Oligotrophic Lake Lucerne Research Articles

Microbial carbon processing in oligotrophic Lake Lucerne (Switzerland): results of in situ 13C-labelling studies

Although lakes play a major role in the storage of organic carbon, processes involved are not yet very well characterized, especially for oligotrophic lakes. Whether a lake functions as a net source or sink for carbon depends on relative rates of primary production, inputs of terrestrial organic matter and respiration. The microbial community will affect the efficiency of carbon cycling and thereby potential carbon storage. Because the organic matter fluxes are smaller in oligotrophic lakes they have been studied less intensively with respect to their carbon cycling compared to eutrophic lakes. Whether they play an appreciable role in freshwater carbon cycling relies on unraveling primary and secondary production. Here we present the results from such a study in oligotrophic Lake Lucerne, Switzerland. Based on in situ carbon isotopic labelling experiments using dark, glucose-labelled and transparent, DIC-labelled bottles positioned at different depths in the water column, we conclude that even though the photic zone was very deep, integrated primary productivity was consistently low. The carbon processing efficiency of the heterotrophic producers was such that photosynthesized organic matter was fully consumed, even during times of maximum productivity. This implies that the heterotrophic producers were well adapted to rapidly respond to a temporary increase in primary productivity, which is in line with calculated bacterial growth efficiencies in the surface water layer. Highest glucose-based productivity, as a measure of the heterotrophic potential, was observed in the deepest parts of the water column. Chemoautotrophy was shown at 60 m water depth and is of relatively minor importance for overall fluxes. Mixotrophy was recognized as a strategy to keep up production when light conditions become less favorable for autotrophic growth. A mesocosm experiment earlier in the year indicated lower primary production, which agrees well with the timing of this experiment preceding the annual spring bloom. During the low-productivity season the coupling between phytoplankton and bacterial production was much weaker and potentially more organic matter could escape recycling at that time, although quantitatively fluxes remained very low.

Interplay of community dynamics, temperature, and productivity on the hydrogen isotope signatures of lipid biomarkers

Abstract. The hydrogen isotopic composition (δ2H) of lipid biomarkers has diverse applications in the fields of paleoclimatology, biogeochemistry, and microbial community dynamics. Large changes in hydrogen isotope fractionation have been observed among microbes with differing core metabolisms, while environmental factors including temperature and nutrient availability can affect isotope fractionation by photoautotrophs. Much effort has gone into studying these effects under laboratory conditions with single species cultures. Moving beyond controlled environments and quantifying the natural extent of these changes in freshwater lacustrine settings and identifying their causes is essential for robust application of δ2H values of common short-chain fatty acids as a proxy of net community metabolism and of phytoplankton-specific biomarkers as a paleohydrologic proxy. This work targets the effect of community dynamics, temperature, and productivity on 2H∕1H fractionation in lipid biomarkers through a comparative time series in two central Swiss lakes: eutrophic Lake Greifen and oligotrophic Lake Lucerne. Particulate organic matter was collected from surface waters at six time points throughout the spring and summer of 2015, and δ2H values of short-chain fatty acids, as well as chlorophyll-derived phytol and the diatom biomarker brassicasterol, were measured. We paired these measurements with in situ incubations conducted with NaH13CO3, which were used to calculate the production rates of individual lipids in lake surface water. As algal productivity increased from April to June, net discrimination against 2H in Lake Greifen increased by as much as 148 ‰ for individual fatty acids. During the same time period in Lake Lucerne, net discrimination against 2H increased by as much as 58 ‰ for individual fatty acids. A large portion of this signal is likely due to a greater proportion of heterotrophically derived fatty acids in the winter and early spring, which are displaced by more 2H-depleted fatty acids as phytoplankton productivity increases. Smaller increases in 2H discrimination for phytol and brassicasterol suggest that a portion of the signal is due to changes in net photoautotrophic 2H fractionation, which may be caused by increasing temperatures, a shift from maintenance to high growth, or changes in the community assemblage. The fractionation factors for brassicasterol were significantly different between the two lakes, suggesting that its hydrogen isotope composition may be more sensitive to nutrient regime than is the case for fatty acids or phytol.

Seasonal and vertical distribution of putative ammonia-oxidizing thaumarchaeotal communities in an oligotrophic lake

The discovery of Archaea carrying an amoA gene coding for the A-subunit of ammonia monooxygenase gave a boost to studies aimed at detecting this gene under diverse conditions. Despite numerous studies describing the archaeal amoA gene abundance and richness in different habitats, the understanding of the freshwater ecology of potentially archaeal ammonia oxidizers, recently positioned in the phylum Thaumarchaeota, is still lacking. In a seasonal and vertical study of deep oligotrophic Lake Lucerne, Switzerland, with high Thaumarchaeota-specific crenarchaeol concentrations, we showed that all archaeal 16S rRNA genes found belong to the thaumarchaeotal phylum. The abundances of archaeal 16S rRNA and amoA genes remained in the same order of magnitude (average 6×mL(-1) ) and displayed matching seasonal dynamics within 16 monthly collected samples at three different water depths (r(2) =0.80, slope of 1.06). The Thaumarchaeota in this lake form a distinct cluster in both the 16S rRNA and amoA gene phylogenies, are affiliated to other thaumarchaeotal freshwater sequences within group 1.1a Archaea, and fall in the low saline cluster of the amoA phylogeny. In accordance with temperature and conductivity, the Thaumarchaeota in the surface water showed a significantly different seasonality and lower abundance than their counterparts in the deeper waters. This study indicates that the ecology of Thaumarchaeota, with their potential in nitrogen cycling, may differ per water depth in deep freshwater ecosystems.

