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.     

A QUARTER CENTURY OF GREAT LAKES RESEARCH IN AFRICA

SUMMARY

The African Great Lakes differ from shallower large African lakes by undergoing seasonal stratification and from large reservoirs in having long residence times, so that the ionic concentration of the lake water differs substantially from that of the inflows.

The African Great Lakes are so large that limnological events are usually local, causing regional differences. They can he regarded as models for oceanic systems, with local processes playing a major part in nutrient cycling, Production, on an area basis, may be high because of the considerable depth of the euphotic zones of many of the lakes, but the production cycle is poorly understood because of temporal and spatial limitations of sampling. Cyclonic upwelling may play an important part in offshore nutrient cycling.

Links between limnology and fisheries are becoming apparent. Catches of Oreochromis in Lake Malawi are correlated with falling lake levels three years previously, which is explained in climatological terms. Different transfer efficiencies between plankton and fish production may reflect differences in the depth of the euphotic zone. The history and seasonal changes in deep stratification appears to have had a large influences in the evolution of demersal fish faunas.

The lakes are so large and so complex that their complete investigation is beyond the resources of any single Government. There is a need for independently funded fundamental research to complement the applied research which must be the first priority of local Governments.     

Spatial distribution of subfossil Chironomidae in surface sediments of a large, shallow and hypertrophic lake (Taihu, SE China)

Spatial heterogeneity of benthic communities has clear implications for estimating lake production, biodiversity as well as identifying representative sites for palaeolimnological studies. This study investigates chironomid variability and the controlling factors (i.e., environmental and spatial variables) in surface sediments from Taihu Lake (2,338 km²), a hypertrophic lake in the Yangtze delta in eastern China. The spatial distribution of chironomids shows distinct heterogeneity. Microchironomus tabarui-type and Tanypus dominate the midge communities around the estuaries, while Cricotopus sylvestris-type and Polypedilum nubifer-type are the predominant taxa in the East Bays and the East Taihu Lake. Redundancy analysis was used for exploring the relationships between chironomid variability and environmental and spatial stressors. Four variables were identified as significant factors that influence chironomid community structures. The high nutrient concentrations around the estuarial areas favor the development of nutrient-tolerant taxa. Water depth-related oxygen depletion in the open lake during algae blooms prohibits the survival of many organisms, except for a few hypoxic-resistant species. High transparency in the East Bays and the East Taihu Lake indirectly creates a favorite microhabitat for macrophyte-associated chironomid species through aquatic plants. Space per se is a significant forcing factor for organism community and distribution at scales of >1,000 km². It might be important to consider spatial variables more explicitly in future studies of chironomids in large lakes where multiple stressors make the interactions within the ecosystem more complicated. This study aims to illustrate the ecological characteristics of specific chironomid taxa related to a “microecosystem” which is contributed by the multiple environmental gradients within a large lake, and to provide empirical support for interpretation of palaeochironomid data.     

Phosphorus dynamics at multiple time scales in the pelagic zone of a large shallow lake in Florida,USA

Phosphorus (P) dynamics in large shallow lakes are greatly influenced by physical processes such as wind-driven sediment resuspension, at times scales from hours to years. Results from long-term (30 year) research on Lake Okeechobee, Florida (area 1,730 km², mean depth 2.7 m) illustrate key features of these P dynamics. Variations in wind velocity result in changes in water column transparency, suspended solids, and total P (TP). In summer there are diurnal changes in TP associated with afternoon winds, and in winter, when strong winds occur for multiple days, monthly average TP remains high compared to summer. The magnitude of daily and seasonal TP changes can exceed 100 μg l⁻¹. Hurricanes and tropical storms also cause extreme changes in TP that are superimposed on seasonal dynamics. When a hurricane passed 80 km south of the lake in October 1999, mean pelagic TP increased from 88 to 222 μg l⁻¹. During large resuspension events, light attenuation is substantially increased, and this influences the biomass and spatial extent of submerged plants, as well as water column TP. In Lake Okeechobee, TP concentrations typically are ∼20 μg l⁻¹ when submerged plants are dense, and soluble reactive P concentrations are reduced below detection, perhaps by the periphyton and plant uptake and by precipitation with calcium at high pH. In contrast, TP exceeds 50 μg l⁻¹ when submerged plants and periphyton are absent due to prolonged deep water, and phytoplankton biomass and algal bloom frequency both are increased. In Lake Okeechobee and other large shallow lakes, complex models that explicitly consider wind-wave energy, hydrodynamics, and sediment resuspension, transport, and key biological processes are needed to accurately predict how lake water TP will respond to different management options.     

