SPATIAL PATTERNS OF ZOOPLANKTON DISTRIBUTION AND ABUNDANCE IN RELATION TO PHYTOPLANKTON, FISH CATCH AND SOME WATER QUALITY PARAMETERS AT SHIRATI BAY, LAKE VICTORIA-TANZANIA

Revania K. Waya1§, Samwel M. Limbu3, Godfrey W. Ngupula2, Chacha J. Mwita3 and Yunus D. Mgaya3*

1Tanzania Fisheries Research Institute, P. O. Box 46, Shirati, Tanzania 2Tanzania Fisheries Research Institute, P. O. Box 475, Mwanza, Tanzania 3Department of Aquatic Sciences and Fisheries, University of Dar es Salaam, P. O. Box 35064, Dar es Salaam, Tanzania *Corresponding author, e-mail: [email protected] §The writing of this paper was completed posthumously.

ABSTRACT Spatial patterns and abundance of zooplankton in aquatic habitats are important determinants for production of fish species, invertebrates and availability of phytoplankton. Weekly monitoring for zooplankton abundance was conducted in Shirati Bay, Lake Victoria, to explore their spatial patterns in relation to phytoplankton, fish catch and some water quality parameters. The vertical distribution of zooplankton was generally higher close to the bottom as compared to surface waters of the lake. Zooplankton vertical distribution positively correlated with water transparency (r = 0.680, p = 0.011). The horizontal abundance of zooplankton was not significantly different amongst the three stations (p = 0.5143). While Copepoda was the dominant group in terms of composition, Rotifera had the highest diversity indices of all the zooplankton groups obtained. The abundance of nauplius larvae was significantly higher than that of the copepodites (p = 0.022). Nile perch, Lates niloticus dominated the total catches (47%) followed by Nile tilapia, Oreochromis niloticus (29%) and haplochromines (21%). The abundance of haplochromines and juvenile fishes correlated significantly with the abundance of zooplankton (r = 0.856, p = 0.002 and r = 0.58, p = 0.038, respectively). The current study revealed that zooplankton vertical distribution at Shirati Bay is mainly controlled by water transparency and predation by juvenile Nile perch, Nile tilapia and haplochromines.

Keywords: chlorophyll-a; calanoid; cyclopoid; nauplius larvae; juvenile fish

INTRODUCTION (Mwebaza-Ndawula 1994, Sitoki et al. 2010, Lake Victoria has received an exceptional Vincent et al. 2012, Ngupula 2013). consideration because of its changes in Eutrophication and pollution are known to limnological conditions caused by nutrients affect the structure of the lake communities inputs and regional climate change (Waya and by modifying its invertebrate species Mwambungu 2004, Ojwang et al. 2014). The composition, distribution and abundance bays of the lake are highly impacted by patterns which in turn affect their overall eutrophication and pollution due to the ecological functions (Mwebaza-Ndawula chronic release of water containing higher 1994, Ngupula et al. 2010). levels of nutrients from urban/industrial effluents and agricultural activities conducted Zooplankton are a major constituent of along the shoreline and in nearby catchments freshwater ecosystems, interceding the energy

