The Role of Allochthonous Inputs of Dissolved Organic Carbon on the Hypolimnetic Oxygen Content of Reservoirs

Hypolimnetic oxygen content in lentic ecosystems has traditionally been modeled as a function of variables measured at the epilimnion, or that are supposed to drive epilimnetic processes, like total phosphorus load. However, in man-made reservoirs the river inflow can plunge into deep layers, directly linking the hypolimnion with the surrounding watershed. In these circumstances, organic matter carried by the river can influence the hypolimnetic oxygen content without important intervention of epilimnetic processes. Taking long-term data from two reservoirs in Spain, we applied an empirical regression approach to show that the dissolved organic matter carried by the river is the main driver shaping the hypolimnetic oxygen content. By contrast, typical variables commonly included in the modeling of the oxygen content in the hypolimnion (nutrient concentrations, chlorophyll a, and dissolved organic carbon measured in the water column) did not show any significant correlation. Interpretations from this regression approach were supported by a comparison between the monthly oxygen consumption in the hypolimnion and the monthly dissolved organic carbon load from the river inflow. We also revisited the prediction of the year-to-year variability of the Nürnberg’s anoxic factor in four reservoirs from Spain and the USA, explicitly including the allochthonous sources in the equations. These sources were significant predictors of the anoxic factor, especially in those systems subject to relatively high human impact. Thus, effects of allochthonous dissolved organic carbon should always be considered in empirical modeling and management of reservoir hypolimnetic processes related to oxygen content (for example, anoxia, nutrient internal loading, or phosphorus cycle resilience).     

A mass balance evaluation of the ecological significance of historical nitrogen fluxes in Lake Kinneret

Lake Kinneret (LK) is a monomictic lake that has undergone significant biological and chemical changes over the last three decades of the twentieth century. The transition between the 1970s and the 1980s attracted a lot of scientific attention as it was marked by significant changes in the ecology of the lake. In the early 1980s, phytoplankton biomass increased, apparently in response to an increase in the external soluble reactive phosphorus (SRP) load. This period was marked by a rise in hypolimnetic levels of ammonium (NH₄) and SRP as well as surface water dissolved oxygen (DO) and pH. Cconcomitantly, in surface waters in winter levels of NH₄ increased and NO₃ decreased. In this study interrelationships amongst these observations were examined with a mass balance modelling approach, including simulation of individual nutrient sources and sinks, focusing on nitrogen fluxes in winter. The step-like rise in phytoplankton biomass in 1981 may have been triggered by the increase in winter external loads of SRP, as P is likely to be the growth-limiting nutrient during this season. The additional P load led to a sequence of changes including greater summer phytoplankton biomass, followed by enhanced sedimentation of organic matter. Furthermore, higher organic matter mineralization fluxes within the hypolimnion resulted in elevated levels of NH₄ and SRP in this layer through the 1980s, with a feedback to productivity in the trophogenic zone following seasonal destratification in early winter. In an apparent transition period (late 1970s to early 1980s), an increase in the modelled rate of nitrate (NO₃) production occurred via nitrification together with increased uptake of the additional nitrate by phytoplankton. These results are consistent with increased phytoplankton abundance and elevated levels of surface water NH₄ and DO during this period. Through this period the increase in phytoplankton uptake of NO₃ predominated over the increase in nitrification, and NO₃ concentrations in the 1980s were reduced compared with the previous decade, with increased partitioning of N in biomass and NH₄.     

Can zebra mussels change stratification patterns in a small reservoir?