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.     

A recently landlocked brackish-water lagoon of Lake Tanganyika: physical and chemical characteristics,and spatio-temporal distribution of phytoplankton

Monthly measurements of physical and chemical characteristics were made at two localities in the eastern part of a recently landlocked lagoon of Lake Tanganyika. Variables analysed were: temperature, pH, conductivity, sodium, potassium, magnesium, calcium, carbonate, bicarbonate, chloride and sulphate. Large seasonal fluctuations of salinity were recorded (1.68–8.21 g l^–1). The seasonal water input controlled algal seasonality mainly through its effect on salinity and indirectly through its influence on nutrient concentration by dissolution and dilution of the excrements of the numerous cattle and other organic matter. Phytoplankton was mainly composed of Cyanophyta and Euglenophyta. Euglenophyta dominated during the dry periods with high salinity and probably very high nutrient levels, while Cyanophyta preferred moderate salt and nutrient concentrations during the rainy periods. The phytoplanktonic community was composed of a large number of perennial and a reduced quantity of annual organisms.A spatial study of the recently landlocked lagoon revealed an ascending salinity gradient, principally due to a sodium bicarbonate/carbonate enrichment, between locations near the lake and more inland situated stations. These facts point to a lake water supply and a salt concentration by evaporation in the swamps. Proportionally lower magnesium, calcium and potassium values were recorded at high salinities, due to chemical precipitation and biotic factors.A blue-green algal bloom was observed in the eastern water-body (salinity: 4.64 g l^–1); simultaneously an important development of diatoms dominated the western water-body (salinity: 2.18 g l^–1). No significant differences in morphometry, exposure, water temperature or nutrient levels (nitrate, nitrite, ammonia, orthophosphate) were observed. The relatively low salinity and high nutrient concentration in the western water-body probably favoured diatom development during the rainy season. The relatively higher salinity in the eastern water-body during the rainy season was probably responsible for the dominance of blue-green algae through its negative influence on silica concentration and notwithstanding the high inorganic nitrogen concentration.     

Above-ground biomass and structure of 260 African tropical forests

We report above-ground biomass (AGB), basal area, stem density and wood mass density estimates from 260 sample plots (mean size: 1.2 ha) in intact closed-canopy tropical forests across 12 African countries. Mean AGB is 395.7 Mg dry mass ha⁻¹ (95% CI: 14.3), substantially higher than Amazonian values, with the Congo Basin and contiguous forest region attaining AGB values (429 Mg ha⁻¹) similar to those of Bornean forests, and significantly greater than East or West African forests. AGB therefore appears generally higher in palaeo- compared with neotropical forests. However, mean stem density is low (426 ± 11 stems ha⁻¹ greater than or equal to 100 mm diameter) compared with both Amazonian and Bornean forests (cf. approx. 600) and is the signature structural feature of African tropical forests. While spatial autocorrelation complicates analyses, AGB shows a positive relationship with rainfall in the driest nine months of the year, and an opposite association with the wettest three months of the year; a negative relationship with temperature; positive relationship with clay-rich soils; and negative relationships with C : N ratio (suggesting a positive soil phosphorus–AGB relationship), and soil fertility computed as the sum of base cations. The results indicate that AGB is mediated by both climate and soils, and suggest that the AGB of African closed-canopy tropical forests may be particularly sensitive to future precipitation and temperature changes.     