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flow between phytoplankton communities and flourishing fish and invertebrate productivity higher trophic levels, such as fish in pelagic of the lake. Unpacking the effects of different habitats (Kelly et al. 2013). The distribution factors that control spatial distribution and and abundance of zooplankton are abundance of zooplankton remains an heterogeneous (Castro et al. 2007, Mulimbwa important challenge for the ecological et al. 2014). In most aquatic ecosystems, the scientists working in freshwater ecosystems spatial distribution and abundance of including Lake Victoria (Lévesque et al. zooplankton are characterized by both vertical 2010). This study, determined the patterns of and horizontal patterns (Yurista and Kelly spatial distribution (vertical and horizontal) 2009). These patterns are important indicators and abundance of zooplankton at Shirati bay, of the ecological status of aquatic ecosystems Lake Victoria (Tanzania) in relation to and thus directly affecting human interests phytoplankton, fish catch, water temperature, (Dietzel et al. 2013). Notably, spatial patterns conductivity and transparency. Understanding and zooplankton abundance determine the the distribution patterns and abundance of production of fish species, invertebrates and zooplankton is an important step in predicting availability of phytoplankton (Casper and their influence on phytoplankton and fish Thorp 2007). production under the changing lake environments. The zooplankton patterns and abundance in large lakes vary spatially (Lévesque et al. MATERIALS AND METHODS 2010). They are affected by many factors Study area including water depth (Irvine 1995), water Water quality parameters, chlorophyll-a, transparency (Semyalo et al. 2009), zooplankton and fish samples were collected conductivity, wastewater discharged into on a weekly basis (March 2005 to March lakes, herbicides and nutrients produced from 2006) from three stations (Nyangoge, 15 m agricultural activities which diffuse into lakes deep; Nyasobu, 5 m deep; and Michire, 7 m either directly through run-off or indirectly deep) at Shirati Bay, located at latitude 1° 09' via polluted rivers (Dietzel et al. 2013). 00" S and longitude 33° 58' 00" E in the Predation by fishes and other invertebrates, as Tanzanian waters of Lake Victoria near the well as availability of algal biomass further border with Kenya (Figure 1). On each modify zooplankton patterns, especially in sampling occasion, almost similar stations shallow lakes that experience constant mixing were visited using the aid of a Global and have uniform temperature and food Positioning System (GPS) model GPSMAP gradients (Semyalo et al. 2009), therefore, the 62S, made in USA. Within each sampling zooplankton distribution patterns and station, samples were taken from three points abundance are a function of physico-chemical and afterwards their mean was estimated as a and biological processes (Semyalo et al. 2009, single value for the station on each sampling Oyoo-Okoth et al. 2011). occasion. The choice of the above sites was done strategically to include varying depths to Spatial patterns and abundance of capture zooplankton variation with depths. zooplankton distribution are an inherent The climate of Lake Victoria basin is characteristic of pelagic freshwater characterized by two distinct seasons; a dry ecosystems. Advancement in understanding season from July to September and a wet spatial patterns of zooplankton distribution in season with two rainfall maxima, i.e., short relation to changes in Lake Victoria rains from October to December and long ecosystem (Ngupula et al. 2010) has lagged rains from March to May. behind despite their role in supporting the

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Figure 1: A map of Shirati Bay showing the three sampling stations.

Sampling of water quality parameters transportation to the Tanzania Fisheries Water temperature, pH and conductivity were Research Institute (TAFIRI) laboratory at measured using a multi-meter probe deployed Sota Centre for analysis. In the laboratory, at the positions where the zooplankton chlorophyll-a pigment was extracted samples were collected. Water transparency according to procedures recommended by was measured using a Secchi disc with a Wetzel and Likens (2000). diameter of 20 cm. Estimation of zooplankton abundance and Determination of chlorophyll-a diversity To obtain the concentration of chlorophyll-a Water samples for determination of (an indirect measure of phytoplankton zooplankton vertical distribution were taken abundance), 150 mL of water sample was at each site during the day using a one-litre filtered through membrane filters of 0.45 µm van Dorn water sampler at five-metre depth pore size. Three to four drops of magnesium intervals between 0800 and 1000 hrs. carbonate slurry were added to aid filtration Replicates of three samples from each site and prevent acid formation during extraction. were sieved through 60 µm mesh size. After filtration, the filter papers were Samples for determination of zooplankton transferred onto absorbent pads, folded in horizontal distribution were taken by making labelled aluminium foil, kept in darkened three vertical hauls at each site. Hauls were desiccators and subsequently stored in frozen made from one metre below the lake’s water conditions using a cool box prior to surface, using an open conical plankton net

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with a 29 cm mouth diameter, 60 μm mesh During counting, adult males and and one metre in length. The samples were females, ovigerous and non-ovigerous preserved in four percent formaldehyde and females as well as copepodite stages one to transported in a cool box to TAFIRI five and the nauplii stages were counted laboratory at Sota Centre. In the laboratory, separately in order to obtain an insight into fixed water samples were diluted and at least the different life stages of zooplankton. 10 mL of water from each sample were Abundance as individuals per cubic metre of examined for zooplankton under an inverted water and as individuals in a given one litre of microscope at 40x magnification. water are analysed using the following Zooplankton were identified using keys and formulae (IFMP 2003-2008) according to manuals by Ruttner-Kolisko (1974), implementation of the fisheries management Korovchinsky (1993) and Maas (1993). plan:-

