Lake Vechten Research Articles

Succession of bacteria and archaea involved in the nitrogen cycle of a seasonally stratified lake.

Human-driven changes affect nutrient inputs, oxygen solubility, and the hydrodynamics of lakes, which affect biogeochemical cycles mediated by microbial communities. However, information on the succession of microbes involved in nitrogen cycling in seasonally stratified lakes is still incomplete. Here, we investigated the succession of nitrogen-transforming microorganisms in Lake Vechten over a period of 19 months, combining 16S rRNA gene amplicon sequencing and quantification of functional genes. Ammonia-oxidizing archaea (AOA) and bacteria (AOB) and anammox bacteria were abundant in the sediment during winter, accompanied by nitrate in the water column. Nitrogen-fixing bacteria and denitrifying bacteria emerged in the water column in spring when nitrate was gradually depleted. Denitrifying bacteria containing nirS genes were exclusively present in the anoxic hypolimnion. During summer stratification, abundances of AOA, AOB, and anammox bacteria decreased sharply in the sediment, and ammonium accumulated in hypolimnion. After lake mixing during fall turnover, abundances of AOA, AOB, and anammox bacteria increased and ammonium was oxidized to nitrate. Hence, nitrogen-transforming microorganisms in Lake Vechten displayed a pronounced seasonal succession, which was strongly determined by the seasonal stratification pattern. These results imply that changes in stratification and vertical mixing induced by global warming are likely to alter the nitrogen cycle of seasonally stratified lakes.

Spatio-temporal dynamics of sulfur bacteria during oxic--anoxic regime shifts in a seasonally stratified lake.

ABSTRACTSulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria drive major transformations in the sulfur cycle, and play vital roles in oxic--anoxic transitions in lakes and coastal waters. However, information on the succession of these sulfur bacteria in seasonally stratified lakes using molecular biological techniques is scarce. Here, we used 16S rRNA gene amplicon sequencing to study the spatio-temporal dynamics of sulfur bacteria during oxic--anoxic regime shifts in Lake Vechten. Oxygen and sulfate were mixed throughout the water column in winter and early spring. Meanwhile, SRB, green sulfur bacteria (GSB), purple sulfur bacteria (PSB), and colorless sulfur bacteria (CSB) exclusively inhabited the sediment. After the water column stratified, oxygen and nitrate concentrations decreased in the hypolimnion and various SRB species expanded into the anoxic hypolimnion. Consequently, sulfate was reduced to sulfide, stimulating the growth of PSB and GSB in the metalimnion and hypolimnion during summer stratification. When hypoxia spread throughout the water column during fall turnover, SRB and GSB vanished from the water column, whereas CSB (mainly Arcobacter) and PSB (Lamprocystis) became dominant and oxidized the accumulated sulfide under micro-aerobic conditions. Our results support the view that, once ecosystems have become anoxic and sulfidic, a large oxygen influx is needed to overcome the anaerobic sulfur cycle and bring the ecosystems back into their oxic state.

New information on the geographical distribution of Piscicola brylinskae Bielecki, 2001 (Hirudinea: Piscicolidae) in Poland and remarks on its systematic position

For the first time, Piscicola brylinskae was described from Lake Vechten in the village of Bunnik, near Utrecht — The Netherlands. Until now, P. brylinskae has been found in Poland in Lake Maroz and in the Łyna River near Olsztyn (the northern part of Warmian-Masurian voivodeship). Thanks to proper conservation the coloration of P. brylinskae was described for the first time. Applying 32 indexes resulted that P. brylinskae clustered to Caspiobdella fadejewi. Analysis based on 113 non-metric characteristics has shown that P. brylinskae is most similar to Piscicola margaritae. Though, P. brylinskae do not form with P. margaritae dichotomic branching nor with any other species from this cluster, it could confirm that it is an individual species of the Piscicola genus.

