The Stratification of Pools in the Alluvium of the River Lužnice

The stratification of two shallow and small pools, situated in a floodplain ecosystem, was investigated. A new methodological approach was applied. The parameter called summarized chemical and biological stratification (SCB), was derived from basic limnological parameters in order to evaluate the global intensity of chemical and biological stratification and to compare it with a thermal one. Three situations were described when the seasonal courses of thermal and SCB stratification were compared: 1. Both SCB and thermal stratification are present. 2. The water column is homogeneous — there is no stratification in the pool. 3. SCB stratification is present, but the water is homoiothermic. The thermal stratification is very unstable because of the small volume of the pools and frequent floods. The SCB stratification is of higher stability and it may persist in a homoiothermic water column. In comparison with temperate deep lakes, the thermal stratification does not play such an important role in initiating the chemical and biological stratification. The surface/volume ratio and the depth in the pools are lower, which cause the “bottom-initiated” quick renovation of stratification even in homoiothermic water. The differences in oxygen regime, volume and localization of both investigated pools cause the differences in stratification patterns. Frequent oxygen depletions in one of the pools increase the intensity of stratification and influence its character. The anaerobic conditions lead to steeper gradients of basic nutrients, pH, alkalinity and water color. The occurrence of a unique phytoplankton community (Cryptophyceae) is probably closely related to the described stratification patterns.     

Vertical distribution of the European sardine (Sardina pilchardus) larvae and its implications for their survival

This study presents results of the vertical behaviour of theEuropean sardine (Sardina pilchardus) larvae as observed atsea off the NW Iberian coast during an oceanographic cruiseconducted in May 2002. Samples were taken in a grid of 38 stations(conductivity-temperature-depth [CTD] measurements and LonghurstHardy Plankton Recorder [LHPR] plankton hauls); a 69-h fixedstation study was also performed (hourly CTD measurements andLHPR/neuston hauls every 2 h). The horizontal distribution oflarvae is closely related to the circulation patterns measuredby a current metre-mooring array deployed during the cruise.Larvae were mainly distributed in the upper 20–25 m ofthe water column, in evident association with the waters ofthe Western Iberia Buoyant Plume (WIBP). Large (older) larvaeare found mainly in the surface layers, and larval size decreaseswith depth. A diel rhythm of migration to the neuston layerwas observed, correlated with the inflation/deflation activityof the swim bladder. Larvae with lengths greater than 12.5 mmand inflated swim bladders were only found in this layer. Consideringthe near surface stratification conditions for food availabilityand Ekman transport in the upper few metres, even small verticalmigrations of larvae can be very important for their survivaland subsequent recruitment success.     

Oxygen cycle in a hippo pool, Serengeti National Park, Tanzania

The vertical distribution of temperature and dissolved oxygen was sampled over 24 h in a pond of the Seronera River inhabited by hippos. The waters were very turbid (visibility < 2 cm) because of the high turbidity from animals trampling sediment and a permanent surface algal bloom sustained by faecal matter. Direct solar heating was restricted to the top few centimetres. This resulted in a strong thermal and density stratification inhibiting aeration of the water column. Waters at mid-depth were aerated only when hippos stirred the water. Anoxic conditions were common in bottom water, these were occasionally ventilated in daytime by mixing due to bottom heating from decaying organic matter and at night by convective cooling. Poor water quality in hippo pools may affect wildlife.     

Catastrophic shifts in vertical distributions of phytoplankton The existence of a bifurcation set

A model of phytoplankton dynamics within a water column was analyzed with special consideration on the existence of a bifurcation set in the parameter space. We considered two resources, light and a limiting nutrient, for phytoplankton growth and assumed that the water column is separated into two layers by thermal and/or density stratification. It was shown that there exists a bifurcation set in the parameter space when the growth function meets several conditions that are general for growth functions of two essential resources. Specifically, these conditions include that a less abundant of the two resources limits the growth while the effect of the other is sufficiently small. Folded structure with two stable states separated by one unstable state appears in the catastrophe manifold when parameters move to a certain direction with a certain curvature from a point in the bifurcation set. These results suggest that occurrence of discontinuous transition between two alternative vertical patterns is possible nature of phytoplankton dynamics within a stratified water column.     