Colonization and proliferation of zebra mussel (Dreissena polymorpha) population in Hargus lake, a small thermally stratified reservoir in Ohio, U.S.A., caused a significant increase in water clarity and a remarkable decrease in phytoplankton biomass during the period from 1993 to 1995. Increased light penetration and reduced organic matter loading to the meta-and hypolimnion were reflected in the lake stratification patterns, particularly in the temperature and oxygen profiles in the metalimnion. The meta- and hypolimnetic water temperature increased significantly over three years, irrespective of variation in surface water temperature. The epilimnion depth (mixing depth) increased by about the same magnitude as did the average Secchi depth. However, the total heat content of the lake did not show a consistent trend to increasing zebra mussel abundance, as it was largely influenced by the temperature of the large water volumes near the surface, which were in turn affected by weather conditions. Concurrent with the thermal structure change, the dissolved oxygen structure also changed over three years, though to a lesser extent. The changes in oxygen stratification pattern were reflected by increased oxygen concentrations in the metalimnion and a lowered depth of 3 mg l^–1 DO isopleth. These observed changes were likely attributed to increased water mixing depth, metalimnion photosynthesis and reduced oxygen consumption by organic matter. With increased epilimnion thickness and improved oxygen conditions in the metalimnion, the habitable space for aquatic macro-organisms (including fish) expanded substantially. Our results suggest that the indirect impacts of zebra mussels on small lake stratification patterns may have much broader implications than do the direct trophic interactions to the whole ecosystem.     

Oxygen consumption and ammonia accumulation in the hypolimnion of Walker Lake, Nevada

Walker Lake (area = 140 km², Z _mean = 19.3 m) is a large, terminal lake in western Nevada. As a result of anthropogenic desiccation, the lake has decreased in volume by 75% since the 1880s. The hypolimnion of the lake, now too small to meet the oxygen demand exerted by decaying matter, rapidly goes anoxic after thermal stratification. Field and laboratory studies were conducted to examine the feasibility of using oxygenation to avoid hypolimnetic anoxia and subsequent accumulation of ammonia in the hypolimnion, and to estimate the required DO capacity of an oxygenation system for the lake. The accumulation of inorganic nitrogen in water overlaying sediment was measured in laboratory chambers under various DO levels. Rates of ammonia accumulation ranged from 16.8 to 23.5 mg-N m^–2 d^–1 in chambers with 0, 2.5 and 4.8 mg L^–1 DO, and ammonia release was not significantly different between treatments. Beggiatoa sp. on the sediment surface of the moderately aerated chambers (2.5 and 4.8 mg L^–1 DO) indicated that oxygen penetration into sediment was minimal. In contrast, ammonia accumulation was reversed in chambers with 10 mg L^–1 DO, where oxygen penetration into sediment stimulated nitrification and denitrification. Ammonia accumulation in anoxic chambers (18.1 and 20.6 mg-N m^–2 d^–1) was similar to ammonia accumulation in the hypolimnion from July through September of 1998 (16.5 mg-N m^–2 d^–1). Areal hypolimnetic oxygen demand averaged 1.2 g O₂ m^–2 d^–1 for 1994–1996 and 1998. Sediment oxygen demand (SOD) determined in experimental chambers averaged approximately 0.14 g O₂ m^–2 d^–1. Continuous water currents at the sediment-water interface of 5–6 cm s^–1 resulted in a substantial increase in SOD (0.38 g O₂ m^–2 d^–1). The recommended oxygen delivery capacity of an oxygenation system, taking into account increased SOD due to mixing in the hypolimnion after system start-up, is 215 Mg d^–1. Experimental results suggest that the system should maintain high levels of DO at the sediment-water interface (10 mg L^–1) to insure adequate oxygen penetration into the sediments, and a subsequent inhibition of ammonia accumulation in the hypolimnion of the lake.     

Trophic relationships between bacteria and protozoa in the hypolimnion of a meromictic mesostrophic lake

The development of ciliated protozoan biomass in the hypolimnion of Piburger See, a small subalpine lake, was demonstrated to depend mainly on two factors. Firstly, the availability of oxygen or nitrate as electron acceptors determines the depth profiles of ciliates. Large quantities of ciliates and even maximum numbers were found at depths where no oxygen could be detected. If nitrate also disappeared during the summer stagnation period, the biomass of protozoa was strongly reduced. Nitrite peaks generally corresponded with ciliate peaks. An extension of Finlay's findings (dissimilatory nitrate reductase within the inner mitochondrial membrane) to other ciliate groups is hypothesized.Secondly, the biomass development of hypolimnetic ciliates was strongly correlated with the bacterial biomass registered approx. 2 weeks before (r² = 0.891, n = 14). The biomass of bacteria, on the other hand, was dependent upon the sedimentation rate of organic carbon (r² = 0.850, n = 15), if a time lag of approx. 2 weeks was taken into account. Therefore a total time lag of approx. 4 weeks was assumed to take place between sedimentation of organic substance and the corresponding increase in ciliate biomass (r² = 0.853, n = 14). Bacteria were shown to be an important intermediate link in the food chain of the hypolimnion. They appear to represent the principal energy source for pelagic ciliates. Sedimentation of organic carbon, nitrogen and phosphorus is the driving force for the establishment of the hypolimnetic microbial community.     

