Long-term trends of epilimnetic and hypolimnetic bacteria and organic carbon in a deep holo-oligomictic lake

We analysed the long-term dynamics (1980–2007) of hypolimnetic and epilimnetic bacterial abundances and organic carbon concentrations, both dissolved (DOC) and particulate (POC), in the deep holo-oligomictic Lake Maggiore, included in the Southern Alpine Lakes Long-Term Ecological Research (LTER) site. During the 28 years of investigation, bacterial abundance and POC concentrations did not decrease with declining phosphorus concentrations, while DOC concentrations showed a pronounced decrease in the epi- and hypolimnion. We used the annual mean total lake heat content and total annual precipitation as climate-related variables, and in-lake total phosphorus as a proxy for trophic state. The model (forward stepwise regression, FSR) showed that reduced anthropogenic pressure was more significant than climate change in driving the trend in DOC concentrations. Bacterial dynamics in the hypolimnion mirrored the fluctuations observed in the epilimnion, but average cell abundance was three times lower. The FSR model indicates that bacterial number variability was dependent on POC in the epilimnion and DOC in the hypolimnion. In the hypolimnion, cell biovolumes for rod and coccal morphotypes were significantly larger than in the epilimnion.     

Epilimnetic nutrient loading by metalimnetic erosion and resultant algal responses in Lake Waramaug,Connecticut

Phosphorus regeneration from lake sediments, and subsequent migration to trophogenic surface water, significantly contributes to the lake nutrient budgets and algal bloom conditions in some lake types. Decomposition of organic matter in deep water and sediments results in the accumulation of regenerated nutrients, alternate electron acceptors (reduced products of anaerobic respiration = COD), carbon dioxide, and depletion of dissolved oxygen (electron acceptor in aerobic respiration). Thermal stratification creates spatial segregation of trophogenic and tropholytic environments in the lake, resulting in gradients between sediments, hypolimnion, and the epilimnion. Exchange of oxygen, nutrients, and reduced alternate electron acceptors between the hypolimnion and epilimnion affects the productivity of a lake. Secchi depth, temperature, and dissolved oxygen profiles were determined twice each week from May 1980 to October 1980 at each of five lake stations. Nutrient concentration profiles, including total soluble and total phosphorus, ammonium-N, nitrate, soluble Kjeldahl, and total Kjeldahl nitrogen were determined twice each month. Epilimnetic algal samples were collected twice each week using Kemmerer and water column ‘straw’ amplers. Cell counts of total, green, bluegreen, and diatom algae groups were made. Three methods were used to describe hypolimnetic-epilimnetic exchange, including coefficients of eddy diffusion (based on lake heat budget), a graphical method of defining thermocline location, and relative thermal resistance to mixing (RTRM, based on density differences). All three methods yeilded comparable estimates of net seasonal transport. The graphical and RTRM methods described events occurring at shorter intervals (greater resolution). We find general agreement between the three methods of describing hypolimnetic-epilimnetic transport. The frequency of sampling resulted in increased resolution of thermal profiles (in time), allowing accurate estimation of short-term nutrient flux into epilimnetic waters. An algal bloom event occurred 5 to 12 days following erosion of the top of the metalimnion to below the aerobic-anaerobic interface. The lag time to peak algal concentration, following such events, decreased through the summer (June = 12 days, September = 5 days)     