Water temperature and stratification depth independently shift cardinal events during plankton spring succession

In deep temperate lakes, the beginning of the growing season is triggered by thermal stratification, which alleviates light limitation of planktonic producers in the surface layer and prevents heat loss to deeper strata. The sequence of subsequent phenological events (phytoplankton spring bloom, grazer peak, clearwater phase) results in part from coupled phytoplankton–grazer interactions. Disentangling the separate, direct effects of correlated climatic drivers (stratification‐dependent underwater light climate vs. water temperature) from their indirect effects mediated through trophic feedbacks is impossible using observational field data, which challenges our understanding of global warming effects on seasonal plankton dynamics. We therefore manipulated water temperature and stratification depth independently in experimental field mesocosms containing ambient microplankton and inocula of the resident grazer Daphnia hyalina. Higher light availability in shallower surface layers accelerated primary production, warming accelerated consumption and growth of Daphnia, and both factors speeded up successional dynamics driven by trophic feedbacks. Specifically, phytoplankton peaked and decreased earlier and Daphnia populations increased and peaked earlier at both shallower stratification and higher temperature. The timing of ciliate dynamics was unrelated to both factors. Volumetric peak densities of phytoplankton, ciliates and Daphnia in the surface layer were also unaffected by temperature but declined with stratification depth in parallel with light availability. The latter relationship vanished, however, when population sizes were integrated over the entire water column. Overall our results suggest that, integrated over the entire water column of a deep lake, surface warming and shallower stratification independently speed up spring successional events, whereas the magnitudes of phytoplankton and zooplankton spring peaks are less sensitive to these factors. Therefore, accelerated dynamics under warming need not lead to a trophic mismatch (given similar grazer inocula at the time of stratification). We emphasize that entire water column dynamics must be studied to estimate global warming effects on lake ecosystems.     

Turbulence and stratification in Priest Pot,a productive pond in a sheltered environment

Priest Pot is an example of the abundant ponds that, collectively, contribute crucially to species diversity. Despite extensive biological study, little has been reported about the physical framework that supports its ecological richness. This article elucidates the physical character of Priest Pot’s water column and thus that of similar water bodies. Vertical thermal microstructure profiles were recorded during summer 2003 and analyzed alongside concurrent meteorological data. During summer stratification, the thermal structure appeared to be dominated by surface heat fluxes. Surface wind stress, limited by sheltering vegetation, caused turbulent overturns once a surface mixed layer was present but appeared to contribute little to setting up the thermal structure. Variations in full-depth mean stratification occurred predominantly over seasonal and ∼5-day time scales, the passage of atmospheric pressure systems being posited as the cause of the latter. In the uppermost ∼0.5 m, where the stratification varied at subdaily time scales, turbulence was active (sensu Ivey and Imberger 1991) when this layer was mixed, with dissipation values ε ∼ 10⁻⁸ m² s⁻³ and vertical diffusivity K_Z = 10⁻⁴ — 10⁻⁶ m² s⁻¹. Where the water column was stratified, turbulence was strongly damped by both buoyancy and viscosity, and K_Z was an order of magnitude smaller. Vertical transport in the mixed layer occurred via many small overturns (Thorpe scale r.m.s. and maximum values were typically 0.02 m and 0.10 m, respectively), and seston were fully mixed through the water column.     

Mixing patterns and deep chlorophyll a maxima in an eutrophic tropical lake in western Mexico

In many temperate oligotrophic lakes, algal accumulations can form below the mixing zone. However, Deep Chlorophyll Maxima (DCM) have also been found in some eutrophic, tropical lakes and in this paper we aim to identify if they are recurrent features in these kinds of lakes and to recognize the factors that favor their formation. We analyzed 5 years of thermal stratification, water quality, and chlorophyll a concentrations in a tropical eutrophic lake in Central Mexico. Thermal stratification patterns were characteristic of warm monomictic lakes. Full water column deoxygenation during winter mixing was recorded in 3 of the analyzed years, and an increase of ~ 1 °C in the hypolimnion was detected between 2011 and 2015. DCM were detected in 4 out of the 5 studied years, at the top of the hypolimnion when the water column was stratified (spring–summer). This study is the first report of recurrent DCM formation in the northern limit of the Neotropics. It confirms that high light penetration is a necessary condition for DCM. Stratified nutrients with epilimnetic P depletion are also factors favoring DCM formation.

