Temporal and vertical distribution of planktonic rotifers in a meromictic crater lake,Lake Nyahirya (Western Uganda)

Quantitative zooplankton sampling was carried out biweekly during 16 months in Lake Nyahirya, a Western Uganda crater lake. Six out of 24 rotifer species recorded from the lake were quantitatively important (Horaella brehmi, Brachionus angularis, B. caudatus, Keratella tropica, Filinia longiseta and F. opoliensis, and three others were common (Brachionus falcatus, Asplanchna sieboldi and Conochiloides natans). Rotifer peaks were correlated with rainy periods and relatively clear water, which coincide with the European spring and autumn. All species were able to deal with hypoxic conditions. A clear niche separation can be observed between different genera and between species of the same genus. The population dynamics of the rotifers seem to be ruled by repeated irregular environmental fluctuations. Rainfall appears to be a primary steering factor.     

Evolution de la composition en acides gras d'un phytoplancton lacustre (Lac Pavin,France)

Fatty Acid Composition of Lacustrian Phytoplankton (Pavin Lake, France) Fatty acid composition of lacustrian phytoplankton was established during a study carried out on Lake Pavin (France) from May to September 1981. Fatty acid spectra point out even numbers of carbon atoms (ranging from 14 to 22) and in particular unsaturated fatty acids preponderance. In addition, the occurrence of some unsaturated fatty acids is directly linked to phytoplankton taxonomic composition, but also to physiological state of the cells. Lastly, the main types of phytoplankton populations differ in the degree of unsaturation of their total fatty acids.     

Phylogenetic diversity of archaea and bacteria in the anoxic zone of a meromictic lake (Lake Pavin, France)

The compositions of archaeal and bacterial populations at different depths (60 m [mixolimnion-chemocline interface], 70 m [chemocline-subchemocline interface], 90 m, and 92 m [the water-sediment interface]) in the anoxic zone of the water column in Lake Pavin, a freshwater permanently stratified mountain lake in France, were determined. Phylogenetic trees were constructed from sequences to assess archaeal and bacterial diversity at the four sites.     

Dissolved oxygen and thermal regimes of a Ugandan crater lake

This paper quantifies the temporal pattern of thermal stratification and deoxygenation in Lake Nkuruba, a small (3 ha), deep (maximum depth = 38 m) crater lake in western Uganda. Dissolved oxygen penetrated to an average depth of 9 m and a maximum depth of 15 m below which the lake was permanently anoxic over the 2 years of study. Although surface oxygen levels were correlated with both surface water temperature and rainfall, seasonal cycles of dissolved oxygen were not well-defined and may have been obscured by the high frequency of short-term fluctuations and by inter-annual variations caused by shifts in rainfall. Surface water temperature averaged 23.3±0.7 °C (S.D.) and varied directly with air temperature. Both diurnal changes and top-bottom temperature differentials were small averaging 1.7±0.7 °C and 1.6±0.8 °C, respectively. Thermal stability ranged from 101.3 to 499.9 g-cm cm^-2 and was positively related to surface water temperature suggesting that this small protected lake responds rapidly to short-term meteorological changes. Because contribution to the annual heat exchange cycle was confined to upper waters, the lake's annual heat budget was low, 1,073.8 cal cm^-2 yr^-1. However, net primary productivity was relatively high averaging 1.3 g C m^-2d^-1. The region where Lake Nkuruba is situated experienced a very strong earthquake (6.2 on the Richter scale) on 4 February, 1994. Subsequently, water levels dropped markedly in the lake, falling 3.14 m over a 5-month period. This revised version was published online in September 2006 with corrections to the Cover Date.     

Contribution of mathematical modeling to lake ecosystem understanding: Lake Bourget (Savoy,France)

This paper demonstrates how mathematical modeling can contribute to improve understanding of lake behavior. Since the 60's Lake Bourget, one of the largest in France, had been suffering from eutrophication which was checked in 1980 by the diversion of the main sewers entering the lake. A research program was implemented between 1987 and 1990, including an on-site sampling campaign conducted concurrently with thermal and biogeochemical modeling of lake behavior. The model helped provide a better understanding of the ecosystem, displaying some processes hitherto misunderstood: (1) Winter overturn does not reach the bottom of the water column when the weather is mild. This leads to a incomplete reoxygenation of the hypolimnion and to redox conditions inducing the release of orthophosphate from the sediment, (2) Grazing by herbivorous zooplankton is getting more important in the control of spring algal growth as eutrophication of the lake regresses, (3) Settling of particulate phosphorus seems a complex and very important process in Lake Bourget, showing high sedimentation rates for particulate mineral phosphorus.     

