Spatial and temporal phytoplankton species diversity in Southern Gulf of Lake Tana , northwestern Ethiopia

To assess phytoplankton species biodiversity in the Southern Gulf of Lake Tana, water samples were collected twice a month from eight sites for six months, from October 2010 to March to 2011. Data were analyzed and compared with one another using SPSS version 16. There were significant differences between months in phytoplankton species composition (p < 0.05). However, there was no significant difference among sites during the study period (p > 0.05). A total of 67 phytoplankton genera belonging to nine classes were recorded in the study area. The study showed phytoplankton species diversity is poor in the study area. Absence of significant differences among sites might be because there were similar human impacts in the gulf on a spatial base. In contrast to this, the significant differences among the six months might be due differences in nutrient inputs into the lake and also possibly due to human impact differences on temporal base. To conserve phytoplankton species composition so that it is sustainable at all times and sites, the local people in general and government in particular should avoid some activities on the gulf.


INTRODUCTION
Microscopic phytoplankton accounts for approximately half the production of organic matter on Earth (Daniel et al., 2010).According to Daniel et al. (2010) many phytoplankton declines have occurred in tropical regions.The reduction in numbers or change in species composition of phytoplankton could lead to a reduction in zooplankton abundance.Since the 1950s, scientists have recorded a decrease in zooplankton in the Californian current.Whether this steady decline is attributable to a decrease in primary production or an increase in predation on the zooplankton is not certain (United Nations Environment Programme, 2000).
Threats to Lake Tana have serious implications in environmental and human security in Ethiopia and raise ecological and cultural questions (Eguavoen, 2009).The lake provides livelihoods to over 3 million people living around it and is a significant source of hydro power (World Bank, 2006).It is a contributor to food security, providing water resources for agriculture and livestock, and has a significant fishing industry.Staple crops such as rice, pulses and teff are grown in the watershed, which is home to Ethiopia's unique cattle breed, the Fogera  (Abebe, 2008).The lake ecosystem and the water resources as a whole are in danger due to erosion, deforestation, sedimentation, water level reduction, flooding of the wetlands, competing uses of water resources, increased pollution and the pressure of the growing population in its catchment area.There is an erosion of Lake Tana water quality and gene pools affecting the stability and resilience of the system and endangering sustainability of the lake and the surrounding wetland resources (Esthete, 2003).
Many research attempts have been done to describe the chemical, physical and biological characteristics of Lake Tana in general and the southern gulf of the lake in particular.Yirgalem and Assefa (2009) have explained Lake Tana's water level fluctuations annually and seasonally following the patterns of changes in precipitation.Similarly, Akoma and Imoobe (2009) reported on a limnological and phytoplankton survey of the Bahir Dar gulf of Lake Tana but there is no research done on algal species biodiversity done so far in the area.The purpose of this research paper was to determine the phytoplankton species composition on a temporal and spatial basis in the study area, the southern gulf of Lake Tana, Ethiopia from November 2010 to April 2011 (Figure 1).

Description of the study area
Lake Tana, situated at an altitude of about 1800 m ASL, is a crater lake formed two million years ago due to the volcanic blocking of the Blue Nile River.Significant inflow to the lake comes from three major rivers (Gilgel Abai, Rib and Gumara) in the south, which carry a large amount of silt resulting from erosion and increase the turbidity of water to the gulf (Berhanu et al., 2001).
Lake Tana serves for local transport, ecological restoration, recreational purposes and dry season irrigation supply (Yirgalem and Assefa, 2010).According to Yirgalem and Assefa (2010), past attempts to observe historical fluctuations of Lake Tana based on a simplistic water balance approach of inflow, out-flow and storage have failed to capture well known events of drawdown and rise of the lake that have occurred in the last 44 years.Bahir Dar town is found on the southern gulf of the lake.According to the Ethiopian Central Statistical Agency (2008), the population living in the town is about 220,000 and is increasing from time to time.

