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.     

The role of plankton biomass in controlling fluctuations of suspended matter in Lake Constance

The concentrations of particulate matter, expressed as dry weight (DW), particulate organic (POM), and inorganic material were measured at regular intervals in Lake Constance between February 1980 and December 1982. Maximum particle concentrations were recorded for the euphotic zone in summer (7 mg l⁻¹), while minima occurred during the early summer and in winter. Annual mean concentrations of DW within the entire water column varied between 0.6 and 0.7 mg l⁻¹. In the euphotic zone nearly 70% of DW is organic material. The inorganic particles originate either from phytoplankton (diatomaceous silicon, biogenic decalcification) or from the tributaries. Although phytoplankton biomass only comprises a relatively small proportion (i.e. 30% at maximum) of organic matter, it is the primary source of POM. Therefore, seasonal variations in phytoplankton control epilimnetic concentrations of POM in Lake Constance. Inorganic material comprises smaller proportions of suspended matter. Seasonal variations are related predominantly to fluctuations in biomass and therefore particulate inorganic matter is suggested to originate mainly from autochthonous sources. At the sampling station concentrations of inorganic particles supported by the main tributary, the Alpenrhein, only occasionally vary concomitantly with runoff.     

Protozoan growth rates in Antarctic lakes

The growth rates of heterotrophic nanoflagellates (HNAN), mixotrophic cryptophytes, dinoflagellates and ciliates in field assemblages from Ace Lake in the Vestfold Hills (eastern Antarctica) and Lakes Fryxell and Hoare (McMurdo Dry Valleys, western Antarctica), were determined during the austral summers of 1996/1997 and 1997/1998. The response of the nanoflagellates to temperature differed between lakes in eastern and western Antarctica. In Ace Lake the available bacterial food resources had little impact on growth rate, while temperature imposed an impact, whereas in Lake Hoare increased bacterial food resources elicited an increase in growth rate. However, the incorporation of published data from across Antarctica showed that temperature had the greater effect, but that growth is probably controlled by a suite of factors not solely related to bacterial food resources and temperature. Dinoflagellates had relatively high specific growth rates (0.0057–0.384 h⁻¹), which were comparable to Antarctic lake ciliates and to dinoflagellates from warmer, lower latitude locations. Temperature did not appear to impose any significant impact on growth rates. Mixotrophic cryptophytes in Lake Hoare had lower specific growth rates than HNAN (0.0029–0.0059 h⁻¹ and 0.0056–0.0127 h⁻¹, respectively). They showed a marked seasonal variation in growth rate, which was probably related to photosynthetically active radiation under the ice at different depths in the water column. Ciliates' growth rates showed no relationship between food supply and mean cell volume, but did show a response to temperature. Specific growth rates ranged between 0.0033 and 0.150 h⁻¹ for heterotrophic ciliates, 0.0143 h⁻¹ for a mixotrophic Plagiocampa species and 0.0075 h⁻¹ for the entirely autotrophic ciliate, Mesodinium rubrum. The data indicated that the scope for growth among planktonic Protozoa living in oligotrophic, cold extreme lake ecosystems is limited. These organisms are likely to suffer prolonged physiological stress, which may account for the highly variable growth rates seen within and between Antarctic lakes. Accepted: 7 December 1999     

Factors controlling the activity of the microbial community of the alkaline Lake Beloe (Transbaikal Region)

