Zooplankton as ecosystem indicators and their effects on eutrophication in Lake Arekit (Ethiopia) – implication for freshwater habitat management

Abstract This paper examines the zooplankton abundance and composition along with various environmental factors in Lake Arekit – a shallow freshwater habitat in Ethiopia’s rift valley system. Zooplankton samples and specific environmental data were collected by a seasonal campaign (rainy season: June to August) and dry season: March to May 2023) from three sites: inlet area (IA), pelagic area (PA), and macrophyte area (MA). In total, 11 zooplankton taxa from four different groups – rotifera (7 taxa), cladocera (2 taxa), copepod (1 taxa), and ostracoda (1 taxa) – were identified in Lake Arekit. Large-bodied copepoda and cladocera comprised 90% of the zooplankton density in the lake, but small-bodied rotifers dominated the zooplankton diversity. The most noticeable zooplankton in the lake were Mesocyclops aequatorialis, Daphnia barbata, and Cypridopsis vidua. Seasonal difference in the total zooplankton abundance was evident (p < 0.01): being high during the rainy season which was accompanied by an increase in the lake’s water temperature and nutrient levels. At MA, where D. barbata predominated, chlorophyll a was relatively low. Chlorophyll a had a strong negative correlation with the abundance of grazing zooplankton, especially with D. barbata (r = −0.563) and C. vidua (r = −0.518). The zooplankton composition and abundance of Lake Arekit reveal a low Shannon diversity index (H’) (1.03) and a predominance of a few species, indicating heavy contamination of the lake’s water. Moreover, Lake Arekit was determined to be hypertrophic (TSI > 70) using Carlson’s trophic index (TSI), which took into account the lake’s water transparency, total phosphorus, and chlorophyll a. Our results indicated that high densities of Daphnia should be the goal of biomanipulation since they can achieve filtration potentials high enough to enhance the water’s clarity in hypereutrophic lake. Even though high D. barbata concentrations cannot address the underlying cause of eutrophication, grazing can assist by lengthening the clear-water stages in a hypertrophic lake – Lake Arekit. The sources of pollution for the lake are the discharge of wastewater from nearby water bottling companies and possibly high nutrient levels from the lake’s geological history. Stopping the discharge of wastewater from the nearby commercial industries (the major source of contaminant) into heavily washed open areas and establishing a buffer zone may help manage the water quality of Lake Arekit.


Introduction
Ethiopia is blessed with a plethora of freshwater habitats due to its unique geographical location and wide-ranging climatic fluctuations (Kebede and Travi 2012;Ayenew et al. 2013).The country's inland waters are estimated to occupy an area of 8800 km 2 bordered by 12 river basins (Ayenew and Legesse 2007;Kebede 2016).Freshwater habitats of Ethiopia provide a variety of useful ecosystem services, including the production of fish, energy, water supply, and ecotourism (Fetahi 2019).These natural ecosystems are, nonetheless, deteriorating and in danger because of anthropogenic activity-related habitat degradation, contamination, overexploitation, and harm to biodiversity (Muhsin et al. 2023).Eutrophication is a leading cause of impairment of freshwater ecosystems in Ethiopia (Fetahi 2019).
Eutrophication of surface waters (i.e.proliferation of algae as a result of high levels of nutrients) (Glibert 2017) upsets the natural balance of aquatic ecosystems and is detrimental to the normal functioning of freshwater habitats of Ethiopia (Menberu et al. 2021).The rapid increase in agricultural expansion and intensification results in increased nutrient concentrations in water bodies through surface runoff, which results in eutrophication (Li et al. 2023).Eutrophication could lead to algal blooms and the associated cyanotoxin production results in hypoxia and subsequently in biodiversity losses and diminished ecosystem services (Fetahi 2019;Omara et al. 2023).In the research lake, Lake Arekit, there are several phytoplankton species (cyanobacteria) that produce toxins.Microcystis aeruginosa and Anabaena spiroides dominate the lake's phytoplankton, which may lead to the detection of cyanotoxins in the water.
The rapid increase in agricultural expansion and intensification in Ethiopia will lead to increased nutrient concentrations in water bodies through surface runoff, which results in eutrophication (Ayele and Atlabachew 2021).Eutrophication of Ethiopia's freshwater ecosystem is regarded as a problem with water quality that damages the aquatic environment (Fetahi 2019).In smaller lakes (as the focus of the present study, Lake Arkeit), zooplankton communities could potentially be used as a biomanipulation tool to reduce the eutrophication of lake ecosystems (Cardella 2017).The zooplankton community is composed of both primary consumers (which eat phytoplankton, bacteria, detritus, etc.) and secondary consumers (which feed on the other zooplankton) (Anbalagan and Sivakami 2019).Zooplankton have a critical role in the development of water quality and trophic levels as well as acting as the main subject of bioindication and environmental condition monitoring; they are vital biotic components of the food webs in any body of water (Chen 2020).
Zooplankton communities are sensitive to anthropogenic impacts and their study may be useful in the prediction of long-term changes in lake ecosystems, as these communities are highly sensitive to environmental fluctuations (Kehayias et al. 2014).Changes in zooplankton abundance, species diversity, and community composition can indicate the change or disturbance of the environment; it has been reported by several studies that zooplankton can serve as an indicator of changes in trophic dynamics and the ecological state of lakes related to changes in nutrient loading and climate (Rogers et al. 2020).
The filtering capacity of zooplankton contributes to the consumption of algae and helps to increase water transparency.If a lake has nutrient loading issues (such as in Lake Arekit, the subject of this study), which can result in high phytoplankton densities in turn results in algal blooms, let Daphnia species feast (Cardella 2017).Using zooplankton species, particularly those with large-bodied like copepods and cladocerans, would be the optimal way to control excessive algal proliferation (i.e.eutrophication of the water) (García-Chicote et al. 2018;Goyat et al. 2023).Moreover, because zooplankton grazes algae, it has been proposed that it may be possible to control the excessive growth of algae (i.e.eutrophication) (Zheng et al. 2022).
Lake Arekit, a highland Ethiopia's freshwater habitat, serves a range of domestic needs in addition to fishing, irrigation, and wildlife.However, because of the lake's enormous anthropogenic impact, it is currently under ecological stress.Eutrophication is one of the issues that Lake Arekit is currently facing.The cyanobacteria-dominated phytoplankton growth and the high nutrient load, both of which have an impact on the lake's ecosystem, are the causes of Lake Arekit's eutrophication (Enawgaw and Wagaw 2023).In Lake Arekit, there are several species of phytoplankton (cyanobacteria) that produce toxins.The dominant phytoplankton species in the research lake were Microcystis aeruginosa and Anabaena spiroides, which are known to produce toxins (Enawgaw and Wagaw 2023).We saw the production of cyanotoxins in the lake as a result of these phytoplankton species.Zooplankton communities have long provided ecological indicators and have been used as eutrophication responses in small lakes.However, no zooplankton investigations have been done on Lake Arkeit.This study set out to evaluate the habitat quality of Lake Arekit using zooplankton communities.