     

Effects of thermal stratification and mixing on reservoir water quality

In this study, the effect of thermal stratification on water quality in a reservoir has been investigated by field observations and statistical analysis. During the summer period, when stratification is evident, field observations indicate that the observed dissolved oxygen concentrations drop well below the standard limit of 5 mg l⁻¹ at the thermocline, leading to the development of anoxia. The reasons for variations in the dissolved oxygen concentrations were investigated. Variations of air temperature and other meteorological factors and lateral flows from side arms of the lake were found to be responsible for the increase of dissolved oxygen concentrations. It was also observed that turbidity peaked mostly in the thermocline region, closely related to the location of the maximum density gradient and thus low turbulence stabilizing the sediments in the vertical water column. Relatively cold sediment-laden water flowing into the lake after rain events also resulted in increased turbidity at the bottom of the lake. Nondimensional analysis widely used in the literature was used to identify the strength of the stratification, but this analysis alone was found insufficient to describe the evolution of dissolved oxygen and turbidity in the water column. Thus correlation of these parameters was investigated by multivariate analysis. Fall (partial mixing), summer (no mixing), and winter (well mixed) models describe the correlation structures between the independent variables (meteorological parameters) and the dependent variables (water-quality parameters). Statistical analysis results indicate that air temperature, one day lagged wind speed, and low humidity affected variation of water-quality parameters.     

Temporal organization of phytoplankton communities linked to physical forcing

The performance of individual phytoplankton species is strongly governed by the thermal stratification’s impact on vertical mixing within the water column, which alters the position of phytoplankton relative to nutrients and light. The present study documents shifts in phytoplankton structure and vertical positioning that have accompanied intensified long-term stratification in a natural ecosystem. Ordination analysis is used to extract gradients in phytoplankton composition in Lake Tahoe, an extremely nutrient-poor lake, over a 23-year period of records. Community structure in the 1980s was associated most strongly with resource availability (low nitrogen to phosphorus ratios, deeper euphotic zone depth), while intensified stratification dominated the phytoplankton structure since the late 1990s. Within diatoms, small-sized cells increased with reduced mixing, suggesting that suppressed turbulence provides them with a competitive advantage over large-sized cells. Among the morphologically diverse chlorophytes, filamentous and coenobial forms were favored under intensified stratification. The selection for small-sized diatoms is accompanied by a shoaling trend in their vertical position in the water column. In contrast, the motile flagellates displayed a deeper vertical positioning in recent years, indicating that optimal growth conditions shifted likely due to reduced upwelling of nutrients. As the thermal stratification of lakes and oceans is strongly linked to climate variables, the present study confirms that climate warming will alter phytoplankton structure and dynamics largely through effects on nutrient availability and sinking velocities. Intensified stratification should favor the expansion of small-sized species and species with the capability of buoyancy regulation, which may alter primary productivity, nutrient recycling, and higher trophic productivity. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Laboratory studies on the effects of thermal change on the behaviour and distribution of juvenile chum salmon in sea water

Schooling chum salmon Oncorhynchus keta were biased towards the water surface (median position <1 m) under isothermal conditions (10° C) in a water column simulator (WCS). Thermal stratification (24/10° C) inhibited upward movement with fish congregating at the thermocline and displaying a clear avoidance of potentially lethal surface waters. A tri-phase model based on piece-wise nonlinear regression was used to describe the distribution shifts of chum salmon during a change from isothermal to thermally stratified conditions. Fish distribution was consistent with thermoregulatory behaviour and exhibited 'attraction', 'preference' and 'avoidance' phases. The thermal preference of 50% of the fish lay between 12·2 and 20·2° C, however, >83·5% of the fish occupied a 'preferred' temperature range of 13·7–17·9° C. The mean temperature at which 50% of chum salmon avoided rising temperature by shifting deeper in the water column and using the cooler thermocline was 20·2° C, and 90% avoidance occurred at 22·9° C. Behavioural responses to thermal stratification were consistent amongst underyearling fish of differing size and age.     