Climate around Lake Tana
The air temperature shows large diurnal but small seasonal changes with an annual average of 20°C.The seasonal distribution of rainfall is controlled by the northward and southward movement of the inter-tropical convergence zone (ITCZ).Moist air masses are driven from the Atlantic and Indian Oceans during summer (June-September).During the rest of the year, the ITCZ shifts southwards and dry conditions persist in the region between October and May.Generally, the southern part of the Lake Tana basin is wetter than the western and the northern parts (Kebede et al., 2005).The climate of Lake Tana is characterized by a major rainy season with heavy rains (June-October).Average annual rainfall in the lake area is 1418 mm.Similarly, the water level of the lake fluctuates with rainfall up to 1 m.Maximum water temperature, as a monthly average, ranged between 21 and 26°C over 1997 to 2000.Water temperatures varied between narrow limits, with lowest values in January, a sharp increase in February, peak values in May and a sharp decline with the big rains in June-July (Esthete, 2003).According to the Ethiopian National Meteorology Agency, Bahir Dar Branch, the maximum and minimum rain fall was recorded in July and August, and in December, February and March in the study period, respectively.Likewise, the minimum and maximum temperatures were recorded in August and April, respectively (Figure 2).

Site selection and sampling
The sites were selected based on human-induced pressures along the southern gulf of Lake Tana.Site one (Debre Mariam) was characterized by impacts from agricultural inputs.Site two (West Gojam Zone Administration Prison Station) was characterized by human waste inputs.Site three (Shum Abo) was characterized by transportation and recreation around the site.Site four (Titu Recreation Center and port for Tana Transport Organization) was characterized by both transportation and hotel construction on the shore line of the lake.Site five (Bahir Dar City Legislative Area) was characterized by only swimming, washing clothes and bathing.Site six (Mohammed Hussein Ali Al -Amoudi Resort Area) was relatively not impacted by humans.Site seven (Felege Hiwot Referral Hospital) was impacted by both human wastes from hospitals and several religious schools.The final site (site eight) (Tana Medhanie Alem Integrated Development Association) was characterized by human-induced impacts from agriculture and lodges.The study sites were named as sites 1, 2, 3, 4, 5, 6, 7 and 8 (Figure 3).
Sampling was done using mesh net (pore size 55 µm) starting from site one to eight in offshore by considering depth and representativeness of the sample for the site as well the gulf and emptied into eight pure sample bottles, for species composition determination.Samples were put into an ice box containing no ice, that is, at room temperature after addition of Lugol's iodine solution (purchased from a local pharmacy).Sampling was done once per month for species composition variation of the southern gulf of Lake Tana starting from November 2010 to April 2011.These samples were transported to the laboratory of Biology Department, Bahir Dar University for species identification and enumeration.The sample reached the laboratory within a maximum of two hours interval after collection and stored at 40°C.

Phytoplankton species composition determination and enumeration
The samples were concentrated following the inverted microscope method (Pxinos and Mitchell, 2000).A 10 L volume of water was concentrated to 100 ml using 50 µm pore plankton net.A subsample of 10 ml was taken from this and poured into a 15 ml centrifuge tube and put at room temperature for more concentration following the Utermőhl method for qualitative algal analysis.The 10 ml of water was again concentrated to 5 ml and a sub-sample of 0.1 ml of water was taken using a dropper and one drop of water was put onto an inverted microscope (XDS series Inverted Microscope) slide to identify and quantify the species present.The identification was carried out using various reference books (Fritsch, 1974;Stern, 1997;Elliott, 1974;Botes, 2001).
Assuming that 0.1ml of water represents two drops, the identified species from one drop were multiplied by two, and to get the number of algal and cyanobacterial species in 5 ml of water, the identified algae and cyanobacteria in 0.1 ml of water were multiplied by 50 following the same logic.Likewise, to estimate the number of algal and cyanobacterial species from 100 ml of water, the previous number was multiplied by 1000; that is, the reverse of the concentration.Then, the species were distributed into their respective classes by site and by time of sample collection.