Study area
This study was carried out in Lake Arekit (Ethiopia).The lake is situated between 38°04'30" and 38°05'0" E of latitude and 7°57'0" and 7°58'0" N of longitude at elevation between 2820 and 2950 m above sea level (Figure 1).It is 2.5 km long (from north to south) and 1.1 km wide (from east to west), respectively.The lake's maximum depth was determined to be 3.2 m.Lake Arekit is a shallow freshwater ecosystem with an average depth of 2.5 m in Ethiopia's Rift Valley basin.It is a lake that is rather small in size, with a surface area of approximately 150 hectares.Lake Arekit gets its name from the township of Arekit, which is located about 240 km south of Addis Ababa, the capital city of Ethiopia.According to Zerga et al. (2021), Gurage land is divided into four drainage basins, namely the Awash Basin, Rift Valley Basin, Bilate Basin, and Omo-Gib Basin.The Western Gurage land drains to the Omo-Gibe Basin (East to West) and covers large areas.Because of its hilly nature, the Gurage Eco region in general has the potential to be a natural surface water source.Enawgaw and Wagaw (2023) listed a total of 44 surface water bodies in the Gurage Eco regions.The majority (about 86%) of which are riverine.Hot springs make nearly 10%; small portions (less than 5%) of the water bodies are lacustrine.Lake Arekit is one of these numerous naturally occurring inland water bodies in the Gurage Eco region.The Lake receives its water from two sources: direct rainfall and runoff from the watersheds.However, it has no defined outlet.
The productivity of Lake Arekit concerning phytoplankton communities and environmental variables was investigated by Enwagaw and Wagaw (2023).The findings showed that the lake is well-oxygenated, firly warm similar to other aquatic ecosystems found in tropical regions, less acidic, turbid with low water transparency, high in nutrients (nitrate and phosphate), and has relatively considerable phytoplankton diversity.Enawgaw and Wagaw (2023) found a diversity of 34 phytoplankton taxa in Lake Arekit, comprising four divisions: Bacillariophyceae, Chlorophyceae, Cyanophyceae, and Cryptophyceae.Anabaena sp., Cosmarium sp., Cylindrospermopsis sp., Microcystis sp., Navicula sp., Nitzschia sp., Pediastrum sp., Peridinium sp., Scenedesmus sp., and Synedra sp. are the top ten most dominant and consistently appearing phytoplankton species in the Lake Arkeit.During the study's reconnaissance survey, it was found that the common carp (Cyprinus carpio) was the only fish species inhabiting Lake Arekit.However, to substantiate this claim, electronic fishing is required.The lake is also known for its outstanding avifauna diversity, with mainly Blue-Winged Goose (Cyanochen cyanoptera) (endemic to Ethiopia) and White-collared Pigeon (Columba albitorques), Thick-billed Raven (Corvus crassirostris), and Wattled Ibis (Bostrychia carunculata) (endemic to Ethiopia) inhabiting the lake (Tilahun et al. 2022).
Considering the wide range of phytoplankton and other biotic communities like fish and avifauna, and certain environmental factors like dissolved oxygen, water tempreture, turbidity, and water trandsparency, and nutrient contents, Enawgaw and Wagaw (2023) concluded that Lake Arekit is biologically productive inland ecosystem.The lake is useful for multidimensional uses.The nearby communities used the Lake for small-scale irrigation, selective fish farming, and domestic use, among other things.Nonetheless, it is affected by various disruptions.Anthropogenic factors such as agricultural activities, the waste from adjacent drinking water bottling plants, hotels, and townhouses, among others, have a major influence on the lake's ecosystem.Furthermore, the lake experiences natural disturbances like floods brought on by the river's entry of sediments and nutrients, which in turn causes the lake to become eutrophic -overpopulating with algae.Consequently, there's evidence of eutrophication -a condition caused by an excess of proliferation of algae -in the lake.

