coexist (Connell, 1978). Spatial patterns in algal community structure often relate tochanges in water chemistry (Crump et al., 2007; Fierer et al., 2007). Lake Hawasa was investigated extensively during the 1980s and early1990s. Phytoplankton biomass and primary production in relation to nutrients and light were studied during this time (Kifle and Belay,1990 and Kebede and Belay, 1994) and some dominant phytoplankton species were identified including Lyngbya nyassae Schmidle, 1902,Botryococcus braunii Kützing, 1849 and Microcystis species. Previous studies were carried out to show temporal and spatial variations ofphytoplankton primary production in lake Hawassa (Kifle, 1985). Microalgae are dominant among microorganism in aquatic habitatswith sufficient nutrients and light available. They are one of the major primary producers in freshwater aquatic ecosystems such as rivers,lakes, ponds and canals[1] (Anahas et al., 2013 and Muthukumar et al., 2007). Microalgal species composition and growth of speciesare useful indicators of nutrient enrichment (Dynes et al., 2006).

They are widely used in biotechnological applications mainly forbioremediation, nutriceutical and pharmaceutical purposes, as well as for bioenergy production (Barra et al., 2014). Microalgal speciesare capable of synthesizing all amino acids; they can also be a source of the high protein content (Sankaran and Thiruneelagandan, 2015).They also help as maintenance of nutrient recycling, trophic structures balance and most importantly harbours diverse floral communitiesin the ecosystem. The green algae play an important role acting as primary producers (Dhyani et al., 2007).

Most developing countries depend heavily on the resources providedby lakes and these resources are found among in very poor rural communities whose livelihood depends on their exploitation becausemost lakes are important natural resources with diverse ecological, economical and aesthetic significance (IUCN, 1996). Among all theAfrican countries Ethiopia is quite unique due to its geographic conditions, rich water resources, extensive green fields, varied animalhusbandry and overall, diversity of flora and fauna (Shonga, 2015). In Ethiopia, the major lakes that are of ecological and economicimportance are concentrated in the Rift Valley (IBC, 2010). These lakes include the southern lakes (Abaya, Chamo, Chew Bahr andsmall portion of Turkana), the central lakes (Hawassa, Shalla, Abijata, Langano, Ziway, Cheleleka /Abaya) and man-made Lake Koka))and the saline Northern lakes (Beseka, Afdera, Asale and Abbe).

There are also crater lakes such as the high plateau Bishoftu group(Lake Hora, Arenguade, Bishoftu, Kiloles and Pawlo) and Lake Chitu in the Rift valley (Tesfaye, 2011). Rift valley lakes gifted with manybeautiful lakes, numerous hot springs, warm and pleasant climate and a variety of wildlife. These lakes considered as one of the mostideal areas for the development of international tourism in Ethiopia and anybody can be relaxing sports for fishing. Nile perch, catfish,tilapia and tiger fish can be fished in these lakes. Lake Hawasa is located in the vicinity of the growing city of Hawasa, and some ofthe many potential adverse effects facing the lake include lack of proper sewage treatment system, poor land-use management andhigh levels of recreational activity. Moreover, the nearby Hawasa Textile Factory drains its effluent into the lake, apparently with littletreatment (Gebremariam and Desta, 2002).

An experiment made byGebremariam and Desta (2002) showed that the effluent from theHawassa textile factory contains relatively high concentrations of heavy metals and other trace elements of toxic nature as well as majorions and plant nutrients which ends up in the Lake Hawassa, will bring about eutrophication to the lake. Runoff from the eastern wall of thecaldera feeds another small lake called Lake Cheleleka( fig.1) Overflow from Lake Cheleleka drains into Lake Hawassa through the Tikur-Wuha River, which is the only major affluent river (Sai, 2014). LakeHawasa has long been exposed to anthropogenic threats including over-fishing, irrigation, deforestation, overgrazing and indiscriminateuse of pesticides and fertilizers in their catchment areas for the last 15–20 years (Tenalem, 2004). Lake Hawassa has an important naturevalue, as it provides habitat for a diverse avifauna and an important population of hippopotamus. The lake also has an important touristicpotential and is a popular resort for local and foreign visitors.

