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To calculate mean abundance, numbers in different samples were summed for each species and averaged across all sampling sites. Results Physical and chemical conditions All physical and chemical properties are within limits for the survival and development of plankton and fish populations.
However, highest values of water temperature and bicarbonate ions were obtained during the dry season while lowest values were recorded during the wet.
Phytoplankton Thirty four major species of phytoplankton from four families were identified. Chlorophyceae green algae were represented by 16 colonial, filamentous and unicellular forms, which was the largest number of species in any of the given families with many quantitatively significant species Table 2.
Bacillariophyceae Diatoms were represented by 10 species but only two species produced large pulses. Cyanophyceae Blue—green algae had four species while Dinophyceae had two. Of highest numerical concentrations were Dictyosphaerium pulchellum, Chlamydomonas ehrenbergii and Scenedesmus quadricauda, all from the family Chlorophyceae that produced the major dry season pulses.
The Dinoflagellates were rare. The seasonal distribution of the three major phytoplankton species was similar at the surface for the three collecting sites Figure 5. The algal concentration at the littoral stations was generally higher than that in the open water Figure 6. Monthly distribution of major phytoplankton species Chlamydomonas chrenbergi, Scenedesmus quadricauda and Dictyosphacrium pulchellum at surface of Ikwori Lake.
Zooplankton Total of sixteen 16 species belonging to seven families were identified during the study. Decapoda were the most diverse taxonomic group with 4 species Table 3. Other important groups were Cladocera, and Copepoda 3 speciesPolychaeta and Rotifera two species while Actinophrys sol and Paramecium were the only Holiozoans and Ciliates present in the lake, respectively. Polychaetes were absent during the wet season while the decapods were scarce being represented by only one species Lucifer hansenii.
Seasonal variation in the occurrence of zooplankton in the Ikwori Lake. Monthly distribution of zooplankton at the water surface showed highest percentage occurrence in the months of April and November while the lowest was in September and December Figure 7.
The seasonal distribution of the three major zooplankton species was similar at the surface for the three collecting sites Figure 8. The percentage occurrence at the littoral regions was generally higher than that in the limnetic region with highest percentage occurring in the months of March and December. The zooplankton occurrence at the littoral stations was generally higher than that in the open water Figure 9.
Monthly variation in the zooplankton at the water surface of the Ikwori Lake. Monthly distribution of major zooplankton species on the surface of the Ikwori Lake. Monthly distribution of zooplankton in the limnetic, littoral 1 and littoral 2 of the Ikwori Lake. Fish species total of fish representing 16 species belonging to 10 families were sampled during both dry and wet seasons, with 7 species Hemichromis fasciatus, H.
Cichlidae recorded the most species accounting for Seasonal variation in the proportion of fish species in Ikwori Lake. Seasonal differentiation in the richness and diversity indices of the plankton and fish species revealed higher values for the dry season samples than wet Table 5.
The equitability index values were low for the two seasons. Seasonal variation in the richness index, diversity function, equitability and the relative proportion of phytoplankton, zooplankton and fish species. Discussion Physico-chemical parameters The higher turbidity in the wet season is contrary to Otobo [ 22 ] and Payne [ 23 ] and can be explained that as flooding occurred due to heavy rainfall and consequent rise in water levels, run-off from nutrient rich agricultural lands increased, some particles and debris were carried along while others were re-suspended under action of wind.
If the rocks contain, calcium salts with mineral containing sodium chloride, it will leach and dissolves into making it salty [ 2223 ]. Low dissolved oxygen observed during dry season could be attributed to oxidation of humic compounds available for decomposition and wind velocity that seemed to be lower thus reducing the movement of the waters by wind action [ 2425 ].
Dissolved oxygen level recorded in this study is lower compared to 4. High organic enrichment of lakes had been suggested by Mason [ 26 ] as the possible reason responsible for such low oxygen values. The values of dissolved oxygen fell below the ranges 5.
The higher value could be due to ground water and surface runoff from the grounding farmlands that might have increased ionic substances such as nitrate, chloride and phosphate from fertilizers [ 29 ]. High values of phosphate ions concentration in the lake in both seasons could be attributed to effluents from agricultural lands with high content of phosphate fertilizers.
The values are within the range commonly found in oligotrophic and eutrophic lakes where typical reading lies between pH 6. High mean value recorded during wet season could be due to combined effects of run-off from agricultural lands with high concentration of lime and photosynthetic activity of macrophytes.
