Phytoplankton communities and environmental variables as indicators of ecosystem productivity in a shallow tropical lake

Abstract The water quality and ecosystem productivity of a tropical lake in Ethiopia (Lake Arkiet) was evaluated using phytoplankton and environmental factors. This was to ascertaining the lake’s potential for various applications. Phytoplankton communities and some selected environmental variables were collected from two predefined sampling sites (open water and littoral) using a seasonal campaign between March to May (dry season) and June to August (wet season) in 2022. The analysis of the physicochemical characteristics of this study showed that the lake’s water was well-oxygenated (6.8–16.7 mg/L), warm (25.8–29.8 °C), turbid (154–317 NTU), had poor water transparency (4.3–16.1 cm), and was alkaline (pH = 7.29–11.31). The concentrations of inorganic nutrients (phosphate and nitrate) were notably high, ranging from 2.12–5.26 and 2.19–10.64 mg/L, respectively. A total of 34 phytoplankton taxa from four divisions were identified in Lake Arekit. Bacillariophyceae (18 taxa) and Chlorophyceae (10 taxa) were the major groups of phytoplankton which together represented the largest (82%) phytoplankton taxa. The total biovolume of phytoplankton in the lake was to be 384.15 mm3/L. Cyanobacteria contributed the largest (41%) phytoplankton biovolume followed by Bacillariophyta (32%). The highest biovolume (about 75%) was constituted in the Microcystis aeruginosa, Cylindrospermopsis raphidiopsis, Anabaena spiroides, Pediastrum duplex, Aulacoseira granulate, Navicula schroeteri, and Nitzschia palea. The distribution of environmental factors and phytoplankton communities showed greater seasonality. High phytoplankton biovolume was observed in the dry season at both sampling sites concurrently with high water transparency and low water temperature. Findings from this study revealed that Lake Arkeit was a moderately polluted but productive lacustrine ecosystem that can still support aquatic life, fish production, irrigation, and aquaculture. The primary source of the pollution is the entry of inorganic and organic wastes from a brewery and other water bottling industries nearby and around the lake, as well as possible excessive nutrient levels due to the lake’s geological background. It is advised that commercial factories stop producing or releasing waste in open spaces that are heavily washed and enter the lake via flooding and tributary rivers and streams. Additionally, the local government should work with the nearby communities to set up a buffer zone where people are prohibited from engaging in agricultural activities to protect and reduce pollution, which will help to preserve the lake’s water and ecosystem.


Introduction
Water quality is an ensemble of physical, chemical and biological characteristics of the given water (Carmack and Wassmann 2006).Plankton communities (phytoplankton and zooplankton) are one of the main indicators of biotic communities for freshwater aquatic ecosystems.Plankton are susceptible to water pollution (Zelalem 2015).Species diversity of plankton is an excellent biological indicator of an environment's health (Ogbeibu and Edutie 2002).Phytoplankton communities give more information on changes in water quality than mere nutrient concentrations or chlorophyll-a concentration (Emmanuel et al. 2008).As phytoplankton assemblages are at the base of the food web, changes in phytoplankton community structure have implications for the whole community (Soares et al. 2007;Edward and Ugwumba 2010;Barupal and Gehlot 2015).Rott et al. (2008) evaluated the water quality in tropical Asian water bodies using the Shannon diversity and evenness indexes of phytoplankton.Peerapornpisal et al. (2007) created a way to evaluate the water quality in bodies of water using a dominating phytoplankton scoring system, i.e.AARL-PP score, which is a straightforward evaluation method devoid of chemical requirements.This index is obtained by score values of the dominant phytoplankton genera representing at least 5% of the total bio-masses.Amphorn and Wanninee (2013) also determined the water quality of Pak Phanang River Basin (Southern Thailand) using phytoplankton as a bio-indicator of Water Quality.They stated that values of the diversity's water quality index >3.0indicate clean conditions, values 1.0 indicate severe contamination and intermediate values indicate moderate pollution.Melosira and Cyclotella phytoplankton are typically found in clean water, but Nitzschia, Microcystis, and Aphanizomenon are typically found in contaminated waterways (Amphorn and Wanninee 2013).The top eight genera that can indicate organic contamination in the waterways are Euglena, Oscillatoria, Chlamydomonas, Scenedesmus, Chlorella, Nitzschia, Navicula, and Stigeoclonium, according to their list.Chlamydomonas, Euglena, Scenedesmus, and Microcystis are indications of eutrophic waters.Anabaena is typically found in waters with low nitrogen content, whereas Aphanizomenon, Microcystis, and Ceratium are typically found in waters with high phosphate levels (Reynolds and Lund 2006).
Eutrophication of freshwater is regarded as a water quality issue which results in the deterioration of the aquatic environment and impacts on water usage (Chislock et al. 2013).Determining trends in the eutrophication process in aquatic ecosystems and the potential responses of the primary indicators of biotic communities are key problems of contemporary research (Sonia et al. 2009).The preliminary survey of this study demonstrated that as a result of high anthropogenic impacts such as small-scale irrigation, plaguing activities with no buffering zone, car washing in the lake's shoreline area, washing clothes, entrance of wastes from house hold and factories, there is an indication of eutrophication of the lake water which has an impact on the water quality and productivity of the lake.Phytoplankton communities have long served as a source of ecological indicators.In Lake Arkeit, however, there is no study on phytoplankton.
The aim of this study was to determine the water quality and productivity of Lake Arqiet based on its phytoplankton communities.This was with a view to ascertaining the potential of the lake water for multidimensional uses.