Study design and characterization of sampling sites
To collect zooplankton and some selected environmental samples, three established sampling locations (inlet area -IA, pelagic area -PA, and macrophyte area -MA) were visited three times biweekly during the wet season (June-August 2023) and another three times each during the dry season (March-May 2023).The selection of sampling sites was made based on the presence or absence of significant human impacts.The first sample was collected from site 1, which is located in a severely impacted area.The second sample was obtained from site 2, which is a moderately impacted area.The third sample was taken from a site (site 3) that had been less severely affected.The level of human impairment at Site 1 (inlet area) was the highest, followed by moderate and low levels at Site 2 (pelagic area) and Site 3 (macrophyte area), respectively (Table 1).

Measurements of selected environmental variables
A select few environmental variables that had an impact on the zooplankton communities in the study lake were measured both in-situ and in the laboratory.Surface water temperature, dissolved oxygen and pH of the lake water were measured in situ using portable millimeter probe (Model HQ 40d Multi Hatch Lange).Turbidity and transparency of the lake water were also measured on site using a digital turbidimeter (Model Oakton: T-100) and a portable standard secchi disc (28 cm in diameter with alternate black and white quarters), respectively.To analyze chlorophyll a and inorganic nutrients that were used to determine the eutrophication of Lake Arekit, regular water sample collection from the lake's surface area was carried out using opaque 1 L plastic bottles.The chlorophyll a of the lake was determined using spectrophotometric analysis (Talling and Driver 1963), and the two main dissolved inorganic nutrients (nitrate: NO 3 -N and phosphate as total phosphorous-TP) were assessed in the laboratory using the standard method of APHA (1995).The level of eutrophication of the Lake Arekit was determined using Carlson's trophic status index (TSI) calculation method for an inland water body, which was developed based on chlorophyll a, total phosphorus, and Secchi disc (water transparency) (Carlson 1977).Carlson determined the trophic state values for oligotrophic lakes with a TSI of less than 40, mesotrophic lakes with a TSI of 40-50, eutrophic lakes with a TSI of 50-70, and hypereutrophic lakes with a TSI of >70.

Zooplankton sample collection, identification and quantitative enumeration
Zooplankton samples were collected using a 30 µm net sampler with vertical hauling at three sampling stations (IA, PA, and MA) during the dry and wet seasons.The sample was immediately fixed with 4% formalin and taken to the laboratory for identification and further enumeration.10 mL of the preserved volume from the homogenized sample was taken to identify the zooplankton taxa.Zooplankton was identified using the lowest taxonomic classification that may have been employed under a stereoscope microscope (magnification 40×), using numerous identification references, including Fernando ( 2002) and Grosjean et al. (2004).From a sub-sample of 10 mL, individuals were counted in the counting grid at random, typically up to 400 individuals.The final estimation of the relative zooplankton abundance (individual per liter) (ind/L) of lake water was done using the equations developed by Edmondson and Vinberg (1971).

Data analysis
Multivariate analysis of variance (MANOVA) was used to determine the significance spatiotemporal differences of environmental variables and the diversity and abundance of zooplankton.A redendundy analysis (RDA) were used to look at zooplankton community structure as a function of environmental factors and how different taxa relate to each other and to environmental factors.RDA was used because the lengths of gradient value was 0.5 which is less than 3 as stated by McArdle and Anderson (2001).Pearson correlation 'r' was also used to check the affinities of various environmental parameters and its correlation with zooplankton abundance.The Shannon diversity index (H') was used to categorize the lake's water quality using zooplankton diversity index;Carlson index was also applied to support this result.IBM SPSS Statistics 20 was used to analyze MANOVA.
A constrained Redundancy Analyses (RDA, CANOCO for Windows 4) was analysed using PAST software (McArdle and Anderson 2001).The significance of the variables was determined using a 95% confidence interval.