In this work, the main environmental factors determining the functioning of the microbial community of the alkaline low-mineralized Lake Beloe during the annual cycle were studied. High numbers of phototrophic and heterotrophic microorganisms (up to 10⁷ cells/mL) and high rates of bacterial processes of organic matter (OM) production and destruction were observed. The highest rate of dark CO₂ assimilation (up to 0.43 mg C dm⁻³ day⁻¹), as well as the peak intensities of the terminal processes of sulfate reduction and methanogenesis (up to 1.81 mg S dm⁻³ day⁻¹ and 0.96 μL CH⁴ dm⁻³ day⁻¹, respectively), detected at the end of summer, were comparable to the rates of these processes detected in the bottom sediments of most soda lakes of the Transbaikal Region. Principal Component Analysis (PCA) allowed us to estimate the effect of environmental factors on the functioning of the microbial community of the alkaline Lake Beloe. Four main components, explaining 98% of variations, were detected. The first one (PC1) explained 63.5% of the seasonal variations and represented the temperature factor consisting of the temperatures of air, water, and bottom sediments. Water temperature and pH were the main contributors to the second component (PC2) and determine 26.2% of the seasonal variations. The PC3 (silt temperature and the concentration of organic matter) and PC4 (salt concentration) components were less important and explained only 6.5 and 2.2% of the variations, respectively.     

Autotrophic energy sources for Paracheirodon axelrodi (Osteichthyes, Characidae) in the middle Negro River, Central Amazon, Brazil

The cardinal tetra (Paracheirodon axelrodi) is the most abundant species of the Brazilian ornamental fish trade, constituting more than 80% of the ornamental fish collected in the middle Negro River basin. Stable isotope analyses were used to identify the autotrophic carbon sources and trophic position for the cardinal in relation to the plant groups at the base of its foodchain. Filamentous algae, tree and plant leaves and cardinals were collected in stream habitats, flooded forest and interfluvial swamps (campos) during peak flood, falling water and low water periods. δ¹⁵N values of the cardinal in relation to the plants at the base of the food chain indicated a trophic position of omnivore. Values of δ¹³C for the plants ranged from −43.1 to −26.4‰, with averages of −37.6, −30.4, and −29.4‰ for filamentous algae, flooded forest leaves, and campo leaves, respectively. The δ¹³C values for the cardinal ranged from −35.0 to −27.9‰, with an average of −31.4‰. Relative contributions of plants to fish carbon were estimated in a two end-member mixing model which determined that the leaves (flooded forest and campo leaves combined) and filamentous algae had average relative contributions to cardinal carbon of 71% and 29%, respectively. However, seasonal variation in the relative contributions was encountered throughout the hydrological cycle. The cardinals least enriched in ¹³C were encountered in November during the falling water period, indicating that they had perhaps recently migrated down from interfluvial campos where filamentous algae production is significant. Considering that algal production has been reported to be less than 1% of total primary production in the Negro River, these results could suggest some evidence of selective herbivory in the cardinal’s food chain. Handling editor: J. M. Melack     

First attempt to apply whole-lake food-web manipulation on a large scale in The Netherlands

Lake Breukeleveen (180 ha, mean depth 1.45 m), a compartment of the eutrophic Loosdrecht lakes system, was selected to study the effects of whole-lake foodweb manipulation on a large scale. In Lake Loosdrecht (dominated by filamentous cyanobacteria), due to water management measures taken from 1970–1984 (sewerage systems, dephosphorization) the external P load has been reduced from 1.2 g m⁻² y⁻¹ to 0.35 g m⁻² y⁻¹. The water transparency (Secchi-depthca. 30 cm), however, has not improved. The aim of the food-web manipulation in Lake Breukeleveen was not only to improve the light climate of the lake, but also to study if the successfull effects observed in small lakes (a few ha) can be upscaled. In March 1989 the standing crop of planktivorous and bentivorous fish populations was reduced by intensive fishery, fromca. 150 kg ha⁻¹ toca. 57 kg ha⁻¹. The lake was made unaccessible to fish migrating from the other lakes and it was stocked with large-sized daphnids and 0⁺ pike. However, water transparency did not increase in the following summer and autumn 1989, which is in contrast with great improvement in the light conditions previously observed in smaller lakes. The main explanations for the negative outcome in Lake Breukeleveen are: 1) the rapid increase of the planktivorous fish biomass and carnivorous cladocerans, predating on the zooplankton community; 2) suppression of the large daphnids by the high concentrations of filamentous cyanobacteria; 3) high turbidity of the lake due to resuspension of bottom material induced by wind, unlike in smaller lakes, and thus inability of submerged macrophytes to develop and to stabilize the ecosystem.     

