Phytoplankton productivity and chlorophyll-A concentration of Oguta Lake in Southeastern Nigeria

The phytoplankton productivity and chlorphyll-a concentration of Oguta Lake, the largest natural lake in south-eastern Nigeria, are presented (Dec. 1983. Nov. 1984). The gross productivity ranged from 1.3 to 3.77 g C.m^–2.day^–1 for the water column, dropped during the period of heavy rainfall and varied with depth. The chlorophyll-a concentration had monthly means ranging from 2.31 to 4.00 mg.m^–3, with a drop during the rains, but little depth variation. Both productivity and chlorophyll-a showed non-significant correlation with the physico-chemical features of the water. The values of the biological parameters showed the lake as mesotrophic. The values are compared with those of other African lakes.     

Geophysical limnology of lake Oguta in Southeastern Nigeria with notes on its possible origin

Lake Oguta is a relatively small, shallow fresh water body (flood season maximum surface area=2.48 km²; max. depth=9.30 m) but the largest natural lentic system in Southeastern Nigeria. The maximum water depth, surface water temperature, Secchi-disc-transparency and conductivity, varied seasonally with rainfall.These parameters had ranges of 7.0–9.3 m, 24.0–31.0 °C, 0.61–4.00 m and 8.6–16.5 S.cm^–1, respectively. Weak, unstable thermal stratification developes towards midday in the warmer months (April–August).The sedimentological data show variation in the lake's particle size distribution; with coarse sandy shores, and fine, silty clay at the deep, open area. The sediment particle size variation is related to relief, transportation and depositional processes. The lake basin is a low energy, aqueous continental depositional environment. The origin of the lake is related to the Quartenary/Holocene Eurasian glaciation which correlate with the pluvials in the tropics.     

The effect of impoundment on the water chemistry of Lake Asejire, Nigeria

SUMMARY. Following the impoundment of the River Oshun to form Lake Asejire, Nigeria, in 1970, changes in the water chemistry were monitored. Chemical stratifications followed closely the pattern of thermal stratifications as the lake was chemically stratified from February to April and destratified from June to September and in January.     

Preliminary Observations on the Nature,Taxonomic Composition,Distribution, Behaviour and Nutrient Requirements of the “Unusual” Algal “Jellies” of Oguta Lake in Southeastern Nigeria

Preliminary observations were made between October 1982 and May 1983 on the nature, taxonomic composition, spatial distribution, buoyancy behaviour and nutrient requirements of some unusual planktonic algal “jellies” in Oguta Lake in Southeastern Nigeria. These “jellies” are macroscopic, blue-green algal masses whose excessive mucilage of high consistency acts as substrata for other epiphytic components viz. diatoms and green algae. They are more abundant in the lentic areas of the lake, undergo diurnal vertical movements and have their growth enhanced by spikes of nitrogen and phosphorus salts into their aquatic medium. The effects of these “jellies” on the ecology and general water use of the lake are discussed. Speculations on the reasons for their occurrence are given and possible control measures outlined.     

Fish distribution in Kainji Lake, Nigeria

A study of the distribution of fish populations in the inshore, surface and bottom water habitats of L. Kainji, and comparison with previous data, showed that there has been a relative increase in the populations of the fish families Cichlidae, Bagridae, Schilbeidae, Centropomidae and Cyprinidae and a relative decrease in the populations of the Characidae, Mochokidae, Citharinidae and Mormyridae. The overall composition of fish families caught in the three habitats varied significantly ( P <0.05). Four of the nine major families showed habitat preference: there was a preponderance of Cichlidae and Centropomidae in the inshore water habitat; Bagridae were evenly distributed in the inshore and bottom waters; and Schilbeidae were the most abundant in the surface water habitat. There were significant differences in the total catch among the various sampling stations. However, there was no significant difference in the catch of various family groups except in Cyprinidae. It was established that overall catches from the three major habitats were inversely proportional to water level in the lake.     

Sex,reproductive cycle and variations in the fecundity of the family schilbeidae (Osteichthyes : Siluriformes) in Lake Kainji,Nigeria.