Thelake is also crucial for the subsistence of the local communities as itis an important fishing ground, including commercial species such as Nile tilapia, Oreochromis niloticus; African catfish, Clarias gariepinusand Labeobarbus species. According to Shonga, (2015) there is large variety of species related to phytoplankton scattered in Lake Hawassaand Lake Abaya. Lake Hawassa is actual and potential sources of food and income for local communities. Furthermore, their rangeof variations in morphometric characterization and physical and chemical features offers opportunities for outstanding comparativelimnological studies. Microalgal diversity and physico- chemical characterstics of the lake information is important to understand thefactor influencing rise, fall and change in algal population and to study the effect of anthropogenic (environmental pollution or pollutantoriginating from human activities) pressure upon aquatic habitats of the lake (Round, 1981). Certain groups of phytoplankton, especiallyblue green algae can degrade recreational value of surface waters and in higher densities can cause deoxygenating of water and diatoms aregood indicators of water quality (Whitton, 2000).

Microalgae biodiversity of water bodies has been studied by severalworkers in Ethiopia (Shrivastava et al., 2014). However, there is limited knowledge on the microalgal diversity of the middle rift valley sodalakes of Ethiopia. The general objective of this research was to studyspatial and temporal variation in microalgal diversity in Lake Hawassa.

Material And Methods

Lake Hawassa is an endorheic basin, located in the main EthiopianRift valley and situated 275 km away from the capital city Addis Ababa towards the south near the city of Hawassa, the capital city of southernprovince (SNNPR) (figure 1)

Figure 1: Map showing location of Lake Hawassa (station 1- station 14)

The lake has 16 kilometers length, 9 kilometers width and it has a meandepth of 11 meters and is located at 7° 3ʹ0ʹʹN and 38° 26ʹ0ʹʹ• E at an elevation of 1,708 m a.s.l. It is a terminal lake with no surface out flowand receives surface inflow through Tikur Wuha River (LFDP, 1997). Lake Hawassa is the most studied of the Rift Valley lakes in Ethiopiabecause it is relatively accessible to scientists (University of Waterloo website, 2006).

Stratified random sampling design was conducted in Lake Hawassa ofEthiopia in winter, spring and summer seasons at 2017G.C to assesstemporal and spatial variability in the diversity of microalgae. 288water samples from 14 sampling stations and three seasons were collected from Lake Hawassa (fig. 1). Geographical parameter datalike longitude, latitude and altitude are the most important steps in identifying the sampling locations for proper sampling. Physicochemical Parameter Measurements such as water pH was measured in situ using a portable pH meter. Water transparency was measuredusing a white color Secchi disc of 20 cm diameter. Altitude was measured using portable GPS (Gpsmap 64s/GARMIN).

Sampling was done using sterile glass bottles starting from site one tofourteen in offshore by considering depth and representativeness of the sample for the sites. The fresh water sample was fixed with 1ml of4% of formaldehyde solution (purchased from a local pharmacy). Then the mixture was allowed to settle for 24 h. Later the supernatant wasdiscarded and the settled part of the solution was transferred to 100ml black capped bottle (Hosmani, 2010). Finally samples were transportedto the laboratory for identification of microalgal species. The preserved water samples were examined using inverted microscope(WILD M 40), at a magnification of x40. Species identification was done by placing six drops of water on a glass slide until no more newmicroalgae species found. The identification to genus or species level was made on the basis of various descriptors of microalgae (John etal., 2002; Janse et al., 2006; John and Robert, 2002 and Bellinger E, and Sigee D, 2010) .

To evaluate the significance of spatial and temporal variations ofmicroalgae, analyzed through one way analysis of variance (ANOVA) the SPSS version 20.0 was used. The abundance of the species were gotby counting microalgae species observed using microscope(frequency of each species). Microalgal diversity was analyzed by calculatingdifferent diversity indices (DI)

The Shannon diversity index was calculated by following Odum(1969) Diversity Index (H) = – Σ Pi ln Pi, Where Pi = S / N
S = Number of individuals of one species
N = Total number of all individuals in the sample
In = Logarithm to base e
Similarly the Evenness Index (EH) was calculated by following Pielou (1967).
EH= H/Hmax,
Where H max is the maximum value of diversity: Hmax= (lnS)

