Some effects of enclosure on the zooplankton in a small lake

Observations on the effect of enclosure on zooplankton by introducing two experimental tubes, each holding some 18 000 m³ of water, into a small lake, showed that a limnetic community could be maintained within tubes of this capacity throughout a period of 22 months. Two species, the copepod Diaptomus gracilis and the cladoceran Daphnia hyalina were dominant both in the open lake and in each tube, but Diaptomus gracilis was relatively more abundant in the tubes than in the lake, whereas Daphnia hyalina, especially in spring and autumn, was more abundant in the lake than in the tubes. Several scarce species became relatively more abundant in the tubes than in the lake. These changes are more probably associated with reduced predation, especially by larvae of Chaoborus, than with artificially induced changes in the phytoplankton, changes only poorly correlated with changes in the zooplankton.     

Ecological studies on Chaoborus (Diptera, Chaoboridae) in Lake George, Uganda

(1) Chaoborus anomalus and Chaoborus ceratopogones were found together throughout the year in Lake George, Uganda, the larvae forming 65-85% by number of the macro-benthos of the central lake area. (2) First and second instar larvae were wholly planktonic and third and fourth instar larvae mainly benthic during the day, migrating into the water column at night. A proportion of the third and fourth instar larvae were found in the water column during the day in the centre of the lake where light attenuation was greatest. C. anomalus showed a greater tendency to be benthic than C. ceratopogones. (3) Planktonic larvae were distributed in dense patches thought to be cohorts resulting from oviposition by swarms of adult females. The maximum density of planktonic larvae recorded was 31.7 × 10²/m² column. Third and fourth instar larvae were more evenly distributed in the benthos of the central lake area, the maximum density recorded was 79.9×10²/m². (4) First and second instar larvae occurred mainly in the inshore regions, very few were found in the centre of the lake. Oviposition is likely to take place inshore. The patches of planktonic larvae were not stationary. Dispersal of larvae away from the shore may be active or may result from complex oscillations of the water mass produced by storms. In C. ceratopogones the floating egg batches could he carried away from the north shore by currents produced by inflows and prevailing winds. (5) Adults of C. ceratopogones were more numerous during the rainy seasons than during the dry seasons. A similar, though less marked seasonality was found in the occurrence of C. anomalus adults. The biomass of benthic larvae of C. ceratopo-gones was estimated at 387.0 mg C/m² in the dry season, June 1970, and 130 0 mg C/m² in the rainy season, October 1970. That of C. anomalus was constant at 241.7 and 254.3 mg C/m² in June and October, 1970 respectively. (6) No difference was found in the horizontal distribution of the larvae, or in the size and weight attained by the various stages. (7) Interspecific differences were found in the mean number of eggs per adult female (309, C. anomalus and 224, C. ceratopogones); in the nature ofthe egg batches; n i the morphology of the larval mouthparts and pharyngeal sphincters, as well as i n the vertical distribution of the larvae and seasonal occurrence of the adults. On the basis of these differences the co-existing species are thought not to be competing for the same resources.     

Tube structures of probable microbial origin in the Neoarchean Carawine Dolomite, Hamersley Basin, Western Australia

The ~2.63 Ga Carawine Dolomite, Hamersley Basin, Western Australia, preserves tube structures of probable microbial origin that formed in a low energy environment. The tubes are 0.4–1.8 cm in diameter and at least 10–16 cm long in outcrop. The tubes are defined by dark, 45-µm-thick dolomicritic walls, whereas the tube fill and host rock are composed of 30 µm, cloudy dolomite crystals and rare 170- to 425-µm-wide, dark well-sorted clasts. Closely spaced, rarely discontinuous laminae coat the insides of tubes; less closely spaced, peaked, discontinuous laminae coat the outsides of tubes. The laminae on the outsides of tubes are often intercalated with mammilate structures. The presence of probable microbial coatings on both the insides and the outsides of the tube walls requires that the tubes formed above the sediment–water interface. These tube structures probably formed during gas-charged fluid escape, similar to tubes observed in ancient and modern hydrocarbon seeps and cylindrical water transfer structures in sandstones. The laminae that coat the tubes have very similar geometries to modern biofilms that form in both turbulent and laminar flow, and their geometries probably reflect flow conditions during the fluid escape. The identification of these structures suggests that the preserved interaction between fluid escape and microbial growth in carbonates may be more common than previously thought.     