The sex, reproductive cycle and variations in the fecundity of the family Schilbeidae (Osteichthyes : Siluriformes) in Lake Kainji, were investigated. Sexual differences in size and morphology were found to exist in all species. Sex ratio varied with species and season. There were more females than males in the population. Size at maturity varied from species to species with the males attaining maturity at a slightly lower length than the females. Breeding condition was attained mainly during the rains in Eutropius niloticus, Schilbe mystus and Siluranodon auritus and in the dry and rainy seasons in Physailia pellucida. Spawning occurred once during the breeding season in E. niloticus and S. mystus; and more than once during the prolonged breeding season in P. pellucida. Condition factors did not show any correlation with maturation of gonads. Fecundity was found to vary from species to species.     

Depth Distribution of Microbial Diversity in Mono Lake, a Meromictic Soda Lake in California

We analyzed the variation with depth in the composition of members of the domain Bacteria in samples from alkaline, hypersaline, and currently meromictic Mono Lake in California. DNA samples were collected from the mixolimnion (2 m), the base of the oxycline (17.5 m), the upper chemocline (23 m), and the monimolimnion (35 m). Composition was assessed by sequencing randomly selected cloned fragments of 16S rRNA genes retrieved from the DNA samples. Most of the 212 sequences retrieved from the samples fell into five major lineages of the domain Bacteria: α- and γ-Proteobacteria (6 and 10%, respectively), Cytophaga-Flexibacter-Bacteroides (19%), high-G+C-content gram-positive organisms (Actinobacteria; 25%), and low-G+C-content gram-positive organisms (Bacillus and Clostridium; 19%). Twelve percent were identified as chloroplasts. The remaining 9% represented β- and δ-Proteobacteria, Verrucomicrobiales, and candidate divisions. Mixolimnion and oxycline samples had low microbial diversity, with only 9 and 12 distinct phylotypes, respectively, whereas chemocline and monimolimnion samples were more diverse, containing 27 and 25 phylotypes, respectively. The compositions of microbial assemblages from the mixolimnion and oxycline were not significantly different from each other (P = 0.314 and 0.877), but they were significantly different from those of chemocline and monimolimnion assemblages (P < 0.001), and the compositions of chemocline and monimolimnion assemblages were not significantly different from each other (P = 0.006 and 0.124). The populations of sequences retrieved from the mixolimnion and oxycline samples were dominated by sequences related to high-G+C-content gram-positive bacteria (49 and 63%, respectively) distributed in only three distinct phylotypes, while the population of sequences retrieved from the monimolimnion sample was dominated (52%) by sequences related to low-G+C-content gram-positive bacteria distributed in 12 distinct phylotypes. Twelve and 28% of the sequences retrieved from the chemocline sample were also found in the mixolimnion and monimolimnion samples, respectively. None of the sequences retrieved from the monimolimnion sample were found in the mixolimnion or oxycline samples. Elevated diversity in anoxic bottom water samples relative to oxic surface water samples suggests a greater opportunity for niche differentiation in bottom versus surface waters of this lake.     

The Phototrophic Community Found in Lake Khilganta (an Alkaline Saline Lake Located in the Southeastern Transbaikal Region)

The structure of the phototrophic community found in Lake Khilganta (the Agin-Buryat Autonomous Area), a shallow saline soda lake (depth, 35–45 cm; water mineralization, 45 g/l; alkalinity, 30 mg-equiv/l; pH 9.5) has been studied. The bottom of the lake is covered with a 10- to 15-mm microbial mat, whose basis is formed by the filamentous cyanobacterium Microcoleus chthonoplastes. The mat exhibits pronounced layering and contains a significant amount of minerals. Six zones, which have characteristic colors and consistencies and are composed of intermittent layers, have been identified along the vertical profile. Live phototrophic bacteria have been found in the three upper zones. The bulk of the cyanobacteria is concentrated in the upper zone. In the lower zones, the development of purple bacteria has been observed. The diurnal dynamics of the vertical distribution of phototrophic microorganisms, which results from variations in the physicochemical environmental parameters, is described. Ectothiorhodospira sp. are dominant among the anoxyphotobacteria present. Their number, determined according to the inoculation method, is 10⁶–10⁷ cells/ml. The purple bacteria of the genera Allochromatium, Thiocapsa, and Rhodovulum are also present. Experiments with isolated pure cultures have shown that the anoxygenic photosynthetic bacteria of Lake Khilganta are halotolerant and alkalitolerant or alkaliphilic. In liquid enrichment cultures, at pH 9.5, the ratio of anoxyphotobacteria species is close to that observed in the lake. When the pH is increased to 10.4, it is Ectothiorhodospira, which is the most adapted to life under increased mineralization and alkalinity, that predominantly develops. Photosynthetic activity has been observed in the three upper mat zones and constitutes, on average, 1.5 g C/(m²h); the share of anoxygenic photosynthesis accounts for 75–95% of the total productivity. The main role in sulfide oxidation belongs to the phototrophic anoxyphotobacteria and cyanobacteria. In terms of the physicochemical conditions and structure of the phototrophic community, Lake Khilganta is similar to shallow saline water bodies of marine origin. The main differences consist in the increased alkalinity and in the consequent prevalence of alkaliphilic and alkalitolerant microorganisms and in the absence of representatives of the neutrophilic group of green sulfur bacteria.__________Translated from Mikrobiologiya, Vol. 74, No. 3, 2005, pp. 410–419.Original Russian Text Copyright © 2005 by Kompantseva, Sorokin, Gorlenko, Namsaraev.     

