Zooplankton associations in some Ethiopian crater lakes

SUMMARY. 1. The zoopiankton associations in five crater lakes in Ethiopia were examined in April 1984. The two least alkaline lakes had the most diverse zoopiankton.
2. The large diaptomid copepod Lovenula africana was present in the lakes with alkalinities between 10 and 63 meq l⁻¹, but was absent from Lake Chitu (239 meq l⁻¹).
3. The contribution of the zoopiankton to the nocturnal deoxygenation of Lake Aranguadi is negligible.     

Pollen input to, and incorporation in, two crater lakes in tropical northeast Australia

Pollen input to the water surfaces of maar crater lakes Barrine and Eacham (1km(2) and 0.5km(2) respectively, ca. 720m a.s.l.), surrounded by rainforest in northeastern Australia, were measured in floating Tauber traps for a variety of periods spanning the years 1978 to 1985. The mean weekly total pollen catch is 75% of that estimated for the surrounding region of varied vegetation. Regardless of their sources, the influxes of pollen taxa are unevenly distributed over the lakes' surfaces. There is similar variation through time, with some indication that this might be smoothed over periods of 5years or more. There are no inflow streams and pollen input down surrounding slopes is negligible. Mean total pollen influx to seston traps close to the lakes' bottoms (1979 to 1988) is <5% of that to the Tauber traps, a discrepancy attributed to trap design. In contrast, mean annual influx measured by the Tauber traps is similar to that estimated from dated sediment samples younger than 1966 AD at Lake Barrine. The proportions of different pollen taxa are less affected by trap position or period of exposure than are their influxes; their mean percentages for Tauber traps, seston traps and sediment samples are all rather similar.The similarity between the mean contribution of exclusively rainforest pollen to the lakes' surfaces (56%) and to Oldfield traps in rainforest interiors (51%) emphasizes the importance of the rainforests, which clothe the crater walls, as sources of pollen to the lakes. The impact of pollen from other vegetation in the region, although present, is much smaller than at interfaces between rainforest and other vegetation types. All this pollen is mixed within and above the crater and delivered to the lakes' surfaces by gravity and frequent rainfall. Limnological processes redistribute it in the water body before it is incorporated into the deep-lake sediments.The value and limitations of current pollen transport and accumulation theory are noted in relation to sites with morphologies similar to these two crater lakes and to the irregular flowering and floristic inhomogeneity of tropical rainforest. The potential for the use of modern pollen input data in the interpretation of pollen analyses from the sediments of such sites is explored.     

The seasonal pattern of thermal characteristics of four of the Bishoftu crater lakes, Ethiopia

Measurements of temperature/depth profiles in four Ethiopian crater lakes were made during 1964, 1965 and 1966 at fairly regular intervals. Several more intensive diurnal studies, with observations at 3-h intervals during periods of about 48 h, were also carried out. Results show a pattern of heating leading to thermal stratification from February to September and cooling from September to December with possible overturn at the end of that period. Thermal stability was never great and thermoclines were often poorly defined and multiple. Net daily changes in heat content during both heating and cooling can be a small fraction (c. 3%) of the diurnal heat flux and depend upon the relative magnitudes of solar radiation and evaporative heat loss. These are such that cooling may occur during periods of highest solar input and vice versa. However, meteorological conditions could easily reverse the cooling process before the December overturn so that complete mixing is by no means a predictably regular annual event.     