Seasonal changes in sediment and water chemistry of a subtropical shallow eutrophic lake

A field study was conducted (May 1981 to June 1982) to develop a data-base on seasonal changes of water and sediment chemistry of Lake Monroe (4 000 ha surface and ca. 2 m deep) located in central Florida, USA. This shallow eutrophic lake is a part of the St. Johns River. Quantitative samples of lake water and sediments were collected on a monthly basis from 16 stations and analyzed for various physico-chemical parameters. Relatively high levels of dissolved solids (mean electrical conductivity (EC) = 1832 µS cm¹) prevailed in the lake water, and seasonal changes in EC were probably associated with hydrologic flushing from external sources, such as incoming water from upstream as well as precipitation. Average monthly levels of total N and P during the study period were 1.82 and 0.21 mg l^–1, respectively. Nutrient concentrations in the water did not show any strong seasonal trends. Organic matter content of lake sediments ranged from 1 to 182 g C kg^–1 of dry sediment, reflecting considerable spatial variability. All nutrient elements in the sediments showed highly significant (P < 0.01) correlations with sediment organic C, though little or no significant relationship appeared at any sampling period between water and sediment chemistry of the lake. Temporal trends in water and sediment chemical parameters may have been concealed by periodic hydrologic flushing of the St. Johns River into Lake Monroe.Florida Agricultural Experiment Stations Journal Series No. 7836.     

Physical and Chemical Limnology of a Subsaline Athalassic Lake in West Greenland

Physical and chemical profiles of a shallow (c. 12-m-deep) subsaline (total dissolved solids 2.3-2.8 g l^–1) closed-basin lake in the continental area of southwestern Greenland are described for the first time. Watercolumn data for every 5th consecutive day between April 20 and October 6, 2001, and continuous recordings of lake water level and meteorological conditions are used to infer controls on contemporary lake functioning, sediment formation and climate-lake interactions. Limnological observations demonstrate the importance of lake-ice formation and its role in haline convection and the development of meromixis. Observed lake cycling suggest that the lake at present is in a state of near-meromixis where stagnant bottom waters de-stratify through deep penetration of weak haline convective cells by the end of June. From this study, the primary reasons the shallow Greenlandic low salinity lakes develop meromixis are:(i) lack of an outflow (ii) meltwater dilution and chemical stratification of surface waters, (iii) insubstantial wind mixing, (iv) a weak winter thermohaline convective cell forced by cryoconcentration, and (v) biogeochemically enhanced solute concentrations near the sediment bed. Throughout the open water period the hydrological balance is dominated by evaporative losses. Lake surface water conductivities change from 2110 to 2890 S cm^–1 due to the combined effects of open water evaporation, meltwater dilution, diffusive exchanges over the seasonal pycnocline, and boundary mixing. Freeze-out of salts and resulting deep haline convection increase overall water column salinity during winter. Owing to deep convective mixing, plant nutrients are relatively high in the upper watercolumn with a dominant internal source of phosphorous. Extreme productivity pulses of phytoplankton are observed as soon as sub-ice radiation levels increase and directly after ice-out when sufficient wind mixing can support an intense monospecific diatom bloom of Diatoma spp. leading to the rapid depletion of dissolved silica.     