Physical and chemical limnology of a wind-swept tropical highland reservoir

Valle de Bravo (VB) is a tropical reservoir located (19°21′30″ N, 100°11′00″ W) in the highlands of Mexico. The reservoir is daily swept by strong (7.4 m s⁻¹ mean speed) diurnal (12:00–19:00 h) winds that blow along its two main arms. As expected from its fetch (6.9 km) and its depth (21.1 m mean), the reservoir behaves as a warm monomictic water body. During 2001, VB was stratified from February to October, and well mixed from November to January. Its mean temperature was 19.9°C; the maximum found was 23.8°C in the epilimnion, while a minimum of 17.8°C was registered during mixing. VB exhibited a thermal regime similar to other water bodies of the Mexican tropical highlands, except for a steady increase of its hypolimnetic temperature during stratification, which is attributed to entrainment of epilimnetic water into the hypolimnion. During stratification, the hypolimnion was anoxic, while the whole water column remained under-saturated (60%) during mixing. The flushing time is 2.2 years. Mineralization and total alkalinity are low, which allows strong changes in pH. Ammonia remained low (2.4 μmol l⁻¹ mean) in the epilimnion, but reached up to 60 μmol l⁻¹ in the hypolimnion. Soluble reactive phosphorous had a mean of 0.28 μmol l⁻¹ in the epilimnion and a mean of 1.25 μmol l⁻¹ in the hypolimnion. Nitrate exhibited maxima (up to 21 μmol l⁻¹) during mixing, and also in the metalimnion (2 μmol l⁻¹) during stratification. Low dissolved inorganic nitrogen indicated nitrogen limitation during stratification. Eutrophication is an emerging problem in VB, where cyanobacteria dominate during stratification. At VB chlorophyll a is low during mixing (mean of 9 μg l⁻¹), and high during stratification (mean 21 μg l⁻¹), when blooms (up to 88 μg l⁻¹) are frequent. This pattern is similar to that found in other eutrophic tropical water bodies. We propose that in VB the wind regime causes vertical displacements of the thermocline (0.58–1.10 m hr⁻¹) and boundary mixing, enhancing the productivity during the stratification period in this tropical reservoir.     

Can zebra mussels change stratification patterns in a small reservoir?

Colonization and proliferation of zebra mussel (Dreissena polymorpha) population in Hargus lake, a small thermally stratified reservoir in Ohio, U.S.A., caused a significant increase in water clarity and a remarkable decrease in phytoplankton biomass during the period from 1993 to 1995. Increased light penetration and reduced organic matter loading to the meta-and hypolimnion were reflected in the lake stratification patterns, particularly in the temperature and oxygen profiles in the metalimnion. The meta- and hypolimnetic water temperature increased significantly over three years, irrespective of variation in surface water temperature. The epilimnion depth (mixing depth) increased by about the same magnitude as did the average Secchi depth. However, the total heat content of the lake did not show a consistent trend to increasing zebra mussel abundance, as it was largely influenced by the temperature of the large water volumes near the surface, which were in turn affected by weather conditions. Concurrent with the thermal structure change, the dissolved oxygen structure also changed over three years, though to a lesser extent. The changes in oxygen stratification pattern were reflected by increased oxygen concentrations in the metalimnion and a lowered depth of 3 mg l^–1 DO isopleth. These observed changes were likely attributed to increased water mixing depth, metalimnion photosynthesis and reduced oxygen consumption by organic matter. With increased epilimnion thickness and improved oxygen conditions in the metalimnion, the habitable space for aquatic macro-organisms (including fish) expanded substantially. Our results suggest that the indirect impacts of zebra mussels on small lake stratification patterns may have much broader implications than do the direct trophic interactions to the whole ecosystem.     

Winter anoxic layer in Lake Hibara

The concentration of dissolved oxygen in waters 0.5–0.6 m above the bottom of Lake Hibara, a dimictic lake, was zero in early spring of 1994 and 1997. The concentrations in early spring of 1992, 1993, 1995, 1996, and 1998 ranged from 3.75 to 10.1 mg l⁻¹. The depth profiles of water temperature suggest that water had not circulated prior to the sample collections of 1994 and 1997, but it had done so in the cases of the other years, suggesting that winter conditions were well preserved in the former years. On the other hand, the dissolved oxygen in the same strata decreased severely in summer. However, more or less titratable amounts of dissolved oxygen still remained (0.01–0.73 mg l⁻¹) at the final stages of summer stratification from 1992 to 1998. These facts show that a completely anoxic condition is not formed in this lake in summer but is sometimes formed in winter. It is interesting to note that in spite of unfavorable winter conditions for oxygen consumption, i.e., shorter duration and lower water temperature, oxygen is exhausted. These facts suggest that ventilation to the depths is much greater in summer than in winter. Received: March 5, 1999 / Accepted: October 18, 1999     

