Hydrobiologia

Mavuti, Kenneth M.;Jaccarini, Victor

doi: 10.1007/BF00014603pmid: N/A

doi: 10.1007/BF00014604pmid: N/A

Kudhongania, A. W.;Twongo, T.;Ogutu-Ohwayo, G.

doi: 10.1007/BF00014605pmid: N/A

Abstract The combined effects of lack of effective management, over-exploitation with destructive fishing gear and interspecific competition, particularly among tilapiines have had profound effects on the fish stocks of lakes Victoria and Kyoga. It has been proposed that these have been more important in the decline of the indigenous fisheries than predation or competition from Nile perch.

Masundire, Hillary M.

doi: 10.1007/BF00014606pmid: N/A

Abstract Limnological measurements during the first year of filling of the newly formed Mazvikadei Reservoir in Zimbabwe showed variations in dissolved oxygen as well as soluble forms of phosphorus and nitrogen. The ratios of calcium to other major cations were: Ca:Mg = 0.38, Ca:Na = 0.50 and Ca:K = 2.0. The low Ca:Mg ratio may suppress development of mollusc populations, thus keeping the reservoir snail free. Conductivity was 160 µScm-1. Daytime pH was 6.95–7.8. The water was dark-brown in colour, 90% of incident light was attenuated in the top 0.75 m in May, and in the top 3.5 m in August. Positive net photosynthesis was only recorded above a depth of 0.1 m, at 0.5 m and 1.0 m depths, it was negative. Zooplankton consisted of nine rotifer, four cladoceran, two cyclopoid and one calanoid species.

Mwangi, R. W.;Mukiama, T. K.

doi: 10.1007/BF00014607pmid: N/A

Abstract A survey of the mosquito fauna in the Mwea-Tebere Irrigation Scheme in Kenya was conducted between 1984 and 1985. Two genera, Anopheles and Culex were found indoors in large numbers. The major species were Anopheles gambiae s.l., Anopheles pharoensis, and Culex quinquefasciatus, comprising 66% and 21% of the total catch, respectively. The large numbers of mosquitoes found resting indoors showed seasonal population fluctuations. The seasonal population increases observed were due to the availability of larval breeding conditions over a long period of time and to man-made environmental changes. These changes included irrigation canals and rice paddies. The irrigation canals and rice paddies were flooded from August till December, thus linking the flooding effects of the two rainy seasons. This enabled mosquitoes to breed continuously for up to 9 months per year.

Mwaura, F. B.;Widdowson, D.

doi: 10.1007/BF00014608pmid: N/A

Abstract Nitrogenase activity (acetylene reduction activity) was found to occur universally in the Cyperus papyrus swamp in Lake Naivasha. Low rates of acetylene reduction activity (0.9–104.9 nmol C2H4 g d.wt. roots-1 h-1) were associated with excised roots of C. papyrus but higher rates of activity (89.0–280.4 nmol C2H4 g d.wt. roots-1 h-1) were associated with intact root systems of the plant. It was estimated that nitrogen fixation associated with young roots alone could supply about 26% of the nitrogen requirements of growing papyrus plants. Acetylene reduction activity in the lake bottom sediments was generally low and associated with adjacent papyrus stands. Plate counts of putative aerobic and facultatively anaerobic N2-fixing bacteria associated with papyrus roots showed the presence of high numbers of diazotrophs (5.4 × 106 CFU g d.wt. roots-1). Fewer numbers of N2-fixing bacteria were detected in the sediments (1.9 × 103-3.2 × 104 CFU g d.wt. sediment-1).

Mavuti, Kenneth M.

doi: 10.1007/BF00014609pmid: N/A

Abstract Diurnal and diel vertical distribution of limnetic zooplankton species in relation to temperature and dissolved oxygen profiles was examined at a central station in Lake Naivasha. During calm days thermal stratification developed gradually from late morning to reach maximum formation at mid-day. Dissolved oxygen concentrations showed similar vertical profiles to temperature. These stratifications were, however, short lived and were broken up in late afternoons by the wind induced poly-holomictic nature of the lake. During the day most zooplankters aggregate at the top 3–4 metre zone of the water column coincident with maximum photosynthetic activity. The pattern of diel vertical distribution of zooplankton in Lake Naivasha is undefinedly even. The absence of significant diel changes in the distribution of the limnetic zooplankton may be related to the absence of permanent physico-chemical boundaries and lack of predation pressure in the open water.

Mollion, J.

doi: 10.1007/BF00014610pmid: N/A

Abstract The red alga Gelidium madagascariense, endemic to SE Madagascar, has been exported on a small scale to Japan for over two decades. The lack of information on the chemical characteristics of the phycocolloid contained in this species is the main reason for limitation of its commercialization. FT IR and GLC analysis show that it is an agar polymer, in which the methylated fraction contains predominant 6-0-methyl galactose and traces of 4-0-methyl galactose.

Mathooko, J. M.;Mavuti, K. M.

doi: 10.1007/BF00014611pmid: N/A

Abstract A survey was carried out to establish the nature and composition of the benthos along the Naro Moru, a tropical river in central Kenya using artificial substrate baskets, from November 1986 to October 1987. A clear longitudinal zonation existed for Diptera and Ephemeroptera which were the major benthic taxa. Maximum colonization took place after ten days of exposure. Seasonal variations in abundance were also observed. All taxa collected from the bottom samples were also collected in the drift samples, but the percentage composition of the benthos showed variations with that of the drift. Simulium sp. dominated the benthos whilst Baetis spp. dominated the drift. There was a positive correlation between drift rate and benthic fauna density.

Moreau, Jacques;Nyakageni, Boniface

doi: 10.1007/BF00014612pmid: N/A

Abstract Over two years, monthly length frequency distributions of Luciolates stapperssi have been obtained from commercial fisheries of Lake Tanganyika using purse seines. Due to low selectivity of this fishing method, the length frequency distributions have been used to get estimates of growth parameters, breeding season and total mortality. Information is provided for the years 1972, 1982, 1983, in the northern part of the lake (Burundi Sector). Growth parameters remain more or less constant, but total mortality between two and six years displays important variations.