Mechanisms regulating zooplankton populations in a high-mountain lake
1. We studied the seasonal succession of phyto- and zooplankton and the potential impact of predation by salmonids on zooplankton population dynamics in a high-mountain Swiss lake.<br/>
2. A comparison of patterns in the abundance, body length, fecundity and age structure in the <I>Daphnia galeata</I> population strongly suggests that trout predation had little impact on the population and was not the cause for a decline in summer.<br/>
3. The dominance in the lake of adult trout that feed mainly on benthic prey may buffer the effect of predation on the larger zooplankton. Further, the relatively high amount of phytoplankton after spring thaw could be important for sustaining the <I>Daphnia</I> population under moderate fish predation.<br/>
4. Partial correlation analyses proved circumstantial evidence for both exploitative and interference competition between some zooplankton taxa. <I>D. galeata</I> depressed performance of other plankton species through exploitative competition.<br/>
5. Our study shows that the impact of fish on zooplankton in high-mountain lakes depends strongly on food web structure and trophic state of the lake. Where fish predation is weak, invertebrate predation combined with competition for food may be responsible for the dominance of large-bodied zooplankton species.
On the cost of vertical migration: are feeding conditions really worse at greater depths?
1. The ultimate explanation for diel vertical migration (DVM) of zooplankton is the avoidance of visual predation in surface waters. Studies on migrating zooplankton have shown that remaining in the cold and food-poor hypolimnion during the day, however, has demographic costs. Higher temperatures and greater food concentrations in the surface waters are thought to be the main reasons why <I>Daphnia</I> species move upwards at night.<br/>
2. In this study, we investigated the growth condition of daphniids raised on seston taken from different depths from a lake with and without a deep-water chlorophyll maximum.<br/>
3. Juvenile growth rates of <I>Daphnia galeata x hyalina</I> from the lake without a deep-water chlorophyll maximum were similar for all treatments. After temperature correction, however, growth rates were significantly higher on seston taken from the surface layers.<br/>
4. In contrast, in the lake with the deep-water chlorophyll maximum, <I>D. galeata</I> growth rates were higher in deeper strata, even after temperature correction. Although this lake had a weak temperature gradient, <I>D. galeata</I> left the food-rich strata at night and migrated into the surface food-poor environment. Invertebrate predation and oxygen depletion are probably not the reasons for the nocturnal upward migration into the surface strata. Therefore, we assume that <I>D. galeata</I> migrates upwards to take advantage of higher temperatures. Using several temperature–egg-development models, we could not, however, fully explain this behaviour.<br/>
Trinkwasser in guter Qualität ist in der Schweiz eine Selbstverständlichkeit.
Dass Leitungswasser ohne Bedenken getrunken und sogar genossen
werden kann, ist schon in Nachbarländern nicht immer garantiert. Ein Blick
auf ein alltägliches Wunder und wie es zustande kommt.
Estimating the precipitation potential in urine-collecting systems
Precipitation in urine-separating toilets (NoMix toilets) and waterless urinals causes severe maintenance problems and can strongly reduce the content of soluble phosphate. In this study, we present a computer model for estimating the precipitation potential (PP) in urine-collecting systems. Calculating the PP enables to predict the composition and mass concentration of precipitates. We used our computer model for investigating how urea hydrolysis and dilution with flushing water affect precipitation. In a previous study, we found that microbial urea hydrolysis (ureolysis) triggers precipitation and that the amount of precipitates is limited by calcium and magnesium. With the present simulations, we could confirm these findings. We determined that only a small fraction of urea has to be hydrolysed for reaching 95% of the maximum PP. Since urease-positive bacteria are abundant in urine-collecting systems, strong precipitation is very likely. In further simulations, we determined that struvite (MgNH<sub>4</sub>PO<sub>4</sub>·6H<sub>2</sub>O) and hydroxyapatite (HAP, Ca<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub>) are the main precipitate compounds. If urine is highly diluted with tapwater, calcite (CaCO<sub>3</sub>) occurs as well. HAP is the only calcium phosphate mineral, although several others were supersaturated. Additionally, the simulations indicated that urine dilution diminishes the risk of blockages, since the mass concentration of precipitates decreases with the volume of flushing water. Rainwater flushing is more effective than flushing with tapwater. Moreover, flushing with tapwater leads to high phosphate fixation, because the total amount of calcium and magnesium ions increases, while the total amount of phosphate keeps constant. Finally, we compared simulation results with field measurements and found good agreement at low and very high urine dilution.