The response of the sedimentological regime in Lake Kinneret to lower lake levels

The surface level of Lake Kinneret is regulated to remain between –209 m and –213 m msl. During the stratified period, soluble reactive phosphorus (SRP), ammonium (NH _inf4 ^sup+ ) and dissolved sulphide (H₂S) accumulate in the hypolimnion. The concentration of these solutes, which are direct and indirect products of the decomposition of organic matter, increase considerably in summers with lower lake levels. A numerical model describing depth-averaged hypolimnion and epilimnion current velocities for high and low lake levels was adapted for Lake Kinneret. Simulated hypolimnetic currents were shown to be stronger for low lake levels as a result of the fact that low lake levels are characterized by a thinner hypolimnion while the thickness of the epilimnion remains unchanged. We suggest that the stronger hypolimnetic currents have the following consequences: 1. turbulence is induced, 2. the enhanced turbulence results in higher resuspension, 3. because SO⁴= is available to bacteria on resuspended particles, mineralization rates are enhanced, and 4. focusing of fine sediments and associated organic matter to the pelagic zone is enhanced.     

Sulfide and methane evolution in the hypolimnion of a subtropical lake: a three-year study

The differential impact of microbial sulfate reduction and methanogenesis on the mineralization of particulate organic carbon (POC) in warm monomictic Lake Kinneret (LK), Israel was studied during three consecutive lake cycles. The hypolimnetic accumulation of total sulfide and dissolved methane was examined in relation to the physical forcing of the water column and the settling flux of particulate matter. With the on-set of thermal stratification in spring, both solutes increased concomitantly with the depletion of oxygen, first in the benthic boundary layer, followed by the upper hypolimnion. Methane production was restricted to the sediments as emphasized by the persistently linear concentration gradient in the hypolimnion. Sulfate reduction occurred both in the sediments and the water column as revealed by the hypolimnetic distribution of sulfide and recurring metalimnetic sulfide peaks. Annual differences in the accumulation pattern of both solutes appeared to be primarily linked to the settling flux of POC and the length of the stratified season. Relatively lower hypolimnetic concentrations of dissolved methane during the stratified season of 2000 coincided with increased ebullition of gaseous methane, likely as the result of a nearly a 2 m drop in the lake level. Overall, sulfate reduction accounted for more than 60% of the POC settling flux, a finding that differs from similar studies made in temperate lakes where methanogenesis was shown to be the primary mode of terminal carbon mineralization. Intensive organic carbon turnover at the sediment water interface and comparatively high sulfate concentrations in LK are the most likely reason.     

Spatial and temporal patterns of temperature,alkalinity, dissolved oxygen and conductivity in an oligo-mesotrophic,deep-storage reservoir in central Texas

Impoundment behavior was determined for alkalinity, temperature, dissolved oxygen, and conductivity from stations located along the length of a bottom-draining, oligo-mesotrophic, hardwater, deep-storage reservoir located in central Texas. The epilimnion deepened the length of the reservoir throughout the summer as a result of drawdown. Bicarbonate alkalinity and conductivity exhibited both longitudinal and vertical stratification. Alkalinity and conductivity in the epilimnion decreased from the riverine reach downreservoir to the dam. This longitudinal progression was attributed to inflow and photosynthetically induced epilimnetic decalcification.Hypolimnetic anoxic conditions first occurred in the sedimentation zone in the upreservoir and riverine reaches and then developed in a downreservoir pattern as summer progressed as a result of drawdown. Alkalinity and conductivity in the hypolimnion increased during anoxic conditions and consequently increased in a downreservoir progression.     