Life histories ofNeomysis integer,and its copepod prey,Eurytemora affinis,in a eutrophic and brackish shallow lake

Data was collected on the population structure and fecundity of the mysidNeomysis integer and the calanoid copepodEurytemora affinis in Hickling Broad, a shallow and eutrophic brackish lake, over a two-year period 1988/89. Standing biomass and production rate estimates were made using estimates of size-specific dry weights and development times obtained from laboratory measurements, field observations and information found in the literature. Both mysid and copepod are capable of a rapid response to favourable conditions and have high rates of birth, growth and production.E. affinis reproduces throughout the year with an estimated annual (May 1988–May 1989) production of 20 g dry wt m⁻². Copepod standing biomass was less in 1989 compared with 1988 owing to an overall reduction in copepod body-size and a reduction of size at maturity. There was a suggestion from the data that this was caused by predation from a greater number of large (>9 mm body-length) mysids in 1989 compared with 1988.N. integer is highly seasonal in its growth with distinct peaks of recruitment in May and July. Annual (May 1988–May 1989) production of the mysid was estimated as 5.8 g dry wt m⁻². AlthoughE. affinis is the only available prey ofN. integer in the lake, the mysid population appears independent of changes in that of the copepod and probably avoids negative predator-prey feedback mechanisms owing to an ability to feed on epiphytic algae.     

Species composition,distribution and seasonal dynamics of Rotifera in a Rift Valley lake in Ethiopia (Lake Awasa)

The species composition of rotifers in Lake Awasa was studied and 40 species recorded. Seven species appeared in large numbers in the plankton seasonally. Brachionus and Keratella species made up more than 50% of the rotifer community by numbers during the observation period (1983–1987). The standing stock numbers of rotifer species are low with a maximum of 50 individuals L^–1, and some possible reasons for this observation are discussed. Most rotifer species are distributed randomly in the lake and show 3-fold fluctuations between consecutive days.The seasonal dynamics of most rotifer species are correlated with mixing periods in the lake, and the amplitude of seasonal fluctuation may be as high as 50-fold. Clear-cut seasonal succession of rotifer species was observed during the study period, but no consistent seasonal pattern for individual species was obvious. Also, observations indicate that rotifer biomass is partly sustained by availability of carbon through the bacterial pathway and that competitive exclusion for food by nauplii and ciliates probably keeps rotifer abundance low.     

Patterns of Ecosystem Metabolism in the Tonle Sap Lake,Cambodia with Links to Capture Fisheries

The Tonle Sap Lake in Cambodia is a dynamic flood-pulsed ecosystem that annually increases its surface area from roughly 2,500 km² to over 12,500 km² driven by seasonal flooding from the Mekong River. This flooding is thought to structure many of the critical ecological processes, including aquatic primary and secondary productivity. The lake also has a large fishery that supports the livelihoods of nearly 2 million people. We used a state-space oxygen mass balance model and continuous dissolved oxygen measurements from four locations to provide the first estimates of gross primary productivity (GPP) and ecosystem respiration (ER) for the Tonle Sap. GPP averaged 4.1±2.3 g O₂ m⁻³ d⁻¹ with minimal differences among sites. There was a negative correlation between monthly GPP and lake level (r = 0.45) and positive correlation with turbidity (r = 0.65). ER averaged 24.9±20.0 g O₂ m⁻³ d⁻¹ but had greater than six-fold variation among sites and minimal seasonal change. Repeated hypoxia was observed at most sampling sites along with persistent net heterotrophy (GPP<ER), indicating significant bacterial metabolism of organic matter that is likely incorporated into the larger food web. Using our measurements of GPP, we calibrated a hydrodynamic-productivity model and predicted aquatic net primary production (aNPP) of 2.0±0.2 g C m⁻² d⁻¹ (2.4±0.2 million tonnes C y⁻¹). Considering a range of plausible values for the total fisheries catch, we estimate that fisheries harvest is an equivalent of 7–69% of total aNPP, which is substantially larger than global average for marine and freshwater systems. This is likely due to relatively efficient carbon transfer through the food web and support of fish production from terrestrial NPP. These analyses are an important first-step in quantifying the resource pathways that support this important ecosystem.     