Seasonality in the environmental variables and their relations
Except for chlorophyll a that had a spatial effect (p < 0.05), the bulk of environmental factors did not significantly differ among the sampling sites (p > 0.05).However, each environmental variables that measured in the study lake had significant seasonal variations (p < 0.01) (Table 2).The surface water temperature in Lake Arkeit ranged from 20.6 ± 0.2 °C (at MA) to 27.4 ± 0.1 °C (at PA).The study lake's water temperature was found to be higher in the wet season than in the dry season.It increased from June, at the start of the rainy season, through August, at the height of the rainy season.Dissolved oxygen (DO) had the opposite effect.With a significant difference between the two seasons (p < 0.001), higher DO was seen in the dry season than in the wet season.The DO of Lake Arekit varied from 4.6 ± 0.3 mg/L (at IA) to 9.3 ± 0.2 mg/L (at PA).The lake's temperature and DO had a significant negative correlation (r = −0.904,p < 0.001).The lake's pH ranged from 5.1 ± 0.2 (at IA) to 6.71 ± 0.2 (at PA).There was a seasonal effect on the pH of the lake (p < 0.001).The uppermost and the bottommost pH values were observed in April (the dried period) and August (the heavy rain month), respectively.pH exhibited a strong negative correlation with water turbidity (r = −0.833,p < 0.001).However, it exhibited a strong positive correlation with water transparency (r = 0.885, p < 0.001).The lake's water was turbid, measuring between 25.5 ± 7.0 NTU (at MA) and 174.5 ± 29 NTU (at IA).It had poor transparency ranging from 4.45 ± 0.3 cm (at IA) to 13.95 ± 0.3 (at PA) cm.Seasonality in both the turbidity and transparency of the lake water was evident (p < 0.001), and they showed a significant negative correlation between them (r = −0.874,p < 0.001).During the wet season, there was always considerably high water turbidity.The water's turbidity was at its lowest point in April (mid-dry period).This month saw the highest water transparency levels.The maximum water turbidity was noted in July (during the middle of the rainy period), while the lowest water transparency was noted in August (during the heavy rainy spell).
Inorganic nutrients, which were among the main environmental variables evaluated in this study, were responsible for the eutrophication and community structure of zooplankton in the study lake.Nitrate (NO 3 ) and phosphate (as a measure of total phosphorous -TP) were the two principal inorganic nutrients measured in Lake Arekit.NO 3 and TP ranged in concentration from 1.37 ± 0.38 mg/L to 5.55 ± 1.4 mg/L and 1.43±.63mg/L to 5.44 ± 1.3 mg/L, respectively.Both nutrients were highly prevalent in the lake's littoral region (shoreline), though there was no statistically significant difference among the sampling sites (p > 0.05).Relatively, high nitrate and phosphate nutrients were found at IA, and low nutrients were seen at MA.The concentrations of the two nutrients varied significantly between the two study seasons (p < 0.001).During the rainy season (June-August), both nutrients were high, which was consistent with large runoff into the lake.
The other significant environmental parameter evaluated in the current study was the level of chlorophyll a.It ranged from 40.5 ± 8.7 μg/L (at IA) to 157.15 ± 28 μg/L (at PA).With a p-value of 0.048, the level of chlorophyll a varied across the sampling sites.Chlorophyll a concentrations at MA were frequently lower than at the other two sampling sites.High levels of chlorophyll a, on the other hand, were more frequently seen at PA.The concentration of chlorophyll a showed a seasonal effect (p < 0.01).At all sampling sites, chlorophyll a was meaningfully high in the dry season coinciding with the presence of high DO and low relative abundance of zooplankton.Chlorophyll a showed a positive and significant correlation with DO (r = 0.778, p < 0.01), but a negative correlation with zooplankton total abundance (r = −0.657,p < 0.05).The concentration of chlorophyll a was negatively linked with the abundance of Daphnia barbata and Cypriodopsis vidua, which are the predominant zooplankton in Lake Arekit, with r values of −0.564 and −0.518, respectively.

Trophic status of Lake Arekit
The trophic status of Lake Arekit was found between 87.3 (at IA) and 104.85 (at PA) for the secchi disc (TSI-SD), 75.13 (at PA) and 94.39 (at IA) for total phosphorus (TSI-TP), and 66.87 (at IA) and 80.16 (at PA) for chlorophyll-a (TSI-CHLa) (Tabe 3).The TSI did not exhibit significant spatial change (p > 0.05).Considering water transparency (i.e.Secchi disc reading) and chlorophyll a, however, the trophic status was higher at PA than it was at IA and MA.The trophic state exhibited the opposite pattern in terms of total phosphorus (TP).It was usually greater at IA than it was at the other sampling sites.In all three parameters (TP, CHLa, and SD), seasonality affected the trophic status (p < 0.05).
Both TSI-SD and TSI-CHLa were high during the dry season.It was peak in April, and showed a sharp decline towards to the rainy season (June to August) (Figure 2).The opposite effect was observed for TSI-TP.Significant TSI-TP occurred during the rainy season inline with increasing runoff from the watershed.In a nutshell, Lake Arekit had an average TSI of 84.6, which could potentially be classified as a hypertrophic (Table 3).