Study area description
This study was conducted in Lake Arqiet which is located the Gurage drainage basins of Ethiopia.The Gurage Zone is located in Central-south Ethiopia with the location between 7040' to 8030' North and 37030' to 38040' East and covers an area of 5,932 km2 (Belay et al. 2021).According to Belay et al. (2021), Gurage Zone is divided by four drainage basins, namely Awash, Rift Valley, Bilate and Omo-Gib.The Western Gurageland (Watersheds) drains to Omo-Gibe Basin and cover large areas.In the western parts of Gurageland, several rivers and streams drain from northeast and east to west that is to Gibe River.With the exception of minor deviations at local level, the streams in the ecoregion have a dendrite drainage pattern (observation of this study).The major streams ultimately drain to west and south-west/east ward following the general inclination of the slope direction of the agro-region.The western watersheds of the Gurage land drain to Omo-Gibe Basin and cover large areas.In this part of the watershed, several inland waters are available, which drains from East to West (Belay et al. 2021).
The reconnaissance assessment for this study revealed that because of its hilly nature, the Gurage drainage basin has the potential to be a natural surface water source.A total of 44 surface water bodies have been noted in the Gurage drainage basins.Only small portions (5%) of the water bodies are lacustrine, the majority (86%) of which are riverine (perennial rivers).Hot springs make for nearly 10% of the drainage system's remaining water bodies.Lake Arekit is one of the numerous naturally occurring inland water bodies in the Gurage drainage basin.The lake is about 240 kilometers away from Addis Abeba, the country's capital.It is situated in the Gumar district, 70 kilometers from the settlement of Wolkite (the capital town of the Gurage zone).Lake Arekit is located between latitudes 70°59 '30" and 80°16'00" N and 370°53'30" and 380°10'00" E, respectively.It is located at an elevation of 2820-2950 meters above sea level.The lake has a surface area of 130 hectares and an average depth of 2.5 meters (Figure 1).The Lake receives its water from two sources: direct rainfall and runoff from the nearby watershed.The lake has no outlet.It is a highland lake system (observation during this study).

Study design and sampling sites
For the analysis of some selected environmental variables (physicochemical parameters) and biotic variables (phytoplankton communities), samples were collected from two predetermined sampling sites over six sampling months, following a seasonal pattern (three times from the dry season: March to May 2021; another three times from the wet season: June to August 2022).The sampling locations were selected based on the presence or absence of significant human effect, with one sample obtained from a location that had suffered less impacted (site 1 -the lake's open water), whereas Site 2 was the lake's littoral region (shoreline region) that had been severely impacted (Figure 1).