Stratification in the filling phase of the man-made lake Brokopondo in Surinam (S. America)

Summary Temperature and dissolved oxygen content measured weekly at various depths during 3 1/2 years in the filling phase of a tropical man-made lake, show that at the dam-site fluctuations of epilimnion values occurred, whereas the hypolimnion remained almost constant in temperature and permanently devoid of oxygen. This confirms previous reports concerning another mid-lake station. Apart from rather steep superficial temperature gradients, classical thermoclines did not occur. It is proposed that the lake could not become fully mixed by wind action because of the great vertical density differences at the prevailing temperatures, and because of the drowning forest still giving shelter by the extending crowns. On the other hand, the stratification was not static. Fluctuations between high and low epilimnion values for temperature, oxygen content and transparency oocurred yearly, but could not be related satisfactorily to the prevailing seasons. The depth of the epilimnion increased each year. It is suggested that the observed kind of stratification was effective though not absolute. Vertical exchange may have been reduced by increasing density differences, but it was not fully eliminated. This explains the varying degree of incorporation of upper hypolimnetic layers in the epilimnion in relation with epilimnetic temperature. Comparison with large African reservoirs accentuates the exceptional limnological development of Lake Brokopondo during ifs filling phase.     

Thermal,chemical, and optical properties of Crater Lake,Oregon

Crater Lake covers the floor of the Mount Mazama caldera that formed 7700 years ago. The lake has a surface area of 53 km² and a maximum depth of 594 m. There is no outlet stream and surface inflow is limited to small streams and springs. Owing to its great volume and heat, the lake is not covered by snow and ice in winter unlike other lakes in the Cascade Range. The lake is isothermal in winter except for a slight increase in temperature in the deep lake from hyperadiabatic processes and inflow of hydrothermal fluids. During winter and spring the water column mixes to a depth of about 200–250 m from wind energy and convection. Circulation of the deep lake occurs periodically in winter and spring when cold, near-surface waters sink to the lake bottom; a process that results in the upwelling of nutrients, especially nitrate-N, into the upper strata of the lake. Thermal stratification occurs in late summer and fall. The maximum thickness of the epilimnion is about 20 m and the metalimnion extends to a depth of about 100 m. Thus, most of the lake volume is a cold hypolimnion. The year-round near-bottom temperature is about 3.5°C. Overall, hydrothermal fluids define and temporally maintain the basic water quality characteristics of the lake (e.g., pH, alkalinity and conductivity). Total phosphorus and orthophosphate-P concentrations are fairly uniform throughout the water column, where as total Kjeldahl-N and ammonia-N are highest in concentration in the upper lake. Concentrations of nitrate-N increase with depth below 200 m. No long-term changes in water quality have been detected. Secchi disk (20-cm) clarity varied seasonally and annually, but was typically highest in June and lowest in August. During the current study, August Secchi disk clarity readings averaged about 30 m. The maximum individual clarity reading was 41.5 m in June 1997. The lowest reading was 18.1 m in July 1995. From 1896 (white-dinner plate) to 2003, the average August Secchi disk reading was about 30 m. No long-term changes in the Secchi disk clarity were observed. Average turbidity of the water column (2–550 m) between June and September from 1991 to 2000 as measured by a transmissometer ranged between 88.8% and 90.7%. The depth of 1% of the incident solar radiation during thermal stratification varied annually between 80 m and 100 m. Both of these measurements provided additional evidence about the exceptional clarity of Crater Lake.     