Phosphorus limitation in some tropical African lakes

Two of three Kenyan lakes studied between November 1979 and October 1980 have very short ³³PO₄ turnover times, indicating a high phosphorus (P) demand throughout the year. The P turnover time in Lakes Oloidien and Sonachi is as rapid as in the most P deficient temperate zone lakes. The third lake, Lake Naivasha, has a lower overall P demand and a wide seasonal range, with lowest demand between November 1979 and February 1980 when a P deficiency was unlikely. On an annual basis the Lake Naivasha status is, however, not statistically different from that recorded during the summer in Lake Memphremagog, a generally P-limited temperate zone lake. Lake Naivasha and Lake Oloidien fit well to the line of best fit for the Dillon-Rigler relationship relating total phosphorus (TP) and chlorophyll a derived in temperate zone lakes. Thus, temperate zone models predicting aspects of lake behaviour on the basis of TP may also be applicable to these two tropical lakes. Saline lake Sonachi had not only a short P turnover time but also responded dramatically to the fertilization of enclosures with P. However, it does not fit the TP-chla or the total nitrogen-chla plots from the temperate zone. This suggests that, in this saline lake at least, much of the TP is unavailable to the algae, with some of it in a particulate form that is readily extracted with boiling water. The epilimnetic N:P ratios also characterize lakes Oloidien and Sonachi lakes as highly P deficient and lake Naivasha as more moderately P limited. A single set of measurements in Winam Gulf (Lake Victoria) also showed a rapid P turnover time and thus P limitation, but as in lake Sonachi much of the TP was in a non-algal particulate form. Occasional measurements in three other hypertrophic and saline lakes suggest them to be primarily light limited on the basis of their very high photosynthetic cover. These findings support the hypothesis of a primary P limitation for those lakes not light limited, and contradicts literature suggestions that nitrogen is the primary limiting element in tropical lakes.     

Thermal and chemical stratification and mixing in a small tropical reservoir, Solomon Dam, Australia

SUMMARY.

• 1 The patterns of distribution of temperature, dissolved oxygen, and filterable iron in the water mass of Solomon Dam were determined at about 30 day intervals over a 2.5 year period (1981–83).
• 2 The thermal stratification, and resultant chemical differentiation of the epilimnion and hypolimnion, was the dominant limnological feature of the reservoir. A pronounced thermoline was present for most of the year and, during the stratified phase, diurnal circulation appeared to be the main mechanism for maxis.
• 3 Evidence for the presence of an internal seiche, a phenomenon commonly associated with stratified water bodies, was also observed.
• 4 An annual overturn in June-July coincided with the minimum in solar radiation. The ‘wet’ season (January-March) caused either partial or complete circulation depending on the volume of the reservoir and the rate of inflow. In 1982 the lake circulated once, fitting the warm monolithic pattern, but in 1983 there were two circulations, the first resulting from flood inflows.
• 5 The study illustrated the inappropriate nature of a thermally derived classification scheme for a small reservoir.

     

Associations of rotifers in Australian crater lakes

Associations of rotifers in 12 Australian crater lakes have been compared by means of indices of similarity and diversity, and by two dimensional ordination. Four of the lakes lay on the Atherton Tableland in tropical Queensland, four were shallow saline lakes in Victoria and the remaining four were fresh lakes in the same general region as the saline lakes.
The total number of species identified was 67, but the maximum found in any one sample was 27. Salinity is an important factor reducing diversity, whilst the development of vegetation increases the diversity of the rotifer fauna. The index of diversity (a) ranged from less than one in the saline lakes to 13 in the richly vegetated Lake Euramoo.
Comparisons with the rotifers living in the open waters of crater lakes in Africa, Indonesia and Europe revealed no marked similarities between the fresh Australian lakes and these other regions, although strong similarities were found between Ausralian saline lakes and those of other regions. In general the rotiferan faunas of the Australian crater lakes resembled each other less than might be expected from the similarities found between crater lakes within other regions.     

Chemical and thermal stratification in lakes

An index that shows chemical stratification strength [IC-i; i = water quality item such as chlorophyll-a (Chl.a) and soluble phosphorus (SP)] was proposed and compared with one of thermal stratification strength indices, Schmidt’s stability index (SSI), in Shiozu Bay and Lake Biwa, Japan. The proposed indices of IC-i can be easily calculated with at least one set of each water quality data in both the epilimnion and the hypolimnion. The SSI was shown to be consistent with the traditional thermocline index of thermocline strength index (TSI), but SSI is used as the stability index of the whole lake, whereas TSI is used as the stability index near the thermocline. Analyses showed that chemical stratification strength is determined largely by thermal stratification strength. Totally different characteristics of IC-Chl.a and IC-phosphate (PO₄) at high SSI in the main North Basin of Lake Biwa and in Shiozu Bay were possibly due to the difference in their volumes and hydrodynamic conditions. The proposed index and relationships are especially useful to roughly determine thermal and chemical stratification when only few water quality data are available.     