Distribution and ecology of <Emphasis Type="Italic">Hemimysis anomala</Emphasis>, the latest invader of the Great Lakes basin

Since 2006, the known distribution of Hemimysis anomala has greatly expanded in the Great Lakes ecosystem, with, to date, 45 sites of occurrence among 91 monitored sites, located in four of the Great Lakes and the upper St. Lawrence River. By means of carbon and nitrogen stable isotopes, a first assessment of the feeding ecology of Hemimysis was completed. The δ¹³C values of 18 individuals collected in Lake Erie (Port Mainland) on a single date (Sept. 23, 2008) ranged from −30.2 to −24.5‰, indicating that Hemimysis could feed on multiple carbon sources including pelagic and littoral autochthonous and terrestrial carbon. In Lake Erie, variation in δ¹³C was related to δ¹⁵N, indicating the importance of food source for determining the trophic position of Hemimysis. The δ¹⁵N signatures of individuals were strongly related to their C/N ratios, suggesting that variations in the nutritional value of Hemimysis may depend on trophic position. Isotopic variation among individuals in Lake Erie was complemented by temporal variation in Lake Ontario. Monthly changes (from June to December 2008) in carbon isotope signatures were observed and related to changes in water temperature, highlighting the variations in the baseline prey signatures that fuel Hemimysis diets. The observed variation in stable isotope signatures occurring among individuals within a localized Hemimysis assemblage and temporally should be considered as a key design feature in further studies attempting to identify the possible effects of Hemimysis on nearshore food webs in the Great Lakes.     

Diel variation in urea decomposing activity in the euphotic zone of brackish Lake Nakaumi

Diel variations in urea decomposing activity in the euphotic zone of brackish Lake Nakaumi were measured under fixed light intensity. The decomposition rate of urea was 17 to 44 μ mol urea m⁻³ h⁻¹ in the light and 10 to 27 μ mol urea m⁻³ h⁻¹ in the dark. Higher decomposition rates were obtained in the upper euphotic zone. A clear diel periodicity in the urea decomposition rate was observed, with high rates from 1200 to 1600 and low rates from 0000 to 0400. Chlorophyll a specific decomposing activity ranged from 12 to 21 μg urea C mg chl.a ⁻¹ h⁻¹ in the light and 7 to 13 μg urea C mg chl.a ⁻¹ h⁻¹ in the dark. In the light, high values were obtained from 1600 to 2000 and low values from 0400 to 0800. The diel change in specific decomposing activity exhibited a similar pattern to that of the photosynthetic assimilation number, following the diel change in photosynthetic activity. Received: March 10, 1999 / Accepted: October 22, 1999     

Can Simple Empirical Equations Describe the Seasonal Dynamics of Bacterioplankton in Lakes: An Eight-Year Study in Shallow Hypertrophic and Biologically Highly Dynamic Lake Søbygård,Denmark