Soluble Reactive Phosphorus Transport and Retention in Tropical, Rainforest Streams Draining a Volcanic and Geothermally Active Landscape in Costa Rica. : Long-Term Concentration Patterns, Pore Water Environment and Response to ENSO Events

Soluble reactive phosphorus (SRP) transport/retention was determined at four sites in three rainforest streams draining La Selva Biological Station, Costa Rica. La Selva is located at the base of the last remaining intact rainforest transect from 30 m above sea level to 3000 m along the entire Caribbean slope of Central America. Steam SRP levels can be naturally high there due to regional, geothermal groundwater discharged at ambient temperature. Monitoring since 1988 has revealed distinctive long-term differences in background SRP and total P (TP) for three streams in close proximity, and identified the impact of ENSO (El Nino Southern Oscillation) events on SRP-enriched reaches. Mean interannual SRP concentrations (± standard deviation) were 89 ± 53μg/l in the Salto (1988–1996), 21 ± 39μg/l in the Pantano (1988–1998), and 26 ± 35μg/l in the Sabalo (1988–1996). After January, 1997 the separate upland-lowland contributions to discharge and SRP load were determined monthly in the Salto. SRP in Upper Salto was low (19 ± 8μg/l, 1997–2002) until enriched at␣the upland-lowland transition by regional groundwater. Mean SRP concentration in Lower␣Salto (108 ± 104μg/l) was typically highest February–April, the driest months, and lowest July–September, the wettest. SRP concentration was positively correlated to the inverse of discharge in Lower Salto when ENSO data were omitted (1992 and 1998–1999), but not in the Upper Salto, Pantano, or Sabalo. TP was positively correlated to the inverse of discharge in all three streams when ENSO data were omitted. High SRP springs and seeps along the Lower Salto contributed 36% of discharge but 85% of SRP export 1997–2001. Annual SRP flux from the total Salto watershed (1997–2001) averaged 2.9 kg/ha year, but only 0.6 kg/ha year from the Upper Salto. A dye tracer injection showed that pore water environments were distinctly different between Upper and Lower Salto. Upper Salto had high surface water–pore water exchange, high dissolved oxygen, low SRP, and low conductivity similar to surface water, and Lower Salto had low surface water–pore water exchange, low dissolved oxygen, high SRP, and high conductivity reflecting geothermal groundwater influence. SRP export from the Salto was controlled by regional groundwater transfer, which in similar volcanic settings could be a significant P source. However, ENSO events modified the SRP concentration in the Salto suggesting that long-term monitoring is required to understand underlying SRP dynamics and P flux to downstream communities.     

Investigation of bathymetry and water quality of Lake Nam Co, the largest lake on the central Tibetan Plateau, China

Comprehensive field investigations have been conducted four times on Nam Co, central Tibet, from September 2005 to September 2008. Here, we present the preliminary results focusing on the bathymetric survey and water quality measurements. The isobathic map shows that Nam Co is a high-altitude, deep lake where a flat and large basin lies in the central part with a water depth of more than 90 m. Water depth data from the northwestern bank areas of Nam Co provide unquestionable evidence of rising water levels in the last 3 decades because of the formation of two small islands that were still peninsulas in the 1970s. Water quality measurements taken at 19 stations during three summer field campaigns (2006, 2007 and 2008) covering almost all of the lake areas showed that the temperature, pH, dissolved oxygen and electric conductivity of surface water are on average 11.43°C, 9.21, 8.90 mg l⁻¹ and 1,851 μS cm⁻¹, respectively. The surface water shows no obvious spatial variability among all the stations. Vertical fluctuations of profiles, however, display some differences in thermocline and related parameters, such as pH and dissolved oxygen. According to the vertical variations of water quality parameters, the water column in relatively deep lake areas of Nam Co could be divided into three layers with distinctly various features: the epilimnion is from the surface to about 18–20 m depth in which the parameters are homogeneous with higher temperature and abundant sunlight; the metalimnion ranged from 20–60 m where a thermocline develops; the deepest layer forms a cold and dark hypolimnion.     