Epilimnetic and hypolimnetic symptoms of eutrophication in Great Mazurian Lakes, Poland

SUMMARY. An increase in the trophic state and productivity of a lake is not necessarily accompanied by a decrease in epilimnetic Secchi disc transparency. In smaller lakes with sharp thermoclines the symptoms of eutrophication may be more evident as a decrease in transparency in the epilimnion but, in larger lakes with poorly defined thermoclines, symptoms of eutrophication may be more evident in the hypolimnion as shown by an increase in the rate of hypolimnetic oxygen consumption.     

An oxygen model for Lake Haukivesi

A simple water quality model for Lake Haukivesi, heavily loaded by pulp and paper mill effluents, has been developed. The main purpose of the model is to predict the concentration of dissolved oxygen in the hypolimnion. The lake is divided into seven sub-basins, and also into epilimnion and hypolimnion. Transfers between sub-basins are calculated using water balance equations. The state variables of the model are dissolved oxygen concentration, biochemical oxygen demand, phytoplankton biomass, and total phosphorus concentration. The effect of temperature on reaction rate coefficients has been taken into account. Temperature is calculated in the model using a second degree polynomial function. The processes affecting hypolimnetic oxygen consumption are BOD decay, decomposition of phytoplankton, benthic oxygen demand, and decomposition of slowly decaying organic matter.     

Effects of alum treatment on phosphorus dynamics in a north-temperate reservoir

Eau Galle Reservoir, Wisconsin, was treated with a hypolimnetic dose of aluminum sulfate (alum) in 1986 to diminish excessive phytoplankton production associated with high phosphorus loading from anoxic, profundal sediments. Prior to treatment, internal total phosphorus (TP) loading was 3 to 6 times greater than external TP loading during summer stratification. Periodic increases in epilimnetic TP mass and chlorophylla concentrations closely corresponded with elevated internal TP loading. For one year following treatment, internal TP loading and concentrations of soluble reactive phosphorus (SRP) in the hypolimnion were substantially reduced. However, abnormally high external TP loading during the stratified period of 1986 resulted in high TP mass and chlorophylla in the epilimnion. During the summers of 1987 and 1988, effects of alum treatment on internal TP loading were essentially negated, and epilimnetic TP mass and chlorophylla remained unchanged from pretreatment years. Multiple potential sources of P input to this reservoir make it difficult to reduce epilimnetic P and phytoplankton growth.     

Effects of hypolimnetic oxygenation on water quality: results from five Danish lakes

Stratified eutrophic lakes often suffer from hypolimnetic oxygen depletion during summer. This may lead to low redox conditions and accumulation of phosphate and ammonia in the hypolimnion. Hypolimnetic oxygenation has been used as a lake management strategy to improve the water quality in five eutrophic dimictic Danish lakes where oxygenation was conducted for 4–20 years. In one lake, the hypolimnetic oxygen concentration clearly improved by oxygenation, whereas the other four lakes still exhibited low mean summer levels (<2.2 mg O₂ l⁻¹). Oxygenation generally increased the hypolimnetic water temperature by 0.5–2°C, but in one lake it increased by 4–6°C. In all lakes, oxygenation significantly reduced the hypolimnetic concentrations of phosphorus and ammonia during stratification. The accumulation of phosphorus and ammonia typically decreased by 40–88%. In two lakes oxygenation was stopped for 1–2 years and here hypolimnion concentrations of both phosphorus and ammonia increased again. Surface water quality only improved in one lake, but was likely also influenced by simultaneously occurring changes in external nutrient loading. Overall, it is concluded that hypolimnetic oxygenation reduces the hypolimnetic accumulation of phosphorus and ammonia and may prevent anoxia in the deeper parts of the lake. However, long-term oxygenation is required and it is uncertain whether the overall lake water quality can be improved by oxygenation. Reduction of the external nutrient loading is still essential to improve lake water quality. Handling editor: Luigi Naselli-Flores     

Studies on ciliated protozoa in eutrophic lakes: 2. Field and laboratory studies on the effects of oxygen and other chemical gradients on ciliate distribution