Zooplankton Dynamics of Lake Escondido (Rio Negro,Argentina), with Special Reference to a Population of Boeckella gracilipes (Copepoda,Calanoida)

The zooplancton dynamics and the populations of Boeckella gracilipes Daday of Lake Escondido (Rio Negro, Argentina) were studied from March 1987 to March 1988. Rotifers and Cladocerans showed high densities during spring and summer, while copepods were densest in autumn. The vertical distribution study showed that rotifers prefer upper levels of the water column, while cladocerans and copepods prefer lower levels, especially during thermal stratification. Boeckella gracilipes had a sex ratio biassed toward female, though changes in the sex ratio were observed. The clutch size was high in winter when the proportion of ovigerous females was low. The instantaneous birth rates were low during winter and high in summer. This result is related to the thermal cycle of the lake, by the influence of temperature on egg development time. The mortality rates showed a similar pattern, and maximum values were registered in spring and summer.     

Thermal anomaly in waters of the Gulf of Marseilles during the summer of 1999. A partial explanation of the mortality of certain fixed invertebrates?

During summer 1999, a massive mortality of attached benthic invertebrates was reported at several points along the French Mediterranean coast, east of the Rh?ne River. Such a wide geographical extension suggested the occurrence of a climatic anomaly as a direct or an indirect cause of this phenomenon. The systematic observations of surface water temperature (1 record/30 min) and vertical thermal structure from 0 to 55 m (semi-monthly frequency) performed by the service of observation of centre d'océanologie de Marseille since 1994 in the Gulf of Marseilles (northwestern Mediterranean) compared with historical data obtained for the summer periods (June till October) indicates a thermal anomaly during summer 1999. Although the surface temperatures do not strictly show exceptional values, the stability during long periods of high temperatures (24.1 degrees C +/- 0.5 in September, n = 1,018) and the thermal structure of the water column are remarkable. The vertical temperature profiles of 1999 reflect a general warming of 2-3 degrees C in the water layer overlying the thermocline, accompanied by its deepening (up to 30-40 m). This situation can be explained by the scarcity of NW winds with a marked drop of their relative frequency during July and October 1999 (13-27% versus 42%, mean value calculated from the previous 12 years) and the short duration of these windy periods (between 30 min and 5 h as a mean). Parallel to this, a large number of no wind registrations (125-250 h/month as a mean) were observed during this period. A similar thermal stratification, of lesser amplitude, associated with comparable meteorological conditions was also observed in 1997. The hypothesis that this summer 1999 temperature anomaly could constitute a partial explanation for the recorded massive mortality is discussed.     

The Potential for Nitrogen Fixation in a Lake Receiving Sewage Effluent (Grasmere, English Lake District)

The potential for nitrogen fixation in the water column and sediment of Grasmere (English Lake District) was investigated using the acetylene reduction technique, with reference to seasonal changes and concentrations of dissolved oxygen and inorganic nitrogen in the epilimnion and hypolimnion. Potential rates of nitrogen fixation and MPN counts of nitrogen-fixing bacteria correlated with each other and with decreases in dissolved oxygen concentration. The results suggested that selective changes in the nitrogen-fixing microflora took place during the period of thermal stratification. Neither light-dependent fixation, nor cyanobacteria could be detected in water column samples, and acetylene reduction was detected, for most of the season, only in samples of water and sediment to which a source of carbon and phosphorus had been added. The possible role of the sewage effluent in suppressing nitrogen fixation is discussed.     

Vertical Distribution of Potentially Pathogenic Free-Living Amoebae in Freshwater Lakes1

The vertical distribution of thermotolerant (37°C and 45°C) free-living amoebae (FLA) in warm monomictic lakes was determined in relation to the onset of thermal stratification and associated physical and chemical changes. The position of abiotic or biotic paniculate layers in the water column was located by using a submersible horizontal beam transmissometer that measures attenuance, or the absorption and scattering of light by participates in the water column. During mixis, the vertical distribution of amoebae was sporadic with significant numbers of FLA only occurring in clay layers caused by runoff after heavy rains. With the onset of thermal stratification in the lakes, phytoplankton layers began to form. Few amoebae were isolated from layers containing flagellated phytoplankton; however, significant (P < 0.005) numbers of FLA were isolated from two paniculate layers dominated by the filamentous blue-green algae Aphanizomenon and Lyngbya, respectively. By late June, a persistent detrital or decomposition layer formed in the lower metalimnion, as well as a hypolimnetic iron layer where the Fe²⁺ state was predominant. In this midsummer period, 13 Naegieria fowleri were isolated, with three from the detrital layer and seven from the iron layer. The presence of attenuation zones was found to be the best indicator of the vertical distribution of FLA in the water column, and such layers represent an important, previously undescribed habitat for potentially pathogenic FLA.     