Respiratory rates in the cladoceran Ceriodaphnia dubia in Lake Rotongaio, a monomictic lake

The experimentally measured oxygen consumption rate by the cladoceran,Ceriodaphnia dubia, showed a linear increase between 5 and 20°C.Oxygen consumption rates of C. dubia were estimated in situfrom respiratory electron transport system (ETS) activity inLake Rotongaio during summer stratification and winter mixing.Oxygen consumption was 0.002 µl O₂ animal^–1 h^–1in the hypolimnion and 0.076 µl O₂ animal^–1 h^–1in the epilimnion during stratification. Implications of respiredoxygen for metabolic carbon requirements are discussed.     

Autochthonous origin of semi-labile dissolved organic carbon in a large monomictic lake (Lake Biwa): carbon stable isotopic evidence

Semi-labile dissolved organic carbon (DOC) plays an important role in the transport and hypolimnetic remineralization of carbon in large freshwater lakes. However, sources of semi-labile DOC in lakes remain unclear. This study used a carbon stable isotope approach to examine relative contributions of autochthonous and allochthonous sources to semi-labile DOC. Vertical and seasonal variations in the concentration and carbon stable isotope ratio (δ¹³C) of DOC were determined in large (surface area 674 km²; maximum depth 104 m), monomictic Lake Biwa. A sharp vertical gradient of δ¹³C of DOC (δ¹³C-DOC) during the stratification period [mean ± standard error (SE) −25.5 ± 0.1 and −26.0 ± 0.0‰ in the epi- and hypolimnion, respectively] indicated the accumulation of ¹³C-rich DOC in the epilimnion. Vertical mixing explained the intermediate values of δ¹³C-DOC (−25.7 ± 0.0‰) measured throughout the water column during the overturn period. Both DOC concentration and δ¹³C-DOC decreased in the hypolimnion during stratification, indicating selective remineralization of ¹³C-rich DOC. Using a two-component mixing model, we estimated the δ¹³C value of semi-labile DOC to be −22.2 ± 0.3‰, which was close to the δ¹³C of particulate organic carbon collected in the epilimnion during productive seasons (−22.7 ± 0.7‰) but much higher than the δ¹³C-DOC in river waters (−26.5 ± 0.1‰). Semi-labile DOC appeared to be mainly autochthonous in origin, produced by planktonic communities during productive seasons. The spatiotemporal uncoupling between production and remineralization of semi-labile DOC implies that hypolimnetic oxygen consumption may be affected by pelagic primary production during productive seasons of the preceding year.     

Population dynamics of bacterioplankton in an oligotrophic lake

The population ecology of bacterioplankton was studied overa 3 year period in Mirror Lake, an oligotrophic lake in thenortheastern USA. Bacterial population density, biomass, andrates of biomass production in the epilimnion and hypolimnionwere examined for their relationship with several environmentalparameters. Bacterioplankton density fluctuated between 0.5and 7 x l0 bacteria ml^–1, with highest values in the anoxichypolimnion. At all depths there was a trend towards a higherdensity of bacteria from spring to midsummer, followed by adecline in late summer to early autumn. Cocci tended to dominatebacterial cell shapes from winter to midsummer, after whichrod-shaped cells became most abundant. Rod-shaped cells contributedthe most to bacterioplankton biomass at all depths and timesof year. The mean annual biovolume of all bacterioplankton was0.12 µ cell^–1. The mean annual areal bacterioplanktonbiomass was 11–12 mmol C m^–2. The percentage ofbacterial to phytoplankton biomass per volume in summertimewas 27% in the epilimnion and 11% in the hypolimnion. Averageannual and summertime bacterial production estimated using the[3H]thymidine method was similar to previous estimates of bacterialproduction measured in Mirror Lake using other methods. Theaverage ratio of bacterial to net phytoplankton production pervolume was 0.34 in the epilimnion, and between 0.65 and 1 1.depending on depth, in the hypolimnion during summer. Of severalvariables considered in regression analyses, only temperatureexplained >50% of the variance in bacterial production inboth the hypolimnion and epilimnion. Above 14°C, however,bacterial production and growth rate in the epilimnion werenot clearly related to temperature. During the period of midsummerhypolimnetic anoxia, despite colder temperatures in the hypolimnion,bacterial production was up to 10 times greater than in theepilimnion.     