Carbon Cycling of Lake Kivu (East Africa): Net Autotrophy in the Epilimnion and Emission of CO2 to the Atmosphere Sustained by Geogenic Inputs

We report organic and inorganic carbon distributions and fluxes in a large (>2000 km²) oligotrophic, tropical lake (Lake Kivu, East Africa), acquired during four field surveys, that captured the seasonal variations (March 2007–mid rainy season, September 2007–late dry season, June 2008–early dry season, and April 2009–late rainy season). The partial pressure of CO₂ (pCO₂) in surface waters of the main basin of Lake Kivu showed modest spatial (coefficient of variation between 3% and 6%), and seasonal variations with an amplitude of 163 ppm (between 579±23 ppm on average in March 2007 and 742±28 ppm on average in September 2007). The most prominent spatial feature of the pCO₂ distribution was the very high pCO₂ values in Kabuno Bay (a small sub-basin with little connection to the main lake) ranging between 11213 ppm and 14213 ppm (between 18 and 26 times higher than in the main basin). Surface waters of the main basin of Lake Kivu were a net source of CO₂ to the atmosphere at an average rate of 10.8 mmol m⁻² d⁻¹, which is lower than the global average reported for freshwater, saline, and volcanic lakes. In Kabuno Bay, the CO₂ emission to the atmosphere was on average 500.7 mmol m⁻² d⁻¹ (∼46 times higher than in the main basin). Based on whole-lake mass balance of dissolved inorganic carbon (DIC) bulk concentrations and of its stable carbon isotope composition, we show that the epilimnion of Lake Kivu was net autotrophic. This is due to the modest river inputs of organic carbon owing to the small ratio of catchment area to lake surface area (2.15). The carbon budget implies that the CO₂ emission to the atmosphere must be sustained by DIC inputs of geogenic origin from deep geothermal springs.     

Sulfur cycling in the water column of Little Rock Lake, Wisconsin

The S cycle in the water column of a small, soft-water lake was studied for 9 years as part of an experimental study of the effects of acid rain on lakes. The two basins of the lake were artificially separated, and one basin was experimentally acidified with sulfuric acid while the other served as a reference or control. Spatial and seasonal patterns of sulfate uptake by plankton (53–70 mmol m^–2 yr^–1), deposition of sulfur to sediments in settling seston (53 mmol m^–2 yr^–1), and sulfate diffusion (0–39 mmol m^–2 yr^–1) into sediments were examined. Measurements of inputs (12–108 mmol m^–2 yr^–1) and outputs (5.5–25 mmol m^–2 yr^–1) allowed construction of a mass balance that was then compared with rates of S accumulation in sediments cores (10–28 mmol m^–2 yr^–1) and measured fluxes of S into the sediments. Because of the low SO₄ ^2– concentrations (µmole L^–1) in the lake, annual uptake by plankton (53–70 mmol m^–2 yr^–1) represented a large fraction (>50%) of the SO₄ ^2– inventory in the lake. Despite this large flux through the plankton, only small seasonal fluctuations in SO₄ ^2– concentrations (µmole L^–1) were observed; rapid mineralization of organic matter (half-life <3 months) prevented sulfate depletion in the water column. The turnover time for sulfate in the water column is only 1.4 yr; much less than the 11-yr turnover time of a conservative ion in this seepage lake. Sulfate diffusion into and reduction in the sediments (0–160 µmole m^–2 d^–1) caused SO₄ ^2– depletion in the hypolimnion. Modeling of seasonal changes in lake-water SO₄ ^2– concentrations indicated that only 30–50% of the diffusive flux of sulfate to the sediments was permanently incorporated in solid phases, and about 15% of sulfur in settling seston was buried in the sediments. The utility of sulfur mass balances for seepage lakes would be enhanced if uncertainty about the deposition velocity for both sulfate aerosols and SO₂, uncertainty in calculation of a lake-wide rate of S accumulation in sediments, and uncertainty in the measured diffusive fluxes could be further constrained.     