In situ survival and growth of zebra mussels (Dreissena polymorpha) under chronic hypoxia in a stratified lake

Experiments and field surveys were conducted in Hargus Lake (Ohio, U.S.) to investigate the effect of lake stratification on the survival, growth and distribution of zebra mussels. During the lake stratification period, relatively stable temperature and dissolved oxygen (DO) gradients persisted across the water column, allowing us to examine the chronic effect of hypoxia on zebra mussels. Zebra mussels were incubated in cages and suspended at different depths in the water column at both pelagic (max. depth = 12 m) and littoral (max. depth = 3.5 m) sites from April 18 to September 28, 1994. No mussel survived to the end of the experiment in cages ≥ 5.5 m, whereas the highest survival rate (76%) occurred at 5 m depth where temperature and DO remained fairly stable for at least 3 months. The threshold oxygen level for survival was between 1.0–1.7 mg l^-1 when water temperature was at about 17–18 °C. While zebra mussels′ survival rate was not affected under the sublethal hypoxic conditions, their growth was greatly retarded by poor water quality. The field survey showed that the zebra mussels and macrophytes had about the same distribution and their biomasses were positively related. The percentage of mussels in aggregates increased towards their maximum distribution depth. The maximum distribution depth of the naturally occurring zebra mussels was only 2.8 m, whereas the adult mussels could survive the entire stratification period when being artificially placed on the 3.5 m bottom, and young mussels could colonize the 3.5 m bottom if solid substrates were provided. We conclude that lack of substrate, rather than hypoxia, was the limiting factor of zebra mussel distribution above 5 m depth in Hargus Lake. This revised version was published online in July 2006 with corrections to the Cover Date.     

Acidolysis of sardine oil by lipase to concentrate eicosapentaenoic and docosahexaenoic acids in glycerides

Lipoprotein lipase from Pseudomonas sp. was the best enzyme to concentrate eicosapentaenoic and docosahexaenoic acids (EPA and DHA) in sardine oil by acidolysis reaction, and acetone was more effective than n-hexane as a solvent for dissolving the reactants and concentrating the two fatty acids. The water concentration in the reaction mixture was a decisive factor governing the enrichment of EPA and DHA and the yield of glycerides. EPA and DHA were more concentrated, but the yield of glycerides decreased, when the water concentration was increased gradually. Thus, the concentration rates of both the fatty acids were low with 0.25% water, although a considerable amount of diglyceride was detectable in the reaction products. The effect of reaction temperature was very slight with the use of acetone; however, the ratio DHA/EPA increased when the temperature was lowered in the presence of n-hexane. When acidolysis was performed at 25°C for 1 h, using 10,000 units of lipase per g of the reactants, the total percentage of EPA and DHA reached 65% in the glycerides and the recoveries of the two acids were 87.4 and 81.3%, respectively, based on the contents in the original sardine oil. The relationship of the enzyme substrate specificity to the reaction results was also investigated.     

Vertical distribution of pelagial zooplankton in a middle-sized dimictic valley reservoir

Our observations indicate the vertical distribution of zooplankton and its seasonal changes in Dubník II reservoir (Slovakia) are determined mainly by the thermal regime of the reservoir, by transparency, and by fish and invertebrate predation. During periods of circulation, zooplankton vertical distribution in the whole water column was more homogeneous, whilst during summer temperature stratification zooplankton concentrated in the epilimnion — rotifers in higher layers than crustaceans. During summer stagnation a steep thermal gradient occurred at the boundary of the epi-and hypolimnion and low temperature and low dissolved oxygen in hypolimnion offered a refuge for Chaoborus flavicans larvae against fish, enabling coexistence of vertebrate and invertebrate predation. This evidence supports our previous findings concerning dominance of rotifers in zooplankton and representation of crustaceans by small-bodied species in the study reservoir. Steep thermal gradient and the presence of Chaoborus larvae caused very low zooplankton abundance in the lower part of the water column and a reduction of cladocerans refuges against fish to layers of thermocline or closely under thermocline where Daphnia cucullata and Daphnia parvula were found. Our previous assumptions about the high density of zooplanktivorous fish in Dubník II reservoir are supported by the fact that these small cladocerans are represented by smaller individuals in the upper layers and bigger individuals in deeper layers.     