An investigation into the spatial distribution of hypolimnetic ciliates in three small eutrophic lakes during the period of summer stratification was carried out. Peak ciliate densities were found to occur at the oxic/anoxic boundary, ciliate numbers declining with increasing depth within the hypolimnion. The ciliates only occurred in aerobic water where oxygen levels were less than about 0.5 mgl^–1 Laboratory experiments demonstrated that the ciliates swim upwards under anaerobic conditions but swim rapidly downwards under aerobic conditions. Further laboratory experiments showed that although the bulk of the population occured within anaerobic water, the hypolimnetic ciliates are aerobes and cannot survive indefinite anoxia. Despite the demonstrable toxicity of high levels of ammonia and sulphide, it was probably excesive distance from an available source of oxygen that excluded the ciliates from the lowest levels of the hypolimnion. Possible mechanisms which allowed these aerobic ciliates to colonise anaerobic water are considered.     

Sedimentation Rate of Seston during the Formation of Temperature Stratification after Ice Break-up in the Partly Meromictic Lake Verevi

The small strongly stratified hard-water hypertrophic lake Verevi (max. depth 11.0 m, surface area 12.6 ha, mean depth 3.6 m) was investigated in 2000 and in 2001. The lake is sheltered from winds, and the role of waves in mixing the water column is minimal. Eutrophication favours the strengthening of stratification. Early warm springs cause a fast stagnation of the water column forming partly meromictic conditions. Seston content of water and in sediment traps in 3 layers was measured several times during the formation of stratification. Besides measuring particulate matter, in 2001, the nutrient content of the trapped sediment was analysed. During the first 7 days of the investigation, 30% of the total particle sedimentation took place. The sedimentation rate of particulate matter was 0.4–6.3 g m^–2 d⁻¹ dry weight in different layers of the water column. Daily average sedimentation loss rate was 27% of the total amount of seston of the epilimnion, whilst from the meta- and hypolimnion the settling was much slower (9.6 and 7.3%, respectively). In our experiments with twin sediment traps, to one of which formaldehyde was added, the PO₄³⁻-P concentration was 19% smaller in the trap without formaldehyde, probably due to planktonic uptake. The relationship between primary and export production is loop-like. The shape was irregular, indicating a high grazing rate of zooplankton.     

Algal and Bacterial Activities in Acidic (pH 3) Strip Mine Lakes

Reservoir 29 and Lake B are extremely acid lakes (epilimnion pHs of 2.7 and 3.2, respectively), because they receive acidic discharges from coal refuse piles. They differ in that the pH of profundal sediments in Reservoir 29 increased from 2.7 to 3.8 during the period of thermal stratification, whereas permanently anoxic sediments in Lake B had a pH of 6.2. The pH rise in Reservoir 29 sediments was correlated with a temporal increase in H₂S concentration in the anaerobic hypolimnion from 0 to >1 mM. The chlorophyll a levels in the epilimnion of Reservoir 29 were low, and the rate of primary production was typical of an oligotrophic system. However, there was a dense 10-cm layer of algal biomass at the bottom of the metalimnion. Production by this layer was low owing to light limitation and possibly H₂S toxicity. The specific photosynthetic rates of epilimnetic algae were low, which suggests that nutrient availability is more important than pH in limiting production. The highest photosynthetic rates were obtained in water samples incubated at pH 2.7 to 4. Heterotrophic bacterial activity (measured by [¹⁴C]glucose metabolism) was greatest at the sediment/water interface. Bacterial production (assayed by thymidine incorporation) was as high in Reservoir 29 as in a nonacid mesotrophic Indiana lake.     