Abstract Seasonal variation in bacterioplankton abundance, biomass, and bacterioplankton production was studied over eight years in hypertrophic Lake S?byg?rd. Biologically, the lake is highly variable; this is due mainly to large interannual variation in fish recruitment. Bacterioplankton production was low during winter, typically 1–3 × 10⁷ cells l⁻¹ h⁻¹, and high during summer, albeit greatly fluctuating with maximum rates typically ranging from 60 to 90 × 10⁷ cells l⁻¹ h⁻¹ (or 0.4 to 0.6 mg C l⁻¹ day⁻¹). Less pronounced variations were found in bacterioplankton abundance, which typically ranged from 3–8 × 10⁹ cells l⁻¹ in winter to 15–30 × 10⁹ cells l⁻¹ during summer. The specific growth rate of bacterioplankton varied from 0.02–0.2 d⁻¹ in winter to 0.5–2.3 day⁻¹ during summer. Interpolated mean bacterioplankton production, in terms of carbon, ranged from 0.08 to 0.16 mg C l⁻¹ day⁻¹, corresponding to 1.6–5.5% of the phytoplankton production, while biomass ranged from 0.28 to 0.36 mg C l⁻¹, corresponding to 1.9–4.6% of the phytoplankton biomass. We conducted regression analysis, relating the bacterioplankton variables to a number of environmental variables, and evaluated the interannual parameter variability. Chlorophyll a and phytoplankton production contributed less to the variation in the bacterioplankton variables than in most previous analyses using data from less eutrophic systems. We suggest that the proportion of phytoplankton production that is channelized through bacterioplankton in lakes decreases with increasing trophic state and decreasing mean depth. This probably reflects a concurrent increase in fish predation on macrozooplankton and loss by sedimentation. An important part of the residual variation in the equations hitherto proposed in the literature could be explained by variation in macrozooplankton biomass and pH > 10.2. A negative effect of high pH on bacterioplankton production was confirmed by laboratory experiments. The impact of different zooplankton varies considerably, with Daphnia seeming to have a negative impact on bacterioplankton abundance and, thereby, indirectly on bacterioplankton production, while Bosmina, rotifers, and cyclopoid copepods seem to stimulate both abundance and production. Bosmina apparently also stimulate the bacterioplankton specific growth rate. Received: 8 February 1996; Accepted: 16 July 1996     

The influence of large water level fluctuations and hurricanes on periphyton and associated nutrient storage in subtropical Lake Okeechobee,USA

The abundance, community structure and nutrient content of periphyton, and the host plant taxa Chara, Hydrilla, Potamogeton, Vallisneria and Scirpus were studied in Lake Okeechobee, USA. Water levels were generally high during the study period (August 2002–January 2006), but substantial fluctuations occurred. All host plant biomass was seasonally variable but only Vallisneria biomass was spatially variable. All submerged plant beds disappeared after the passage of two hurricanes in September 2004, and a third hurricane passed over the lake in October 2005. Periphyton assemblages were statistically separated most by substrate and then by season. Prior to the hurricanes, annual maxima of periphyton biovolumes and those of summer submerged plant coverage coincided. During all study years, the diatom taxa dominated periphyton total biovolumes. Periphyton biomass was generally highest during the summer or prior to the hurricanes (in the case of epiphytes) and was spatially variable in the case of both Scirpus and Vallisneria. Epiphytic nutrient contents within each host plant group seasonally varied except for nitrogen and carbon in the Vallisneria epiphytes. Epipelic nutrient contents were spatially variable and seasonally variable for carbon. Nutrient contents in epipelon were significantly higher than that in Scirpus epiphytes and were similar but lower among all epiphytic communities. The total annual areal potential epiphytic phosphorus storage extrapolated during this study (2.0 × 10⁻⁴ metric tons ha⁻¹ year⁻¹) was underestimated because storage estimates for epipelon, Chara and Hydrilla-associated epiphytes were omitted. The Chara and Hydrilla-associated epiphytic nutrient storage values were omitted because of limited data, whereas the epipelic data may have not been spatially representative. For periphyton biovolume, host substrate type, water level fluctuation and hurricane impacts on host substrates appear to be more important than seasonal variation in such factors as temperature and nutrients. Epiphytic nutrient storage appears to be influenced most by water level fluctuation and hurricane-related impacts, while the host substrate type appears to be a less important factor than it is for periphyton biovolume. Maximum periphyton biomass and high nutrient storage in shallow subtropical and tropical eutrophic lakes may only occur at consistently lower water levels and during infrequent periods of disturbance, which enhance host substrate colonizable area.     

