The ecology of Lake Nakuru (Kenya)

Summary The Cichlid fish Tilapia grahami (-Sarotherodon alcalicum grahami) was introduced into Lake Nakuru (Kenya) in about 1960 and is now one of the main herbivores. Spatial distribution and biomass changes were estimated from lift net catches from 1972–1974 which were partly continued until 1976. The length/weight relationship is represented by the equation W=0.008·l ^2.98 (W=dry weight=24% of fresh-weight; l=standard length=85.1% of total length). The fish distribution is very patchy (aggregation coefficient 5.2–12.2). The density decreased and the mean fish size increased from in-shore to off-shore regions. At noon the fish concentrate near the shore and at hight they move off-shore, a migration pattern probably reflecting a preferance for higher temperatures. 70% of Tilapia concentrate in the top 50 cm and 80% in the top 100 cm. The total ichthyomass of the lake had a mean of 90 t dry weight (=2.1 g/m²) in 1972 and it increased to a mean of 400 t dry weight (=10.2 g/m²) during 1973. Possible causes for the spatial distribution and the biomass variations are discussed. The high density of Spirulina platensis makes nutritional competition among the herbivores unlikely. The main impact of Tilapia grahami on the lake's ecosystem is a substantial increase in diversity by extending the food chains to fish eating birds, of which the Great White Pelican is dominating. The breeding of Pelicans at a neighbouring lake causes a considerable nutrient export (13 t phosphorus/year).     

Aquatic biodiversity and saline lakes: Lake Bogoria National Reserve,Kenya

Lake Bogoria, in the Rift Valley of Kenya is an extreme saline lake (conductivity 40–80 mS cm^–1, alkalinity 1500 m equ l^–1). It is hydrologically more stable than the other, endorheic lakes in Kenya, because it is deep – maximum depth at present just over 10 m in an area of 3000 ha – and so does not have periods when it is dry. It is ecologically simple, with only one species dominating the phytoplankton – the cyanobacterium `spirulina', Arthrospira fusiformis. Its biomass and productivity were very high – biomass between 38 and 365 g l<sup>–1</sup> chlorophyll `a' and 3.4–21 × 10³ coils ml^–1 and net production between 0.24 and 1 gm C m³ h, the latter in a narrow zone of less than a metre. There were no macro-zooplankton in the plankton and the only grazer of A. fusiformis was the lesser flamingo, Phoeniconaias minor,which occurred irregularly in very high concentrations (in excess of 1 × 10⁶). Detritivory in the benthos was effected by a single chironomid species, Paratendipes sp., at a maximum density of 4 × 10⁴ m^–2. The mean daily emergence of adult chironomids was estimated to be 1 × 10³ m^–2, the maximum 3. There was no littoral plant community within the lake but 44 dicotyledonous and 31 monocotyledonous plant species in the drawn-down zone and adjacent to it. A diverse draw-down terrestrial invertebrate fauna, only superficially described here, processed the flamingo feathers and carcasses, with other detritus such as chironomid pupal exuviae and decaying A. fusiformis scum. About 50 bird species depended upon the chironomids, either as they emerged through the water column as flying adults or later on the shoreline as floating pupal exuvia and dead adults. The lake has high conservation value because of three bird species in particular – lesser flamingo, Cape teal and black-necked grebe. The former provides real economic value in a region otherwise impoverished, because of the spectacle of tens of thousands of flamingos set against the landscape of hot springs and fumaroles at the lake edge, which draws 15000 visitors per annum. P. minor has experienced three periods during the past ten years when major mortalities have occurred, the last of which killed 700 birds day^–1. This could have involved as many as 200000 birds (about 1/5th of the maximum population at this lake) if mortality was at a constant rate for the nine months it was observed. Causes of mortality have been suggested as avian tuberculosis, poisoning from cyanobacterial toxins or from heavy metal contamination at Lake Nakuru, but it is still not yet clear what contribution each makes to the problem.     

The ecology of Lake Nakuru (Kenya)