Spatiotemporal changes in zooplankton diversity and relative abundance regarding key environmental factors in Lake Arekit
A total of 11 zooplankton taxa from four different groups -rotifera, cladoceran, copepoda, and ostracoda -were found in Lake Arekit as a result of this study (Table 4).Rotifera (7 taxa), followed by cladocera (2 taxa), had the most taxonomic taxa richness.
Copepoda and Ostracoda were each represented by only a single taxon.There was no apparent variation in the number of zooplankton taxa (diversity) across the sampling sites (p > 0.05).IA, however, had fewer zooplankton taxa than the other two sampling locations (Table 4).The bulks of the rotifera taxa (such as Brachionus calyciflorus, Filinia opoliensis, Polyarthra vulgaris, and Trichocerca sp.) as well as a few large cladocera (such as Diaphanosoma excisum)were not found at IA (Table 4).Seasonality in the number of taxa was evident (p < 0.05).In each sampling month, only two species -Brachionus falcatus and Keratella tropica -were spotted; the bulk of rotifera taxa (70%) were only present during the dry season.The majority of Rotifera taxa did not reset, during the main rainy season when there was a substantial entry of waste into the lake.Copepods and cladocerans, large-bodied microcrustaceans, dominated the zooplankton species during the rainy season.The most noticeable zooplankton species in Lake Arekit, generally, were the large-bodied microcrustaceans zooplankton such as Daphnia barbata, Mesocyclops aequatorialis and Cypriodopsis vidua.Also, these species are more common in the wet season than in the dry season.This study provided more evidence that the zooplankton in Lake Arekit was dominated by a small number of common species and had a low Shannon diversity index (H') (1.03), pointing to the lake's ecosystem's pollution.
The relative abundance of zooplankton in Lake Arekit was showed to change both spatially and seasonally (Tables 5 and 6).The abundance of zooplankton (ind./L) in Lake Arekit ranged from 41 (at IA) to 195 (MA).The largest portion of zooplankton abundance was given by the large-bodied copepods and cladocerans.Together, they made for about 80% of the lake's total zooplankton abundance (Figure 3).M. aequatorialis (copepods) and D. barbata (cladocera) were the most noticeable and dominantly responsible for the highest percentage of zooplankton abundance in Lake Arekit.M. aequatorialis was the only copepod that contributed the lion's share (45%) of the zooplankton abundance in the study lake.D. barbata, the zooplankton species that was most frequently observed at all sample sites, also made a sizable contribution (about 30%) to the overall zooplankton abundance in Lake Arekit.C. vidua, the biggest species of zooplankton in the lake (with lengths ranging from 0.42 to 0.76 mm), also made a meaningful contribution to the total abundance of zooplankton in the study lake.This species significantly increased the zooplankton abundance by roughly 12% (Figure 3).The small-bodied rotifers made up a minor proportion (less than 10%) of the total zooplankton abundance while having a greater taxonomic richness than the three large-bodied microcrustaceans (copepods, cladocerans, and ostracods).B. falcatus and K. tropica were the two most Note.the letter 'b' in the superscript denotes zooplankton groups that dominated zooplankton abundance at that particular study seasons.*denotes significance at p < 0.01.**denotes significant at p < 0.001.notable rotifera in Lake Arekit.The two rotifers together accounted for nearly 80% of the zooplankton abundance within the group.However, their contribution to the overall abundance of zooplankton in the study lake was negligible (less than 3%).In general, the findings revealed that M. aequatorialis and D. barbata made up about 75% of the zooplankton abundance, while other taxa made up the remaining 25% (Figure 3).Significant spatial heterogeneity was evident in the relative densities of rotifera and ostracoda (p < 0.01) (Table 5).High rotifer relative abundance was computed at PA, the site with less anthropogenic influence, while high ostracod abundance was seen at MA, the area with substantial macrophyte coverage.At this sampling point, chlorophyll a levels, a marker of eutrophication, were frequently low.Seasonal differences in the total zooplankton abundance were evident (p < 0.01) (Table 6).Overall, the zooplankton abundance was higher during the main rainy season than during the dry season.The highest total zooplankton abundance (448 ind/L.) was found in July, which saw heavy rainfall, while the lowest total zooplankton abundance (206 ind/L.) was found in the mid-dry  month of April.The relative abundance of the large-bodied microcrustaceans -copepods, cladocerans, and ostracods -was lower in the dry season than in the wet season when there was a rise in lake water temperature and a major nutrient intake into the lake.Their relative abundance increased dramatically between April and July (Figure 4).The correlation analysis revealed a positive association between the abundance of the prominent zooplankton taxa, the lake's surface water temperature, water transparency, and the levels of nitrate and phosphate nutrients (Table 7).The Pearson correlation (r) showed, however, that the variability of the lake water temperature and water transparency had a greater impact on zooplankton abundance than did nutritional fluctuations (Table 7).The Lake's most prominent zooplankton taxa -D.barbata and M. aequatorialis -had densities that positively linked with the lake's water temperature, with r values of 0.511 and 0.619, respectively (Table 6).For rotifer abundance, the opposite impact was noted.During the dry season, rotifers had a high relative abundance along with high levels of dissolved oxygen and water transparency.From April to August, the abundance of the dominant rotifers significantly dropped (Figure 4).The highest rotifer abundance (48 ind/L) was recorded in April, coinciding with an increase in the lake's transparency.
The lowest rotifera abundance (8 ind/L) was established in August when both DO levels and water transparency were low.The abundance of the rotifer was positively correlated with the lake water transparency (r = 0.663), pH (r = 0.60), and DO (r = 0.493) (Table 7).