Vertical distributions of zooplankton and population dynamics of Daphnia in a meromictic lake

Vertical distributions of zooplankton were studied in relation to profiles of temperature, oxygen and chlorophyll a in Roi Lake, a small meromictic lake in central Alberta. Zooplankton were distributed fairly evenly through the oxygenated part of the water column in early summer, but a gradual descent of several species became evident in June. The vertical distribution of chlorophyll was dominated by a huge peak at the 8- to 9-m-deep chemocline. the location of a plate of photosynthetic sulfur bacteria. Ambient concentration of chlorophyll was a poor predictor of the numbers of zooplankton and the fecundity of Daphnia pulicaria at different depths, and per capita birth rates of Daphnia were usually highest in the surface waters. The reproductively disadvantageous restriction of daphnids to deep water by late summer and their catastrophic decline in the face of high ambient concentrations of chlorophyll suggest that factors other than temperature and food supply are important in influencing the dynamics and distribution of zooplankton in this lake.     

Vertical diffusion and nutrient transport in a tropical lake (Lake McIlwaine,Rhodesia)

Measurements of the vertical temperature in tropical Lake McIlwaine were used to calculate the time-averaged ( 6 months) vertical diffusivity coefficient (K_z) in the metalimnion and hypolimnion. The mean value of K_z (0.21 cm² s^–1) was correlated with the lake surface area. The mass transport rates of PO₄-P and NH₄-N, upward from the hypolimnion to the metalimnion, were calculated using K_z and measured values of the nutrient concentration gradients. During a period of 4.5 months when the water was stably stratified, PO₄-P was transported upward at a mean rate of 42 kg day^–1 and NH₄-N at a mean rate of 162 kg day^–1 over the entire lake.     

Evidence for a diurnal variation in the carotenoid content of acanthodiaptomus denticornis (Crustacea,copepoda) in Lac Pavin (Auvergne,France)

In Lac Pavin Acanthodiaptomus denticornis was found to be intensely reddish-orange coloured by keto-carotenoids of the astaxanthin type. Such pigments are not normally found in phytoplankton algae, and apparently these carotenoids result from the metabolism of pigments of dietary origin. The carotenoid content of the zooplankton showed a distinct 2.5-fold diurnal variation, with a minimum at night time and a maximum in the early morning. The possible impact of a diurnal difference in zooplankton feeding activity is discussed.     

Seasonal changes in alkaline phosphatase activity of bacteria and microalgae in Lake Pavin (Massif Central,France)

Seasonal changes in alkalinephosphatase activity of bacteria and microalgae werestudied in the dimictic Lake Pavin (Massif Central,France), to test whetherthis activity is primarily algal or bacterial andwhether the APA presents seasonal variations coupledwith abiotic and biotic variables. Samples werecollected at different depths from May 1992 to May1993. The specific phosphatase activities wereanalysed spectrophotometrically with p-NitrophenylPhosphate (p-NPP) as substrate and were related to theprotein concentrations. No correlation was foundbetween alkaline phosphatase activity and solublereactive phosphorus (SRP) concentrations across anannual cycle. The specific activities of the smallclass (0.2–1.2 m) were the highest and thecontribution of this picoplanktonic size class(0.2–1.2 m) increased with depth. In addition, thelinear correlations between alkaline phosphataseactivity and protein concentration seemed to indicatethat most of these enzymes are constitutive. However,it cannot be excluded that the high phosphorusconcentrations repress APA.Finally, the measure of APA does not seem to be avalid quantitative test of the deficiency ofphosphorus for aquatic microorganisms.     