Viral dynamics and patterns of lysogeny in saline Antarctic lakes

Antarctic lakes are extreme ecosystems with microbially dominated food webs, in which viruses may be important in controlling community dynamics. A year long investigation of two Antarctic saline lakes (Ace and Pendant Lakes) revealed high concentrations of virus like particles (VLP) (0.20–1.26 × 10⁸ ml⁻¹), high VLP: bacteria ratios (maximum 70.6) and a seasonal pattern of lysogeny differing from that seen at lower latitudes. Highest rates of lysogeny (up to 32% in Pendant Lake and 71% in Ace Lake) occurred in winter and spring, with low or no lysogeny in summer. Rates of virus production (range 0.176–0.823 × 10⁶ viruses ml⁻¹ h⁻¹) were comparable to lower latitude freshwater lakes. In Ace Lake VLP did not correlate with bacterial cell concentration or bacterial production but correlated positively with primary production, while in Pendant Lake VLP abundance correlated positively with both bacterial cell numbers and bacterial production but not with primary production. In terms of virus and bacterial dynamics the two saline Antarctic lakes studied appear distinct from other aquatic ecosystems investigated so far, in having very high viral to bacterial ratios (VBR) and a very high occurrence of lysogeny in winter.     

Short-term variation in extracellular phosphatase activity: possible limitations for diagnosis of nutrient status in particular algal populations

Species-specific extracellular phosphatase activity (PA) was studied in natural phytoplankton with an emphasis on short-term variation in PA and its consequences for the diagnosis of inorganic phosphorus (P) deficiency. Extracellular PA was measured over 3 days at 4-h intervals to compare variations in algal PA in two distinct aquatic environments: in the Římov reservoir and acidified Plešné Lake. PA was directly detected in an epifluorescence microscope using fluorescently labelled enzyme activity; the fluorescence signal was quantified using image analysis. High short-term variation was found both in the total PA of the samples and specific PA of Ankyra ancora (G. M. Smith) Fott, Monoraphidium dybowski (Wolosz.) Hind. and Kom.-Legn., and Chlorogonium fusiforme Matwienko. The specific PA of these species varied between zero and 157 fmol cell⁻¹ h⁻¹. The PA of Chlorogonium fusiforme showed a significant relationship with the time of day. During the study period, populations of the certain species were present that differed in physiological status and activity due to diurnal or random changes caused by mixing in the reservoir or lake, or both. Therefore, species-specific extracellular PA should be interpreted carefully, as an indicator of P deficiency in freshwater phytoplankton if based on limited number of samplings.     

Differences in rotifer communities in two freshwater bodies of different trophic degree (Lake Ohrid and Lake Dojran,Macedonia)

We investigated monogonont rotifers in two natural Macedonian lakes that greatly differ in age, size and trophic state: Lake Ohrid and Lake Dojran. A main characteristic of Lake Ohrid is the scarcity of nutrients and consequently a low level of primary production. Lake Dojran represents a typical eutrophic lake. Results clearly indicate that species numbers are negatively correlated with trophic degree. Qualitative analyses of rotifer compositions in Lakes Ohrid and Dojran showed the presence of 70 and 55 taxa, respectively. Rotifer assemblages differed in their community structure, population densities, and the occurrence pattern of dominant species. The density of rotifers increased with increasing nutrient concentration, varying from min. 0.67 ind. L⁻¹ in June, 2006 to max. 8.2 ind. L⁻¹ in July, 2004 in Lake Ohrid, whereas min. 28.8 ind. L⁻¹ (in December, 2005) and max. 442.5 ind. L⁻¹ (in September, 2005) were recorded in Lake Dojran. Gastropus stylifer and Keratella cochlearis were the most abundant species in the pelagic zone of Lake Ohrid, averaging monthly densities of 1.2 ind. L⁻¹ and 0.6 ind. L⁻¹, respectively, thereby contributing 29% and 15% to rotifer abundance. In contrast, Lake Dojran rotifers were dominated by Brachionus spp. Brachionus diversicornis and Brachionus calyciflorus f. amphiceros were most abundant, comprising 40% and 25% of the total rotifer density. These results corroborate our idea, that the trophic state is an important factor in determining the composition of rotifer communities.     