Physicochemical parameters sampling and measurements
A monthly routine water sample collection was made from the lake's surface area at the two predetermined sampling sites (sites 1 and 2) for the investigation of specific physicochemical variables using opaque 1 L plastic bottles and it chilled in the icebox on site and transported to the Limnology Laboratory of Addis Ababa University for further chemical analysis.In-situ measurements of water temperature, dissolved oxygen, and pH were measured using a portable multimeter probe (Model HQ 40d Multi Hach Lange) in the early mornings between 8:00 and 11:00 am.Turbidity and water transparency of the lake water was measured using a portable digital turbidimeter (Model Oakton: T-100) and a standard Secchi Disc (28 cm in diameter with alternating black and white quarters), respectively.The major inorganic nutrients (NO 3 and PO 4 ) were determined using the APHA's (1995) suggested standard determination approach.

Phytoplankton sampling, identification and enumeration
For identification and quantitative analysis, phytoplankton samples were collected using a seasonal campaign from the two selected sampling locations.To qualitatively analyses the phytoplankton, 1 litre sample of water was collected from the near surface.The water samples were immediately fixed in Lugol's iodine solution.To make 10 times more concentrated samples, the preserved samples were concentrated in 1 L measuring cylinders in the dark for 48 h.Then, the upper 900 ml of the sample were siphoned off, and the remaining 100 ml were homogenized.Samples were cleaned using a cold H 2 O 2 washing technique) to easily identify the diatoms (Taylor et al. 2011(Taylor et al. , 2016)).From the homogenized sample, 1 mL sample was pipette into Sedgwick Rafter cell (1 mm 2 ) to identify the possible lowest taxonomic groups of phytoplankton.Identification was done using identification keys of Cronber and Komarek (2004), Komárek and Anagnostidis (2000), and crosschecked to other relevant sources available in the web.The biovolume (cell volume or area or space covered by cell area or space covered by cell) of each individual taxon was determined by measuring the dimensions of the cell directly and then applying similar geometric shapes to the cell forms (Hillebrand et al. 2002;Sun and Liu 2003).

Data analysis
The seasonal variability in the phytoplankton taxa and the various environmental variables were determined using independent t-test.The association between phytoplankton taxa and the environmental factors were ascertained using Pearson correlation (r) analysis.Binary regression model was used to cheek the effect of environmental factors on the existence and bioolume of phytoplankton communities.Shannon diversity index (H') was used to classify the water quality of the lake based on phytoplankton diversity index.SPSS software were used to analyses data at 0.05% statistical significant.

Seasonal variations of environmental variables
The results of the analyzed environmental factors that contributed to the seasonal variability of phytoplankton communities in the study lake were presented in Table 1.The results of the physicochemical properties revealed that the bulk of the measures had a significant seasonal variation (p < .001)(Table 1).The surface water temperature and dissolved oxygen (DO) of the lake Arkeit had inverse negative relationships (r= − 0.876).Both parameters showed a seasonal effect (p =.001).The average temperature was higher during the wet season than during the dry season at both sample locations (Table 2).At both study sites, DO was meaningfully greater during the dry season.The peak temperature of the lake was seen in August alongside the occurrence of heavy rain.In contrast, DO levels peaked in March and dropped in August.The pH scale used to determine how acidic or alkaline of the lake's water was ranged in value from 7.29 to 11.31.The mean pH value was higher in the dry season than the wet season with a significant difference between the two seasons (p= .001).The uttermost pH value was observed in March (the mid dry period) and the bottommost pH value of the lake water was seen in August (the heavy rain month).pH had negative and significant correlation with temperature (r= −.889) and the two inorganic nutrients-NO 3 (r= −.844) and PO 4 (r= −.865).pH, however, was positively correlated with water clarity (r= .842).The lake's water had a turbidity range of 154 to 317.0 NTU and a transparency range of 4.3 to 16.1 cm.Water turbidity and transparency showed a stronger seasonal variations at a P value of .017and .004,respectively.Turbidity and transparency had a negative and inverse correlation (r= −.752) between them in which high water turbidity was observed in the main rainy period.Contrary, high water transparency was observed during the dry season (Table 1).The two main inorganic nutrients that were detected in Lake Arkeit during this study were nitrate (NO 3 ) and phosphate (PO 4 ).Despite there being no statistically significant difference between the sampling sites, both nutrients were rather high in the lake's littoral region (shoreline) (Table 1).Nonetheless, there was a significant difference in the two nutrients' amounts between the two study seasons (p =.002).Both nutrients were high during the rainy season (June to August) consistent with significant runoff into the lake.