Redundancy analysis (RDA)
Redundancy analysis (RDA) revealed the structure of zooplankton communities in the study lake as a function of environmental conditions and the relationships between each other.The RDA table (Table 8) illustrates that axis 1 explained 60.7% of the cumulative percentage variance in the species-environment relation, contributing more to the explanation of the RDA graph than axis 2 (which explained 39.3%).Based on the RDA, turbidity (0.83), chlorophyll a (0.56), NO3 (0.99), and TP (0.53) all had a high correlation with Axis 1.A weak positive correlation was also seen between axis 1 and DO (0.14).In contrast, there was a significant negative connection between axis 1 and both water transparency (−0.99) and temperature (−0.59).Figure 5 shows that Axis 2 had a negative correlation with the majority of the environmental factors.There was a significant positive correlation between the presence of M. aequatorialis and water transparency.Although the correlation was weak, D. barbata, C. vidua, and D. excisum showed positive correlations with temperature and water transparency.Temperature and the prevalence of the dominant rotifers -K.tropica, B. falatus, B. angularis, and B. calyciflorus -were strongly correlated.Large-bodied crustaceans such as D. barbata, D. vidua, and D. excisum were more drawn to MA.As the RDA graph showed, this large-bodied zooplankton's abundance had a negative correlation with chlorophyll a.All rotifer species -small-bodied zooplankton -and M. aequatorialis, a kind of relatively small-sized crustacean -had a predisposition towards PA (Figure 5).