Summary Abiotic factors, standing crop and photosynthetic production were studied in the equatorial alkaline-saline closed-basin Lake Nakuru (cond. 10,000–160,000 S). Meteorological conditions and abiotic factors offer suppositions for a high primary productivity: mean solar radiation is 450–550 kerg·cm^-2·s^-1, with little seasonal variation, regular winds circulate the lake every day and nutrient concentrations are usually high (>100 g P–PO₄·l^-1). Oxygen concentrations near sediments were <1 gO₂·m^-3 for at least 6 h·d^-1 in 1972/73, resulting in a release of 45 mg P–PO₄·m^-2·d^-1. Attenuation coefficients vary from 3.6–16.5 according to algal densities and mean depth from 0–400 cm. Algal biomass was 200 g·m^-3 (d.w.) in 1972/73, due to a lasting Spirulina platensis bloom (98.5% of algal biomass). In 1974 algal biomass suddenly dropped to 50 g·m^-3 (d.w.). Spirulina and several consumer organisms almost vanished, but coccoid cyanobacteria, Anabaenopsis and diatoms increased. Several causes for this change in ecosystem structure are discussed. The use of the light/dark bottle method to measure photosynthetic production in eutrophic alkaline lakes is discussed and relevant experiments were done. Oxygen tensions of 2–35 gO₂·m^-3 do not influence primary production rates. Net photosynthetic rates (mgO₂·m^-3·h^-1; photosynthetic quotient=1.18) reached 12–17.7 in 1972/73 and 2–3 in 1974, but vertically integrated rates were only 1–1.4 in 1972/73 and 0.8 in 1974, and daily net photosynthetic rates (gO₂·m^-3·24 h^-1) 3.5 in 1972/73 and 1 in 1974. 50% of areal rates were produced within the 10 most productive cm of the depth profile. The disproportion between high algal standing crops and relatively low production rates is due to self-shading of the algae, reducing the euphotic zone to 35 cm in 1972/73 and 77 cm in 1974. Efficiency of light utilization is 0.4–2%, varying with time of day and phytoplankton density. In situ efficiencies show an inverse relationship to light intensities. Photosynthetic rates of L. Nakuru remain within the range of other African lakes (0.1–3 gO₂·m^-2·h^-1). The relation of O₂ produced/Chl a of the euphotic zone is 50% lower then in tropical African freshwater lakes and conforms to lakes of temperate regions.     

The ecology of Lake Nakuru

Summary The major pathways of energy flow in Lake Nakuru (East Africa) are presented. The trophic structure of this equatorial alkaline-saline lake shows no predictable long term continuity. During the five years of this study it had a bloom of Spirulina platensis that persisted at least two years, it had periods with low algal densities and in addition it had various transitional phases with dramatic fluctuations of species composition and density.The Spirulina platensis bloom is characterized by a rich and almost unialgal bloom of the cyanophyte Spirulina platensis minor, with a mean biomass of 3,500 kJ m^-3 (20 kJ 1 g dry weight). Net photosynthetic rates were very high at depths with optimal light conditions (230 kJ m^-3 h^-1), but algal self-shading made integrated rates modest (45 kJ m^-3 24 h^-1) relative to the high biomass. Of the eight primary consumers only five species contributed significantly to the consumer biomass of 220 kJ m^-3: the flamingo Phoeniconaias minor, the cichlid fish Sarotherodon alcalicus grahami, the copepod Lovenula africana, the dipteran larva Leptochironomus deribae, and the rotifer Brachionus dimidiatus. Consumption rates were 50% of net photosynthetic rates, production rates 10%. Secondary consumers (90% being the pelican Pelecanus onocrotalus and the Greater Flamingo Phoenicopterus ruber) had a biomass of about 6.8 kJ m^-3. Pelicans consumed almost the whole fish production (7.5 metric tons wet weight/day).At low algal densities the lake had a more diverse algal population but a reduced mean biomass of 1,500 kJ m^-3 and mean net photosynthetic rates of 12 kJ m^-3 24 h^-1. Primary consumer species diversity and biomass were also reduced. Consumption rates sometimes exceeded primary production rates. Rotifers probably contributed 50% to total consumption and 75% to total secondary production but the estimates of their role is speculative as the relative contributions of algae, bacteria and detritus to rotifer consumption are not known. Transitional phases are characterized by rapidly changing abiotic and biotic conditions with algal breakdowns and sudden population peaks at all levels. Rotifers dominated secondary consumers, they contributed 25% to the total biomass of 380 kJ m^-3, 90% to the total consumption rate of 290 kJ m^-3d^-1 and 95% to the total production of 41 kJ m^-3d^-1.The discussion focusses on problems of measuring primary production in alkaline-saline lakes, and the control of producer and consumer densities. The difficulty in assessing the importance of bacteria and rotifers is emphasized. Also questions of ecological stability and efficiency are addressed. Finally, some recommendations for conservational policy are included.     

Evaluation of a fluorescent microparticle technique for measuring filtering rates of Daphnia

The paper demonstrates the applicability of fluorescent microparticles (FMP) for the determination of cladoceran feeding rates. For this purpose we examined the relation between filtering rate, ingestion rate, gut passage time and body length of Daphnia pulex at different food concentrations in laboratory experiments, and diurnal changes of filtering rates of Daphnia longispina in in situ experiments, using FMPs as tracer particles. Good agreement of our results with published data based on cell counts and radiotracer methods proves the suitability of the FMP-method. Potentials and limitations of the method are discussed.     