Thermal stratification and the stability of meromixis in the Pretoria Salt Pan,South Africa

The Pretoria Salt Pan, South Africa, a small (0.076 km²), shallow (Z_max = 2.85 m), hypersaline, maar lake, lies within a clearly-defined crater and is fed by a perennial, slightly saline (3 g l^-1) artesian spring. The lake has two distinct solar-heated peaks in its temperature profile, each of these peaks located in a highly turbid (>80 JTU) layer below a steep chemocline. The upper thermal peak, located at a depth of 10 cm, was transient, with a distinct diel pattern of diurnal heating and nocturnal cooling. The lower thermal peak, located below a steep chemocline and centred at approximately 60 cm, was stable and showed a seasonal pattern of winter heating (maximum: 38.5°C) and summer cooling (minimum: 27.4°C). The unusual bathymetry of the lake, combined with the sheltering effect of the crater rim and steep salinity gradient between the surface (30–80 g l^-1) and bottom water (280–310 g l^-1) prevented windmixing of surface waters beyond a depth of approximately 50 cm. During a 28 month study all water deeper than 55 cm remained anaerobic, and the lake appeared to be meromictic.     

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.     

Disturbance events affecting phytoplankton biomass,composition and species diversity in a shallow,eutrophic, temperate lake

The seasonal changes in phytoplankton biomass and species diversity in a shallow, eutrophic Danish lake are described and related to different disturbance events acting on the phytoplankton community.Both the spring diatom maximum and the summer bloom of the filamentous blue-green alga, Aphanizomenon flos-aquae (L.) Ralfs, coincided with low values of phytoplankton species diversity and equitability. Diatom collapse was mainly due to internal modifications as nutrient depletion (Si, P) caused by rapid growth of phytoplankton, and increased grazing activity from zooplankton. A large population of Daphnia longispina O.F. Müller in June effectively removed smaller algal competitors, thus favouring the development of a huge summer bloom (140 mm³ l^–1) of Aphanizomenon flos-aquae. Heavy rainfall and storms in late July increased the loss of Apahnizomenon by out-flow and disturbed the stratification of the lake. These events caused a marked decline in phytoplankton biomass but had no effect on species diversity. A second storm period in late August circulated the lake completely and was followed by a rapid increase in phytoplankton diversity, and a change in the phytoplankton community structure from dominance of large, slow-growing K-selected species (Aphanizomenon) to small, fast-growing r-selected species (cryptomonads).     

Variations and controls of nitrogen stable isotopes in particulate organic matter of lakes

Nitrogen stable isotope (δ¹⁵N) data of particulate organic matter (POM) from the literature were analyzed to provide an understanding of the variations and controls of δ¹⁵N_POM in lakes at the global scale. The δ¹⁵N_POM variability characterized by seasonal mean, minimum, maximum, and amplitude (defined as δ¹⁵N_POM maximum − δ¹⁵N_POM minimum) from 36 lakes with seasonal data did not change systematically with latitude, but was significantly lower in small lakes than in large lakes. The seasonal mean δ¹⁵N_POM increased from oligotrophic lakes to eutrophic lakes despite large variations that are attributed to the occurrences of nitrogen fixation across the trophic gradient and the differences in δ¹⁵N of dissolved inorganic nitrogen (DIN) in individual lakes. Seasonal mean δ¹⁵N_POM was significantly correlated with DIN concentration and δ¹⁵N_DIN in two subsets of lakes. Seasonal minimum δ¹⁵N_POM in individual lakes is influenced by nitrogen fixation and δ¹⁵N_DIN while seasonal maximum δ¹⁵N_POM is influenced by lake trophic state and δ¹⁵N_DIN. As a result of the dominance of non-living POM in the unproductive surface waters, seasonal δ¹⁵N_POM amplitude was small (mean = 4.2‰) in oligotrophic lakes of all latitudes. On the other hand, seasonal δ¹⁵N_POM amplitude in eutrophic lakes was large (mean = 10.3‰), and increased from low to high latitudes, suggesting that the seasonal variability of δ¹⁵N in the phytoplankton-dominated POM pool was elevated by the greater spans of solar radiation and thermal regimes at high latitudes. The δ¹⁵N_POM from 42 lakes with no seasonal data revealed no consistent patterns along latitude, lake area, and trophic gradients, and a greater than 2‰ depletion compared to the lakes with seasonal data. Along with the large seasonal variability of δ¹⁵N_POM within lakes, these results provide insightful information on sampling design for the studies of food web baseline in lakes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Abundance,Distribution, and Diversity of Viruses in Alkaline,Hypersaline Mono Lake,California