Seasonal variation in the chemical composition of two tropical seaweeds

The chemical composition of red seaweed Gracilaria cervicornis and brown seaweed Sargassum vulgare from Brazil was investigated. In this study, the relationship between the nutritive components of each species and the environment was established. Protein content varied from 23.05+/-3.04% to 15.97+/-3.04%. The highest value was found in G. cervicornis. The protein levels were positively correlated with nitrogen content and negatively with water temperature and salinity. Carbohydrate contents of both species varied significantly (p<0.01) and the values observed were superior to others chemical constituents. Contrary to carbohydrates, the lipid concentrations were the lowest recorded chemical component and varied slightly between the two species. Ash content was greater in S. vulgare (14.20+/-3.86) than in G. cervicornis (7.74+/-1.15). In general the variation in chemical composition was related to environment.     

Seasonal succession of phytoplankton and its diversity in two highly eutrophic lakes with different conditions of stratification

Seasonal succession of phytoplankton biomass, its diversity and its photosynthetic activity in two highly eutrophic lakes have been compared. In order to test the intermediate disturbance hypothesis, the lakes have been chosen with almost the same level of trophy but different conditions of stratification, through two ice-free periods of open water with different weather conditions.High phytoplankton diversity throughout the period of investigation was characteristic for the shallower Lake Lobardzu. The number of species here was usually more than 30 and the Shannon diversity changed from 1.2 to 4.2. Owing to the frequent external disturbances, periods characterized by autogenic succession with establishing dominance and declining diversity alternated with periods of biomass reduction and rises of diversity and photosynthetic activity. In the warmer summer of 1983, with more intense warming of bottom layers and predominance of blue-greens, phytoplankton biomass was higher and diversity lower than in the cold summer of 1982.In stratified Lake Rudusku, phytoplankton diversity and number of species were usually much lower. During the long summer stratification up to three-four dominant species of blue-greens and dinoflagellates become established and competitive exclusion leading to low diversity advanced. Some changes in biomass and diversity, were caused by zooplankton activity.     

Practical aspects of mycorrhizal technology in some tropical crops and pastures

Summary Greenhouse and field experiments were conducted on the effect of VA mycorrhiza (VAM) on the growth of cassava, various tropical grass and legume species, as well as beans, coffee and tea. A large number of VAM fungal species were evaluated for effectivity in increasing cassava growth and P uptake in acid low-P soils. The effectivity of VAM species and isolates was highly variable and dependent on soil pH and fertilizer applications, as well as on soil temperature and humidity. Two species,Glomus manihotis andEntrophospora colombiana were found to be most effective for a range of crops and pastures, at low pH and at a wide range of N, P, and K levels. At very low P levels nearly all crops and pasture species were highly mycorrhizal dependent, but at higher soil P levels cassava and several pasture legumes were more dependent than grass species. Mycorrhizal inoculation significantly increased cassava and bean yields in those soils with low or ineffective indigenous mycorrhizal populations. In these soils cassava root yields increased on the average 20–25% by VAM inoculation, both at the experiment station and in farmers’ fields. VAM inoculation of various pasture legumes and grasses, in combination with rock phosphate applications, increased their early growth and establishment. Agronomic practices such as fertilization, crop rotations, intercropping and pesticide applications were found to affect both the total VAM population as well as its species composition. While there is no doubt about the importance of VA mycorrhiza in enhancing P uptake and growth of many tropical crops and pastures grown on low-P soils, much more research is required to elucidate the complicated soil-plant-VAM interactions and to increase yields through improved mycorrhizal efficiency.     