Urea hydrolysis and precipitation dynamics in a urine-collecting system
Blockages caused by inorganic precipitates are a major problem of urine-collecting systems. The trigger of precipitation is the hydrolysis of urea by bacterial urease. While the maximum amount of precipitates, i.e. the precipitation potential, can be estimated with equilibrium calculations, little is known about the dynamics of ureolysis and precipitation. To gain insight in these processes, we performed batch experiments with precipitated solids and stored urine from a urine-collecting system and later simulated the results with a computer model. We found that urease-active bacteria mainly grow in the pipes and are flushed into the collection tank. Both, bacteria and free urease, hydrolyse urea. Only few days are necessary for complete urea depletion in the collection tank. Two experiments with precipitated solids from the pipes showed that precipitation sets in soon after ureolysis has started. At the end of the experiments, 11% and 24% of urea was hydrolysed while the mass concentration of newly formed precipitates already corresponded to 87% and 97% of the precipitation potential, respectively. We could simulate ureolysis and precipitation with a computer model based on the surface dislocation approach. The simulations showed that struvite and octacalcium phosphate (OCP) are the precipitating minerals. While struvite precipitates already at low supersaturation, OCP precipitation starts not until a high level of supersaturation is reached. Since measurements and computer simulations show that hydroxyapatite (HAP) is the final calcium phosphate mineral in urine solutions, OCP is only a precursor phase which slowly transforms into HAP.
Interaction of phenolic uncouplers in binary mixtures: concentration-additive and synergistic effects
The uncoupling activities of 14 binary mixtures of substituted phenols and of 4 binary mixtures of phenols and anisols were investigated at different pH values. Experiments were performed with time-resolved spectroscopy on membrane vesicles (chromatophores) of the photosynthetic bacteria <I>Rhodobacter sphaeroides</I>. Phenols are known to destroy the electrochemical proton gradient in energy-transducing membranes by a protonophoric mechanism. Anisols do not have protonophoric activity but disturb membrane structure and functioning as a nonspecific baseline toxicant. It was postulated in the literature that, for certain substituted phenols, the formation of a dimer between the phenoxide and the neutral phenol may contribute significantly to the overall protonophoric activity. In 13 of 14 mixtures of substituted phenols but in none of the mixtures of phenols with anisols, such a dimer appears to be formed between two different mixture partners. An extended shuttle mechanism of uncoupling, which includes a term for the contribution of such a mixed dimer, provided a good description of all experimental data. Opposite speciation favors interaction and ortho substituents abate interaction, which adds evidence for the dimer formation via a hydrogen bond between the phenol-OH and the phenoxide. These findings are significant not only regarding the mechanism of protonophoric action but also for the risk assessment process of chemical mixtures in the environment. When assessing the effect of mixtures, concentration addition is regarded as a reference concept to estimate effects of similarly acting compounds. The substituted phenols in this work act according to the same action mechanism of uncoupling. Nevertheless, the overall effect of four of the investigated mixtures, which exhibit stronger dimer formation as compared to the single compounds or for which the resulting dimer is intrinsically more active, exceeded the effect calculated according to concentration addition considerably. In future work, this synergistic effect observed in-vitro has to be validated in-vivo to deduce its implications for the risk assessment process.
The physical structure and dynamics of a deep, meromictic crater lake (Lac Pavin, France)
The characteristic feature of the physical structure of Lac Pavin is a distinct and permanent chemically induced density increase between about 60 and 70 m depth. This chemocline separates the seasonally mixed mixolimnion from the monimolimnion, which is characterized by elevated temperature and salinity as well as complete anoxia. Previously published box-models of the lake postulated substantial groundwater input at the lake bottom, and consequently a short water residence time in the monimolimnion and high fluxes of dissolved constituents across the chemocline. We present a new view of the physical structure and dynamics of Lac Pavin, which is based on the results of high-resolution CTD profiles, transient and geochemical tracers (tritium, CFCs, helium), and numerical modeling. The CTD profiles indicate the existence of a sublacustrine spring above rather than below the chemocline. A stability analysis of the water column suggests that vertical turbulent mixing in the chemocline is very weak. A numerical one-dimensional lake model is used to reconstruct the evolution of transient tracer distributions over the past 50 years. Model parameters such as vertical diffusivity and size of sublacustrine springs are constrained by comparison with observed tracer data. Whereas the presence of a significant water input to the monimolimnion can clearly be excluded, the input to the mixolimnion – both at the surface and from the indicated sublacustrine spring – cannot be accurately determined. The vertical turbulent diffusivity in the chemocline is well constrained to <I>K</I> ≈ 5 × 10<sup>-8</sup> m<sup>2</sup> s<sup>-1</sup>, about a factor of three below the molecular diffusivity for heat. Assuming thus purely molecular heat transport, the heat flow through the chemocline can be estimated to between 30 and 40 mW m<sup>-2</sup>. With respect to dissolved constituents, the very weak turbulent diffusive exchange is equivalent to a stagnant monimolimnion with a residence time of nearly 100 years. Based on these results and vertical concentration profiles of dissolved species, diffusive fluxes between monimolimnion and mixolimnion can be calculated. A large excess of helium with a <sup>3</sup>He/<sup>4</sup>He ratio of (9.09 ± 0.01) × 10<sup>-6</sup> (6.57 <I>R</I><sub>a</sub>) is present in the monimolimnion, clearly indicating a flux of magmatic gases into the monimolimnion. We calculate a flux of 1.0 × 10<sup>-12</sup> mol m<sup>-2</sup> s<sup>-1</sup> for mantle helium and infer a flux of 1.2 × 10<sup>-7</sup> mol m<sup>-2</sup> s<sup>-1</sup> (72 t year<sup>-1</sup>) for magmatic CO<sub>2</sub>. The monimolimnion appears to be in steady state with respect to these fluxes.