Mono Lake is a large (180 km²), alkaline (pH ~10), moderately hypersaline (70–85 g kg^–1) lake lying at the western edge of the Great Basin. An episode of persistent chemical stratification (meromixis) was initiated in 1995 and has resulted in depletion of oxygen and accumulation of ammonia and sulfide beneath the chemocline. Although previous studies have documented high bacterial abundances and marked seasonal changes in phytoplankton abundance and community composition, there have been no previous reports on the occurrence of viruses in this unique lake. Based on the high concentrations and diversity of microbial life in this lake, we hypothesized that planktonic viruses are also abundant and diverse. To examine the abundance and distribution of viruses and bacteria, water samples were collected from four stations along 5 to 15 vertical depths at each station. Viral abundance ranged from 1 × 10⁸ to 1 × 10⁹ mL^–1, among the highest observed in any natural aquatic system examined so far. Increases (p < 0.1) in viral densities were observed in the anoxic bottom water at multiple stations. However, regression analysis indicated that viral abundance could not be predicted by any single environmental parameter. Pulsed field gel electrophoresis revealed a diverse viral community in Mono Lake with genome sizes ranging from ~14 to >400 kb with most of the DNA in the 30 to 60 kb size range. Cluster analysis grouped the anoxic bottom-water viral community into a unique cluster differentiating it from surface and mid-water viral communities. A hybridization study using an indigenous viral isolate as a probe revealed an episodic pattern of temporal phage distribution with strong niche stratification between oxic and anoxic waters.     

Temperature dependence of sediment-water exchange in Lake Grevelingen,SW Netherlands

Extended abstract Lake Grevelingen is a brackish water lake in the SW Netherlands. The lake has an area of 108 km², a mean depth of 5.3 m (maximum 48 m), a mean chlorinity of 13 to 16%0 Cl⁻, and a hydraulic residence time of about 8 years. Mass budget studies have shown a consistent seasonal pattern in the phosphorus sediment-water exchange in Lake Grevelingen (Kelderman 1980). From May to August a P mobilization from the sediment takes place, estimated atca. 12.5 mg P · m⁻² · day⁻¹. The sediment accumulatesca. 5.5 mg P · m⁻² · day⁻¹ during the rest of the year. Temperature may be an important factor in establishing this pattern. Sediment-water exchange was studied by means of laboratory experiments under specified conditions. Sediment cores (30 cm depth, 11 cm diameter) were taken at four stations in the lake, with sediment types varying from medium- to muddy sand (Fig. 1). The cores with overlying water (ca. 21) were placed in the dark at 5 °C in thermostatically controlled water baths. After a week's incubation time the temperature was slowly raised, such that after three weeks eight cores (four sediment types, duplicates) were at 5 °C, eight were at 10 °C, eight at 15 °C and eight at 20 °C. The same procedure was applied to the four control cores, containing lake water.     

Interactive effects of season and temperature on enzyme activities,tissue and whole animal respiration in roach,Rutilus rutilus