Seasonality in phytoplankton taxa richness and biovolume
A total of 34 phytoplankton taxa from four divisions-Bacillariophyceae, Chlorophyceae, Cyanophyceae, and Cryptophyceae-were identified in Lake Arekit (Table 2).The Bacillariophyceae (the diatoms) were the most widely diverse groups of phytoplankton.With 18 taxa, about 53% of the phytoplankton's total taxa were represented by this group.Chlorophyceae (the green algae) were the second most diverse class of phytoplankton after the diatoms.Around 29.4% of the phytoplankton taxonomic richness in the study lake was made up of green algae, which had 10 taxa.The number of phytoplankton taxa showed a seasonal effect (p=.002).High number of phytoplankton species (23 taxa) was seen in the dry season coincided with the high water transparency.The total biovolume of the dominant phytoplankton that was computed in Lake Arkeit varied from a minimum value of 1.02 mm3/L occurred at the shoreline site to a maximum value of 90.73 mm3/L that occurred in the open water site (Table 3).Seasonality Note. the species with astrix "a" are phytoplankton that occurs regularly, whereas "b" is phytoplankton that occur rarely in the study lake.
in phytoplankton total biovolume was observed (p=.025).It displayed a dramatic decrease pattern from dry to rainy months (Figures 2 and 3).In March (mid of the dry period), the largest total biovolume was calculated at both sampling sites (Figure 3).The least total biovolume was reported in June (Table 3).The correlation analysis as the total biovolume of phytoplankton had a positive correlation with the two inorganic nutrients -NO 3 (r=.076)and PO 4 (r=.819) and important physicochemical parameters like DO (r=.593), temperature (r=.794), and pH (r=.753) concentrations.Cyanobacteria (blue-green algae) contributed to the greatest phytoplankton biovolume, followed by Bacillariophyta (the diatoms).The two phytoplankton groups together contributed roughly about 72% of the total phytoplankton biovolume in the study lake.Next to cyanobacteria and diatoms, chlorophytes (the green algae) contributed a comparatively high proportion of total phytoplankton volume (Figure 4).Anabaena sp., Cosmarium sp., Cylindrospermopsis sp., Microcystis sp., Navicula sp., Nitzschia sp., Pediastrum sp., Peridinium sp., Scenedesmus sp., and Synedra sp. were the top ten most dominant and consistently appearing phytoplankton species in the Lake Arkeit.More than 95% of the total phytoplankton biovolume in Lake Arkeit was accounted for by these phytoplankton species (Table 4).Among cyanobacteria, Anabaena sp., Cylindrospermopsis sp., and Microcystis sp. were the most prominent taxa.These taxa accounted for roughly 34% of the total phytoplankton biovolume in the study lake.Microcystis sp.dominated the biovolume of phytoplankton within and between phytoplankton groups among these blue-green algae.It accounted for 53.36% of the biovolume of blue-green algae.This species also contributed the greatest percentage (17.32%) of the total phytoplankton biovolume (Table 4).Anabaena sp. and Cylindrospermopsis sp. were also contributed a sizable share of total phytoplankton biovolume.They accounted for about 20% of the total biovolume of phytoplankton (Table 4).
Thalassiosira sp., Aulacuoseria sp., Fragilaria sp., Navicula sp., Nitzschia sp., Peridinium sp., Synedra sp., and Asterionella sp. were the most visible diatom taxa in Lake Arkeit.These taxa accounted for approximately 95% of total diatom biovolume).The most prevalent species of the diatom were Navicula, Nitzschia, Peridinium, and Synedra, accounting for approximately 31% of total phytoplankton biovolume (Table 4).These four phytoplankton species accounted for around 90% of the biovolume of the diatom communities.In terms of biovolume, chlorophytes (green algae) were the third most important category of phytoplankton after cyanobacteria and diatoms.Closterium sp., Cosmarium sp., Merismopedia sp., Oocystis sp., Pediastrum sp., and Scenedesmus sp. were the most common green algae species in the study lake.The total biovolume of green algae was dominated by Cosmarium sp., Pediastrum sp., and Scenedesmus sp.These three species accounted for around 98% of the total biovolume of green algae.They also contributed significantly (almost 25%) to the total phytoplankton biovolume in the study lake (Table 4).