Evidence for Isotopic Exchange during Metabolism of Stable-Isotope-Labeled Formate in a Methanogenic Sediment

The disappearance of stable-isotope-labeled formate in freshwater sediment from Lake Vechten was studied by gas chromatography-mass spectrometry. Both deuterium- and (sup13)C-labeled formate were removed from pore water at a high rate (400 (plusmn) 35 [standard deviation] and 1,700 (plusmn) 600 (mu)M h(sup-1), respectively), with concomitant formation of nonlabeled formate. The removal rate of labeled and nonlabeled formate amounted to 240 (plusmn) 15 and 400 (plusmn) 40 (mu)M h(sup-1), respectively. The formation of nonlabeled formate is explained by isotopic exchange due to the activity of formate dehydrogenase rather than by the turnover of formate.

Comparison of acetate turnover in methanogenic and sulfate-reducing sediments by radiolabeling and stable isotope labeling and by use of specific inhibitors: evidence for isotopic exchange

Acetate turnover in the methanogenic freshwater anoxic sediments of Lake Vechten, The Netherlands, and in anoxic sediments from the Tamar Estuary, United Kingdom, and the Grosser Jasmunder Bodden, Germany, the latter two dominated by sulfate reduction, was determined. Stable isotopes and radioisotopes, inhibitors (chloroform and fluoroacetate), and methane flux were used to provide independent estimates of acetate turnover. Pore water acetate pool sizes were determined by gas chromatography with a flame ionization detector, and stable isotope-labeled acetate was determined by gas chromatography-mass spectrometry. The appearance of acetates with a different isotope labeling pattern from that initially added demonstrated that isotopic exchange occurred during methanogenic acetate metabolism. The predominant exchange processes were (i) D-H exchange in the methyl group and (ii) (sup13)C-(sup12)C exchange at the carboxyl carbon. These exchanges are most probably caused by the activity of the enzyme complex carbon monoxide dehydrogenase and subsequent methyl group dehydrogenation by tetrahydromethanopterine or a related enzyme. The methyl carbon was not subject to exchange during transformation to methane, and hence acetate with the methyl carbon labeled will provide the most reliable estimate of acetate turnover to methane. Acetate turnover rate estimates with these labels were consistent with independent estimates of acetate turnover (acetate accumulation after inhibition and methane flux). Turnover rates from either radioisotope- or stable isotope-labeled methyl carbon isotopes are, however, dependent on accurate determination of the acetate pool size. The additions of large amounts of stable isotope-labeled acetate elevate the acetate pool size, stimulating acetate consumption and causing deviation from steady-state kinetics. This can, however, be overcome by the application of a non-steady-state model. Isotopic exchange in sediments dominated by sulfate reduction was minimal.

Methane oxidation at the sediment-water interface of shallow eutrophic Lake Loosdrecht and deep meso-eutrophic Lake Vechten

Methane (CH4) oxidation and CH4 release into the water column were determined at the sediment-water interface of a shallow eutrophic lake (Lake Loosdrecht) and a deep meso-eutrophic lake (Lake Vech...

Zooplankton as a compound mineralising and synthesizing system: phosphorus excretion

Data on phosphate excretion rates of zooplankton are based on measurements using the pelagic crustacean zooplankton of Lake Vechten and laboratory-cultured Daphnia galeata. In case of Daphnia sp we measured the effects of feeding on P-rich algae and P-poor algae (Scenedesmus) as food on the P-excretion rates at 20°C. The excretion rates of the natural zooplankton community, irrespective of the influence of the factors mentioned, varied by an order of magnitude: 0.025–0.275µg PO4-Pmg−1C in zooplankton (C zp ) h−1. The temperature accounted for about half the observed variation in excretion rates. The mean excretion rates in the lake, computed for 20°C, varied between 0.141 and 0.260 µg Pmg−1C zp h−1. Based on data of zooplankton biomass in the lake the P-regeneration rates by zooplankton covered between 22 and 239% of the P-demand of phytoplankton during the different months of the study period. In D. galeata, whereas the C/P ratios of the Scenedesmus used as food differed by a factor 5 in the experiments, the excretion rates differed by factor 3 only. Despite the higher P-excretion rates (0.258± 0.022 µg PO4-P mg−1 C h−1) of the daphnids fed with P-rich food than those fed with P-poor food (0.105 ± 0.047 µg PO4-P mg−1 C hp−1), both the categories of the animals were apparently conserving P. A survey of the literature on zooplankton excretion shows that in Daphnia the excretion rates vary by a factor 30, irrespective of the species and size of animals and method of estimation and temperature used. About two-thirds of this variation can be explained by size and temperature. A major problem of comparability of studies on P-regeneration by zooplankton relates to the existing techniques of P determination, which necessitates concentrating the animals several times above the in situ concentration (crowding) and prolonged experimental duration (starving), both of which manifest in marked changes that probably lead to underestimation of the ‘real’ rates.