Carbon and water cycles in tropical papyrus wetlands

Highly productive papyrus (Cyperus papyrus L.) wetlands dominate many permanently flooded areas of tropical East Africa; however, the cycling of carbon and water within these ecosystems is poorly understood. The objective of this study was to utilise Eddy Covariance (EC) techniques to measure the fluxes of carbon dioxide and water vapour between papyrus vegetation and the atmosphere in a wetland located near Jinja, Uganda on the Northern shore of Lake Victoria. Peak, midday rates of photosynthetic CO₂ net assimilation were approximately 40 μmol CO₂ m⁻² s⁻¹, while night time losses through respiration ranged between 10 and 20 μmol CO₂ m⁻² s⁻¹. Numerical integration of the flux data suggests that papyrus wetlands have the potential to sequester approximately 0.48 kg C m⁻² y⁻¹. The average daily water vapour flux from the papyrus vegetation through canopy evapotranspiration was approximately 4.75 kg H₂O m⁻² d⁻¹, which is approximately 25% higher than water loss through evaporation from open water.     

A Seasonal Study of Bacterial Community Succession in a Temperate Backwater System,Indicated by Variation in Morphotype Numbers,Biomass, and Secondary Production

Abstract The investigation of the bacterial community in the Kühw?rter Wasser, a macrophyte-dominated arm of the River Danube backwater system near Vienna, revealed that variation in microbial densities and biomass could be related to a characteristic sequence in morphotype composition over the seasons. Maximal bacterial cell numbers and biomass occured in early summer, with values of up to 9 × 10⁹ cells l⁻¹ and 122 μg C l⁻¹, respectively, caused by a massive increase of vibrio-shaped cells. On the other hand, in early spring, filamentous bacteria were responsible for a marked increase in bacterial biomass, making up 40% of the total bacterial biomass. Over the year, rod-shaped cells were the dominating morphotype, while the biomass of cocci was rather negligible. In winter, cell numbers and biomass showed minimal values with 2.0 × 10⁹ cells l⁻¹ and 28 μg C l⁻¹, respectively, and bacteria were considered to be substrate and temperature limited during this period. Saturation values of the incorporation of ³H-thymidine into DNA, for the estimation of bacterial secondary production, varied seasonally, ranging from 5 nm to 40 nm. Thus, saturation experiments needed to be conducted on a regular basis. Also, the amount of labeled thymidine in the DNA, as a percentage of labeled thymidine in the TCA precipitate, varied over the year. Minimum values of 45% were recorded during the cold season, while maximum values of 75–80% at the beginning of June coincided with high chlorophyll a values and minimal K _m-values derived from saturation experiments. The potential role of the nitrogen-rich nucleoside thymidine as a readily utilizable substrate for bacteria during labeling experiments, under varying conditions of substrate availability, is discussed. Bacterial secondary production rates ranged from 0.3 μg C l⁻¹ h⁻¹ in winter to values of 10 μg C l⁻¹ h⁻¹ in August, where phytoplanktonic biomass reached the summer maximum, and bacterial biomass was calculated to be renewed 3 times per day. An estimation of the bacterial carbon demand showed that for the major part of the year, with the exception of early spring, the bacterioplankton community in the Kühw?rter Wasser was dependent on carbon sources other than phytoplanktonic primary production. Received: 22 March 1996; Revised: 1 August 1996     