Associations of zooplankton in six crater lakes in Arizona, Mexico and New Mexico

Zooplankton associations in six crater lakes have been compared by means of indices of similarity and diversity, and by k -dominance curves. The lakes covered a wide range of salinities and vegetational characteristics.
The total number of species identified were: Protozoa, 34; Rotifera, 48; Gastrotricha, 7; Crustacea, 24; and Chaoborus larvae, 2; giving an overall total of 115. The numbers of zooplankton species in each lake varied from 68, in a richly vegetated fresh crater lake in Arizona, to one in a highly saline lake in New Mexico.
Testate rhizopods and gastrotrichs were not found in the more saline lakes. A comparison of the rotifer associations with those of crater lakes in other parts of the world shows that there is a general tendency for a reduction in the total number of species when the conductivity rises above 400 μS cm⁻¹ 20°C, and when the conductivity reaches 10,000 μS cm⁻¹ 20°C the number of rotifer species is usually reduced to two. The two species found at high salinities show some variation in different parts of the world, but generally include species of Brachionus and Hexarthra.
There is a general tendency for the number of rotifer species in a crater lake to increase with increasing vegetation, at least up to the stage where there is a dense, zoned vegetation, and before a complete mosaic of vegetation covers the lake.     

Alpha-adrenoceptors: recent development and some comparative aspects

On anatomical and functional bases, alpha-adrenoceptors have been divided into pre- and postsynaptic alpha-adrenoceptors. Recently, alpha-adrenoceptors have been classified as alpha 1 and alpha 2 according to their pharmacological responses, irrespective of their anatomical location. The presynaptic alpha-adrenoceptors, which have been recognized as alpha 2, determine the frequency of the nerve impulses travelling along the axon and also the amount of transmitter released per nerve impulse from the varicose terminal. Postsynaptic alpha-adrenoceptors have been recognized in various tissues including smooth muscle, pancreatic islets, fat cells, platelets and other tissues. Both alpha 1- and alpha 2-adrenoceptors have been located postsynaptically. alpha-Adrenoceptors have been found also in the central nervous system. Generally, they fall into the same categories (alpha 1 and alpha 2) as the peripheral alpha-adrenoceptors. A new class of drugs, the so called calcium blockers, inhibit the postsynaptic response to alpha 2 stimulation but not the alpha 1-mediated response, indicating that the alpha 2-adrenoceptors are dependent on Ca2+ ions for their function. In the most primitive group of vertebrates, the fishes, alpha-adrenoceptors seem to be different in as much as they do not respond to many of the classical drugs employed to distinguish between alpha-adrenoceptors in mammals. In reptiles and amphibians alpha 2-adrenoceptors have been shown to exist. These receptors are involved in the regulation of melanocytes. In the most advanced non-mammalian vertebrates (birds) both peripheral and central alpha-adrenoceptors seem to be qualitatively similar to the mammalian types.     

Biological observations on the crater lakes of Jebel Marra, Sudan

Two lakes lie in a caldera at the top of Jebel Marra. One is shallow and highly saline, with a dense bloom of planktonic blue-green algae and a zooplankton consisting entirely of rotifers. The other is over 100 metres deep, is less saline, with a sparse phytoplankton of diatoms, and the dominant zooplankter is Eucyclops gibsoni. The marginal fauna of the deep lake includes a number of widespread insects, but is unusual in that it includes an aquatic glowworm (Family Lampyridae).
In the shallow lake there is a superabundance of oxygen in the upper two metres, but no oxygen below six metres, except when strong winds mix the lake so that there is then about 40% saturation throughout the water column. In the deep lake it was found that after strong winds there was only about 12% saturation in the top 30 metres of water. The restricted fauna of these lakes appears to be a result of the combined effects of isolation, salinity and a variable oxygen regime.     