ANATOMICAL AND PHYSIOLOGICAL STUDIES OF FLORAL TUBE ELONGATION OF CROCUS VERNUS (IRIDACEAE)

Intact floral tubes of Crocus vernus grown under controlled conditions elongated 50 mm in 8 days. Mitoses of the epidermal cells did not occur during the growth of the intact tube; however, cells did elongate from 50 μm to 150 μm, a three-fold increase in cell length. When the floral buds were excised and maintained in distilled water, elongation of floral tubes was inhibited by 46%. The presence of the ovary or the addition of nutrients had no significant effect upon the elongation of the floral tubes of excised buds. When the excised floral buds were placed in 10^–6 m indoleacetic acid, the final tube lengths exceeded that of the water controls by 30% and achieved 91% of the elongation of the intact tubes. Gibberellic acid and kinetin had no effect on floral tube elongation. As with the intact floral tubes, mitoses of the epidermal cells did not occur during the elongation of the excised floral tubes.     

Studies ofNajas pollen tubes. Fine structure and the dependence of chlorotetracycline fluorescence on external free ions

Summary The distribution of membrane-associated calcium was investigated in pollen grains and tubes of the underwater pollinated angiospermNajas marina L. using chlorotetracycline (CTC). Tubes grown in distilled water (pH 6) showed the highest fluorescence in a subapical region that tapered basally into a fluorescent strand centrally located in the tube and extending back towards the pollen grain. The apical cap had low fluorescence as did the cytoplasm surrounding the fluorescent strand, the tube base and the pollen grain. Tubes grown in different pond waters (pH 8) revealed no intracellular CTC fluorescence. Instead there was an external fluorescence forming a distinct layer around the whole tube, frequently enhanced in a subapical region to form an external collar.Modification of the patterns of fluorescence could be induced by manipulation pH of the growth media and content of specific ions. For example tubes grown in distilled water with 10^–3 M Mg²⁺ salts showed a similar CTC fluorescence as those grown in pond water. In contrast, Ca²⁺ enrichment had no visible influence on the patterns of fluorescence. The pattern of fluorescence displayed by tubes grown in distilled water, could be reproduced in pond water if the pH was artificially reduced to pH 6.Ultrastructurally, there was no detectable difference in the markedly polar distribution of organelles between pollen tubes grown in the various growth media. The secretory vesicles found in the pollen grain prior to germination become distributed throughout the pollen tube but are least concentrated in regions that show highest internal CTC fluorescence. These regions appear to have large amounts of endoplasmic reticulum and include mitochondria.These results are discussed in relation to the significance of calcium gradients for tip growth and limitations in the use of CTC.Abbreviations CTC chlorotetracycline - SV secretory vesicle - ER endoplasmic reticulum - PIXE proton induced X-ray emissions     

Nonmetric multidimensional scaling analyses of temporal variation in the structure of limnetic zooplankton communities

Nonmetric multidimensional scaling analysis was used to graphically summarize and analyze seasonal changes in the structure of limnetic zooplankton communities in Blelham Tarn, English Lake District and two large experimental enclosures (tubes A and B) therein. Species abundances in weekly samples taken from June–December, 1976 from the three sites were ordinated. Paths through species space describing temporal changes in community structure indicated that the tube B community oscillated around some average composition and was dominated by small zooplankters whereas those from tube A and the tarn changed quite distinctly through the season and had higher predator densities. Examination of other characteristics of the sites indicated that the observed differences in seasonal cycles could be related to variations in the intensity of predation, the quality of food for herbivorous zooplankters and the dynamics of nutrient input. The importance of considering time variation in lake properties, particularly in assessing lake responses to stress and in typing lakes for experimentation or management, is emphasized.     

Leaf litter decomposition in an ephemeral karst lake (Chaney Lake,Kentucky, U.S.A.)

Litter decomposition in temporary aquatic environments has not been experimentally studied as much as it has in perennial systems. However, litter is likely a critical resource for organisms inhabiting ephemeral aquatic habitats. In this study, we used litterbags under different conditions of submergence and water physical and chemical properties/characteristics to study mass and nutrient losses of terrestrial materials in an ephemeral karst lake in south-central Kentucky (USA). In the first experiment, which was designed to compare decomposition rates in submerged and dry sites, total mass and carbon declined more rapidly in the litter at fully submerged sites than in dry sites. In the second experiment, which was designed to compare decomposition rates in two different submerged environments, total mass and carbon showed similar decomposition trends between the two submerged areas with different seasonal temperature patterns. Nitrogen patterns were variable but in general nitrogen levels increased in the litter in both experiments over a period of several months. These results are similar to those found in some perennially inundated systems and indicate that litter decomposition dynamics in this temporary lake can be greatly affected by lake hydrology. Year-to-year variations in hydrology may thus have strong impacts on nutrient and energy release within this system, which may affect the organisms within this karst lake and in other areas of the karst ecosystem that are ecologically connected to it.     