Occurrence of dissolved and particle-bound taste and odor compounds in Swiss lake waters

The occurrence of algal taste and odor (T&O) compounds was investigated in three Swiss lakes which exhibit different nutrient levels from eutrophic to oligotrophic (Lake Greifensee, Lake Zurich and Lake Lucerne). Apart from dissolved T&O compounds, the study also encompassed particle-bound compounds, i.e., compounds that can be released from damaged algal cells during drinking water treatment. A combined instrumental (SPME–GC–MS) and sensory method was applied that allowed to detect and quantify T&O compounds in natural waters in the sub ppt to low ppt-range. In addition to the prominent T&O compounds geosmin and 2-methyl-isoborneol (MIB), four other T&O compounds could be detected in the lake waters, though all at relatively low concentrations (maximum concentrations of geosmin 19 ng L −1, MIB 3 ng L −1, β-ionone 27 ng L −1, β-cyclocitral 7 ng L −1, 2-isobutyl-3-methoxypyrazine 2 ng L −1, 2-isopropyl-3-methoxypyrazine 16 ng L −1). The concentration peaks typically occurred in the epilimnion during summer concurrent with a high phytoplankton biomass. Consistently, the concentration levels for most of the compounds varied substantially between the three lakes and generally decreased in the order eutrophic Lake Greifensee > mesotrophic Lake Zurich > oligotrophic Lake Lucerne. Furthermore, our data revealed that the occurrence of β-ionone was largely influenced by Planktothrix rubescens. This is the first time that a correlation between β-ionone and this cyanobacterium has been reported for natural waters.

Cycling of calcite in hard water lakes of different trophic states

Based on oxygen (O2), pH, calcium (Ca2+), and carbonate (CO32‐) sediment pore‐water concentration profiles, we compare the benthic dissolution of autochthonous calcite in oligotrophic Lake Lucerne and eutrophic Lake Sempach. Despite their difference in trophic state, the two lakes have similar benthic O2 regimes because L. Sempach is artificially oxygenated. This peculiarity enabled the study of the direct effect of trophic state on benthic calcite dissolution. Although areal benthic O2 fluxes did not differ, O2 penetrated deeper into the sediment of the oligotrophic L. Lucerne, where most organic matter (OM, 96%) was degraded aerobically, whereas in L. Sempach, aerobic and anaerobic decomposition contributed about equal amounts to the biogenic carbon dioxide production. Release rates of bicarbonate from L. Sempach sediments exceeded those of L. Lucerne nearly twofold. The HCO3− : Ca2+ ratio of benthic fluxes was 3.7 in L. Lucerne compared to 10.3 in L. Sempach, suggesting that in L. Sempach most of the released HCO3− did not originate from calcite dissolution. Furthermore, benthic calcite dissolution in L. Lucerne exceeded that in L. Sempach by two‐fold, despite lower pH values in the pore water of L. Sempach. Differences in benthic microbial decomposition of OM and redissolution of calcite crystals are explained by the longer residence time of OM and calcite in the oxic sediment layer of the oligotrophic lake and the larger weight‐specific surface area of its smaller autochthonous calcite crystals. We suggest that trophic state and O2 supply to the sediment are key parameters controlling the cycling of calcite and organic carbon in lakes.

The Role of Autotrophic Picocyanobacteria in Calcite Precipitation in an Oligotrophic Lake

A 1-year field study monitoring depth profiles of picoplankton and physicochemical data in the oligotrophic Lake Lucerne (Switzerland) showed that picocyanobacteria play an important role in the CaCO3 precipitation process. Laboratory experiments with Mychonastes and Chlorella, isolated from Lake Lucerne and Synechococcus using ion selective electrodes, scanning electron microscopy and X-ray powder diffraction clearly demonstrated the potential of picoplankton for fast and effective CaCO3 precipitation. The combination of a field study with laboratory experiments confirmed the previous reports of picocyanobacteria triggering the CaCO3 precipitation in hardwater oligotrophic lakes. Electron micrographs of particles from the water column often reveal the size and shape of picoplankton cells covered by calcite. In addition the results from the laboratory approach indicated that algae and bacteria induced different precipitation mechanisms. Experiments with Mychonastes and Chlorella produced crystalline calcite completely covering the cells. Experiments with the cyanobacteria Synechococcus, however, yielded amorphous, micritic CaCO3, indicating a different precipitation process.