A dynamic simulation model for the blooming of Oscillatoria agardhii in a monomictic lake

The occurrence of phytoplankton blooms is a common problem in water bodies. The quantitative description of the algal population growth is of primary importance to understand the mechanisms which lead to these phytoplankton blooms, hence also for the development of a successful water-quality management. Due to the high degree of complexity of phytoplankton blooms, dynamic simulation models may be a powerful tool to study and elucidate the mechanisms leading to these blooms. In the present study, a simulation model for the blooming of a natural population of Oscillatoria agardhii in Lake Vechten (lat. 52°04′N, long. 5°05′E, the Netherlands) using measured temperature, filament length and nutrient concentrations as external drives, is presented. The lake was sampled weekly from January to the middle of May, 1992. Temperature, nutrient concentration (nitrate and phosphate in the water) and filament length were recorded. Because cell size was constant, filament length could be converted to number of cells per litre to estimate algal biomass. To implement the model additional information, e.g. some algal growth parameters, was obtained from the literature. The model simulated well the growth of the Oscillatoria agardhii population in the lake during the research period. According to the model, temperature was the most important rate-limitating factor for growth at the beginning of the year. Nutrient limitation became increasingly important while the importance of temperature limitation decreased during the season. During the whole period, light was strongly limiting and became the most important rate-limiting factor in the second week of April. Via sensitivity analysis, the model showed to be highly affected by temperature-related factors, and it was useful in the identification of research needs.

Algal and bacterial pigments in non-laminated lacustrine sediment: Studies of their sedimentation, degradation and stratigraphy

Natural phyto- and bacterioplankton populations of Lake Vechten (the Netherlands) were subjected to darkness under oxic and anoxic conditions at in situ temperatures in order to test the stability in time of their photosynthetic pigments. Furthermore, sedimentary fluxes and concentrations of pigments were estimated in (respectively) sediment trap catches and at the sediment-water interphase in order to measure the pigment breakdown upon burial into the sediment. The chlorophylls and most of the xanthophylls showed substantial losses of 20% to 60% in the incubation experiments as well as in the surficial sediment. β-Carotene, okenone and echinenone were most stable (2–10% losses); fucoxanthin and peridinin were degraded extensively; alloxanthin and zeaxanthin held an intermediate position as did the bacteriochlorophylls. Trends in sediment profiles of pigments were compared with limnological data obtained during the enhanced eutrophication of the lake. Evidence was provided that the β-carotene profile closely followed the increase of phytoplankton biomass. Although susceptible to substantial degradation, several profiles of pigments and of pigment ratios could be related in a qualitative way to biomass and to shifts in species composition which occurred as a result of the changing ecological conditions in the lake.

Kinetic studies of acetate in freshwater sediments: Use of stable isotopic tracers

The kinetics of acetate uptake in the sediment of Lake Vechten, a meso-eutrophic, monomictic freshwater lake, was investigated by monitoring the disappearance of deuterated acetate tracers. Steep acetate concentration gradients were observed in the top cm when the sediment was analyzed at the millimeter scale. The gradients revealed not only the dynamic nature of the acetate pools but also indicated substantial mass transport between sediment layers. Acetate uptake could conveniently be modeled using first order reaction kinetics. Uptake rates ranged from 30 μM day −1 (winter) to 546 μM day −1 (early spring) in the sulfidogenic horizon (2 cm) and from 30 μM day −1 (winter) to 1176 μM day −1 (late summer) in the methanogenic horizon (5 cm). The integrated acetate uptake rate in the top 7 cm was 39.8 ± 20 (2 × SE) mmol acetate m −2 day −1 during early spring. More than 40% of the total acetate pool was reversibly adsorbed in oxic sediment slurries. The presence of hydrous Fe(III) oxides appeared to determine the occurrence of acetate adsorption.