Statistical data analysis
Spatial and temporal phytoplankton species composition was analyzed through one way analysis of variance (ANOVA) based on total phytoplankton cell counts for all species identified to determine if there were spatial and temporal variations in their abundance, using SPSS 16 software (Statistical Procedures Companion, Maija J. NoruŠis).Similarly spatial and temporal distribution patterns of the algae and cyanobacteria were converted to percent to compare specific species percentage out of the total phytoplankton community.The mean, range, minimum and maximum abundance of each class were calculated using MS Excel 2007.Windspeed (m/s) 0.9 0.8 1.3 1.5 1.3 1.1 0.9 0.8 0.8 0.8 0.8 0.7

Phytoplankton species diversity in Southern Gulf of Lake Tana
A total of 67 phytoplankton genera, in nine classes namely: Bacillariophyceae (17 genera), Chlorophyceae (22 genera), Chrysophyceae (5 genera), Cryptophyceae (1 sgenera), Cyanophyceae (10 genera), Dinophyceae (3 genera), Euglnophyceae (1 genera), Zygnematophyceae (5 genera) and Xanthophyceae (3 genera) were recorded in the gulf.The names of the genera together with their classes are presented in Table 1.There were no significant differences among the months of the study (pvalue > 0.05), but, there were significant variations among sites based on cell counts (p-value < 0.05).Figure 4 provides a summary of overall phytoplankton presence at each site and month sampled.At site 1, there was an algal increment towards December and then a decrement towards February and again an increment towards April.At site 2, there was wave like structure where in one month, there was an increment and then a decrement in the next.At sites 3, 5 and 6, there were progressive decrements till March and a higher increment (except site 6) in April.At site 4, there was progressive increment each month relative to the other sites.At site 7, the lowest abundance was recorded in February and remaining months looked similar to each other and had a higher abundance than February.At sites 1 to 5, phytoplankton abundance increased in April.This increment was contributed to mostly by a single phytoplankton class, the Cyanophyceae (comprising more than 60% of the total abundance).

Spatial phytoplankton species diversity
The dominant classes based on abundance (average abundance >350 cells/L) were Chlorophyceae, Cyanophyceae and Xanthophyceae and in some sites, Bacillariophyceae.Bacillariophyceae (commonly diatoms) was neither dominant nor rare; however, it varied from site to site (range from 100 to 500 cells/L) (Figure 5).
Chlorophyceae was present without changing or varying at most sites (sites of 2, 4, 5 and 6).The maximum number of this class and Chrysophyceae was recorded at Tana Medhaniealem Integrated Development Asociation (TMIDA) (Site 8) (Figure 5).The maximum number of Chrysophyceae was recorded near TMIDA.Cryptophyceae was the only class that was not ubiquitous in the southern gulf of Lake Tana.It was only recorded at four sites (sites of 2, 3, 4 and 5).The minimum and maximum number of this class were 0 cells/L (sites 1, 6, 7 and 8) and 233 cells/L (site 4), Table 1.genera and relative abundance of southern gulf of Lake Tana in 2010/11 (where d = less than 20 cells/L, c = between 20 and 50, b = between 50 and 100, a between 100 and 200, a+ = between 200 and 500, a++ = between 500 and 1500 and a+++ = more than 1500 cells/L).respectively (Figure 5).