Fish composition and biomass problems and exhaustion occurred. It has been Fish sampling was done using beach seine demonstrated that pre-chilling prior to with cod-end nets of two, one and 0.5 inches. slaughter is a minor stressor (Sneddon 2012). On each sampling occasion, the seine nets were set to cover an area of approximately Eight samples per week of juvenile fish 200 m2 and left for two hours before hauling. species (L. niloticus, O. niloticus, T. rendalli) Hauling of nets was done three times a day. with relatively similar size and The fish caught from different hauls and nets haplochromines species from each haul were were lumped together to make catch per net analyzed for stomach contents. A total 1536 per day per sampling occasion. stomachs were removed soon after capture and preserved in specimen bottles containing Stomach content analysis of juvenile fish 4% formalin and instantly transferred to the species laboratory for analysis. In the laboratory, the The procedures and the experiments in this stomachs were analyzed for food contents study were done in respect of the based on the modified Amundsen et al. (1996) internationally recognized guidelines for point method. Each individual fish was ethical use of (Håstein 2004, dissected, stomach opened and its content Grigorakis 2010). The fish used for analysis emptied into a petri dish and uniformly mixed of stomach contents when caught alive were with some freshwater. The mixture was then sacrificed by hypothermia through immersing transferred to a slide for microscopic them in an ice-slurry to avoid causing stress examination. Identification and biomass of and pain before death (Sneddon 2006). By plankton food items were done as explained using ice water, it was possible to calm down for phytoplankton and zooplankton above. the fish for several hours until osmoregulatory The food items observed were identified and 23

Waya et al. – Spatial patterns of zooplankton distribution …

assigned percentages on the basis of their zooplankton diversity (Shannon and Wiener relative abundances. 1949). Significant differences were judged at a probability level of p ≤ 0.05. All statistical Data analyses analyses were performed using SPSS version Results are presented as mean ± standard 15 for windows. error of the mean (SE). To safeguard against violation of the assumptions of parametric RESULTS statistics, data were tested for homogeneity of Water quality parameters variances using Levene’s test. One-way Generally, with the exception of water analysis of variance (ANOVA) was used to transparency, water temperature, pH and test for differences in abundance and water conductivity were comparatively higher at the quality parameters among stations. Specific nearshore station (Nyasobu) than at the significant differences were detected using offshore station (Nyangoge) (Table 1). The Tukey’s multiple comparisons test. mean water temperature in the bay ranged Significant differences between female and between 25.65 ± 0.32 °C and 26.36 ± 0.20 °C male calanoid and cyclopoid as well at Nyangoge and Nyasobu, respectively. No as nauplius larvae and copepodites were significant difference in water temperature tested using independent samples t-test. A was found among the sampling stations Spearman rank correlation (r) was used to test during the study period (F = 2.499, p = for relationships between zooplankton and 0.096). The average conductivity of the water chlorophyll-a, and water temperature, ranged from 113.32 ± 5.82 µS/cm to 125.33 ± transparency, conductivity and pH. It was 7.29 µS/cm at Nyangoge and Nyasobu further used to relate the abundance of Nile stations, respectively. Water conductivity did perch and haplochromines with that of not differ significantly among sampling zooplankton. The Shannon-Wiener diversity stations during the study period (F = 1.033, p index was used for the analysis of = 0.434).

Table 1: Variations of water quality parameters (mean ± standard error) measured during the study Sampling stations Water quality parameter Nyangoge Michire Nyasobu Temperature (°C) 25.65 ± 0.32a 26.23 ± 0.17a 26.36 ± 0.20a Conductivity (µS/cm) 113.32 ± 5.82a 119.25 ± 6.29a 125.33 ± 7.29a Transparency (m) 1.71 ± 0.12a 1.64 ± 0.11a,b 1.34 ± 0.06b pH 9.08 ± 0.13a 9.39 ± 0.08a,b 9.76 ± 0.05b a,b: Means in each row sharing the same superscript are not significantly different (p > 0.05).

The three stations along the bay showed 0.13 and 9.76 ± 0.05 recorded at Nyangoge significant differences in transparency values and Nyasobu stations, respectively. The pH (F = 3.779, p = 0.032). Tukey multiple differed significantly among stations (F = comparisons test showed significantly higher 13.656, p < 0.001; Table 1). Post hoc test transparency at the offshore station showed significantly higher values in pH at (Nyangoge), than the nearshore station Nyasobu than Nyangoge (p < 0.001) and (Nyasobu) (p = 0.036; Table 1). Insignificant Michire (p = 0.022). However, no significant differences in transparency were obtained difference in pH was detected between between Nyangoge and Michire (p = 0.879) Nyangoge and Michire (p = 0.052). and Michire and Nyasobu (p = 0.104; Table 1). The average water pH values were 9.08 ±