Oxygen-consuming processes at the profundal and littoral sediment-water interface of a small meso-eutrophic lake (Lake Vechten, The Netherlands)

Oxygen consumption in the profundal gyttja and littoral sands of Lake Vechten was determined with pore-water analyses and core incubations. Methane (58–64%), ammonium (10–13%), iron (2–6%), and sulfide (<12%) oxidation accounted for ∼75% of the oxygen consumed in the profundal sediments. Almost the complete diffusive flux of CH4 out of the anoxic layer into the oxic surface layer (7.9–9.4 mmol m‒2 d‒1) was oxidized, while only 27–36% of the NH4+ flux (5.1–5.2 mmol m‒2d‒1) was oxidized in the oxic surface layer and 64–73% diffused into the overlying water. In contrast to the profundal sediments, oxidation of reduced end products of anoxic respiration was of minor importance (<15%) in littoral sandy sediments. Also, sediment oxygen consumption rate was higher in the profundal gyttja (29 mmol m‒2 d‒1 at 7°C) during overturn than in the littoral sandy sediments in both winter (8.7 mmol m‒2 d‒1 at 7°C) and summer (18.6 mmol m‒2 d‒1 at 22°C). Poisoning the sediment cores with formaldehyde stopped bacterial activity, but oxygen profile measurements showed that this treatment does not reveal the bacterial or chemical nature of the oxidative processes.

DNA Synthesis and Tritiated Thymidine Incorporation by Heterotrophic Freshwater Bacteria in Continuous Culture

Continuous cultivation of heterotrophic freshwater bacteria was used to assess the relationship between DNA synthesis and tritiated thymidine incorporation. The bacteria were grown on a yeast extract medium with generation times of 0.25 to 3.7 days. In six different continuous cultures, each inoculated with a grazer-free mixed bacterial sample from Lake Vechten (The Netherlands), tritiated thymidine incorporation into a cold trichloroacetic acid precipitate and bacterial cell production were measured simultaneously. Empirical conversion factors were determined by division of both parameters. They ranged from 0.25 x 10 to 1.31 x 10 cells mol of tritiated thymidine (mean, 0.60 x 10 cells mol of tritiated thymidine). In addition, DNA concentrations were measured by fluorometry with Hoechst 33258. The validity of this technique was confirmed. Down to a generation time of 0.67 day, bacterial DNA content showed little variation, with values of 3.8 to 4.9 fg of DNA cell. Theoretical conversion factors, which can be derived from DNA content under several assumptions, were between 0.26 x 10 and 0.34 x 10 cells mol of thymidine (mean, 0.30 x 10 cells mol of thymidine). Isotope dilution was considered the main factor in the observed discrepancy between the conversion factors. In all experiments, a tritiated thymidine concentration of 20 nM was used. Control experiments indicated maximum incorporation at this concentration. It was therefore concluded that the observed difference resulted from intracellular isotope dilution which cannot be detected by current techniques for isotope dilution analysis.

Studies on decomposition ofCeratophyllum demersum litter under laboratory and field conditions: losses of dry mass and nutrients, qualitative changes in organic compounds and consequences for ambient water and sediments

A study was made of decomposition ofCeratophyllum demersum litter over a 17-day period under controlled conditions of temperature and oxygen (5, 10 and 18 °C; aerobic and anaerobic) and over a 169-day period in the field (Lake Vechten, The Netherlands). Litter, water and sediment were sampled on the 0, 2, 4, 7 and 17th day under controlled conditions and on the 0, 17, 49, 127 and 169th day in the field. The litter was analyzed quantitatively for dry mass, ash, carbon, nitrogen, phosphorus and qualitatively of organic composition by pyrolysis mass spectrometry. The water was analyzed for the elemental concentrations of organic carbon (total and dissolved), nitrogen (total, ammonia and particulate) and phosphorus (total and orthophosphate) and for the concentrations of photosynthetic pigments and bacteria. The sediment was analyzed for the elemental concentrations of nitrogen, carbon and phosphorus, and for bacterial numbers.

