The ecological relationships of aquatic plants at Lake Naivasha,Kenya

The distribution and abundance of the aquatic flora of Lake Naivasha has been constrained by two ecosystem-level processes. One is the natural and unpredictable fluctuation of water levels which the lake experiences, resulting in a drawdown zone of several vertical meters. The other is the consequence of herbivory by several alien species, linked with competition between rooted aquatic plants and phytoplankton. The effects of alien introductions in the 1960s and 1970s was to eliminate submerged vegetation from the lake completely by 1982, principally by crayfish herbivory. Vegetation has been progressively returning since 1984 and this return is coincident with a decline in crayfish population density. Concentrations of phytoplankton have progressively increased since 1982 as a result of nutrient increase caused by a decline in water level and papyrus swamp clearance for agriculture. The relationship between high levels of phytoplankton and extensive littoral weed beds is discussed.     

Phytoplankton community composition in Lake Xolotlán (Managua)

Results of two years of research on phytoplankton species composition in Lake Xolotlán (Managua) are presented. A summary of the planktonic algae from other Nicaraguan lakes is also included. One hundred and twenty phytoplankton taxa were identified. A list of species and comments about relevant properties of the most frequent taxa are given. The qualitative composition of Lake Xolotlán phytoplankton is compared with that of other tropical waterbodies.     

The 1974 biological survey of Nyumba ya Mungu reservoir, Tanzania Concluding remarks

Hydrobiological events in Nyumba ya Mungu reservoir, a recent man-made lake in Tanzania, are reviewed in the light of those described for larger impoundments in other parts of Africa. An initial phase of rapid eutrophication and of high fish production has been replaced by a slower stabilization phase. At present the inflowing rivers supply a nutrient rich water and the level of primary production remains high. In spite of the risk of an encroachment of Typha domingensis across the shallow end and events associated with overfishing, the lake retains most of the essential requirements to continue as a useful tilapia fishery. The introduction of new fish into the reservoir, to fill gaps in the trophic structure of the fish community is not recommended.     

The importance of the littoral epiphyton as food for commercial fish in the recent African man-made lake, Nyumba ya Mungu reservoir, Tanzania

Nyumba ya Mungu reservoir is a man-made lake in the Kilimanjaro region of northern Tanzania. The commercial fisheries are principally based on the cichlids, Sarotherodon pangani Lowe, S. jipe Lowe, S. esculentus Graham, and Tilapia rendalli Boulenger.
Examination of gut contents revealed that the dominant species, S. pangani and S. jipe , fed on the epiphyton attached to the vegetation in the littoral zone, S. esculentus was a phytoplankton filter feeder, and T. rendalli consumed macrophytes and associated epiphytes.
A fish exclosure quantitatively demonstrated that the epiphyton community was appreciably depleted by the grazing of fish. After 35 days the mean epiphyton density on the submerged surfaces of Cyperus articulatus L. within the exclosure was 285 times 10³ individuals cm^-2 shoot, but only 181 times 10³ individuals cm^-2 shoot on grazed surfaces outside the exclosure. The differences were statistically significant, and corresponded to a difference in biomass of about 6.5 g organic dry weight m^-2 lake surface. Vertical profiles of periphyton density on Cyperus indicated that fish grazed mainly below a depth of 100 mm from the water surface.
The littoral epiphyton → commercial fish food chain, which may be unique, is considered to predominate in Nyumba ya Mungu, and is compared with the major food chains of commercial fish in some other African natural and man-made lakes.     

Vertical distribution of phytoplankton in Lake Xolotlán (Managua)

A study of the vertical distribution of phytoplankton was carried out at the end of the rainy season (October–November), when wind activity is lower and the probability for phytoplankton stratification is enhanced. Samples were taken every other day at 0, 0.5, 1.0 m and, thereafter, for every meter up to the bottom. The number of individuals and their biomass were determined for each sampling depth and expressed as wet weight and chlorophyll-a, respectively. The results indicate that there were no significant differences (ANOVA one way, P<0.05) with regard to the vertical distribution of phytoplankton biomass over the 15 m of the water column. This result reflects the polymictic condition of Lake Xolotlán (Managua).     