Horizontal variations in associations of zooplankton in Lake Kariba

The horizontal distribution of the zooplankton in Lake Kariba was sampled by means of vertical hauls using 55 and 250 μm meshed nets. Crustacea were sparse in the main lake, but increased dramatically in the very shallow water at the end of Crocodile Creek. The composition of the zooplankton is influenced by the presence in the main lake of the Tanganyika sardine, Limnothrissa miodon , and by the large size of the dominant diatom in the phytoplankton. In the creeks the distribution can be explained by the zooplankton occurring in open spaces into which planktivorous fish do not enter because of the possibility of predation by Tiger fish ( Hydrucyori ) and birds.
The systematic composition of the zooplankton in L. Kariba is reviewed and compared with two natural lakes and another man-made lake. It is concluded that the zooplankton and the fishes in L. Kariba have reached approximately the specific diversity that one would expect from a similar sized natural lake in the same geographical area.     

Successional patterns of aquatic macrophytes in Jebba Lake, Nigeria

Macrophyte succession in Jebba Lake, Nigeria, was observed through the first four years after impoundment. Two distinct serai phases, a floating fern phase and a draw-down flora phase could be recognized. The floristic composition of the macrophyte vegetation was also investigated to check earlier predictions (Obot & Mbagwu, 1986) on the probable structure of the macrophyte flora. The failure of obligate floating macrophytes such as Pistia, Azolla and Salvinia to establish in the lake as predicted is discussed. However, rooted emergent macrophytes were observed to be more successful than would be expected from the predictions.     

The macrophytic vegetation of the draw-down area of Lake Kainji, Nigeria

Recent publications on the vegetation of Lake Kainji, Nigeria, give the impression that Echinochloa pyramidalis is the dominant macrophyte species of the lake. However, our experience of working on the lake since 1979 suggests a different floristic composition of the macrophyte vegetation of the lake. The most frequent macrophyte in Lake Kainji is Echinochloa stagnina. At all sites investigated, emergent macrophytes are more important than floating macrophytes. There is a possible successional shift from an Echinochloa pyramidalis sere to an Echinochloa stagnina stage. This argument does not, however, preclude the possibility of a mis-identification by early workers.     

The fish fauna changes and the place of clupeids in Lake Kainji,Nigeria

The fish fauna changes and the place of the clupeids was examined. It was found that the conditions in the lake since impoundment have favoured the growth and development of these little sardines. The studies undertaken prior to impoundment and their continuation right from the time of the closure of Kainji Dam in August 1968 has made it possible for these changes to be followed sequentially.     

The food of adult copepods from Lake Kainji, Nigeria

SUMMARY. Gut contents of adult copepods from Lake Kainji were analysed. The calanoid copepod Tropodiaptomus banforanus was exclusively herbivorous with Melosira and Peridinium forming a large proportion of the diet. Of the cyclopoid copepods, Mesocyclops leuckarti was mainly carnivorous while the smaller Thermocyclops neglectus was mainly herbivorous. Experiments with Diplocystis (Microcystis) showed that it was not ingested by these three copepod species even after starvation and possible explanations for the results are discussed.     

Depth distribution of oligochaetes in Lake Baikal (Siberia - Russia)

In the course of a preliminary sampling program, oligochaetes were collected along two transects in soft sediments in Lake Baikal. The number of oligochaetes present in the samples was counted, without distinguishing between species. The results suggest an exponential decrease in number of individuals (N) relative to depth (11,165 N m^–2 at 21 m, 265 N m^–2 at 1200 m). Most oligochaetes were found in the top 7 cm of sediment. The orange colour of the sediments suggests a high oxygen availability, even at the greatest water depths.