Results And Discussion

In Lake Hawassa pH varied from 7.2-8.0; 8.5-9.3; and 7.1-7.8 during the winter, spring and summer seasons respectively (Table 1)

Table 1:Seasonal variation of pH, transparency, altitude and GPS data during rainy, spring and dry seasons

PH values showed a seasonal trend of variation with higher values in spring then showed a slight reduction through winter and summerseasons. The highest pH values were recorded from lewi resort (S5) and Haile resort opposite (S10) during summer. According to Elizabethand Amha (1994) the pH of surface water of lake Hawassa was slightly decreased from January to April. In all sites, transparency values werevery high throughout the year; but also showed a seasonal trend of variation. In all three seasons, transparency was lowest in the winterseason but was found to gradually rise in spring and summer seasons. Annual mean transparency was (56.9-83.4) in all three seasons.There were observed temporal fluctuations in water transparency at the station of Tikur Wuha (S14), which were slightly associated withthe changes in the water level of the lake due to seasonal rainfall distribution pattern during the winter season. Water transparencywere considerably lower at Tikur Wuha site in all seasons probably due to easy re-suspension of sediments/mud due to wind actionand/or wading flamingo and other aquatic birds and also inflows of organic materials from rivers feeding the lake during winter seasonwhich may have contributed to low Secchi disc readings. Pearson and spearman Correlation between pH and transparency with season weresignificant at p 0.01(Annex 2). This observation is supported by Oduor (2000) who associated the high turbidity with the daily re-suspensionof the sediments by the winds coupled with shallowness of lake. In contrast, the comparatively high Secchi disc readings recorded at therest of sites were may be the sites had less influence of water inflow (no sediment/mud) of rivers and also less anthropogenic effect.

A total of 63 microalgal species belonging to five classes were identifiedat fourteen stations of the lake during the three seasons (winter, spring and summer) of the study period. Chlorophyceae was the most abundantmicroalgae with 29 species representing 46 %. The dominant (The species that predominates in a community) species of Chlorophyceaewere Cosmarium sp (RA= 8.9%), Tetraedron sp (RA= 10.6%) and Chlamydomonas sp (RA= 10.5%). Cyanophyceae was the second mostabundant group with 17 species representing 26.9 %. It was dominated by Cylindrospermopsis sp (RA= 33.7%), Microcystis aeruginosa (RA=21.2%),Merismopedia sp (RA=13.6%) and Gloeocapsa sp (RA= 13.5%). Bacillariophyceae was the third abundant with 15 species (23.81%). Therelative abundance of some species in this class was Synedra sp (26.2%), Cycotella sp (17.6%), Thallasoria sp (14%), Navicula sp (12.9%), Melisorasp (8.7%) and Nitzschia sp (5.4%). two species (3.17 %) in the division Euglenophyceae identified. relative abundance of the species were Phacussp (90%) and Euglenopsis vorax (10%). Cryptomonas ovate was the only species in the group of Cryptophyta (1.59 %) identified. The dominance ofChlorophyta species was also observed by other researchers, Elizabeth and Amha (1994) and Sai (2014) in the same lake.

The temporal and spatial distributions of microalgae showed thatCylindrospermopsis and Microcystis aeruginosa were abundant in all the sampling sites and seasons (figure 2).

Figure 2: The temporal and spatial distributions of microalgae in all seasons and sampling stations

Figure 3: Species diversity in all seasons and sampling stations (sp. diversity 1, 2, 3= species diversity at rainy season, spring season and dry season respectively).