Vertical stratification of physical,chemical and biological components in two saline lakes Shira and Shunet (South Siberia,Russia)

A feature of meromictic lakes is that several physicochemical and biological gradients affect the vertical distribution of different organisms. The vertical stratification of physical, chemical and biological components in saline, fishless meromictic lakes Shira and Shunet (Siberia, Russia) is quite different mainly because both mean depth and maximum depth of lakes differ as well as their salinity levels differ. The chemocline of the Lake Shira, as in many meromictic lakes, is inhabited by bacterial community consisting of purple sulphur and heterotrophic bacteria. As the depth of the chemocline is variable, the bacterial community does not attain high densities. The mixolimnion in Lake Shira, which is thermally stratified in summer, also creates different habitat for various species. The distribution of phytoplankton is non-uniform with its biomass peak in the metalimnion. The distribution of zooplankton is also heterogeneous with rotifers and juvenile copepods inhabiting the warmer epilimnion and older copepods found in the cold but oxic hypolimnion. The amphipod Gammarus lacustris which can be assigned to the higher trophic link in the fishless lake’s ecosystem, such as Lake Shira, is also distributed non-uniformly, with its peak density generally observed in the thermocline region. The chemocline in Lake Shunet is located at the depth of 5 m, and unlike in Lake Shira, due to a sharp salinity gradient between the mixolimnion and monimolimnion, this depth is very stable. The mixolimnion in Lake Shunet is relatively shallow and the chemocline is inhabited by (1) an extremely dense bacterial community; (2) a population of Cryptomonas sp.; and (3) ciliate community comprising several species. As the mixolimnion of Lake Shunet is not thermally stratified for long period, the phytoplankton and zooplankton populations are not vertically stratified. The gammarids, however, tend to concentrate in a narrow layer located 1–2 m above the chemocline. We believe that in addition to vertical inhomogeneities of both physicochemical parameters, biological and physical factors also play a role in maintaining these inhomogeneities. We conclude that the stratified distributions of the major food web components will have several implications for ecosystem structure and dynamics. Trophic interactions as well as mass and energy flows can be significantly impacted by such heterogeneous distributions. Species spatially separated even by relatively short distances, say a few centimetres will not directly compete. Importantly, we demonstrate that not only bacteria, phytoflagellates and ciliate tend to concentrate in thin layers but also larger-sized species such Gammarus (amphipods) can also under certain environmental conditions have stratified distribution with maxima in relatively thin layer. As the vertical structure of the lake ecosystem is rather complex in such stratified lakes as ours, the strategy of research, including sampling techniques, should consider potentially variable and non-homogeneous distributions.     

Cyanobacteria and cyanobacterial toxins in the alkaline crater lakes Sonachi and Simbi, Kenya

The phytoplankton communities and the production of cyanobacterial toxins were investigated in two alkaline Kenyan crater lakes, Lake Sonachi and Lake Simbi. Lake Sonachi was mainly dominated by the cyanobacterium Arthrospira fusiformis, Lake Simbi by A. fusiformis and Anabaenopsis abijatae. The phytoplankton biomasses measured were high, reaching up to 3159 mg l⁻¹ in L. Sonachi and up to 348 mg l⁻¹ in L. Simbi. Using HPLC techniques, one structural variant of the hepatotoxin microcystin (microcystin-RR) was found in L. Sonachi and four variants (microcystin-LR, -RR, -LA and -YR) were identified in L. Simbi. The neurotoxin anatoxin-a was found in both lakes. To our knowledge this is the first evidence of cyanobacterial toxins in L. Sonachi and L. Simbi. Total microcystin concentrations varied from 1.6 to 12.0 μg microcystin-LR equivalents g⁻¹ DW in L. Sonachi and from 19.7 to 39.0 μg microcystin-LR equivalents g⁻¹ DW in L. Simbi. Anatoxin-a concentrations ranged from 0.5 to 2.0 μg g⁻¹ DW in L. Sonachi and from 0 to 1.4 μg g⁻¹ DW in L. Simbi. In a monocyanobacterial strain of A. fusiformis, isolated from L. Sonachi, microcystin-YR and anatoxin-a were produced. The concentrations found were 2.2 μg microcystin g⁻¹ DW and 0.3 μg anatoxin-a g⁻¹ DW. This is the first study showing A. fusiformis as producer of microcystins and anatoxin-a. Since A. fusiformis occurs in mass developments in both lakes, a health risk for wildlife can be expected.     