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Vertical distribution of zooplankton positively correlated with water transparency The vertical distribution of zooplankton (r = 0.680, p = 0.011). However, zooplankton varied with depths. Generally higher vertical abundance was insignificantly abundance was found close to the bottom of inversely correlated with chlorophyll-a (r = - the lake as compared to surface waters 0.640, p = 0.360), water temperature (r = - (Figure 2). On the contrary, water temperature 0.258, p = 0.750) and conductivity (r = - and conductivity generally decreased with 0.800, p = 0.333) during the study period. pH depth. Chlorophyll-a, varied with depths but was insignificantly positively correlated with without any notable distinct trends (Figure 2). zooplankton abundance (r = 0.161, p = 0.600). Zooplankton vertical distribution significantly

Figure 2: Vertical distribution of zooplankton, chlorophyll-a and some physical water quality parameters at Shirati Bay during the study period.

Zooplankton horizontal distribution species (107,985 ± 15,156 ind. m–2; Figure 3). composition and diversity However, there was no significant difference On average, Nyangoge station had the highest in horizontal zooplankton abundance among zooplankton mean abundance of 171,512 ± the three stations (F = 0.668, p = 0.5143). 14,624 ind. M–2 followed by Michire (122,383 ± 11,156 ind. m-2) and Nyasobu

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250

) 3

200

x 10 -2 150

100

50 Abundance (no m 0 Nyangoge Michire Nyasobu Stations

Figure 3: Horizontal distribution of zooplankton at Shirati Bay during the study period.

A total of 41 zooplankton species were Table 2: Shannon-Weaver (H') diversity identified from the three sampling stations. indices for the different groups of The Copepoda was the most dominant group zooplankton at Shirati Bay. (94%) at all the three stations with Stations Thermocyclops neglectus and Thermocyclops Nyangoge Michire Nyasobu Zooplankton emini being the most common species. The H' H' H' abundance of Rotifera was 4.6% and groups Cladocera 0.479 -0.011 0.603 Cladocera, which was the least abundant Copepoda 1.844 1.813 1.701 group, had an abundance of 1.4%. The Rotifera 1.985 2.455 2.489 abundance of female copepods dominated that of males throughout the study period. Female Fish catch composition calanoid copepods (9061.04 ± 1624.43 ind. Fish catches at Shirati Bay comprised mainly m–2) were significantly more abundant than Lates niloticus (Nile perch), Oreochromis that of male calanoid copepods (4526.33 ± niloticus (Nile tilapia), Oreochromis –2 1055.82 ind. m ; t = 2.341, p = 0.023). leucostictus, Tilapia rendalli and Equally, female cyclopoid copepods haplochromines. L. niloticus dominated the abundances were significantly higher total catches (47%) followed by O. niloticus –2 (4046.97 ± 844.38 ind. m ) than those of (29%) and haplochromines (21%). The O. males (1297.85 ± 229.88 ind. m–2; t = 3.141, p leucostictus (2%) and T. rendalli (1%) were = 0.002). Furthermore, cyclopoid copepods the least dominant fish species. There was no contributed more to copepod biomass significant correlation between zooplankton followed by calanoids and then the abundance and total fish abundance (r = harpacticoids. In general, the abundance of 0.172, p = 0.5760). However, abundance of nauplius larvae was significantly higher haplochromines and juvenile fishes correlated –2 (20955.42 ± 1805.223 ind. m ) than that of significantly with that of zooplankton – the copepodites (14854.24 ± 1953.511 ind. m (Haplochromines: r = 0.856, p = 0.002 and 2 ; t = 2.294, p = 0.022). juvenile fishes: r = 0.580, p = 0.038). Stomach contents analysis of juvenile Nyasobu station had the highest diversity (H') planktivorous fishes revealed that, all fishes for Cladocera and Rotifera at 0.603 and 2.489, sampled included zooplankton in their diet respectively (Table 2). Further, Nyangoge (Table 3). station had the highest diversity indices for Copepoda followed by Michire and Nyasobu DISCUSSION (Table 2). Generally, Rotifera had the highest The results showed higher water transparency diversity indices among the dominant groups at the offshore station than the nearshore at the three sampled stations. station, corroborating the results obtained by

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Kishe (2009) and Gikuma-Njuru et al. (2013) transparency. This is similar to results at different sites in Lake Victoria. The lower obtained by Akindele (2013). The positive water transparency at the inshore station is relationship between zooplankton and water related to flushing of sediments from transparency is due to the significant role anthropogenic activities in the catchment area. played by water transparency during Due to their proximity to the land, the zooplankton diel vertical movement and nearshore stations received higher amounts of feeding (Williamson et al. 2011) as well as sediment laden runoff compared to the visual predator avoidance (Hu et al. 2014). offshore station. The results further showed Thus, based on these results, the vertical that, zooplankton vertical distribution was zooplankton distribution at sampled sites is significantly positively correlated with water controlled by water transparency.