Environmental variables
Most environmental variables did not differ significantly between sampling sites; this is feasible given the occurrence of total mixing in top-down and horizontal water  movement.This is evident from a study of most shallow Ethiopian water bodies (Abnet and Seyoum 2018;Muluken et al. 2020).On the other hand, seasonality of several environmental factors in the Lake Arkeit has been prominent because there is an environmental oscillation between the two seasons of Ethiopia.The surface water temperature of the study lake is relatively high indicating, the lake water is fairly warm as in any other tropical lake system in Ethiopia.The lake's temperature makes it necessary for a variety of multifaceted applications, including irrigation, fishing, and animal consumption.Fish, especially tropical species, are harmed by water temperatures below 14 °C and above 39.5 °C (Fahmida et al. 2022).Natural tropical water has a temperature range of 25 to 30 0 C, which is ideal for aquatic life survival (Francis and Unique 2017).Since Lake Arkeit's highest temperature is within the typical range of temperatures for tropical waters, it is ideal for aquatic life.Lake Arkeit is well oxygenated and suitable of the survival of most of the aquatic live.The occurrence of relatively high DO in the study lake could be attributed to well macrophyte coverage in the eastern lake's littoral region.Complete oxygen depletion has not been observed in the study lake, apparently because of significant water movement through the lake due to frequent top-down mixing (Tamire and Mengistou 2012).The standard DO value for fisheries and aquatic life is between 5.0 and 9.0 mg/L (EU 2016;ESA 2013).WHO (World Health Organization) ( 2004) recommended that DO level greater than 5 mg/L are essential for aquatic life to survive.Fish and other species move to another place as the DO content in the water drops.
Based on this, the DO concentration of the Lake Arkeit is suitable for aquatic life  Note.the taxa with small letter "a" represents the species which were relatively contributed significant value (10% and above) for the total biovolume of phytoplankton in the study water body survival including fish.The pH level in Lake Arkeit is fairly high and appears to be fairly productive based on the pH levels.WHO (2008) claims that water bodies with a pH lower than 4 have a sour taste.The pH range between 6.5 to 8.5 is necessary for the majority of aquatic life to exist (BSI (British Standards Institute) 2003).The pH values of Lake Arkeit fall within the range of the maximum permissible limits for usage in water for agriculture, fish production, and other aquatic life established by the WHO in 2008.
The Lake Arkeit water is turbid and has relatively poor water transparency.This is may be the result of heavy rainfall, which transported sediment-filled waters from surface runoff via Feeder Rivers.Turbidity is limited to 5 NTU or less when used for drinking (WHO (World Health Organization) 2004; ESA (Ethiopian Standard Agency) 2013).Lake Arkeit' water turbidity is significantly high the minimum turbidity value is over 40 times higher than the allowed maximum drinking level.As a result, water should not be consumed directly without first undergoing a specific chemical or biological treatment, such as Solar Water Disinfection (SODIS) (Regula 2002).On the other end of the scale, turbid water is a sign of productive water (Yirga and Brook 2018).Turbidity can aid in the survival of aquatic life as long as there is no eutrophication (the formation of an algal bloom-the root of illness) because a variety of solid and liquid wastes may enter with vital nutrients (e.g.nitrate and phosphate).Nutrient enrichment of water bodies fosters the growth and survival of micro and macroorganisms like fish, phytoplankton, zooplankton, and macro-invertebrates.
The nutrients (nitrate-NO 3 -N and phosphate-PO 4 ) in Lake Arkeit's water are equitably high.This could be attributed to the use of fertilizers in agricultural fields along the shore.Nitrate (NO 3 -N) concentrations below 5 mg/L had no effect on flora (ESA (Ethiopian Standard Agency) 2013).The maximum concentrations of NO 3 required for livestock and irrigation were 100 and 30 mg/L, respectively (WHO (World Health Organization) 2004).The nitrate concentration in Lake Arket is within acceptable limits for livestock, irrigation, and aquatic flora survival.Irrigation water phosphorous concentrations should not exceeded 2 mg/L (ESA (Ethiopian Standard Agency) 2013).Based on this fact, the phosphate range in Lake Arkeit exceeded the permissible limit for irrigation purposes as well.Phosphorus concentrations less than 300 µg/L, indicated that the water body was not disturbed.The minimum concentration for phosphorus in Lake Arkeit is more than seven times higher than the maximum level of EPA stated.This clearly shows that anthropogenic factors (such as small-scale irrigation, wastes from commercial factories, car washing, etc) have adversely affected the water body and its watershed area in the present study.