Methane and oxygen dynamics in a shallow floodplain lake: the significance of periodic stratification

Temperature, dissolved oxygen and dissolved methane profiles were measured during autumn and summer, in a shallow floodplain lake in south-eastern Australia to determine the effects of water-column stability on methane and oxygen dynamics. The water column was well mixed in autumn. Strong thermal stratification developed in the late afternoon in summer, with top-to-bottom temperature differences of up to 6 °C. Methane concentrations in surface waters varied over a daily cycle by an 18-fold range in summer, but only by a 2-fold range in autumn. The implication of short-term temporal variation is that static chambers deployed on the water surface for short times (less than a day) in summer will significantly underestimate the diffusive component of methane emissions across the water–atmosphere interface. There was a marked diel variation in dissolved oxygen concentrations in summer, with the highest oxygen values (commonly 5–8 mg l^–1) occurring in the surface waters in late afternoon; the bottom waters were then devoid of oxygen (< 0.2 mg l^–1). Because of high respiratory demands, even the surface water layers could be nearly anoxic by morning in summer. The concentration of dissolved oxygen in the surface waters was always less than the equilibrium value. When the water column became thermally stratified in summer, the dissolved oxygen and methane maxima were spatially separated, and planktonic methanotrophy would be limited to a moving zone, at variable depth, in the water column. In summer the whole-wetland rates of oxygen production and respiration, calculated from long-term (5 h) shifts in dissolved oxygen concentrations over a diel period, were approximately 6–10 and 3–6 mmol m^–3 h^–1, respectively. These values correspond to net and gross primary production rates of 0.7–1.2 and 1.0–1.9 g C m^–3 day^–1, respectively.     

Seasonal dynamics,typhoons and the regulation of lake metabolism in a subtropical humic lake

1. We used high‐frequency in situ dissolved oxygen measurements to investigate the seasonal variability and factors regulating metabolism in a subtropical alpine lake in Taiwan between May 2004 and October 2005, specifically exploring how the typhoon season (from June or July to October) affects lake metabolism. 2. Gross primary production (GPP) and ecosystem respiration (R) both peaked in early summer and mid‐autumn but dropped during the typhoon season and winter. Yuan‐Yang Lake is a net heterotrophic ecosystem (annual mean net ecosystem production ?39.6 μmole O₂ m^?3). 3. Compared to the summer peaks, seasonal averages of GPP and R decreased by approximately 50% and 25%, respectively, during the typhoon season. Ecosystem respiration was more resistant to external disturbances than GPP and showed strong daily variation during typhoon seasons. 4. Changes in the quality and quantity of dissolved organic carbon controlled the temporal dynamics and metabolic regulation. External disturbances (typhoons) caused increased allochthony, increasing DOC and water colour and influencing lake metabolism. 5. Seasonal winter mixing and typhoon‐induced water mixing in summer and autumn play a key role in determining the extent to which the lake is a seasonal carbon sink or source to the atmosphere.     

Spatial and temporal dynamics of the steady-state phytoplankton assemblages in a temperate shallow hypertrophic lake (Lake Manyas,Turkey)

Spatial and temporal dynamics of phytoplankton biomass and species composition in the shallow hypertrophic Lake Manyas, Turkey, were studied biweekly from January 2003 to December 2004 to determine steady-state phases in phytoplankton assemblages. Steady-state phases were defined when one, two or three coexisting species contributed to at least 80% of the standing biomass for at least 2 weeks and during that time the total biomass did not change significantly. Ten steady-state phases were identified throughout the study peiod. During those periods, Achnanthes microcephala (Kützing) Cleve twice dominated the phytoplankton biomass alone and contributed to more than 50% of the total biomass in seven phases. Microcystis aeruginosa (Kützing) Kützing, Anabaena spiroides Klebahn, Cyclotella stylorum Brightwell, Pediastrum boryanum (Turpin) Meneghini and Phacus pusillus Lemmermann were also represented once in steady-state phytoplankton assemblages. A. microcephala was dominant usually during cold periods of the year, while M. aeruginosa and A. spiroides were usually dominant in warm seasons. The total number of species showed a clear decrease during steady-state phases at all stations. All stations were significantly different in terms of the measured physical and chemical parameters (P < 0.05) and phytoplankton biomass (F = 117, P < 0.05).     

Co-occurrence of denitrification and nitrogen fixation in a meromictic lake, Lake Cadagno (Switzerland)