Pigment stratigraphy and trophic status: An evaluation of radionuclide‐dated lacustrine sediment

Abstract Sediment cores from the profundal region of relatively young (ca. 45 years), warm‐monomictic Lake Vechten were dated with Cs and 210Pb and analyzed for major carotenoids, chlorophyll, and pheophytin. Vertical sediment accretion rates determined from a clay/sand horizon and from the radionuclide datings varied between 0.60 and 0.74 cm/year. Sedimentation rates based on paniculate matter collected in sediment traps agreed with results of the 137Cs method with average values of respectively 2.9 and 2.3 kg dry weight m‐2 year‐1. It was concluded that the profundal sediment is fairly undisturbed. Pigments showed a severalfold increase from the deepest to the superficial sediment layers. Their profiles were compared with limnological data obtained during previous studies of Lake Vechten. Evidence was provided that the distribution of pigments reflected grossly the trophic history of the lake, which became more eutrophic during the last two decades. Pigment analyses of sediment cores may be a useful too...

Protozoan Grazing and Bacterial Production in Stratified Lake Vechten Estimated with Fluorescently Labeled Bacteria and by Thymidine Incorporation

In stratified Lake Vechten, The Netherlands, protozoan grazing was estimated on the basis of uptake of fluorescently labeled bacteria and compared with bacterial production estimated on the basis of thymidine incorporation. By using a grazer-free mixed bacterial population from the lake in continuous culture, an empirical relationship between cell production and thymidine incorporation was established. Thymidine incorporation into total cold-trichloroacetic-acid-insoluble macromolecules yielded a relatively constant empirical conversion factor of ca. 10 (range, 0.38 x 10 to 1.42 x 10) bacteria mol of thymidine at specific growth rates (mu) ranging from 0.007 to 0.116 h. Although thymidine incorporation has been assumed to measure DNA synthesis thymidine incorporation appeared to underestimate the independently measured bacterial DNA synthesis by at least 1.5- to 13-fold, even if all incorporated label was assumed to be in DNA. However, incorporation into DNA was found to be insignificant as measured by conventional acid-base hydrolysis. Methodological problems of the thymidine technique are discussed. Like the cultures, Lake Vechten bacteria showed considerable thymidine incorporation into total macromolecules, but no significant incorporation into DNA was found by acid-base hydrolysis. This applied not only to the low-oxygen hypo- and metalimnion but also to the aerobic epilimnion. Thus, the established empirical conversion factor for thymidine incorporation into total macromolecules was used to estimate bacterial production. Maximum production rates (141 x 10 bacteria liter h; mu, 0.012 h) were found in the metalimnion and were 1 order of magnitude higher than in the epi- and hypolimnion. In all three strata, the estimated bacterial production was roughly balanced by the estimated protozoan grazing. Heterotrophic nanoflagellates were the major consumers of the bacterial production and showed maximum numbers (up to 40 x 10 heterotrophic nanoflagellates liter) in the microaerobic metalimnion.

Determination of total sulfur in freshwater sediments by ion chromatography

A method to determine total sulfur in freshwater sediments has been developed. This method involves oxidation of the sediment by a half fusion reaction with a Na 2CO 3/KNO 3 mixture, purification with cation exchange chromatography and detection of the released sulfate by indirect photometric chromatography. Total sulfur estimates from the sediment of Lake Vechten were similar to those obtained by induced coupled plasma spectrometry ( r = 0.998, n = 35) and roentgen-fluorescence spectrometry. The method gives reliable results with difficult-to-oxidize RSR bonds such as those present in methionine. The detection of sulfate by the sensitive technique of ion-chromatography (detection limit 5 pmol) enables sulfur analysis in μg quantities. The method can be combined with trace determination of radioactive 35S. Total sulfur in the 47-year old sediment of Lake Vechten varied between 1100 and 1600 mg 1 −1 wet sediment, representing approx. 0.4% of the dry weight. The total sulfur pool is comprised of 3% elemental sulfur, 8% pyrite, 60% acid volatile sulfide and 12% ester sulfates, leaving 17% non-identified. From a comparison of total sulfur estimates in the 28 cm thick sedimented layer (125 g S m −2), sulfate reduction rates (11.4 g S m −2 year −1) and sedimentation rates (3.21 g S m −2 day −1), it appears that most of the sulfur reduced during the year is re-oxidized.