Rippey and Rippey [ 37 ] observed that there was increase in pH with photosynthesis. Low pH value in dry season was attributed to anthropogenic acidification of allochthonous organic matter. The onset of the rains caused increase in pH, this dilution effect of rains ameliorated biological conditions in lakes [ 38 ].
The low biological oxygen demand value recorded contradicts observation by Nwankwo et al. This may be a reflection of the low amount of decomposing materials within the water arising from the surrounding derived savannah vegetation. Low alkalinity during dry season is supported by Nwadiaro et al.
The high level of carbon dioxide is as a result of high respiration from aquatic biota, during aerobic and anaerobic heterotrophic decomposition of suspended and sedimented organic material. The high value for colour in the study site was an indication of the higher level of biological activity [ 42 ]. Carbonate value range UNEP observed that weathering process of rock contributes about 50 percents of carbonate and bicarbonate in natural rocky waters.
Phytoplankton Phytoplankton communities in tropical lakes and reservoirs represent summer communities of temperate lakes with a large number of tropical taxa including pan-tropical and regional endemic elements [ 43 ]. Progressive decline in phytoplankton diversity towards tropics has been suggested by Lewis [ 44 ]. The phytoplankton composition of the lake in this study agreed with reports of Adebisi [ 2 ] and Ayodele and Ajani [ 3 ] that blue-green algae, green algae dominate most tropical African Lakes.
The dominance of Chlorophyceae in respect of species number and population density in Ikwori Lake had also been observed elsewhere [ 45 - 47 ] and the overwhelming presence in the dry season had been attributed to the presence of bright sunshine, isothermal water column and extensive catchment area draining calcium rich agriculture land [ 4849 ].
Physiological and behavioural flexibility of Chlorophyceae can accommodate environmental stresses better than most fast growing species [ 50 ]. Second to Chlorophyceae in prominence was Cyanophyceae, which were also found to be prominent in Bulgaria [ 51 ], in Hungary [ 52 ] and in Sanabria Lake Spain [ 53 ].
Cyanophyta dominance, and sometimes bloom are amongst the most visible symptoms of accelerated eutrophication of lakes [ 55 ]. The observation in Ikwori Lake is similar to findings by Adeniyi [ 56 ] that the abundance of phytoplankton increases with increase in transparency, which normally associated with black flood dry seasonwhile the high turbidity associated with the white flood wet season results in a decrease in its abundance.
It was also reported, during monsoon, mostly in ponds and reservoirs in Asia, where phytoplankton minimum can be observed during the wet months [ 47 ]. According to Sugunan [ 57 ], most of the reservoirs in India have three plankton pulses coinciding with the post-monsoon October to Novemberwinter December to February and summer March to May seasons, all within the dry season.
During the wet months June-August flushing disturbs the standing crop of plankton. However, when the destabilising effects wean away, the nutrient input favours an accelerated plankton growth in November. The annual black and white flood pattern of the lake is the most important factor regulating phytoplankton production [ 5859 ]. Phytoplankton abundance, seasonality of aquatic invertebrates [ 6061 ] and annual colonisation of shoreline by aquatic macrophytes in Kainji Lake [ 62 ] vary with flood condition.
In Ikwori Lake dry season mostly starts in November. The high temperature, bright sunlight and rapid tropholytic activities by the decrease in water level and the movement of the deep, nutrient rich areas into the fold of tropholytic zone, increase plankton biomass during dry month of November and April.
Increased in phytoplankton biomass was not observed in December despite being dry month probably due to the North-East Trade wind Hammatan and comparatively low amount of nutrients indicated by low values of Alkalinity, TDS and Conductivity during this period. The considerable fluctuations in the seasonal distribution of total phytoplankton concentrations at the 2m depth as against the similar pulse pattern observed at surface and 1m levels, may be as a result of the settling of plankters from the upper levels which tend to increase their numbers after the phytoplankton development of the upper strata.
Zooplankton The zooplankton population dominated by decapods followed by copepods and cladocerans was similar to Egborge [ 63 ] documentatioon. There may be alternation in abundance between crustaceans and rotifers as reflected in the distribution and abundance of zooplankton in the sampled lake.