Phytoplankton communities as indictors of ecosystem productivity
The phytoplankton taxa found in Lake Arkeit are moderate and comparable to those found in other productive freshwaters in Ethiopia (L.Bishoftu −22 species; Tadesse 2007, L. Kuriftu −25 species;Zelalem 2007, L. Babogaya-32 species;Yeshiemebet 2006. and L. Hayq-40 species;Tadesse 2010).The presence of relatively moderate level of phytoplankton diversity in Lake Arkeit could be the presence of high inorganic nutrients, physicochemical factors like warm water temperature, well dissolved oxygen contents, the water is not acidic, frequency of mixing, and related factors.In most tropical lakes of Ethiopia, the taxonomic families of blue-green algae and green algae were species-rich (Girma 2006).Ethiopia's Rift Valley Lakes Ziway, Hawassa, and Chamo have the highest species richness contributions from green algae, followed by blue-green algae (Girma and Ahlgren, 2010).Diatoms are dominated the diversity of phytoplankton in Lake Arkeit.This could be associated with the high concentration of nitrogen and phosphorus nutrients that came from the absence of other competitive algae species.Chia et al. (2011) claimed that diatoms predominate in water columns with considerable mixing.The exceptional taxonomic richness of diatoms in Lake Arkeit has been attributed to the same tenable explanation.
The most noticeable and commonly occurring phytoplankton in Lake Arkeit are Anabaena sp., Microcystis sp., Oscillatoria sp., Pediastrum sp., Clostridium sp., Cosmarium sp., Scenedesmu sp.s, Asterionell sp.a, Aulacuoseri sp.a, Peridiniu sp., Synedra sp., and Ankistrodusmus sp.These species of phytoplankton may prefer the lake water as a result of high nutrients and the relatively high water temperature.In Ethiopia's most productive lakes, blue-green algae like Anabaena sp. and Microcystis sp., diatom phytoplankton like Navicula sp., Ankistrodusmus sp., Pediastrum sp., Cymbella sp., and Aulacuoseria sp., and green algae like Clostridium sp., Cosmarium sp, and Scenedesmus sp. are the most common (Girma 2006).The fact that these species are primarily found in Lake Arkeit, may indicate that the lake is productive based on the phytoplankton communities.
The biovolume of phytoplankton that we figured out in Lake Arkeit is also relatively high, suggesting that the lake's water is suitable for primary productivity and the ecosystem's next food chain.The study lake's comparatively high phytoplankton biovolume is likely caused by the lake's high inorganic nutrient content as well as other abiotic factors like sufficient DO levels, the lake's alkaline water, and the lake's relatively warm surface water temperature.Seasonality in the biovolume of phytoplankton is more noticeable in Lake Arkeit.Most allegations demonstrate a large bivolume of phytoplankton during the dry season.This is due to relatively high water transparency in this dry period.As a result, the lake's water column receives more light, which in turn increases phytoplankton output.On the other hand, during the wet season, there is great biovolume of cyanobacteria, which contributes the most to its total biovolume of phytoplankton in the lake.This is due to the fact that there is a greater concentration of nutrients and full mixing, which allowed species of cyanobacteria to easily access those nutrients for growth and development.Recent research in some Ethiopian lakes (L.Hawasa, L. Ziway, L. Chamo, and L. Tinishu Abya) demonstrates that the dominance of cyanobacteria is the result of other phytoplankton groups experiencing disproportionately higher losses (Girma and Ahlgren, 2010).
Microcystis aeruginosa is consistently the most noticeable phytoplankton population, and it once again contributed the largest bivolume of phytoplankton.This species is most prevalent during the main rainy season (July and August), which may be caused by the increasing water mixing.In the nearby rift valley Lakes Lake Hwassa, Chamo, and Ziway as well as the tropical high land Lake Lake Hayq and other Ethiopian creator lakes L. Kuriftu and L. Bishoftu, it was also found that M. aeruginosa predominated during mixing and rainy periods rather than a period of thermal stability.High temperature promotes cyanobacterial supremacy.In lakes and reservoirs of the tropical regions, high temperatures encourage the best growth of Cyanobacteria, particularly Microcystis sp.(Yirga and Brook 2018).In Ethiopian lakes and reservoirs, cyanobacterial power is a frequent occurrence (Girma and Ahlgren, 2010).Other studies have emphasized the association of cyanobacterial dominance with a variety of factors, including luxury phosphorus consumption, ability to reduce grazing and buoyancy regulation (Carvalho and Kirka, 2003;Pinto-Coelho et al. 2005).Following M. aeruginosa, Anabaena spiroides made a noteworthy contribution to the lake's overall phytoplankton biovolume.Anabaena sp. is the nitrogen-fixing blue-green phytoplankton that forms as plankton in shallow water.This species prefers conditions with strong nitrogen nutrient levels.Nitrate level in Lake Arkeit is reasonably high.This contributed the dominance of A. spiroides in Lake Arkeit next to M. aeruginos.In addition to the two cyabobacterial phytoplankton species (M.aeruginosa and A. spiroides), Navicula species such as Navicula abdita and Navicula abelioensi (diatoms) and Pediastrum species such as Pediastrum simpex and Pediastrum duplex (green algae) contributed significantly to the total biovolume of phytoplankton for Lake Arkeit.All of these species are more prevalent in the dry season when there is a high concentration of dissolved oxygen and an increase water transparency.
Phytoplankton are very susceptible to the stress that they produce because of their small size, fast rate of growth and spatiotemporal instability in reaction to environmental conditions (Sultan et al. 2003;Sosnovsky et al. 2010;Nwonumara et al. 2016).The Shannon-Wiener Diversity Index (H′) is one of the popular diversity indices that are frequently used in environmental monitoring.On the basis of H' , there are three classes of pollution status.Water bodies with H′ values greater than 3 have no contaminants, H′ in between 1-3 have a moderate level of contaminants, and H′ less than 1 indicates a high level of pollution.Lake Arkeit is moderately polluted based on this classification.