Table 3: Gut contents of juvenile planktivorous fish of Shirati Bay, March 2005 to March 2006 Lates Oreochromis Food item Haplochromine Tilapia zillii niloticus niloticus Asplanchna spp. + - + - Caridina nilotica + - - - Cyclopoid copepodite + + + + Cyclopoid + + + + Calanoid + + + + Ceriodaphnia cornuta - - + - Chaobrid larvae - - + - Chironomid larvae - - + + Filinia opoliensis - - + - Insects - - + + Lecane spp. - - + + Macrothrix spp. - - + - Moina micrura - - + - Nauplii - - + + Ostracods - - + - Phytoplankton - + + + Key, + = Present, - = Absent

The results have shown variations of triggers their diurnal vertical migrations zooplankton distribution and abundance with (Lampert et al. 2003, Waya 2004). Most depths whereby higher values were found zooplanktivorous fish and invertebrates close to the bottom. Similar results have been (visual predators) concentrate at the water obtained by White (1998) in Lake Miramar, surface during the day and vice-versa at night Isumbisho et al. (2006) in Lake Kivu and (Mgana et al. 2014). To counteract predation, Lévesque et al. (2010) in Lake zooplankton have developed avoidance Memphremagog. The existence of high strategies (Masson et al. 2001) which include abundance of zooplankton close to the bottom diel vertical migration by concentrating in the of the lake is due to predator avoidance deep dark layers during the day light hours behaviour (Lampert 1993, Waya 2004) which and in surface waters at night when visual

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predators are inactive and predation threats copepods to flourish in the bay resulting in the have been reduced (Lampert 1993, Semyalo observed higher dominance. et al. 2009, Mgana et al. 2014). This is a normal behaviour in many zooplankton Moreover, copepods are known to have the species including rotifers, cladocerans and toughest exoskeleton, the longest and the copepods which lead to the observed spatial strongest appendages among all zooplankton distribution patterns. The relatively higher species (Savitha and Yamakanamardi 2012). composition of Nile tilapia and This body form helps them to swim faster haplochromines obtained in the current study during the search for food and predator might be controlling the vertical distribution avoidance than any other zooplankton species. of zooplankton. Therefore, predator avoidance The ability to swim faster makes them the influenced the observed vertical distribution most successful grazers and predators among pattern in zooplankton during the current zooplankton species (Waya and Mwambungu study. Other migration cues, e.g. food 2004). Thus, due to their relatively smaller concentrations and moonlight were not size, physical structure and effective feeding investigated in the present study. capacity copepods were able to flourish at Shirati bay leading to their dominance. The results of the current study depicted dominance of the zooplankton community by The study has revealed significantly higher copepods. Similar results in Lake Victoria abundance of nauplius larvae than were obtained by Waya and Mwambungu copepodites. This results concurred with those (2004), Semyalo et al. (2009) and Ngupula et by de Azevedo and Bonecker (2003), al. (2010). Elsewhere, Isumbisho et al. (2006) Semyalo et al. (2009) and Mgana et al. obtained similar results in Lake Kivu, Mageed (2014). The higher abundance of nauplii is and Heikal (2006) in Lake Nasser, Sellami et due to their small size which helps them to al. (2011) in a Kasseb Reservoir, Tunisia and avoid predators. It has been shown that Mgana et al. (2014) in Lake Tanganyika. In nauplii are not very susceptible to fish view of their dominance, Savitha and predation, due to their small sizes which Yamakanamardi (2012) concluded that provide a potential defence against predators copepods constitute a dominant zooplankton (Castro et al. 2007). As noted earlier, group for both freshwater and marine habitats, predation by fish on zooplankton is size- and play a vital role as primary consumers in dependent whereby larger-sized prey is the aquatic ecosystems. The high dominance consumed first. It is more likely that of copepods at Shirati Bay is due to their copepodites were preferred by predators relatively small size and being successful leaving the smaller-sized nauplius larvae to feeders. Fish species exert size-selective thrive. Thus, the size of nauplius larvae predation on zooplankton species by helped them to avoid predators resulting in preferentially preying on large-sized ones higher abundance than copepodites at Shirati (Hansson et al. 2007b, Oguz et al. 2013). Bay during the study. Based on selective feeding, Helenius et al. (2015) found that, cladocerans rather than The zooplankton community composition copepods were efficiently removed by indicated that rotifers ranked second in terms predation, consequently altering zooplankton of diversity at all study stations of Shirati community. Due to their relatively smaller Bay. High diversity of rotifers is a size compared to cladocerans, it is possible characteristic of tropical lakes (Ghadouani et that fish predators consumed more al. 1998, Oueda et al. 2007, Badsi et al. 2010, cladocerans leaving the smaller-sized Okogwu 2010) including Lake Victoria (Waya and Chande 2004, Mwebaza-Nadwula