Limnological description of the Lakes Zürich, Lucerne, and Cadagno

This introductory article of the special GAP issue gives an overview on general limnological characteristics of the prealpine Lakes Zürich and Lucerne and the alpine Lake Cadagno and reports on the specific situation of primary production parameters during the international GAP Workshop in mid September 1999. Furthermore, it describes methods used for water analysis and fieldwork in these lakes.¶A comparison of data related to primary production in the three lakes in September 1999 during stratification shows that (i) phytoplankton community structure varied considerably between the lakes. The dominating algae were Planktothrix rubescens in Lake Zürich, various chrysophytes and diatoms in Lake Lucerne, and Echinocoleum elegans in Lake Cadagno, (ii) the euphotic zone in Lake Lucerne was considerably deeper (app. 15m) than in the other two lakes (app. 10 m), (iii) chlorophyll a standing crop was highest in mesotrophic Lake Zürich (August: 121 mg m-2), followed by oligotrophic Lake Lucerne (August: 75, September: 34 mg m-2) and mesotrophic Lake Cadagno (August: 33, September: 25 and 14 mg m-2), and (iv) areal primary production was highest in Lake Zürich (August: 105, September: 124 mg C m-2 h-1), followed by Lake Cadagno (August: 102, September: 52 mg C m-2 h-1) and Lake Lucerne (August: 90, September: 52 mg C m-2 h-1). Physiological parameters, determined in situ from P versus I relationships, showed a lower initial slope α in Lake Lucerne (August: 0.03, September: 0.02 mg C mg-1 chl a h-1μmol-1 m2 s) than in the other two lakes (Lake Zürich in August: 0.05, in September: 0.11; Lake Cadagno in August: 0.05, in September: 0.11 and 0.28 mg C mg-1 chl a h-1μmol-1 m2 s). Lake Zürich showed the lowest AN max (August: 2.6, September: 3.2 mg C mg-1 chl a h-1, as compared to 5.9 - 7.4 mg C mg-1 chl a h-1 in the Lakes Lucerne and Cadagno), while in Lake Cadagno the highest inhibitory effects of C-assimilation were found (highest slopes of inhibition β, 0.007-0.011, as compared to 0.0003-0.0026 in the other two lakes), due to a higher UV-exposure in this alpine lake.

Diel and seasonal patterns of food intake and prey selection by Coregonus sp. in re‐oligotrophicated Lake Lucerne, Switzerland

Feeding intensity by whitefish Coregonus sp., in oligotrophic Lake Lucerne in Switzerland was high during dusk when the bulk of potential prey items were in the depth zone occupied by the fish. Diet composition was fairly uniform throughout the day but changed substantially over the seasons. The fish fed opportunistically; differences between seasons reflected changes in prey availability. During the intensive feeding and growing period (May‐September), fish were found in the upper 20 m of the lake feeding primarily on cladocerans. Large and non‐evasive species, Daphnia spp. and Bythotrephes longimanus, were the most numerous and frequent organisms in the diet during the major part of the growing season. Smaller (Bosmina spp.) as well as evasive species (cyclopoid copepods) were consumed in large numbers when larger, non‐evasive species were rare in the lake. The fish showed strong preference for the least abundant crustacean, B. longimanus, while the most abundant crustaceans, calanoid copepods, were rare in the diet. The fish not only selected particular species but, within each species, selected the larger individuals. Diel vertical migration of the prey items in this lake could be, at least in part, attributed to fish predation pressure. The observed selectivity patterns shown by the fish are explained in terms of prey visibility, escape ability, the overlap in distribution of predators and prey in time and space, the profitability of the prey and the present trophic state of the lake.

Measurements and significance of bio-optical parameters for remote sensing in two subalpine lakes of different trophic state

Bio-optical measurements were carried out on eutrophic Lake Zug and oligotrophic Lake Lucerne, to provide data and models for thein situ calibration of multispectral imagery collected in 1991, during an AVIRIS flight over Central Switzerland. The results indicate that Secchi depth, chlorophyll and total suspended solid concentrations, vertical extinction coefficient, absorption coefficient and irradiance reflectance in the PAR region can be used to discriminate between the two lakes. Dissolved organics concentration, scattering and total attenuation coefficients estimates, as well as backscattering probability, are less acceptable, because of instrumental limitations. Relationships between optical and water quality parameters were investigated and found to behave according to accepted theoretical frameworks. Both lakes exhibited vertical and horizontal heterogeneities in chlorophyll and turbidity, and had contrasting mean bio-optical characteristics: although suspended solid concentrations were similar, transparency was lower in Lake Zug, but reflectance was substantially higher in Lake Lucerne. Water colour determined by chromaticity analysis was blue-green in Lake Lucerne and green in Lake Zug. Reflectance spectra simulated through a three-component optochemical model did not completely match those derived fromin situ measurements, because of lack of site-specific optical cross-sections for suspended minerals and dissolved organics. The monitoring of water quality in subalpine lakes by remote sensing, both with present and future technology, is discussed and considered as possible, provided that their optical behaviour is known, and preferably expressed by a valid optochemical model.