Discussion
Based on some selected environmental factors, Lake Arekit was found to be a productive freshwater ecosystem.The water temperature of Lake Arkeit was between the typical range for tropical fish (14-35 °C) (Enawgaw and Wagaw 2023).The lake's continuous top-down mixing, produces an oxygenated water column.Aquatic life requires DO at least 5 mg/L to live (WHO 2004).Considering DO level, Lake Arkeit was consequently appropriate for aquatic life.Lake Arekit seemed to be fairly acidic (pH less than 7).The majority of tropical aquatic life (such as zooplankton and nektons) needs a pH between 6.5 and 8.5 to survive (WHO 2004;BSI 2003).The average pH level at Lake Arekit was below the BIS-recommended ranges for the majority of aquatic life, indicating that the lake's water is unsuitable for the growth of biotic communities that are sensitive to acidity.A detailed review of the zooplankton literature found strong evidence that lake acidification consistently causes declines in crustacean and rotifer species richness, and the relative abundance of cyclopoids and daphnids.This is evident in Lake Arekit.Lake Arkeit's water was turbid and had lower transparency.This could be the result of heavy rainfall that sends silt-contaminated surface runoff waters to Feeder Rivers.The turbidity of Lake Arkeit's water is too high compared to the drinking water standard limit.As a result, direct consumption is not recommended.However, turbid water is a sign of productive water since it may include necessary nutrients along with essential solid and liquid wastes for aquatic life.
Lake Arkeit contains significant concentrations of the nutrients nitrate (NO 3 -N) and phosphorus (TP).This is evidently connected to the use of fertilizers in the adjacent agricultural areas.The concentration of nitrate-nitrogen in uncontaminated water is less than 0.1 mg/L (Brönmark and Hansson 2002).Less than 300 mg/L of phosphorus suggests that the water body is not disturbed (ESA 2013).The minimal nitrate and phosphorus concentration in Lake Arkeit demonstrates unequivocally how anthropogenic factors -such as irrigation, industrial waste, and car washing have harmed the ecology of Lake Arekit.
The other key environmental element used to assess the degree of eutrophication in the Lake Arekit ecosystem is chlorophyll a. Lake Arkeit's chlorophyll a is thought to be high.It is comparable to be higher based on data from other shallow rift valley lakes in Ethiopia, such as Lake Hawassa (10-25 g/L; Fetahi 2019; 13-26 g/L; Tilahun and Ahlgren 2010) and Lake Ziway (27.8-45.2g/L; Benberu 2005).Chlorophyll a is influenced by 'bottom-up' elements such as temperature, light, and nutrition, as well as 'top-down' factors such as zooplankton predation (Lemma 2009).These variables -water transparency, temperature, inorganic nutrients, and zooplankton occurrence -have a significant impact on chlorophyll in Lake Arekit, as demonstrated by the RDA graph of this study.The primary biotic and abiotic factors that regulate the production of chlorophyll a in the lake are, at most, temperature and water transparency.The seasonality of Lake Arekit's chlorophyll reveals that it drops during periods of heavy rainfall.This is largely due to the increased turbidity and decreased light penetration brought on by the lake's high silt load following the heavy rains.Moreover, zooplankton's consumption of phytoplankton may be the cause of the low levels of chlorophyll a during the rainy season.Large-bodied zooplankton, especially D. barbata and M. aequatorialis, were abundant throughout the rainy season.Although it is modest, the RDA graph shows that there is a negative link between the abundance of these large-bodied zooplankton and chlorophyll a.This negatively impacted the growth of phytoplankton communities, which in turn indirectly caused chlorophyll levels to drop.Low water transparency restricted zooplankton, but during the rainy season, phosphorus and nitrogen nutrient levels increased, making these species more common.The reason behind this could be that the species favors inorganic nutrients above those that give water its transparency, or it could be due to their capacity to filter water, which enables them to endure and proliferate throughout the wet season.Because of their poor transparency, which limits their growth, rotifers are rare during the rainy season.Chlorophyll a in Lake Arekit increased over the dry season as a result of the water being more transparent allowing more light to penetrate.The temporal dynamics of chlorophyll a are generally acknowledged to be influenced by loss activities, such as grazing by zooplankton (Tilahun and Ahlgren 2010) and water transparency, or light penetration.
Based on the results of the TSI values using water transparency, the concentration of phosphorous nutrient, and the amount of chlorophyll a, Lake Arekit is classed as hypertrophic under Carlson's (1977) inland water trophic state categorization.The major cause of Lake Arekit's significant eutrophication is the watershed's large input of nutrients rich in phosphorus and nitrogen, which promotes the growth of algae.The lake's abundance of blue-green algae, algal scum, a limited number of macrophytes, summer fish kills, and macrophytes are visible.There are species of toxic phytoplankton (cyanobacteria) in Lake Arekit.The two most common phytoplankton species in the research lake were M. aeruginosa and A. spiroides, both of which are known to produce toxins (Enawgaw and Wagaw 2023).This causes the lake to become eutrophic, which causes the scum, or algal bloom, that is seen in Lake Arekit.Seasonal variations in the determining factors (Secchi disc, total phosphorus, and chlorophyll a) affect the trophic status of Lake Arekit.During the dry season, elevated TSI-SD and TSI-CHLa metrics have been seen due to increased water transparency.During the dry season, the increased transparency of the water allows light to penetrate, leading to a high production of chlorophyll a and a high TSI-CHLa.On the reverse, high TSI-TP, which was recorded during the rainy season, is connected with high runoff from the watershed.In short, there is an increased trend in the eutrophication of Lake Arekit.In light of this, it needs to be managed using two strategies: first, human activities in the watershed should be prohibited by offering them alternate sources of income, such as getting the local communities involved in lake fishing and using aquaculture; second, the growth of algae that contribute to the eutrophication of the lake should be controlled by using large zooplankton, in particular D. barbata.
It is discovered that Lake Arkeit's zooplankton diversity is limited.Lake Arkeit's zooplankton diversity was significantly less than that of other nearby freshwater rift valley lakes, such as Lake Hawassa (53 species; Mengestou and Fernando 1991) and Lake Tinishu Abaya (37 species; Enawgaw and Lemma 2018).High siltation that create unfavorable conditions for the growth of phytoplankton (a major source of zooplankton food) and unsuitable environmental factors (such as the lake's acidic pH, which restrict the growth of environmentally sensitive zooplankton), may contribute to the lack of more zooplankton diversity in the study lake.
The diversity of zooplankton in Lake Arekit reflects typical tropical characteristics, with rotifers being by far the dominating taxon in terms of species richness (Fetahi et al. 2011;Enawgaw and Lemma 2018;Wagaw et al. 2023).Green (1993) in his study on zooplankton groups from East African lakes reported that rotifers have been more specific in the plankton than copepods and cladocerans.According to Green and Mengestou (1991), the mean number of transient rotifer species in water samples from Ethiopia was higher than the global average.It has been demonstrated in the literature that plankton and fish have relationships based on size.For instance, rotifers and crustacean zooplankton populations are consistently smaller than 0.75 mm in lakes with resident salmonoid fish as opposed to lakes without fish.Lemma et al. (2001) studied the fish predation pressure on and interactions between cladoceransusing enclosures in three temperate German lakes (Lakes Feldberger Haussee, Groge Fuchskuhle, and Dagowsee) and one tropical lake in Ethiopia (Lake Bishoftu), found that fish indirectly ensure the proliferation of small-bodied zooplankton, which are protected from fish predation by their small sizes.The reconnaissance survey of this study revealed that Common Carp (Cyprinids carpio), was the predominated fish species reported in Lake Arekit.Even if it is benthivore (largely consume benthic invertebrates), this fish could viciously devour the larger crustaceans, causing a top-down effect those results in a great diversity of small-sized rotifers.The density of zooplankton in Lake Arekit is dominated by a few large-bodies crustacean communities, such as D. barbata and M. aequatorialis.This may be because these communities could adapt to high turbidity, the lake contains high level of nutrients, or there aren't many different types of predator fish present.
In this study area, the relative abundance of zooplankton has been dominated by cyclopoid copepods.This is in line with the findings of a number of other authors who have described the predominant abundance of copepods in tropical and sub-tropical lakes (Fetahi et al. 2011, Enawgaw andLemma 2018).Copepods may have had a competitive advantage in Lake Arekit, where they outnumbered cladocerans in terms of abundance.In the presence of blue-green filamentous algae (dominated phytoplankton in Lake Arekit) (Enawgaw and Wagaw 2023), feeding experiments did demonstrate that smaller zooplankton (such as copepods and rotifers) were dominant (Fetahi et al. 2011).The presence of high rotifers compared to large-bodies zooplankton may results the occurrence of copepods.Cyclopoid copepods are a predator -they would not directly be influencing phytoplankton through predation.They could be eating rotifers though -which is consistent with the seasonal patterns for both groups.
In shallow lakes with frequently poor water transparency, diel vertical migration is hardly observed (Lemma 2009) and this might be also true for Lake Arekit which is known for its shallow and turbid nature.In shallow lakes of this type, large-bodied zooplankton migrate horizontally to seek refuge from predators, moving from the open water to the littoral zone and ultimately into the macrophytes (Burks et al. 2002).Large-bodied zooplankton (like D. barbata and M. aequatorialis) were primarily found around the Lake Arekit shoreline where macrophytes were prevalent.This may be due to the nearby availability of food or the capacity of the macrophytes to filter out nutrients.
The occurrence of zooplankton and their relative abundance were subjected to significant temporal change.This noticeable temporal variation was likely caused by a few close-by factors, like shifts in resource availability and predation pressure.The temporal changes of tropical zooplankton are linked to turbidity, water level, temperature, and stratification (Isumbisho et al. 2006;Melo et al. 2007).Similarly in this study, the key factors contributed to the high zooplankton abundance seen in Lake Arekit during the main rainy season were high ambient inorganic nutrient levels, which indirectly supported the growth of phytoplankton to thrive, and high water turbidity, which provided them with protection from fish predation.The most noticeable zooplankton species, M. aequatorialis and D. barbata had been seen to be more common during times of nutrient enrichment and high water turbidity.The impact of predation on the temporal variation of zooplankton cannot be ruled out in Lake Arekit since zooplankton plays a significant role in planktivorous fish diets (Lemma 2001).There was a significant grazing pressure on phytoplankton, as evidenced by the negative correlation between the relative abundance of copepods and cladocerans in the Study Lake and chlorophyll a (food).In the case of other Ethiopian lakes, this is a well-established fact (Mengestou and Fernando 1991;Dejen et al. 2004;Dagne et al. 2008;Wondie and Mengistou 2014;Enawgaw and Lemma 2018).In contrast to the other variables, the abundance of copepods and cladocerans was highly connected with the lake's water temperature and turbidity, indicating that these two environmental conditions significantly influenced the seasonality of the abundance of large-bodied zooplankton.On the other side, the number of small-bodied rotifers was high during the dry season, perhaps as a result of high phytoplankton biomass (chlorophyll a) and rising lake water transparency.However, water transparency was the most important element for controlling the seasonal variations in rotifer abundance in Lake Arekit.
Diversity is a component of community structure that is correlated with species diversity and abundance.The Shannon-Wiener Diversity Index (H′) is one of the popular diversity indices that are frequently used in environmental monitoring.Based on H' , there are three different levels of pollution status.H′ values greater than 3 indicate the absence of contaminants in a water body, H′ values of 1-3 indicate a moderate level of contaminants, and H′ values of 1 denotes a high level of pollution (Salusso and Moraña 2000).Given this classification and the zooplankton H' index score, Lake Arekit was considered to be severely contaminated.The primary sources of pollution were the entry of garbage from adjacent water firms and potentially excessive nutrient levels as a result of the lake's geological history.