The nitrogen cycling of Lake Cadagno was investigated by using a combination of biogeochemical and molecular ecological techniques. In the upper oxic freshwater zone inorganic nitrogen concentrations were low (up to ～3.4 μM nitrate at the base of the oxic zone), while in the lower anoxic zone there were high concentrations of ammonium (up to 40 μM). Between these zones, a narrow zone was characterized by no measurable inorganic nitrogen, but high microbial biomass (up to 4 × 10⁷ cells ml^?1). Incubation experiments with ¹⁵N‐nitrite revealed nitrogen loss occurring in the chemocline through denitrification (～3 nM N h^?1). At the same depth, incubations experiments with ¹⁵N₂‐ and ¹³C_DIC‐labelled bicarbonate, indicated substantial N₂ fixation (31.7–42.1 pM h^?1) and inorganic carbon assimilation (40–85 nM h^?1). Catalysed reporter deposition fluorescence in situ hybridization (CARD‐FISH) and sequencing of 16S rRNA genes showed that the microbial community at the chemocline was dominated by the phototrophic green sulfur bacterium Chlorobium clathratiforme. Phylogenetic analyses of the nifH genes expressed as mRNA revealed a high diversity of N₂ fixers, with the highest expression levels right at the chemocline. The majority of N₂ fixers were related to Chlorobium tepidum/C. phaeobacteroides. By using Halogen In Situ Hybridization‐Secondary Ion Mass Spectroscopy (HISH‐SIMS), we could for the first time directly link Chlorobium to N₂ fixation in the environment. Moreover, our results show that N₂ fixation could partly compensate for the N loss and that both processes occur at the same locale at the same time as suggested for the ancient Ocean.     

Abundance and distribution of a brown trout (Salmo trutta, L.) population in a remote high mountain lake

Brown trout (Salmo trutta) population of a remote high mountain lake (Lake Redó, Pyrenees, 2240 m above sea level) were studied during autumn using hydroacoustic techniques. This acoustic technique was for the first time used on fish at such high altitude in Spain. Sampling using multimesh nets fish catches and echosounding recording were carried out in September 1998. Mean density of fish was estimated to be 1.82 fish per 1000 m³ (597 fish ha^–1). The results exhibited mainly a littoral habitat, with the brown trout being preferentially in the 10–25 m deep layer, where the water was warmer and the richness and diversity of macroinvertebrates higher. The sampling by hydroacoustic technique found a length frequency range of fish higher than the multimesh gill nets but both of them estimated similar fish proportion for the common length range. The more frequent target strength (TS) for the population was –38 dB (TS range –37 to –39 dB). Good correlation was found between TS frequency distribution obtained by echosounding and that predicted by means of a model based on the log of the fish total length from multimesh gill nets captures.     

Seasonal dynamics of phytoplankton and phytobenthos,and associated chemical interactions,in a shallow upland lake (Malham Tarn,northern England)

Seasonal changes of phytoplankton were followed over 3 years (1985–87) in a shallow, unstratified and calcareous upland lake.The phytoplankton was of low to moderate abundance and generally dominated by phytoflagellates. Seasonality involved a winter minimum of abundance, a spring maximum of diatoms, and often brief increases in summer that included blue-greens, especially the colonial Gloeotrichia echinulata. Some components were of benthic origin. Seasonal growth of the main component of the phytobenthos, Chara globularisvar. virgata, caused a regular summer depletion in lake water of Ca²⁺ and HCO₃ ^- (alkalinity) by associated CaCO₃ deposition, and a more extreme (and unusual) depletion of K⁺. Chemical analysis of Chara biomass and of underlying sediments indicated a large benthic nutrient stock, much surpassing that represented by the phytoplankton. Growth in this biomass, and the magnitude of water-borne inputs, influenced the removals of Ca²⁺, K⁺ and inorganic N. The phytoplankton was probably limited by a low-P medium, to which co-precipitation of phosphate with CaCO₃ may have contributed. A vernal depletion of Si was probably limiting to diatom growth, and appeared to be mainly induced by benthic rather than planktonic diatoms. Examples of long-term change in composition of the phytoplankton and phytobenthos are noted and discussed in relation to the interaction of these components, nutrient enrichment, and possible alternative stable states.     

Nutrient metabolism (C,N, P,and Si) in the trophogenic zone of a meromictic lake

The dynamics of seston and dissolved elements in a meromictic lake with high concentrations of manganese and iron in the monimolimnion were studied through an annual cycle. This publication presents results for assimilation, sedimentation and recovery of nutrients (C, N, P, and Si) in the trophogenic zone. Phosphorus deficiency kept the productivity of the diatom dominated phytoplankton at an oligotrophic level. High concentrations of iron in influent streams and redistribution followed by precipitation of iron during periods of partial turnover removed phosphorus from the water. High concentrations of manganese and sulfate did not have the anticipated fertilizing effect, and recovery of nutrients from the depth of the lake was negligible. Mass balance calculations indicate that liberation of phosphorus from the sediments in the trophogenic zone was most important for the maintenance of primary production. 75% of carbon, 80% of nitrogen and 25% of phosphorus assimilated by the phytoplankton was mineralized in the trophogenic zone. Silica was effectively regenerated from the littoral zone during the decline of diatom blooms. Nitrogen and silica retention was 45% of the external load compared to 66% for phosphorus.Dept. of Limnology University of Oslo     