Lake microbial communities are resilient after a whole-ecosystem disturbance

Disturbances act as powerful structuring forces on ecosystems. To ask whether environmental microbial communities have capacity to recover after a large disturbance event, we conducted a whole-ecosystem manipulation, during which we imposed an intense disturbance on freshwater microbial communities by artificially mixing a temperate lake during peak summer thermal stratification. We employed environmental sensors and water chemistry analyses to evaluate the physical and chemical responses of the lake, and bar-coded 16S ribosomal RNA gene pyrosequencing and automated ribosomal intergenic spacer analysis (ARISA) to assess the bacterial community responses. The artificial mixing increased mean lake temperature from 14 to 20 °C for seven weeks after mixing ended, and exposed the microorganisms to very different environmental conditions, including increased hypolimnion oxygen and increased epilimnion carbon dioxide concentrations. Though overall ecosystem conditions remained altered (with hypolimnion temperatures elevated from 6 to 20 °C), bacterial communities returned to their pre-manipulation state as some environmental conditions, such as oxygen concentration, recovered. Recovery to pre-disturbance community composition and diversity was observed within 7 (epilimnion) and 11 (hypolimnion) days after mixing. Our results suggest that some microbial communities have capacity to recover after a major disturbance.     

Nitrate Accumulation in Aerobic Hypolimnia: Relative Importance of Benthic and Planktonic Nitrifiers in an Oligotrophic Lake

Both nitrate and nitrous oxide accumulate in the hypolimnion of the oligotrophic Lake Taupo, New Zealand, throughout stratification. The two forms of oxidized nitrogen increase in concentration with increasing depth toward the sediments, where the dissolved concentrations of reduced nitrogen are two orders of magnitude higher than concentrations in the overlying water. Nitrification rates were measured by dark [¹⁴C]CO₂ assays with and without the inhibitor nitrapyrin. The fastest rates were recorded for planktonic nitrifiers in the epilimnion and benthic species in the surficial 2.5 mm of the sediments. Nitrifying bacteria were least active in the deep hypolimnion. Deepwater accumulation of NO₃⁻ in Lake Taupo must therefore be a product of benthic rather than planktonic nitrification.     

Spatial and temporal patterns of temperature,alkalinity, dissolved oxygen and conductivity in an oligo-mesotrophic,deep-storage reservoir in central Texas

Impoundment behavior was determined for alkalinity, temperature, dissolved oxygen, and conductivity from stations located along the length of a bottom-draining, oligo-mesotrophic, hardwater, deep-storage reservoir located in central Texas. The epilimnion deepened the length of the reservoir throughout the summer as a result of drawdown. Bicarbonate alkalinity and conductivity exhibited both longitudinal and vertical stratification. Alkalinity and conductivity in the epilimnion decreased from the riverine reach downreservoir to the dam. This longitudinal progression was attributed to inflow and photosynthetically induced epilimnetic decalcification.Hypolimnetic anoxic conditions first occurred in the sedimentation zone in the upreservoir and riverine reaches and then developed in a downreservoir pattern as summer progressed as a result of drawdown. Alkalinity and conductivity in the hypolimnion increased during anoxic conditions and consequently increased in a downreservoir progression.     