Observations on the zooplankton and littoral macroinvertebrates of Nyumba ya Mungu reservoir, Tanzania

Eight species of Crustacea and 28 species of Rotifera were identified in zooplankton samples from Nyumba ya Mungu reservoir. In all localities the dominant organism was the copepod Thermocyclops hyalinus. Cladocerans and rotifers made a variable contribution. Insects, among which the Chironomidae were pre-eminent, formed the major component of littoral macroinvertebrate communities. Oligochaetes, crustaceans and gastropods, including vectors of Schistosomiasis, were rare or restricted in distribution. In contrast to the deep end of the lake, where submerged trees were the poorest substrate, those at the shallow end supported a high biomass of invertebrates, since prolonged exposure had accelerated rotting and facilitated colonisation by the burrowing nymphs of Povilla. The variety of invertebrates on stony and muddy shores was increased when rooted emergent plants were present. The richest benthic community was found in submerged beds of Ceratophyltum. It is suggested that drawdown and the restricted development and movement of floating vegetation, have limited the spread of gastropods in the lake. Zooplankton, chironomid larvae and pupae, and ephemeropteran nymphs were shown to be important in the diet of small carnivorous and omnivorous fish species.     

Phytoplankton production and growth rate in Lake Tanganyika: evidence of a decline in primary productivity in recent decades

1. This study focused on phytoplankton production in Lake Tanganyika. We provide new estimates of daily and annual primary production, as well as growth rates of phytoplankton, and we compare them with values published in former studies.
2. Chlorophyll- a (chl- a ) in the mixed layer ranged from 5 to 120 mg chl- a  m⁻² and varied significantly between rainy and dry seasons. Particulate organic carbon concentrations were significantly higher in the south basin (with 196 and 166 mg C m⁻³ in the dry and the rainy season, respectively) than in the north basin (112 and 109 mg C m⁻³, respectively).
3. Carbon : phosphorus (C : P) ratios varied according to season. Phosphorus limitation seemed to occur more frequently than nitrogen limitation, especially during the rainy season. Severe P deficiencies were rare.
4. Measured particulate daily primary production ranged from 110 to 1410 mg C m⁻² day⁻¹; seasonal contrasts were well marked in the north basin, but less in the south basin, where primary production peaks occurred also in the rainy season. Estimates of annual primary production, based on daily primary production calculated from chl- a and water transparency, gave values lower than those reported in previous studies. Picophytoplankton accounted on average for 56% of total particulate production in the south basin during the wet season of 2003.
5. Phytoplankton growth rates, calculated from primary production, ranged from 0.055 to 0.282 day⁻¹; these are lower than previously published values for Lake Tanganyika.     

Underwater light penetration,phytoplankton biomass and photosynthetic activity in Lake Xolotlán (Managua)

We measured underwater light penetration, phytoplankton biomass and photosynthetic activity during three years (1987–1990) in Lake Xolotlán (L. Managua), Nicaragua. Phytoplankton biomass governed the light climate of the photic zone, but as biomass also was composed of a varying proportion of dead algae, light availability for the potential biomass of actively photosynthesizing algae (170 mg Chl-a.m^–2) was reduced. The concentration of chlorophyll-a within the photic zone was thus lower and ranged between 58 and 141 mg Chl-a.m^–2. Still, photosynthetic activity was high (2,162 mg 02.m-2.h^–1) due to an extremely high specific rate of photosynthesis; light was the only factor that limited growth. As also other conditions in Lake Xolotlán, beside light limitation, met with the requirements of the models that have been used to analyse production and photosynthetic characteristics in tropical lakes there was a striking agreement between observed and predicted values.     