Synopsis This paper reviews investigations on the ecophysiology of a population of roach, Rutilus rutilus, from a subalpine oligotrophic lake in the Austrian Tirol. Metabolic responses to season and temperature were studied in whole animals, tissues and selected enzymes. The exponent of the relationship between body mass and three levels of the metabolic rate of acclimated fish was 0.82 ± 0.02, 0.60 ± 0.15, and 0.75 ± 0.01 at 4, 12, and 20° C respectively. Various combinations of long-term acclimation to constant or seasonally fluctuating temperatures and long-term (up to 14 days) monitoring of O₂ at the acclimation temperature led to the conclusion that the aerobic power of fish swimming in the routine mode does not show any sign of being temperature compensated. On the other hand, there are several indications that the energy expenditure of spontaneously swimming fish is adjusted to the seasonal pattern of environmental change and that these responses of metabolism and behaviour are controlled by both endogenous and exogenous factors. The rate of oxygen consumption of gill and muscle tissue brei from fish caught during a seasonal cycle and measured at 15° C appears to follow closely the reproductive and gonadal cycle of the living fish. The same holds for the activities of phosphofructokinase, acetoacetyl-CoA thiolase, and cytochrome oxidase. On the other hand, the Na⁺, K⁺-ATPase of the kidney shows near perfect temperature compensation when fish acclimated to 5 and 25° C are compared, whereas an equally pronounced case of inverse temperature acclimation has been reported for the activity of digestive enzymes in the gut. Summarizing these data it is pointed out that the temperature relationship of a poikilothermic organism is the sum of often very diverse temperature relationships of specific metabolic and behavioural functions. In the case of the roach, strong effects of acclimation temperature on the molecular level, sometimes in the opposite direction, combine with seasonal effects on enzyme activities and tissue respiration. However, on the whole animal level the fish behave as strictly non-compensating poikilotherms, the reproductive cycle being the only detectable influence capable of modulating the basic temperature relationship of energy expenditure.     

Influence of net ecosystem metabolism in transferring riverine organic carbon to atmospheric CO<Subscript>2</Subscript> in a tropical coastal lagoon (Chilka Lake,India)

Studies on biogeochemical cycling of carbon in the Chilka Lake, Asia’s largest brackish lagoon on the east coast of India, revealed, for the first time, strong seasonal and spatial variability associated with salinity distribution. The lake was studied twice during May 2005 (premonsoon) and August 2005 (monsoon). It exchanges waters with the sea (Bay of Bengal) and several rivers open into the lake. The lake showed contrasting levels of dissolved inorganic carbon (DIC) and organic carbon (DOC) in different seasons; DIC was higher by ∼22% and DOC was lower by ∼36% in premonsoon than in monsoon due to seasonal variations in their supply from rivers and in situ production/mineralisation. The DIC/DOC ratios in the lake during monsoon were influenced by physical mixing of end member water masses and by intense respiration of organic carbon. A strong relationship between excess DIC and apparent oxygen utilisation showed significant control of biological processes over CO₂ production in the lake. Surface partial pressure of CO₂ (pCO₂), calculated using pH–DIC couple according to Cai and Wang (Limnol and Oceanogr 43:657–668, 1998), exhibited discernable gradients during monsoon through northern (1,033–6,522 μatm), central (391–2,573 μatm) and southern (102–718 μatm) lake. The distribution pattern of pCO₂ in the lake seems to be governed by pCO₂ levels in rivers and their discharge rates, which were several folds higher during monsoon than premonsoon. The net CO₂ efflux, based on gas transfer velocity parameterisation of Borges et al. (Limnol and Oceanogr 49(5):1630–1641, 2004), from entire lake during monsoon (141 mmolC m⁻² d⁻¹ equivalent to 2.64 GgC d⁻¹ at basin scale) was higher by 44 times than during premonsoon (9.8 mmolC m⁻² d⁻¹ ≈ 0.06 GgC d⁻¹). 15% of CO₂ efflux from lake in monsoon was contributed by its supply from rivers and the rest was contributed by in situ heterotrophic activity. Based on oxygen and total carbon mass balance, net ecosystem production (NEP) of lake (−308 mmolC m⁻² d⁻¹ ≈ −3.77 GgC d⁻¹) was found to be almost in consistent with the total riverine organic carbon trapped in the lake (229 mmolC m⁻² d⁻¹ ≈ 2.80 GgC d⁻¹) suggesting that the strong heterotrophy in the lake is mainly responsible for elevated fluxes of CO₂ during monsoon. Further, the pelagic net community production represented 92% of NEP and benthic compartment plays only a minor role. This suggests that Chilka lake is an important region in biological transformation of organic carbon to inorganic carbon and its export to the atmosphere.