Nutrient dynamics linked to hydrological condition and anthropogenic nutrient loading in Chaohu Lake (southeast China)

Chaohu Lake, a large (770 km²) shallow lake in the Yangtze River basin, has experienced serious eutrophication over the past three decades. To track its nutrient history and the causes, multi-proxies (diatom, geochemical indicators, magnetic susceptibility, and grain size) were analyzed on a 120-cm long core from the lake. Nutrient dynamics of the past 500 years were reconstructed using sedimentary diatom assemblages and an established diatom-inferred total phosphorus (DI-TP) transfer function. Between about 1500 and 1740 AD, the lake remained in an upper-mesotrophic state with the DI-TP oscillating around 60 ??g l^?1. Together with enhanced agricultural activities, a drier and warmer climate led to a decrease in water level and water exchange volume with the Yangtze River, thus triggering the first phase of eutrophication (ca. 1740?C1820 AD). After this eutrophic episode, the lake had exhibited a recovery in nutrient status until the 1960s. However, the lake became susceptible to nutrient input due to a sharp decrease in water exchange volume after its impoundment in 1962. A large amount of anthropogenic nutrient input has exacerbated eutrophication in the lake since the late 1970s. Redundancy analysis using a range of sedimentary proxies indicated that the two eutrophication phases were mainly attributed to anthropogenic nutrient loading and altered hydrological conditions. The hydraulic connection with the Yangtze River should be the primary self-adjustment mechanism of the lake against anthropogenic nutrient input.     

The Seasonal Succession of Biotic Communities in Wetlands of the Tropical Wet-and-Dry Climatic Zone: II. The Aquatic Macrophyte Vegetation in the Pantanal of Mato Grosso,Brazil.

The seasonal succession of macrophyte communities on temporarily flooded wetlands in the Pantanal of Poconé was investigated by general surveys of the region and by semi-quantitative surveys conducted on an area of 2500 m² with a gradient of water depth steep enough to permit a correlation of the species diversity with the depth and duration of flooding. The tropical wet-and-dry climate is the main determinant of the regional ecological conditions, and the seasonal variations of the physical and chemical factors, including the alternation between aquatic and terrestrial conditions over most of the area, produce a pronounced seasonal succession. During all of the seasons, the occurrence and growth of the plant species on the area surveyed was found to be inversely correlated with the water depth, but the statistical significance of the findings varied during the course of the year. The plant species are discussed individually, since little or nothing had previously been reported about the autecology of several of them. In addition, a general survey is made of the common aquatic and wetland plant species encountered in the region. On sites temporarily inundated during the course of the year, there is usually a succession of vegetation involving purely terrestrial and aquatic species as well as short-lived annuals that appear only during the transition period from wet to dry and perennials with seasonal periods of dormancy during periods of excessive dryness.     

Seasonal stable isotope variations of the modern Amazonian freshwater bivalve Anodontites trapesialis

In a floodplain lake of the Amazon River near the city of Iquitos, northeastern Peru, a one-year monitoring experiment was conducted during which water samples and living bivalves (Anodontites trapesialis) were collected with the aim to investigate seasonal δ¹⁸O variation in and fractionation between bivalve aragonite and host water. Both host water and molluscan growth increments show more than 8‰ seasonal variation in δ¹⁸O. In the floodplain lake under study the δ¹⁸O variation of the water is controlled by contrasting dry and wet season evaporation-precipitation regimes. Molluscan δ¹⁸O appears to be in equilibrium with the host water. Although an approximately 4.0‰ offset occurs, δ¹³C records of water and bivalves are in good agreement, suggesting that both δ¹⁸O and δ¹³C of the shells of freshwater bivalve A. trapesialis are good recorders of (palaeo-)environmental conditions. The δ¹³C of Dissolved Inorganic Carbon (DIC) is governed by plant growth and/or by changes in aquatic chemistry, affecting the DIC pool.     