Nyumba ya Mungu reservoir, Tanzania The general features

Nyumba ya Mungu reservoir was completed in 1965 in northern Tanzania. By 1970 there was a thriving tilapia fishery but it declined catastrophically in subsequent years. A team of biologists surveyed the lake in 1974, and this paper outlines their hydrological, geographical, and climatic findings.
The dam, constructed across a north/south Neogene fault-trough 80 km south of Mt. Kilimanjaro, has produced a reservoir about 180 km² with a mean depth of 6 m. The catchment area is extensive but the main source of water is from Mt. Kilimanjaro. The lake is situated in an arid area, and over one quarter of the input water evaporates from the lake's surface. The outflow is regulated by a hydro-electric power station and is practically constant. At full capacity the retention time of the lake is one year but due to abnormally low rainfall, the reservoir was not full, and the retention time was nearer nine months.
The lake is polymictic and a deep wind-driven current is maintained by the Trade Winds for most of the year.
The general features of Nyumba ya Mungu are briefly compared with other man-made and natural lakes in Africa.     

张掖人工池塘浮游植物群落研究

对张掖具代表性的五个人工地塘的浮游植物四季采样,共获７９种（未鉴定到种的按一种计）,其中蓝藻１２种,绿藻３２种,硅藻１８种,甲藻３种,裸藻８种,黄藻、隐藻和金藻各２种。浮游植物的年平均密度为５７８．２３６万个／Ｌ,生物量为３．１０４０ｍｇ／Ｌ。不同池塘中的现存量有一定的差别,种类以流泉最高,生物量以市鱼场最高。文中对浮游植物的季节变动作了分析,并对浮游植物的产鱼潜力进行了初步估算。     

The ecology of the fishes in Nyumba ya Mungu reservoir, Tanzania

Twenty species of fish were found in Nyumba ya Mungu reservoir on the River Pangani which, with two exceptions, were indigenous to the river basin. Their distribution is described and some information on their growth, reproduction and feeding habits is presented. The highest densities of fish were found in the littoral and the broad, shallow, upstream region of the lake. The lacustrine environment has favoured the endemic tilapias, Sarotherodon jipe and S. pangani , which graze periphyton, and the introduced species, S. esculentus which feeds on phytoplankton. Males of the endemic forms may grow to a large size, but the introduced species, including Tilapia rendalli , were heavier for a given length. A reduction in tilapia stocks since 1970 is reflected in experimental catches during 1972-74 and was attributed to the combined effects of heavy commercial exploitation and a loss of spawning grounds. In the same period a striking increase in the populations of small carnivores, Rhabdalestes leleupi and Haplochromis gr. bloyeti , was recorded. Kully plantivorous and piscivorous fish are absent but omnivores are represented by Barbus species and Synodontis punctulatus. Tilapias exhibited a high incidence of nematode infection and the available evidence indicates that fish-eating birds are important predators of cichlids in Nyumba ya Mungu.     

The filling phase of Mazvikadei Reservoir,Zimbabwe

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.     

The structure of phytoplankton assemblages in Sicilian reservoirs: some hyphoteses on the driving factors

Abstract

From the analysis of the results of several researches carried out into Sicilian reservoirs to investigate their phytoplankton assemblages, it was highlighted that there is not any clear relationship between the trophic spectrum and the structure and composition of phytoplankton. Only a general increase of total biomass is detectable as the trophic state rises up. Anyway, such increase may cause several secondary modifications in the physical and chemical environment that may act as multidimensional segregating factors for phytoplankton organisms. In these secondary modifications, coupled with the peculiar morphology and hydrology of the water bodies, have to be sought the mechanisms governing the structure of phytoplankton assemblages.     