Conclusions
Lake Arekit is advantageous for a range of uses, including irrigation, the production of fish and livestock, and the survival of many aquatic life forms, depending on environmental parameters like temperature, dissolved oxygen level, and nutrient availability.However, the lake's zooplankton diversity is comparatively low, and the majority of its abundance is accounted for by a few species.Zooplankton communities are the most trustworthy group of microscopic organisms that can be used for determining the level of water quality in the lake.Our results imply that dense Daphnia populations are optimal for biomanipulation as a tool for controlling water quality.Higher filtration potentials and a more pronounced effect on chlorophyll a are observed when Daphnia is dominant.High Daphnia barbata concentrations cannot address the underlying cause of eutrophication, but grazing can assist by lengthening the clear-water stages in a eutrophic lake -Lake Arekit.High Daphnia abundance may also accelerate the recovery of the Lake Arekit ecosystem and increase its ability to purify itself, provided that restoration efforts can made in the lakes' drainage basins.

Figure 1 .
Figure 1.Map of lake arekit with sampling sites (black spots).

Figure 3 .
Figure 3. Percentage contributions of the dominat zooplankton taxa for the total abundance of zooplankton in lake arekit.

Table 2 .
seasonality in the environmental variables measured in lake arekit.

Table 3 .
trophic state index (tsI) for lake arekit in terms of secchi disk (sD), total phosphorous (tP) and chlorophyll a (chla) and the average of each tsI's (ctsI-carlson's trophic state index) (mean and range value).

Table 4 .
Zooplankton communities identified in lake arekit at each sampling sites.
Note.Ia: inlet area; Pa: pelagic area; Ma: Macrophyte area; a taxa conspicuous at that particular site.

Table 5 .
spatiality in the relative abundance of zooplankton (ind/l) in lake arekit.Note.the letter 'a' in the superscript denotes zooplankton groups that dominated zooplankton abundance at that particular sampling site.*denotes significance at p < 0.05.**denotes significant at p < 0.01.

Table 6 .
seasonality in the relative abundance (ind/l) of zooplankton in lake arekit.

Table 7 .
the correlation between environmental variables and the abundance of dominat zooplankton in lake arekit using Pearson correlation (r) analysis.

Table 8 .
summary of the statistics of rDa diagram.