Conclusion
The findings of the various environmental variables reveal that the lake's water is well-oxygenated, warm, turbid, low in water transparency, alkaline, and with nutrients enrichment.The diversity and their volume of phytoplankton in Lake Arkeit might be seen as being on a medium level.In a nutshell, the water quality of Lake Arkeit in terms of environmental and phytoplankton diversity could be a eutrophic and moderately-polluted, but a productive lacustrine ecosystem that may still support aquatic life, fish production, irrigation, and aquaculture.The primary source of the pollution is wastes from a brewery and other water bottling industries around the lake.It is advised that commercial factories stop releasing waste in heavily washed open areas that link to the lake through flooding and tributary rivers and streams.Additionally, the local government should work with the neighboring local communities to construct a buffer zone where agricultural operations are restricted in order to safeguard and prevent pollution, conserve the lake's water, and preserve the ecology.
study.He currently holds the position of Assistant Professor in the Department of Biology at Wolkite University.He teaches a variety of biological classes and advisory graduate and undergraduate students, and participates in a range of volunteer activities for the community.He is the first and the correspondence author of this manuscript.
Solomon Wagaw received his Ph.D. in Fisheries, Aquaculture and Aquatic Sciences from Addis Abeba University in 2020.He had about nine articles in peer-reviewed journals.Dr. Solomon is currently working by Wolkite University as an Assistant Professor and researcher.The manuscript's associated author is him.He took part in the writing, gathering, and analysis of the data for this manuscript.

Figure 1 .
Figure 1.Map of the study area and the sampling sites (site 1-one water and site 2-shoreline).

Figure 2 .
Figure 2. total biovolume (mm3/l) at open water and shoreline sampling sites during the study months in lake arkeit.

Figure 4 .
Figure 4. Percentage contribution (%) of each phytoplankton divisions for the total volume of phytoplankton in the study lake.

Table 1 .
seasonal variations in the environmental variables in lake arkeit.
Note. n-number of sampling periods and values with letters (a, b) within a column are significant different at different p values (values with letter a are significant at p < 0.001 and values with letter b are significant at p < 0.05).

Table 3 .
the biovolumeof phytoplankton divisions (biovolume -mm3/l) in lake arkeit at two sampling sites (open water and coastline area) and throughout the sampling months (March-august).

Table 4 .
the percentage contribution of the dominant phytoplankton taxa to total volume in lake arkeit.