S/N
The most dominant class in study area by cell number count was Cyanophyceae.Variations in this class were observed at most sites, except at Taitu Recreation Center (site 4).The minimum and maximum numbers of cells/L were recorded at Shum Abo (site 3) and at West Gojjam Prison Station (site 2), respectively (Figure 5).Similarly, the maximum abundance of Class Dinophyceae was recorded near Debre Mariam (Site 1).Relative to other classes, this class was constant in all sites throughout the study periods (Figure 5).
The most dominant class in the study area was class Cyanophyceae.Variations in this class were observed at most sites except at Taitu Recreation Center (site 4).The minimum and maximum numbers of cells/L were recorded at Shum Abo (site 3) and at West Gojjam Prison Station (site 2), respectively (Figure 5).Similarly, the maximum abundance of Class Dinophyceae was recorded near Debre Mariam (Site 1).Relative to other classes, this class was constant in all sites throughout the study periods (Figure 5).
Class Euglenophyceae was not found near Shum Abo (site 3), Taitu Recreation Center (site 4) and Bahir Dar City Legislative Council Office (site 5).The highest and second highest presence was recorded near Felege Hiwot Referral Hospital (site 7) and TIMIDA (site 8) (Figure 5).The second least dominant class in the southern gulf of Lake Tana was the Class Zygnematophyceae.Relatively, the least variations were observed near Debre Mariam and Shum Abo.The maximum cell number of this class was recorded at TMIDA (Figure 5).
The least dominant class in the study area was Xanthophyceae.However, it was persistent throughout the six months of assessment.Its dominancy ranges from nil (sometimes) to third dominant on other occasions.It was counter partner of Cyanophyceae in abundance which means that Xanthophyceae increased and Cyanophyceae decreased (Figure 5).

Temporal phytoplankton species diversity in South Gulf of Lake Tana
In this gulf, the algal classes of Bacillariophyceae and Dinophyceae were almost constant throughout the 6 months of study period of in 2010/11 having the highest prevalence in March and April and the lowest prevalence in December and January respectively.Cryptophyceae and Euglenophyceae were very low in all the months.Class Cyanophyceae presence increased progressively from November February and became constant in March and April even though it slightly increased in April (Figure 5).
The detailed information on each class for all months is presented separately.Bacillariophyceae was a generally persistent class.Generally, this class was relatively higher in November and April.In April, it was sometimes second and third most dominant class in the southern gulf of Lake Tana.The minimum and maximum cell numbers of this class were recorded in December and respectively.Chlorophyceae presence varied from two peaks in November and April and its lowest abundance in February.It gradually decreased from November to February and then gradually increased from February to April.Usually outliers were common in terms of this class in the months of November, December and March.Similar to Chlorophyceae, the maximum abundance of Chrysophyceae was recorded in November and April.In contrast to Chlorophyceae, it neither decreased nor increased in between these two months which means it was constant from December to March even though outliers were common in February and March.There were almost nil Cryptophyceae in the southern gulf of Lake Tana.It was only recorded in January and February.The range of variation on this class ranged between zero (in November and December, March and April) and 234 cells/L in February.
Cyanophyceae was, as in its spatial distribution described above, dominant throughout the 6 months of study period in the southern gulf of Lake Tana.It gradually increased from November to December and again decreased in January.It then increased from January to April (maximum abundance in February).Dinophyceae was constant throughout the 6 months study period.This class can be divided in to two on a monthly basis: relatively higher abundance and abundance months.November and April were the months when the maximum growth of Dinophyceae was recorded.The remaining months were constant and slightly lower growth of Dinophyceae was observed.The highest abundance in Dinophyceae was recorded in April, 2011 relative to the remaining months.
Euglenophyceae and Cryptophyceae had a similar distribution and abundance in the southern gulf of Lake Tana.Both of them had a low abundance and were found at some sites and months of the phytoplankton assessments.The 6 monthly minimum and maximum abundances of Euglenophyceae were in November and December of 2010, respectively.
The Zygnematophyceae attained the minimum cell counts in March and maximum cell counts in April, respectively.Relatively, this class fluctuated from month to month.Xanthophyceae were opposite to Cyanophyceae in their dynamics and structure in the phytoplankton community of the southern gulf of Lake Tana.The maximum abundance of this class was recorded in November and decreased in December.It then increased from December to January and again gradually decreased in April.