Microscale vertical distribution of algal and bacterial plankton in Lake Vechten (The Netherlands)

Depth profiles of horizontal light-beam attenuation and in situ fluorescence recorded at 1 to 2 cm depth intervals in Lake Vechten contained pronounced maxima related to phytoplankton and bacterioplankton layers. This was confirmed by microscopy and HPLC analysis of pigments in discrete samples. Algae were sparse throughout the oxic water column compared with green and purple sulphur bacteria in the anoxic zone of the lake, but plankton layers comprising diatoms, green algae, euglenophytes and cryptomonads were observed. Maximum concentrations of phototrophic bacteria, with dominant representatives Synechococcus, Thiopedia, Chloronema and Chlorobium, occurred very close to one another and within the anoxic zone at depths of pigment maxima and major fluorescence peaks. Plankton layers affected downwelling irradiance in two ways: first, by markedly increasing overall light extinction in the vicinity of the layers, e.g. between 4 m and 8 m the minimum wavelength-specific extinction coefficient, which was at 580 nm ( k 580) increased from 0.5 m −1 to 1.5 m −1; secondly, changes in the shape of extinction ( k) spectra corresponded to in vivo absorbance characteristics of photosynthetic pigments of plankton in the layers. It is inferred that field microbial ecological studies may greatly benefit from simultaneous use of in situ optical techniques.

Microscale vertical distribution of algal and bacterial plankton in Lake Vechten (The Netherlands)

Depth profiles of horizontal light-beam attenuation and in situ fluorescence recorded at 1 to 2 cm depth intervals in Lake Vechten contained pronounced maxima related to phytoplankton and bacterioplankton layers. This was confirmed by microscopy and HPLC analysis of pigments in discrete samples. Algae were sparse throughout the oxic water column compared with green and purple sulphur bacteria in the anoxic zone of the lake, but plankton layers comprising diatoms, green algae, euglenophytes and cryptomonads were observed. Maximum concentrations of phototrophic bacteria, with dominant representatives Synechococcus, Thiopedia, Chloronema and Chlorobium, occurred very close to one another and within the anoxic zone at depths of pigment maxima and major fluorescence peaks. Plankton layers affected downwelling irradiance in two ways: first, by markedly increasing overall light extinction in the vicinity of the layers, e.g. between 4 m and 8 m the minimum wavelength-specific extinction coefficient, which was at 580 nm (k580) increased from 0.5 m−1 to 1.5 m−1; secondly, changes in the shape of extinction (k) spectra corresponded to in vivo absorbance characteristics of photosynthetic pigments of plankton in the layers. It is inferred that field microbial ecological studies may greatly benefit from simultaneous use of in situ optical techniques.

Microelectrode Measurements of Nitrate Gradients in the Littoral and Profundal Sediments of a Meso-Eutrophic Lake (Lake Vechten, The Netherlands)

NO(3) concentration profiles were measured in the sediments of a meso-eutrophic lake with a newly developed microelectrode. The depth of penetration of NO(3) varied from only 1.3 mm in organic-rich profundal silty sediments to 5 mm in organic-poor littoral sandy sediments. The thickness of the zone of denitrification in the organic-rich sediments was <500 mum. Oxygen profiles measured simultaneously revealed that the zone of denitrification was directly adjacent to the aerobic zone. The results demonstrate high denitrification rates (0.26 to 1.31 mmol m day) at in situ nitrate concentrations in the overlying water (0.030 mmol liter) and limitation of denitrification by nitrate availability.

Instar-dependent mortality rates of coexisting Daphnia species in Lake Vechten, The Netherlands

Instar-dependent mortality rates of coexisting <i>Daphnia</i> species in Lake Vechten, The Netherlands