Seasonal Variation of Virioplankton in a Eutrophic Shallow Lake

Lake Donghu is a typical eutrophic freshwater lake in which high abundance of planktonic viruses was recently revealed. In this study, seasonal variation of planktonic viruses were observed at three different trophic sites, hypertrophic, eutrophic, and mesotrophic regions, and the correlation between their abundances and other aquatic environmental components, such as bacterioplankton, chlorophyll a, burst size, pH, dissolved oxygen, and temperature, was analyzed for the period of an year. Virioplankton abundance detected by transmission electron microscope (TEM) ranged from 5.48 × 10⁸ to 2.04 × 10⁹ ml⁻¹ in all the sites throughout the study, and the high abundances and seasonal variations of planktonic viruses were related to the trophic status at the sampled sites in Lake Donghu. Their annual mean abundances were, the highest at the hypertrophic site (1.23×10⁹ ml⁻¹), medium at the eutrophic site (1.19×10⁹ ml⁻¹), and the lowest at the mesotrophic site (1.02×10⁹ ml⁻¹). The VBR (virus-to-bacteria ratio) values were high, ranging from 49 to 56 on average at the three sampled sites. The data suggested that the high viral abundance and high VBR values might be associated with high density of phytoplankton including algae and cyanobacteria in this eutrophic shallow lake, and that planktonic viruses are important members of freshwater ecosystems.     

Factors Affecting Predation by Cyclidium sp. and Euplotes sp. on PAH-Degrading and Nondegrading Bacteria

Abstract If predators select for or against contaminant-degrading bacteria, it will affect bacterial survival and has important implications for bioremediation. Protozoa are important predators of bacteria. In order to determine whether protozoa preyed differentially on bacteria with different degradation abilities, two ciliates (Euplotes sp. and Cyclidium sp.) and three strains of PAH-degrading bacteria (Vibrio spp., degrading naphthalene, anthracene, or phenanthrene) were isolated from sediment from New York/New Jersey Harbor. By manipulating growth conditions, bacterial strains with different PAH-degradation abilities and different cell properties were produced. Stepwise regression models were used to analyze how clearance rates on suspended bacteria and grazing rates on bacteria attached to particles were affected by bacterial size, hydrophobicity, C:N ratio, protein content, and PAH-degradation ability. Clearance rates ranged from 0 to 49 nl ciliate⁻¹ h⁻¹ for Euplotes sp. and from 0 to 1.7 nl ciliate⁻¹ h⁻¹ for Cyclidium sp. Clearance rates of both ciliates were positively correlated with bacterial size, hydrophobicity, and protein content, and negatively correlated with C:N ratio. PAH degradation ability had no (for Euplotes sp.) or small (for Cyclidium sp.) effects on clearance rates. The models accounted for 63–75% of the variation in clearance rates on different bacteria. Only Euplotes sp. grazed on attached bacteria, at rates from 3 to 176 bacteria ciliate⁻¹ h⁻¹. A regression model with only C:N ratio and protein content explained 45% of the variation in grazing rates. These models indicate that multiple properties of bacteria affect their susceptibility to predation by ciliates, but PAH-degradation ability per se has little effect. Received: 5 May 1998; Accepted: 14 September 1998     

Assessing Primary and Bacterial Production Rates in Biofilms on Pebbles in Ishite Stream, Japan

Various measurements of microbial productivity in streambed pebble biofilms were analyzed almost monthly for 1 year to quantify the importance of primary production as an autochthonous source of organic matter utilized to support heterotrophic bacterial production in the dynamic food web within this natural microbial habitat. Bacterial density varied from 0.3 × 10⁸ to 1.4 × 10⁸ cells cm⁻², and chlorophyll a concentration ranged from 0.7 to 25.9 μg cm⁻², with no coupled oscillation between seasonal changes in these two parameters. In bottle incubation experiments, the instantaneous bacterial growth rate of bacteria was significantly correlated with their production rate [measured by frequency of dividing cells (FDC)] as follows: ln μ = 0.138FDC − 3.003 (n = 15, r ² = 0.445, p < 0.001). FDC values in the pebble biofilms increased with fluctuations during the study period, ranging from 3.6% to 9.2%. Bacterial production rates largely fluctuated between 0.15 to 0.92 μg C cm⁻² h⁻¹, and its seasonal pattern was similar to that of bacterial density. Net primary production measured between May 2002 to November 2002 attained minimum level (0.5 μg C cm⁻² h⁻¹) in June and maximum level (1.9 μg C cm⁻² h⁻¹) in August. Percentages of bacterial production to net primary production ranged between 21% and 120%. Because this ratio extends both below and above 100% for these parameters, it is likely that both autochthonous and allochthonous supplies of organic matter are important for production of bacteria in the pebble biofilms that develop in rapidly flowing fresh water streams.     