DISCUSSION
According to Talling and Heaney (1988), the dominance of phytoplankton species in water bodies is determined by complex interactions between biological, physical and chemical variables.In the southern gulf of Lake Tana, Xanthophyceae was the dominant class in November and it presence slowly decreased to April.This may be due to grazing by fish and competition from Cyanophycae for resources at the same time as nutrient loads in water inflow to the lake from various sources within the catchment decreases.Fish production, for example, is low in early summer (Tesfaye, 1998).Nutrient loads due to run off into the lake are high in the summer season (June-Sept) and low for the rest of the year (Yirgalem and Assefa, 2009).In contrast to this, Cyanophyceae consequently increased from November to April.This may be due to taste and odor of compounds and perhaps toxins may deter grazers, so that it cannot be eaten mostly by grazers.
As reported by Shapiro (1997), the ability of cyanobacteria to use bicarbonate as a carbon source is one of the reasons for their dominance in alkaline aquatic systems.This could be applied for the southern gulf of Lake Tana, which has an average pH of 6.83 to 8.3 (Eshetye, 2003).In this study, it was found that at almost all sites, the dominant phytoplankton species were Microcystis (cyanobacteria), Tribonema, Aphanocapsa, Peridinum and Cymbella species.This might be bicarbonate sources in the gulf.
Swimmers have reported rashes, hay fever-like symptoms and even pneumonia associated with blue green algae blooms and their toxins (Fleming and Stephan, 2001).According to Fleming and Stephan (2001), abdominal cramps, nausea, diarrhea and vomiting may occur if the swimmer swallows the untreated water.Even water-skiing, showering or cleaning with this water may make people sick because the toxins may be absorbed from water breathed into the nose.Unfortunately, boiling water does not remove or destroy these toxins (Fleming and Stephan, 2001).Similarly, those people swimming in the southern gulf of Lake Tana, if inflows of untreated sewage and other untreated runoff occur, they could be subject to a lot worse disease and gastrointestional problems.In addition, the dominance of single class Cyanophyceae may be there is imbalance in species diversity in the study area.

Conclusions
Incidents attributed to cyanobacteria are far more numerous and, in most cases, have been caused by species of cyanobacteria that may accumulate as surface scums of extremely high cell density.As a result, the toxins they contain ("cyanotoxins") may reach concentrations likely to cause health effects (World Health Organization, 2000).In the southern gulf of Lake Tana, the phytoplankton species diversity is poor.However, there was a very high density of cyanobacteria.Cyanophyceae species can cause severe problems to the people living around the gulf, especially swimmers and people drinking untreated water from the lake.
Different factors can influence the growth of phytoplankton resulting in the dominance of different phytoplankton taxa.There was a significant difference in phytoplankton species composition in the southern gulf of Lake Tana.Thus, there might be human impacts that makes some phytoplankton to be dominant (Microsystis sp. and Tribonema sp.) and some others which have low abundance or sometimes not present at all.Absence of significance differences among sites might be due to a possible similarity in anthropogenic impacts at all sites around the southern gulf of Lake Tana or that the gulf is well mixed so that impacts become more widely and evenly dispersed across the gulf.The presence of significance variations between the monitored months in terms of phytoplankton abundance showed that there were very high anthropogenic impact differences from time to time in gulf or simply seasonal effects especially inflows.

RECOMMENDATIONS
During the study period, there were many constraints regarding resources (finance, apparatus and man power, especially skilled man power).If these constraints were absent in the study, there might be at least 100 phytoplankton species identified.Therefore, there is a need for more work to be done in the southern gulf of Lake Tana to accomplish the target objectives.In the gulf generally and Felege Hiowt Referral Hospital and West Gojjam Prison Stations areas in particular, there was reckless use of the gulf by adding waste and hence nutrients may increase which is favorable for a few species to become dominant.Hence in future work, there would be a better opportunity for removing inputs (pressures) from areas surrounding the gulf ecosystem.Such work can include policy, decision and plan making and thus bring sustainable growth and protection of the aquatic biodiversity and resources of the gulf for future generations.
Since the gulf receives nutrients from the inflow rivers during the flood season and contaminated runoff from various sources like the hospitals and prison stations, there is a need to balance, remove or at least minimize the hydrological changes.It is better, for Bahir Dar City people in general and for individuals who use water of the gulf in particular, to avoid drinking, cooking or bathing with untreated water from the gulf.

Figure 1 .
Figure 1.Location map of the study area.

Figure 4 .Figure 5 .
Figure 4. Phytoplankton distribution and abundance in the study area for November 2010 to April, 2011.