The ecology of Lake Nakuru (Kenya)

Summary The shallow, alkaline pan of Lake Nakuru (conductivity 15,000–25,000 mho/cm, 20°C) usually maintains an exceptionally high standing crop of the cyanophyte Spirulina platensis (150–200 mg DW/l; DW=dry weight), the main food of a large population of the lesser flamingo (Phoeniconaias minor). The abundance and feeding of the lesser flamingo were studied in an attempt to quantify the lake's energy flow. Some data on other rift valley lakes with similar chemical and biological conditions are included, since they are inhabited by flamingos as well. The spatial distribution and total population of the flamingos were monitored on a monthly basis. The birds were counted automatically from aerial photographs by a particle counter. The mean was 915,000 in 1972 and 1973, and in 1974 the population dropped to a mean of 113,000. The population also showed pronounced short time fluctuations that are correlated with algal densities. Other possible causes for flamingo migrations are discussed. Flamingos feed by filtering planktonic organisms from the water with their bill. Feeding experiments with caged birds gave a clearing rate of 31.8±1.3 l/h (SE; SE=standard error) for an adult flamingo, a pumping rate of 17.5 strokes/s and a feeding rate of 5.6 g DW/h at the mean algal concentration of 180 mg DW/l in 1972/73. The mean feeding time in that period was 12.5 h/d, which gave a daily feeding rate of 72±6.5 g DW for an adult bird and 66±6 g DW for the average bird (juveniles included). Therefore the whole flamingo population extracted per day 60 t DW of algae (0.7 g DW/m³/d or 3 kcal/m³/d) from the lake. This is 50–94% of the daily primary production or 0.4 to 0.6% of the algal biomass and two to three times the amount all other primary consumers are feeding. About 0.75 kcal/m³/d are returned by fecal and urinary wastes. These feeding rates are slightly lower than calculations based on basic metabolic rates of birds.     

Dressur von Bienenarbeiterinnen und Drohnen auf Pheromone und andere Duftstoffe

Zusammenfassung Mit Hilfe des Rüsselreflexes gelang es, nicht nur Arbeiterinnen, sondern auch Drohnen auf Duftstoffe zu dressieren. Jeder der sechs verwendeten Duftstoffe, darunter Königinsubstanz und Sterzelduft, konnte als Dressurduft von den übrigen Düften sicher unterschieden werden.

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Conditioning of worker and drone honeybee with pheromones and other odourous substances

Summary The proboscis extension reflex was used for conditioning worker and drone bees to odours. Six odours, including queen substance and the scent to the Nassanoff gland were used as conditioning stimuli. The animals could clearly distinguish the odours from one another.

     

The ecology of Lake Nakuru (Kenya)

Summary Consumer biomass and spatial distribution in the equatorial alkaline-saline Lake Nakuru were studied from 1972–1976. These data will provide the basis for estimating feeding and production rates and for quantifying energy flow at the consumer level. Two of the main consumers, the Lesser Flamingo (Phoeniconaias minor) and the Soda Tilapia (Sarotherodon alcalicus grahami), were covered by earlier papers. The biomass of the only planktonic crustacean, the copepod Lovenula (=Paradiaptomus) africana was very high (1.5 gDW (dry weight)·m^-3, mean in 1972/73) in comparison with other tropical lakes. Lovenula was absent in 1974 and 1975, and at very low levels (0.1–0.4 gDW·m^-3) in part of 1976. Among the rotifers Brachionus dimidiatus dominated in 1972/73 (0.2 gDW· m^-3), but was outnumbered by B. plicatilis throughout most of 1974 to 1976 (mean total rotifer biomass 1.4 gDW· m^-3, peak densities 7 gDW·m^-3); during high salinity periods (>20) Hexarthra jenkinae occurred in very low numbers. For short periods rotifers can be the dominant species of L. Nakuru. Aquatic heteroptera (four species) played a minor role: they contributed 0.4% to total consumer biomass in 1972/73; in 1974–1976 the lake had no aquatic heteroptera. Benthic biomass (0.4 gDW·m^-2) was within the range of other tropical lakes, it consisted almost exclusively of Leptochironomus deribae. Bird counts of the twelve most important species are given for the years 1972–1974: Pelecanus onocrotalus accounts for 90% of the biomass (0.44 gDW·m^-3, mean 1972/73) with peak densities of almost 20,000 birds.-The consumer organisms covered by this and the two preceding papers represent >99% of L. Nakuru's consumer biomass. Population dynamics of various consumer species are discussed.