Abundance,population dynamics and production of Chironomidae (Diptera) in an ul traoligotrophic lake in South Greenland

Macrozoobenthos of the ultraoligotrophic Lake 95 (61°N, 46°W, 8 ha, z_max=18 m,) is composed of about 14 taxa dominated by 12 Chironomidae species. Abundance, life cycle, biomass and production were estimated for the six dominant taxa. Abundance declined fromca. 4150 at 2.5 m depth toca. 1400 ind m^–2 at 16 m depth and averagedca. 3200 ind m^–2 on a lakewide basis. By numbers,Heterotrissocladius changi andH. oliveri dominated the average fauna.H. changi was common at the 2.5 m and 5 m depth stations, whereasH. oliveri dominated from 5 m depth downwards. Chironomids showed mainly a 1-yr life cycle, but apparently bothHeterotrissocladius species had two contemporary cohorts with emergence in midsummer and late autumn/early spring, respectively. Average annual ratio was 4.2 and 4.6 forH. oliveri andH. changi, respectively. Annual production varied from 0.3 g ash-free dry weight (AFDW) m^–2 y^–1 at 16 m depth to 1.6 g AFDW m^–2 y^–1 at 2.5 m depthH. changi contributed 45%, fiveMicropsectra spp. 17% andH. oliveri 15% to total average production, which on a lakewide basis wasca. 1.1 g AFDW or 25 kJ m^–2 y^–1. Lake 95 thus belongs at the very low end of measured lake zoobenthic productions, which range from 10 kJ m^–2 y^–1 in Arctic lakes toca. 1600 kJ m^–2 y^–1 in highly eutrophic shallow lakes.     

Collapse of a Planktothrix agardhii perennial bloom and microcystin dynamics in response to reduced phosphate concentrations in a temperate lake

Planktothrix agardhii dynamics, microcystin concentration and limnological variables were monitored every 2 weeks for 2 years (2004-2006) in a shallow hypereutrophic artificial lake (BNV, Viry-Chatillon, France). Time-series analysis identified two components in the P. agardhii biomass dynamics: (1) a significant decreasing trend in P. agardhii biomass (65% of the overall variance) and (2) a residual component without significant seasonal periodicity. A path-analysis model was built to determine the main factors controlling the P. agardhii dynamics over the period studied. The model explained 66% of P. agardhii biomass changes. The decreasing trend in P. agardhii biomass was significantly related to a decrease in the PO4(3-) concentration resulting from an improved treatment of the incoming watershed surface water. The residual component was related to zooplankton dynamics (cyclopoid abundances), supporting the hypothesis of a top-down control of P. agardhii, but only when the biomass was low. Forty-nine percent of the variability in the microcystin (MC) concentration (min:<0.1 microg equivalent MC-LR L(-1); max: 7.4 microg equivalent MC-LR L(-1)) could be explained by changes in the P. agardhii biomass. The highest toxin content was observed when P. agardhii biomass was the lowest, which suggests changes in the proportion of microcystin-producing and -nonproducing subpopulations and/or the physiological status of cells.     

The distribution and abundance of young (I +–II +) perch, Perca fluviatilis L., in a deep eutrophic lake, England

Seasonal changes in the vertical and horizontal distribution of young (I +–II +) perch in Rostherne Mere, a deep (max. depth 31.0 m) eutrophic lake are described using sonar. During the summer (June–August), perch were present within the thermocline (9–14° C) between 7 and 10 m. There was no significant difference between samples during this period, nor was there any evidence of shoaling. From September onwards perch moved into deeper waters. By late winter fish had become patchy in their distribution. Total numbers, during the summer, were estimated to be 988 ha ¹ with 95% confidence limits of ±115 (biomass 98.8 kg ha ⁻¹). Densities (no. m⁻³) fell by over 90% after the autumn overturn. The implications for predators are discussed. Acoustic sampling provides a useful, non-destructive method of sampling eutrophic lakes on a routine basis. Information obtained is of ecological, behavioural and physiological interest.