鄱阳湖湿地水位变化的景观响应

根据《湿地公约》和《全国湿地资源调查技术规程(试行)》关于湿地内涵和界定条件,利用现代遥感技术和地理信息技术,提取鄱阳湖湿地面积为3886km²。在此基础上,利用多时段遥感影像,分析了湿地在相似水位条件和不同水位条件下的景观变化。结果表明,在相似水位条件下,鄱阳湖湿地景观变化主要表现为居民地和裸地面积增加,水域面积变化不明显;在不同水位条件下,鄱阳湖湿地景观变化比较显著。景观变化驱动力分析表明,湿地水位变化是湿地景观变化的主要因素。     

Spatial and seasonal distribution of phytoplankton in an African Rift Valley lake (L. Albert,Uganda, Zaire)

An account is given of the seasonal succession and spatial distributionof phytoplankton derived from a one-year sampling programme (1961–62)on Lake Albert, a large African rift lake. There is evidence of regulationby both physical and nutrient factors. These are influenced by a markedseasonality of temperature/density stratification (temperature range <3°C) and of water input that induces some polarizationalong the axis of the lake. Prevailing concentrations of soluble reactivephosphorus are high (>100 µg l^-1), but those ofinorganic nitrogen are low and, with large depletions of silicate-silicon,may limit algal production. The diatom and cyanophyte components of thephytoplankton show different relationships to the seasonal cycle of limitedstratification and vertical mixing. There are possible relationships, bygrazing, to the described seasonal and spatial abundance of planktonicCladocera. Comparisons are made with other African lakes, especially LakeTurkana which is of similar shape and size and shows end to end polarizationof the aquatic biota.     

The ecology of Biomphalaria sudanica in Lake Ziway, Ethiopia

A longitudinal study was made over a period of 24 months from January 2001 to December 2002 in Lake Ziway, Ethiopia, to elucidate factors influencing changes in the population density of Biomphalaria sudanica and to identify seasons of peak transmission of Schistosoma mansoni. Between January 2001 and December 2002, bimonthly visits were made to the lake to collect data on snail population density and schistosome infection in snails, vegetation and lake water level. Water samples were collected during the dry and rainy seasons and chemical analyses were made for major cations, anions, electrical conductivity, total hardness and pH using standard methods for the examination of water and wastewater. The seasonal fluctuation in the population density of B. sudanica was found to be highly associated with rainfall, water level of the lake and vegetation availability and abundance. Water chemical analysis showed that major cations and anions were all within the range reported for other African freshwaters. Both malacological and meteorological data show that snail population density in Lake Ziway peaks at the end of the long rainy season (June–August), coinciding with a seasonal transmission peak of schistosomiasis.     

Changes in the amino acid composition and protein modification in phloem sap of rice

Pure phloem sap was collected from insects feeding on rice (Oryza sativa L.) leaves by a laser technique similar to the aphid stylet technique. Rapid circulation of nitrogen in the sieve tubes was demonstrated directly using ¹⁵N as a tracer. Application to the roots of the metabolic inhibitors of amino acids, aminooxyacetate and methioninesulfoximine, changed the amino acid composition in the sieve tubes. Feeding methionine to leaf tips resulted in its bulk transfer into the sieve tubes. In vitro experiments confirmed the existence of protein kinases in the pure rice phloem sap. The phosphorylation status of the sieve tube sap proteins was affected by the light regime. The possibility that changes in chemical composition or protein modification such as phosphorylation in the sieve tubes might affect plant growth are discussed.Analysis of pure phloem sap collected from rice plants by insect laser technique has shown dynamic changes in the chemical composition and the quality of proteins in the sap.     