Seasonality and abundance of some dominant crustacean zooplankton in Lake Awasa,a tropical rift valley lake in Ethiopia

The zooplankton of a Rift Valley lake in Ethiopia, Awasa, was sampled at 3 stations for 2 years (1986 and 1987) concurrently with various meteorological and limnological measurements. The spatial and temporal variation in abundance of some numerically dominant crustaceans, Mesocyclops aequatorialis similis (Copepoda), Thermocyclops consimilis (Copepoda) and Diaphanosoma excisum (Cladocera) is discussed. Temporal (months, sampling dates) rather than spatial (station) variability accounts for more than 50% of the total variance in zooplankton abundance but horizontal patchiness exists during periods of high zooplankton density. Sampling errors were generally low, except for counts of cyclopoid nauplii (subsampling) and Diaphanosoma (inter-replicate variance). Zooplankton showed distinct seasonality associated with the mixing cycle of the lake. Total numbers increased to more than 200 000 m⁻³ during the unstratified period (July to September). Low numbers were evident during stratification (February to May) when zooplankton numbers did not exceed 15 000 m⁻³. Individual zooplankton species and age classes showed variable seasonal amplitudes, ranging from 6.4 (nauplius 3) to 44.8 (copepodite 3 of Mesocyclops). We discuss some possible causes for zooplankton seasonality in Lake Awasa, and also review zooplankton seasonal cycles in other tropical lakes, especially African ones.     

Island biogeography, diversity and dominance of zooplankton in crater lakes on the Azores

In July and August 1988 samples of zooplankton were collected from 17 lakes on four islands. Five species of Cladocera, three species of Copepoda and 30 species of Rotifera were found. The maximum numbers of species in any one lake were four planktonic Cladocera, two Copepoda and seven planktonic Rotifera (with up to six additional non-planktonic species). The smallest, most distant island had fewer species than the largest island nearest to the mainland, but the number of species in each lake was determined more by the size of the lake than by its location. This results in the most distant island having lakes containing the same number of species as lakes of similar size on islands nearer to the mainland. The numbers of zooplankton species in each of these lakes are significantly lower than the world average, and the dominance ratios are significantly higher.     

Cannabidiol: an overview of some chemical and pharmacological aspects. Part I: chemical aspects

Over the last few years considerable attention has focused on cannabidiol (CBD), a major non-psychotropic constituent of Cannabis. In Part I of this review we present a condensed survey of the chemistry of CBD; in Part II, to be published later, we shall discuss the anti-convulsive, anti-anxiety, anti-psychotic, anti-nausea and anti-rheumatoid arthritic properties of CBD. CBD does not bind to the known cannabinoid receptors and its mechanism of action is yet unknown. In Part II we shall also present evidence that it is conceivable that, in part at least, its effects are due to its recently discovered inhibition of anandamide uptake and hydrolysis and to its anti-oxidative effect.     

Ecological studies on crater lakes in West Cameroon The blood of endemic cichlids in Barombi Mbo in relation to stratification and their feeding habits

The West Cameroonian crater lake Barombi Mbo is stratified, with no detectable oxygen below 20 m. Of the endemic cichlid fishes in this lake Konia dikume is exceptional in having a mean haemoglobin concentration of 16.55 g/100 ml, while the other ten endemic cichlids range from 5.55 to 8.70 g/100 ml blood. The erythrocyte count is also higher in K. dikume than in the other species.
The phytoplankton in the lake is most abundant between 10 and 15 m from the surface.
The single species of cyclopoid copepod and the rotifers in the zooplankton remain in the top 20 m throughout the day and night, but the larvae of Chaoborus spend the daytime at depths below 20 m, mostly between 30 and 70 m. At night these larvae ascend into the upper layers, and feed on rotifers.
Konia dikume feeds on the larvae of Chaoborus , and the high concentration of haemoglobin in the blood of this fish, coupled with a large blood volume, appears to be an adaptation for the storage of oxygen. This would enable the fish to dive into the deoxygenated hypolimnion and extend the time available for feeding when the Chuoborus larvae are ascending at dusk and descending at dawn. Konia dikume appears to be a visual feeder, and in the short tropical twilight any extension of the feeding period could be of survival value.