The Role of Allochthonous Inputs of Dissolved Organic Carbon on the Hypolimnetic Oxygen Content of Reservoirs

Hypolimnetic oxygen content in lentic ecosystems has traditionally been modeled as a function of variables measured at the epilimnion, or that are supposed to drive epilimnetic processes, like total phosphorus load. However, in man-made reservoirs the river inflow can plunge into deep layers, directly linking the hypolimnion with the surrounding watershed. In these circumstances, organic matter carried by the river can influence the hypolimnetic oxygen content without important intervention of epilimnetic processes. Taking long-term data from two reservoirs in Spain, we applied an empirical regression approach to show that the dissolved organic matter carried by the river is the main driver shaping the hypolimnetic oxygen content. By contrast, typical variables commonly included in the modeling of the oxygen content in the hypolimnion (nutrient concentrations, chlorophyll a, and dissolved organic carbon measured in the water column) did not show any significant correlation. Interpretations from this regression approach were supported by a comparison between the monthly oxygen consumption in the hypolimnion and the monthly dissolved organic carbon load from the river inflow. We also revisited the prediction of the year-to-year variability of the Nürnberg’s anoxic factor in four reservoirs from Spain and the USA, explicitly including the allochthonous sources in the equations. These sources were significant predictors of the anoxic factor, especially in those systems subject to relatively high human impact. Thus, effects of allochthonous dissolved organic carbon should always be considered in empirical modeling and management of reservoir hypolimnetic processes related to oxygen content (for example, anoxia, nutrient internal loading, or phosphorus cycle resilience).     

The Potential for Nitrogen Fixation in a Lake Receiving Sewage Effluent (Grasmere, English Lake District)

The potential for nitrogen fixation in the water column and sediment of Grasmere (English Lake District) was investigated using the acetylene reduction technique, with reference to seasonal changes and concentrations of dissolved oxygen and inorganic nitrogen in the epilimnion and hypolimnion. Potential rates of nitrogen fixation and MPN counts of nitrogen-fixing bacteria correlated with each other and with decreases in dissolved oxygen concentration. The results suggested that selective changes in the nitrogen-fixing microflora took place during the period of thermal stratification. Neither light-dependent fixation, nor cyanobacteria could be detected in water column samples, and acetylene reduction was detected, for most of the season, only in samples of water and sediment to which a source of carbon and phosphorus had been added. The possible role of the sewage effluent in suppressing nitrogen fixation is discussed.     

Distribution of dissolved organic carbon in Lake Baikal and its watershed

 Concentrations of dissolved organic carbon (DOC) in Lake Baikal ranged from 90 to 110 μM C, considerably higher than those in oceanic environments. The DOC concentrations in the epilimnion were higher than those in the hypolimnion. Since particulate organic carbon (POC) concentrations in the pelagic waters of Lake Baikal were <10–40 μM C in the epilimnion and 2–5 μM C in the hypolimnion, DOC constitutes a major component of the organic carbon pool in Lake Baikal, especially in the deep layers. The DOC concentrations downstream of the Barguzin and Selenga Rivers were quite high (400–500 μM C). Probably because of the high concentrations of DOC in these rivers, the DOC levels in Barguzin Bay and offshore at the mouth of the Selenga River were higher than those in the pelagic regions of the central and south basins of Lake Baikal. The relationship between DOC and electric conductivity revealed the transport of DOC from rivers to the pelagic area in Lake Baikal. The spatial distribution of DOC suggested that a major part of DOC in the lake was allochthonous (land-derived). Received: July 26, 2002 / Accepted: September 16, 2002 Present address:Research Institute for Humanity and Nature, 335 Takashima-cho, Marutamachi Kawaramachi, Kamigyo-ku, Kyoto 602-0878, Japan Tel. +81-75-229-6167; Fax +81-75-229-6150 e-mail: yos@chikyu.ac.jp Acknowldgments The authors wish to thank Director Prof. M. Grachev and Dr. O. Timoshkin of the Limnological Institute, Siberian Branch, Academy of Science, Russia, for arranging the cruise on Lake Baikal. We are also indebted to Drs. V. Sinyukovich, I. Khanaev, and A. Zhdanov for their kind assistance during the expeditions. We wish to thank Ms. Y. Ito for measuring the DOC concentrations. This work was supported and financed by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (No. 09041159) and for Scientific Research of Priority Area B (No. 11213101) and by the International Geosphere-Biosphere Programme (IGBP) at Nagoya University. Correspondence to:T. Yoshioka     