Short-term variations in the abundance and cell volume of bacterioplankton in an artificial tropical lake

Circadian and spatial fluctuations in bacterioplankton abundance and cell volume were examined, for the first time, in the Municipal Lake located in the down town area of Yaoundé (Capital of Cameroon, Central Africa, ca 3° 52′ N, 11° 31′ E). Bacterial cell volumes (range, 0.05 to 0.2 μm3) were consistent with those reported for other aquatic systems while bacterial densities (0.8 to 2 × 108 cells ml-1) were among the highest values reported in pelagic systems. These variables and chlorophyll a and dissolved oxygen concentrations within a single depth-zone varied from 13 to 61%, while water temperature fluctuated only from 2 to 6%. Spatial fluctuations of physical-chemical and biological variables were generally higher during the day-time than during the night-time. A significant diel variation was provided for bacterial cell volume in the surface waters where synchronized cell division was occurring during the night. The measured bacterial abundances in this study were 4 to 17 fold higher than values known from other lakes of similar trophic status, and both cell abundance and volume were not correlated with chlorophyll. We conclude that this was due to the dependence of bacterial populations to different sources of allochthonous substrates, including untreated sewage from the major influents of the lake, resuspension of benthic material, and substrate releasing from macrophytes which are prevalent in the littoral zone of the lake. This revised version was published online in September 2006 with corrections to the Cover Date.     

Cobalt determination in serum and urine by electrothermal atomic absorption spectrometry

Cobalt determinations in biological fluids are of great interest in biological or toxicological research programs. Cobalturia is often chosen as an indicator for a biological monitoring program in occupational exposure to cobalt dusts. The method described here derives from the IUPAC reference method for nickel determination. It enables cobaltemia and cobalturia to be measured in small samples (1 mL). The mean usual values for cobalt in biological fluids are very low (2.7 nmol L⁻¹ for serum and 6.7 nmol L⁻¹ for urine), and therefore, thus require an analytical procedure with preconcentration and extraction. The sample is mineralized by wet acid digestion. After digestion, inorganic cobalt is extracted in form of ammonium pyrrolidine dithiocarbamate complex into isobutyl methyl ketone and measured in the organic layer by electrothermal atomic absorption spectrometry. The analytical parameters are described in detail. The extraction output is about 99%. The detection limits are 1.93 and 1.89 nmol L⁻¹ for serum and urine, respectively. Sensitivity (expressed as the concentration that gives a 0.044 absorbance) is 3.4 nmol L⁻¹ for serum and 3.3 nmol L⁻¹ for urine. Within-run precision ranged between 3.9 and 2.5% (coefficients of variation) for serum and 4.2 and 1.1% for urine, at 87 and 136 nmol L⁻¹ levels, respectively. Between-run precision ranged between 4.3 and 3.3% (coefficients of variation) for serum and 4.2 and 2.3% for urine, at 87 and 136 nmol L⁻¹ levels, respectively. At very low concentration, 5.7 nmol L⁻¹ for serum and 2.5 nmol L⁻¹ for urine, the between-run precision is, respectively, 19.5 and 28%. Linearity is effective between 0 and 272 nmol L⁻¹. Interferences and matrix effects are negligible for urine, serum, or plasma samples without hemoglobin. The method is easily applicable for routine determinations.