There were large seasonal differences in microalgae species composition during the study period (2017). The lowest species diversity showed a decline at the onset of the winter season at S14, S5, S6, S9 and S10 sites. This is probably due to low transparency and high inflow of flood/ erosion in to the lake during this period.According to Talling and Lemoalle, (1998) during this period heavy precipitation seems to change the thermal stability and nutrientstatus of the water column. The heavy rainfalls thicken the mixed layer depth by eroding at least the upper part of the metalimnetic region and injecting nutrients into the water column. Low density of microalgae during the winter season is due to high influx of flood water and rain washings and ultimately much of it was also lost in the heavy draw-down (Escaravage and Prins, 1999). The highest species diversity was observed in the summer and spring season at the site of S3 and S9 (fig. 1). This may be due to nutrient enrichment, low turbidity and sufficient sunlight (Sugunan, 2000). In addition to this, the occurrence of Bacillariophyceae and Chlorophyceae increased may be due to available nutrients in the lake water by inflow of water from rivers during spring and summer season. This study supported by Elizabeth and Amha (1994), that total microalgae biomass increased with the onset of stratification after the mixing of the water column in December, and in the beginning of destratification in May at lake Hawassa. This may reflect redistribution of algae and change in the temperature and light, gradient and nutrient availability and climate variability.

Figure 4: Lake Hawassa (H), (Hmax) and EH index during rainy season

Figure 5: Lake Hawassa (H), (Hmax) and EH index during spring

Figure 6: Lake Hawassa (H), (Hmax) and (EH) index during winter

The seasonal and spatial variation of microalgae showed a defined pattern. During the winter season H and EH values increased at S12 (2.19) and S14 showed lower diversity (1.49). S4 showed high diversity (4.13) and S7 showed low diversity index (1.65) during spring. Also S6 and S7 sites showed high diversity and low diversity index (2.79, 1.67) among all stations. In all three seasons high diversity index was recorded during spring season and lower diversity was showed in winter season. There is significance difference in H and EH between seasons and among sampling sites (p< 0.05) (annex 1). Correlation between sampling H and EH among the sampling sites were significant at p 0.01(annex 1). Species diversity measured by Shannon index is directly proportional to the number of species in the sample and the uniformity of the species distribution in the total abundance. According to Kajak (1983) in the lakes species diversity was relatively high; this indicates good environmental conditions favorable to the growing of many species. Cyanobacteria dominance in lakes is an increasing problem that impacts of recreation, ecosystem integrity, and human and animal health. Some Cyanobacteria produce toxins during growth or decay that kills aquatic animals (Sivottctt 1996). The following figures show the Shannon diversity (H), maximum diversity (Hmax) and evenness index (EH) values for fourteen stations and three seasons.

This study shows that there was a positive relation between water PH, transparency and species diversity during the study period, when water pH and transparency values decreases during winter seasons, the species diversity will also decreases. The pH value will also affect the growth rate of microalgae, it will be easier for microalgae to capture CO2 in the atmosphere when the growing condition is alkaline, which can produce more biomass (Zang et al., 2011). Water transparency depends on the amount of particles in the water. In other words, when the water is murky or cloudy and contains a lot of particles, the light cannot penetrate as deeply into the water column and there was lower species diversity in winter. The low water transparency observed, due to re-suspension of sediments favored cyanobacteria species over other microalgae because of their ability to float on water surface as they possess gas vacuoles for buoyancy regulation (Walsby, 1978).

Table 2: List of all the species encountered in the study and their abundances

Table 2: List of all the species encountered in the study and their abundances

Conclusion

The current results indicated dominance and relative abundance ofChlorophyta and Cyanophyta both spatially and temporally in Lake Hawassa, which Chlorophyta are indicators of good water quality.Temporal and spatial distributions of microalgae showed that Cylindrospermopsis and Microcystis aeruginosa species were dominantin all the sampling sites and seasons. The two species are primary toxinproducing cyanobacteria can have a negative impact on aquatic foodwebs and human use of freshwaters. The highest species diversity was observed in the dry and spring season at the site of Amora Gedel andHaile resort and the lowest species diversity showed a certain declineat the onset of the rainy season and at the site of Tikur Wuha as well as Lewi and Haile resort and their opposites. In all three seasons highdiversity index was showed during spring and dry season and lower diversity was showed in rainy season. Significance difference in Hand EH were recorded between seasons and among sampling sites. Correlation between H and EH are significant at p 0.01. Spring anddry season were favorable for the growth of microalgae. This might be due to availability of nutrients from inflow of river water. The findingof this study provides necessary theoretical and data support for the diversity of microalgae in Hawassa Lake. However, further studies arestill needed on the species composition, quantity characteristics and distribution characteristics of the microalgae species in Hawassa Lakefor the upkeep of biodiversity.

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