Artificial destratification of a small tropical reservoir: effects upon the phytoplankton

Seasonal changes in the phytoplankton community of a small tropical reservoir were monitored over a four year period comprising of an initial two seasonal cycles during which the water column stratified strongly for extended periods each year, and two further seasonal cycles after installation of a mechanical aeration system to induce artificial destratification. In the unmanaged reservoir, the concentration of chlorophyll a at 0.5 m reached maximum values (on one occasion > 90 mg m⁻³) when the water column was stratified and the epilimnion was very shallow (ca 2 m depth). The hypolimnion at this time was anoxic (less than 2% oxygen saturation) and had a high concentration of bacteriochlorophyll (100–200 mg m⁻³). The phytoplankton community of the unmanaged reservoir was generally dominated by cyanobacteria (Cylindrospermopsis raciborskii, Anabaena tenericaulis) during the warmer months of the year (November–March) (but replaced by chlorophyta, dinophyceae and euglenophyceae after periods of intense rain) and by bacillariophyceae (Synedra ulna var. chaseana, S. tenera) during the cooler, dry months. In the artificially destratified reservoir (8 h aeration day⁻¹), the phytoplankton community was largely dominated by diatoms except after depletion of the silica content of the water column which caused diatoms to be replaced by cyanobacteria (dominated by A. tenericaulis) and a range of chlorophytes. The changing pattern of stratification and circulation of the water column in the unmanaged reservoir caused repeated disruption of the established phytoplankton assemblage with peaks of high biomass associated with transient cyanobacterial blooms. Continuous aeration and the consequent increase in the ratio mixed: euphotic depth provided conditions suitable for dominance of the phytoplankton by diatoms, as long as silica was available, and resulted in average chlorophyll levels higher than in the unmanaged reservoir (120 ± 10 v. 64 ± 9 mg m⁻²). Hierarchical fusion analysis based on the biomass of species differentiated the phytoplankton samples into cluster groups that could be related primarily to stratification or mixing of the water column.     

Seasonal changes in the chemistry and biology of a meromictic lake (Big Soda Lake,Nevada, U.S.A.)

Big Soda Lake is an alkaline, saline lake with a permanent chemocline at 34.5 m and a mixolimnion that undergoes seasonal changes in temperature structure. During the period of thermal stratification, from summer through fall, the epilimnion has low concentrations of dissolved inorganic nutrients (N, Si) and CH₄, and low biomass of phytoplankton (chlorophyll a ca. 1 mgm ^-3). Dissolved oxygen disappears near the compensation depth for algal photosynthesis (ca. 20 m). Surface water is transparent so that light is present in the anoxic hypolimnion, and a dense plate of purple sulfur photosynthetic bacteria (Ectothiorhodospira vacuolata) is present just below 20 m (Bchl a ca. 200 mgm^-3). Concentrations of N H₄ ⁺, Si, and CH₄ are higher in the hypolimnion than in the epilimnion. As the mixolimnion becomes isothermal in winter, oxygen is mixed down to 28 m. Nutrients (NH₄ ⁺, Si) and CH₄ are released from the hypolimnion and mix to the surface, and a diatom bloom develops in the upper 20 m (chlorophyll a > 40 mgm^-3). The deeper mixing of oxygen and enhanced light attenuation by phytoplankton uncouple the anoxic zone and photic zone, and the plate of photosynthetic bacteria disappears (Bchl a ca.10mgm^-3). Hence, seasonal changes in temperature distribution and mixing create conditions such that the primary producer community is alternately dominated by phytoplankton and photosynthetic bacteria: the phytoplankton may be nutrient-limited during periods of stratification and the photosynthetic bacteria are light-limited during periods of mixing.