水体热分层对万峰湖水环境的影响

2009年9月(秋季)和2010年1月(冬季)对南盘江流域峡谷型水库--万峰湖的水温和水化学(DO、pH、总磷)进行监测,结果表明:万峰湖水体在秋、冬季节均存在明显分层,秋季.水体分为3层,0～10 m为混匀层,10～50 m为斜温层,50 m以下为滞水层,这种温度分层阻止了水的对流混合,引起显著的水化学分层,形成底部厌氧层.冬季,水温下降,在水下50 m左右分层,表层随深度增加水温下降,下层为滞水层,水温较为均匀.相关性分析表明,万峰湖在2009年9月(秋季)和2010年1月(冬季)水温和DO、pH、总磷之间均为极显著相关,水化学分层与温度分层同步.     

Effect of cyanobacterial blooms on thermal stratification

Enclosure experiments were performed at Akanoi Bay, Lake Biwa, in 1995 to determine whether the blooms of cyanobacterial algae changed thermal stratification in the lake. We used four rectangular enclosures, each 10 m × 10 m, with a volume of 200 m³, which were open to the sediments. Two enclosures, A and B, were mixed artificially by aquatic pumps from 1000 to 1400 every day, and the other two enclosures, C and D, were controls with no mixing. The experiment was conducted during late summer from August 3 to September 27. Chlorophyll a concentrations were highest in enclosure D, followed by enclosure C, both of which were controls without mixing. Enclosure A had lower concentrations than enclosures C and D, and enclosure B had the lowest concentrations. No large cyanobacterial algae blooms of Anabaena sp. and Microcystis sp. were seen in the mixed enclosures A and B. In enclosures C and D, blooms of Anabaena sp. occurred in the middle of August, and Microcystis sp. later became dominant in enclosure D at the end of August. In enclosure D, the water temperature changed over the diel cycle before August 17, with thermal stratification during the day and complete mixing at night. After August 17, as Anabaena sp. and Microcystis sp. became dominant, the temperature at the bottom of the enclosure did not change clearly over the 24-h cycle. The APE (available potential energy) density (a measure of water column stability) in the enclosures increased by almost 100% when the biovolume of Anabaena sp. + Microcystis sp. exceeded 20 mm³ l⁻¹. These results indicate that blooms of Anabaena sp. and Microcystis sp. can increase the available potential energy in the water column and create more stable stratification for their growth. Received: September 25, 1999 / Accepted: January 6, 2000     

Computer simulations of seasonal outbreak and diurnal vertical migration of cyanobacteria

Algal blooms caused by cyanobacteria are characterized by two features with different time scales: one is seasonal outbreak and collapse of a bloom and the other is diurnal vertical migration. Our two-component mathematical model can simulate both phenomena, in which the state variables are nutrients and cyanobacteria. The model is a set of one-dimensional reaction-advection-diffusion equations, and temporal changes of these two variables are regulated by the following five factors: (1) annual variation of light intensity, (2) diurnal variation of light intensity, (3) annual variation of water temperature, (4) thermal stratification within a water column and (5) the buoyancy regulation mechanism. The seasonal change of cyanobacteria biomass is mainly controlled by factors, (1), (3) and (4), among which annual variations of light intensity and water temperature directly affect the maximum growth rate of cyanobacteria. The latter also contributes to formation of the thermocline during the summer season. Thermal stratification causes a reduction in vertical diffusion and largely prevents mixing of both nutrients and cyanobacteria between the epilimnion and the hypolimnion. Meanwhile, the other two factors, (2) and (5), play a significant role in diurnal vertical migration of cyanobacteria. A key mechanism of vertical migration is buoyancy regulation due to gas-vesicle synthesis and ballast formation, by which a quick reversal between floating and sinking becomes possible within a water column. The mechanism of bloom formation controlled by these five factors is integrated into the one-dimensional model consisting of two reaction-advection-diffusion equations.