Phytoplankton succession in Lake Valencia,Venezuela

Phytoplankton counts and supporting physical and chemical data were taken on Lake Valencia, Venezuela, over a five-year interval. The data are used to test the validity of a successional paradigm for class-level taxa. According to the paradigm, formulated from previous studies of Lake Lanao, Philippines, and from data on temperate lakes, the order of taxa from early to late succession is: diatoms, chlorophytes, blue-green algae, dinoflagellates. A successional episode is considered to begin when stability of a water column is restored after deep mixing. As the episode progresses, there is a steady decrease in concentration of the limiting macronutrient (in this case, N). In a test of the validity of the paradigm for Lake Valencia, dates of exceptional population increase or decrease were obtained for each taxon. Since nitrate concentration declines steadily as succession progresses, the entry of a given taxon into the successional sequence is indicated quantitatively by the mean nitrate concentration on dates of exceptional increase in population density, and exit from the successional sequence is indicated by mean nitrate concentration on dates of exceptional population declines. The successional position of each major taxon, bounded by its entry and exit in the sequence, can be mapped on the complete spectrum of nitrate concentrations observed in the lake. For Lake Valencia, the nitrate mapping procedure agrees exactly with the predictions based on the successional paradigm. Conformance of Lake Valencia phytoplankton with predictions made a priori suggests that there is a generalized pattern in the phytoplankton succession of the mixed layers of temperate and tropical lakes.     

Abundance and distribution of picoplankton in tropical, oligotrophic Lake Kivu, eastern Africa

1. We used flow cytometry to characterize freshwater photosynthetic picoplankton (PPP) and heterotrophic bacteria (HB) in Lake Kivu, one of the East‐African great lakes. Throughout three cruises run in different seasons, covering the four major basins, phycoerythrin‐rich cells dominated the PPP. Heterotrophic bacteria and PPP cell numbers were always high and spatial variations were modest. This represents an important difference from temperate and high latitude lakes that show high fluctuations in cell abundance over an annual cycle. 2. Three populations of picocyanobacteria were identified: one corresponded to single‐cells (identified as Synechococcus by epifluorescence microscopy, molecular methods and pigment content), and the two other that most probably correspond to two and four celled colonies of the same taxon. The proportion of these two subpopulations was greater under stratified conditions, with stronger nutrient limitation. 3. High PPP concentrations (c. 10⁵ cell mL^?1) relative to HB (c. 10⁶ cell mL^?1) were always found. Lake Kivu supports relatively less bacteria than phytoplankton biomass than temperate systems, probably as a consequence of factors such as temperature, oligotrophy, nutrient limitation and trophic structure. 4. A review of PPP concentration across aquatic systems suggests that the abundance of Synechococcus‐like cyanobacteria in large, oligotrophic, tropical lakes is very high. 5. Photosynthetic picoplankton cell abundances in the oligotrophic tropical lakes Kivu and Tanganyika are comparable to those of eutrophic temperate lakes. This apparently contradicts the view that PPP abundance increases with increasing eutrophy. More data on PPP in tropical lakes are needed to explore further this particular pattern.     

Applicability of phosphorus budget models to southern African man-made lakes

An investigation of the phosphorus loading characteristics of 31 southern African man-made was lakes made. The lakes were characterized by low water retention times, with most of the lakes having retention times of less than one year. Catchment phosphorus export rates showed wide variation (1–162 mg P m^-2 y^-1) with those lakes experiencing excessive municipal wastewater inputs having export rates in excess of 53 mg m^-2 y^-1. The phosphorus data were tested against the Vollenweider (1976) and Dillon & Rigler (1974) phosphorus budget models which predict in-lake steady state concentrations of phosphorus. It was found that both models displayed good potential for the prediction of steady state concentrations of phosphorus, with better results being obtained from the Dillon & Rigler (1974) model. However, because phosphorus concentrations within these lakes may not necessarily be related to trophic status the use of these models as a predictive tool for eutrophication control still requires further development.     

Skeletal development in Hermesinum adriaticum Zacharias, a flagellate from the Salton Sea, California

Hermesinum adriaticum is a rarely reported unicellular biflagellated organism with a solid siliceous skeleton. Live specimens were not observed but many skeletons were found in sediments and, in small numbers, in cyanobacterial mats and the water column of the Salton Sea, a salt lake in California, U.S.A. Stages of the developing skeleton were studied with scanning electron microscopy, and the progression from small tetraxial daughter skeletons to complete asymmetrical adult skeletons is presented. Some variability in the adult skeletons is illustrated.