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et al. 2005, Vincent et al. 2012) and other dominance of copepods and rotifers at Shirati lakes (Özçalkap and Temel 2011). The high Bay signifies more eutrophication in the lake diversity of rotifers is related to their ability to thus creating a difficult environment for other survive in a harsh environment (Mwebaza- zooplankton species to survive; consequently, Ndawula 1994), their versatile feeding habit altering the zooplankton species composition. (Savitha and Yamakanamardi 2012) and The predation by Nile perch, Nile tilapia and reproductive success in the eutrophic waters haplochromines at Shirati Bay influenced the of Lake Victoria. Rotifers are more tolerant to vertical zooplankton distribution pattern adverse environmental conditions than observed in the current study. These results cladocerans and copepods (Hansson et al. call for monitoring programmes at the 2007a, Okogwu 2010). They are more watershed areas to reduce the levels of responsive to water quality changes and sediments flushed into the lake. The changes exhibit a wide tolerance range for turbidity in water quality parameters are likely to affect compared to cladocerans and copepods the distribution patterns and abundance of (Vincent et al. 2012). Their tolerance to zooplankton which form important food items environmental variables has enabled rotifers for fish in the lake. to flourish in most eutrophic waters than copepods and cladocerans (Waya and Chande ACKNOWLEDGMENTS 2004, Waya and Mwambungu 2004). We wish to recognize and highly appreciate Furthermore, the feeding habits of rotifers the Russell E. Train Education for Nature enable them to thrive in eutrophic lakes. Their Programme, World Wide Fund for Nature unspecialized feeding (Joseph and (WWF) and Tanzania Fisheries Research Yamakanamardi 2011) enables them to ingest Institute (TAFIRI) for sponsoring this study. small particles such as bacteria and organic We also equally appreciate the collaboration detritus that are often abundant in eutrophic of TAFIRI-Sota staff for their valuable environments (Badsi et al. 2010). Due to the support during data collection. position of the study site, sedimentation of the organic matter from storm water at Shirati bay REFERENCES and its degradation might have resulted in the Akindele E 2013 Relationships between the production of high density of bacteria and physico-chemical water parameters and detrital matter which were effectively zooplankton fauna of Tiga Lake, Kano, consumed by the rotifers to build up their high Nigeria. Bayero J. Pure and Appl. Sci. 6: diversity. 95-100. Amundsen PA, Gabler HM, Staldvik FJ 1996 In addition, rotifers have frequent A new approach to graphical analysis of parthenogenetic reproduction that is favoured feeding strategy from stomach contents in unstable and eutrophic environments data-modification of the Costello (1990) (Joseph and Yamakanamardi 2011). method. J. Fish Biol. 48: 607-614. Consequently, the high diversity of rotifers at Badsi H, Ali HO, Loudiki M, El Hafa M, Shirati Bay could be explained by their ability Chakli R and Aamiri A 2010 Ecological to live in harsh environments, unspecialized factors affecting the distribution of feeding and reproductive success in the zooplankton community in the Massa eutrophic conditions of the lake. Lagoon (Southern Morocco). Afr. J. Environ. Sci. Technol. 4: 751-762. CONCLUSIONS Casper AF and Thorp JH 2007 Diel and The present study revealed that the vertical lateral patterns of zooplankton distribution zooplankton distribution at Shirati Bay is in the St. Lawrence River. River Res. Appl. mainly controlled by water transparency. The 23: 73-85.

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