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Résumé Au cours d'un programme d'échantillonnage préliminaire, les oligochètes du lac Baïkal ont été récoltés dans le sédiment mou prélevé le long de deux transects. Les oligochètes présents dans les échantillons ont simplement été dénombrés, sans identification déspèces. Les résultats suggèrent une diminution exponentielle du nombre d'individus en fonction de la profondeur du lac (11165 N m^–2 à 21 m, 265 N m^–2 à 1200 m). La plupart des oligochètes ont été trouvés dans les 7 premeirs cm de la couche supérieure du sédiment. La couleur orangée du sédiment suggère une grande disponibilité en oxygène, même aux profondeurs les plus grandes.

     

Diurnal variations in the carbon chemistry of two acidic peatland streams in north-east Scotland

1. Two acidic peatland upland streams in north‐east Scotland draining catchments of 1.3 and 41.4 km² were sampled each season for 2 years to investigate diurnal variations in dissolved and gaseous forms of carbon. Stream metabolism, alkalinity, discharge, pH, air and water temperatures were measured to aid data interpretation. 2. Free CO₂ showed marked diurnal variation with lowest concentrations during the period from late morning to early afternoon and highest during the hours of darkness. Although alkalinity and pH also showed some diurnal fluctuations, in comparison with other more productive alkaline systems, variation was small. Dissolved organic carbon (DOC) showed no significant diurnal pattern. However, significant changes in stream discharge influenced DOC concentrations, as well as over‐riding diurnal patterns of free CO₂, alkalinity and pH. 3. The highest diurnal ratios (maximum concentration/minimum concentration) in CO₂, gross primary productivity (GPP) and community respiration (CR) occurred in spring and summer and the lowest in autumn and winter. Variation in biotic in‐stream processes caused changes in CO₂ concentrations and temperature affected both the solubility of CO₂ and changes in up‐stream CO₂ inputs. There was no significant difference in diurnal fluctuations between the two orders of stream studied. 4. The mean GPP (as CO₂) was 0.81 g CO₂ m^?2 day^?1 and mean CR 2.67 g CO₂ m^?2 day^?1. The mean primary production/respiration (P/R) ratio was 0.26 ± 0.09 and 0.33 ± 0.15 in the first and second order streams, respectively. These values are low compared with published data because these heterotrophic headwater streams are dominated by benthic respiration and upstream allochthonous inputs with little autotrophic metabolism, particularly during the colder autumn and winter months. 5. The results have implications for the calculation of dissolved inorganic carbon (DIC) fluxes in streamwater. Samples taken during daylight hours tend to have lower concentrations of free CO₂ and HCO₃^? than samples taken during darkness. During spring, concentrations of free CO₂ were measured up to 2.4 (annual mean 1.8) times higher at night than during the day at a similar discharge. It is suggested that fluxes based on daytime measurements alone will under‐estimate the annual flux of these determinands in streamwater by as much as 40%.     

Food and feeding of three Citharinus species in Lake Kainji, Nigeria

This paper provides information on the food and feeding habits of three Citharinus species in Lake Kainji. A total number of 2357 specimens made up of 1628 Citharinus citharus , 705 C. distichodoides and 24 C, latus were examined. The majority of the fish caught by gill-netting had empty stomachs unlike those caught by cast-netting and electrofishing. The main food items were planktonic and epipelic blue-green and green algae. All the three species had similar feeding habits and also fed on the same items; their food varied with the seasons. Citharinus spp. have high gut length/standard length ratios which are adaptations to the microphagous and planktivorous feeding habits.     

Seasonal and annual variations in the abundance of the clupeid Limnothrissa miodon in Lake Kivu

There was no clear seasonal cycle in the abundance of the sardine Limnorhrissa miodon in Lake Kivu hut average values suggest that the fish are most numerous in the south from October-April and in the north from April4ctoher. The possibility that there may be a migration from south to north is considered. The catch per unit effort appears to be independent of the present level of fishing effort but may be determined by the annual rainfall two years earlier. In this respect, Lake Kivu is similar to Lake Kariba where nutrient supply is determined by river flow.