Effects of microbial activity on the hydrochemistry and sedimentology of Lake Logipi,Kenya

Lake Logipi is a saline soda and alkaline lake which marks the northern termination of the Suguta River drainage system. It also receives waters from streams, possible seepage from Lake Turkana, and hot springs. Present hydrochemistry and sedimentology is controlled by numerous factors including seasonal variations, composition of incoming waters, water depth and, above all, bacterial activity. Given the scarcity of Ca²⁺ and Mg²⁺ in the lake waters, bacterial activity seems to intensify the alkalinization of the waters which inhibits the deposition of organic matter and leads to the genesis of a poorly organic, zeolitic mud that reaches 1.5 m in tickness in the deepest part of the lake. This black layer may be overlaid with thin crusts of trona and halite which prograde over the basin from its southern bank when the lake is drying out and which are dissolved in the lake waters during the rainy season.     

Sediment diatom assemblages and composition of pore-water dissolved organic matter reflect recent eutrophication history of Lake Peipsi (Estonia/Russia)

A paleolimnological approach was used for the assessment of the recent eutrophication history and identification of possible reference conditions in the large, shallow, eutrophic Lake Peipsi. Lake Peipsi is the fourth largest lake by area, and the largest transboundary lake in Europe, being shared between Estonia and Russia. Lake Peipsi has been anthropogenically impacted over a longer time-scale than that covered by instrumental limnological monitoring. The ²¹⁰Pb record and down-core distribution of fly-ash particles in the 40-cm core from the middle part of the lake suggest 130 years of sediment accumulation. Diatom assemblages indicate alkaline mesotrophic conditions and a well-illuminated water column, sediment pore-water fluorescence index values suggest low autochthonous productivity and a stable aquatic ecosystem similar to natural reference conditions during the second half of 19th and early 20th century. Near-synchronous stratigraphic changes including the expansion of the eutrophic planktonic diatom Stephanodiscus parvus, the appearance of new species associated with eutrophic lakes and the decrease in the relative abundance of littoral diatoms, together with changes in the fluorescence properties of sediment pore-water dissolved organic matter, imply increased nutrient availability, enlarged phytoplankton crops, reduced water-column transparency and the onset of human-induced disturbances in the lake since the mid-20th century. The most conspicuous expansion of eutrophic planktonic diatoms and maximum concentration of siliceous microfossils occur simultaneously with changes in the fluorescence indexes of pore-water dissolved organic matter, indicating a pronounced increase in the contribution of autochthonous organic matter to the lake sediment. This implies that nutrient loading and anthropogenic impact was at a maximum during the 1970s and 1980s. Sedimentary diatom flora may reflect a reduction of phosphorus loading since the 1990s. However, the absolute abundance of planktonic diatoms and sediment pore-water fluorescence index values vary greatly implying that the lake ecosystem is still rather unstable.     

Ecological determinants of rotifer vertical distribution in a coastal karst lake (Vrana Lake,Cres Island,Croatia)

Vrana Lake is a coastal karst lake on Cres Island within the eastern Adriatic coast. It appears to have formed as a cryptodepression on karstified carbonate rocks, and is filled with fresh water. It is classified as an oligotrophic hydrosystem and serves as a natural water supply reservoir (max. depth 74.5 m; surface area 5.5 km²; water volume 220 × 10⁶ m³). The aim of this study was to assess the influence of seasonally and spatially varying ecological conditions (i.e., thermal stratification, dissolved oxygen, orthophosphate and chlorophyll a concentrations) on the vertical distribution of the rotifer plankton community in this geomorphologically and hydrologically specific coastal lake. The rotifer community was surveyed monthly over a one year period. During the study period, a total of 21 rotifer taxa were identified. Among them Ascomorpha spp. (including A. ecaudis, A. ovalis and A. saltans), Filinia terminalis, Keratella cochlearis, Synchaeta gr. tremula-oblonga and Polyarthra vulgaris prevailed, representing 92% of the total rotifer abundance. Measured seasonal and spatial patterns of the rotifer assemblage along a vertical profile of Vrana Lake were considerably affected by temperature, food availability and biotic (competition, predation) interactions in the zooplankton community.