These alternation in the abundance of species in Lake Asejire was regarded as a booster of all year round food for fish in the lake [ 64 ]. This study showed that the seasonal variation in zooplankton concentration could largely be due to the Rotifera, which normally constitute major diet items of larger zooplankton. Species of rotifers and crustaceans considered good indicators of the trophic state of the lakes were identified in the zooplankton community.
Rotifers species recorded were typical of oligotrophic to mesotrophic systems and these include Keratella tropica and Epiphanes macronna. The crustacean zooplankton community was made up of copepods and cladocerans. Copepod abundance was caused by increase in Cyclopoid copepodids and Mesocyclops which are indicative of good water quality. Zooplankton occurrence is generally high during the dry season because temperature and the availability of food are about the most important factors controlling the abundance of zooplankton in lakes [ 58 ].
In this study, with higher temperature regimes pervading the entire dry season, the high level of food in the water as a result of high primary productivity can be responsible for the high populations of zooplankton. With the beginning of the rainy season, nutrients are carried to the Lake allowing the growth of phytoplankton [ 66 ] and this is associated with high temperature that increases zooplankton breeding and consequently a high production occurred during the dry season.
In the dry season, the zooplankton population appeared to have great stability and in the rainy season, the population is suspected to lack stability. This may depend on the residence time of water and on the abrupt water change, which occur frequently during the rainy season [ 67 ]. The increased turbidity of the white flood dry season destroys the periphytic algae and causes a decline in the amount of phytoplankton [ 6361 ], which in turn reduces the standing crop of the herbivorous invertebrate fish food [ 63 ].
Zooplankton and fish distribution is restricted to aerate upper water layers and littoral regions of the lake during dry season [ 6367 ]. Fish species Seasonal differentiation in the ichthyofauna, evident in higher number of species and individuals caught during dry months of the study period, agree with results from Kainji Lake [ 68 ], Asejire Lake [ 65 ], Jebba Lake [ 61 ] and Asa Lake [ 69 ] who described larger ichthyofaunal densities in water bodies in the dry season.
Reasons for the variation were ascribed to the large volume of water during the wet season, available fish were now dispersed over a wider area, and fishing became more difficult. Also during wet season the high level of water and subsequent flood favored reproductive activities, hence fish species show restricted movement making them less vulnerable to catch.
Diversity The average diversity function calculated by Shannon-Wiener Diversity index, was higher for the dry season than wet for all species studied, revealing that dry season samples were more diversified and stable.
Based on the present observation, Ikwori Lake is rich in plankton and the nutrient status is high enough to support plankton populations in both seasons.
If the integrity of such lakes in the tropics is protected, they will support fish growth and survival especially in the dry season.
This lake appeared to be under pollution pressure due to high phosphate concentration in both seasons. Management efforts must therefore be directed towards ensuring that only treated effluents are allowed into Ikwori Lake. In addition, waste facility must be provided for the surrounding communities by the managers. Activities whose by-products result in organic effluents into lakes must be discouraged. Otherwise, the authors declare that they do not have any competing interests. EA sampled and analyzed the phytoplankton, GI worked on the zooplankton aspects, SO sampled and analyzed the fish while FA carried out the statistical analysis.
Acknowledgement The authors are grateful to the Senate of the Cross River University of Technology for the research grant awarded. Seasonal variation in water quality of the Cross River State Nigeria.
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Journal of Zoology, The hydrology and plankton of Asejire Lake. Paradoxically, oceanic areas adjacent to unproductive, arid land thus typically have abundant phytoplankton e. While plankton are most abundant in surface waters, they live throughout the water column.
At depths where no primary production occurs, zooplankton and bacterioplankton instead consume organic material sinking from more productive surface waters above. This flux of sinking material, so-called marine snowcan be especially high following the termination of spring blooms. Food chain[ edit ] Aside from representing the bottom few levels of a food chain that supports commercially important fisheriesplankton ecosystems play a role in the biogeochemical cycles of many important chemical elementsincluding the ocean's carbon cycle.
Organic material tends to be denser than seawaterso it sinks into open ocean ecosystems away from the coastlines, transporting carbon along with it. This process, called the biological pumpis one reason that oceans constitute the largest carbon sink on Earth. However, it has been shown to be influenced by increments of temperature.
However, this technique may not be practical at a large scale. Ocean oxygen depletion and resultant methane production caused by the excess production remineralising at depth is one potential drawback.