Stratification in the filling phase of the man-made lake Brokopondo in Surinam (S. America)

Summary Temperature and dissolved oxygen content measured weekly at various depths during 3 1/2 years in the filling phase of a tropical man-made lake, show that at the dam-site fluctuations of epilimnion values occurred, whereas the hypolimnion remained almost constant in temperature and permanently devoid of oxygen. This confirms previous reports concerning another mid-lake station. Apart from rather steep superficial temperature gradients, classical thermoclines did not occur. It is proposed that the lake could not become fully mixed by wind action because of the great vertical density differences at the prevailing temperatures, and because of the drowning forest still giving shelter by the extending crowns. On the other hand, the stratification was not static. Fluctuations between high and low epilimnion values for temperature, oxygen content and transparency oocurred yearly, but could not be related satisfactorily to the prevailing seasons. The depth of the epilimnion increased each year. It is suggested that the observed kind of stratification was effective though not absolute. Vertical exchange may have been reduced by increasing density differences, but it was not fully eliminated. This explains the varying degree of incorporation of upper hypolimnetic layers in the epilimnion in relation with epilimnetic temperature. Comparison with large African reservoirs accentuates the exceptional limnological development of Lake Brokopondo during ifs filling phase.     

Bacterioplankton Abundance and Activity in a Small Hypertrophic Stratified Lake

Bacterioplankton abundance and production were followed during one decade (1991–2001) in the hypertrophic and steeply stratified small Lake Verevi (Estonia). The lake is generally dimictic. However, a partly meromictic status could be formed in specific meteorological conditions as occurred in springs of 2000 and 2001. The abundance of bacteria in Lake Verevi is highly variable (0.70 to 22 × 10⁶ cells ml⁻¹) and generally the highest in anoxic hypolimnetic water. In 2000–2001, the bacterial abundance in the hypolimnion increased probably due to meromixis. During a productive season, heterotrophic bacteria were able to consume about 10–40% of primary production in the epilimnion. Our study showed that bacterioplankton in the epilimnion was top-down controlled by predators, while in metalimnion bacteria were dependent on energy and carbon sources (bottom-up regulated). Below the thermocline hypolimnetic bacteria mineralized organic matter what led to the depletion of oxygen and created anoxic hypolimnion where rich mineral nutrient and sulphide concentrations coexisted with high bacterial numbers.     

Acid-base chemistry and aluminum transport in an acidic watershed and pond in New Hampshire

Cone Pond is one of the few acidic, clear-water ponds in the White Mountains of New Hampshire, a region dominated by high inputs of strong acids from atmospheric deposition and low base content of bedrock. Monitoring was conducted for 13 months to compare and contrast the acid-base chemistry of the terrestrial and aquatic portions of the watershed. Variations in Al concentration and speciation in drainage waters were correlated with changes in the supply of naturally occurring organic ligands. During the study period, the pond retained 28% of Al inputs, including nearly half of the inputs of organically complexed Al. Chemical equilibrium calculations indicated that the entire water-column was oversaturated with respect to the solubility of synthetic gibbsite during summer, as was the hypolimnion during winter. Retention of Al resulted from an increase in pH in the hypolimnion concomitant with SO₄ ^2– reduction, and from loss of organic anions in epilimnetic waters. Acid neutralizing capacity (ANC) generated in the pond primarily through SO₄ ^2– reduction and base cation (C _B ) release was balanced by ANC consumed as a result of Al retention.     

The dual influences of dissolved organic carbon on hypolimnetic metabolism: organic substrate and photosynthetic reduction

We investigated the effect of dissolved organic carbon (DOC) on hypolimnetic metabolism (accumulation of dissolved inorganic carbon (DIC) and methane (CH₄)) in 21 lakes across a gradient of DOC concentrations (308 to 1540 mol C L^–1). The highly colored nature of the DOC in these lakes suggests it is mostly of terrestrial origin. Hypolimnetic methane accumulation was positively correlated with epilimnetic DOC concentration (Spearman rank correlation = 0.67; p < 0.01), an indicator of allochthonous DOC inputs, but not with photic zone chlorophyll a concentration (Spearman rank correlation = 0.30; p = 0.22). Hypolimnetic DOC concentrations declined in 19 of 21 lakes during the stratified period at rates that ranged from 0.06 to 53.9 mmol m^–2 d^–1. The hypolimnetic accumulation of DIC + CH₄ was positively correlated with, and, in most cases of comparable magnitude to, this DOC decline suggesting that DOC was an important substrate for hypolimnetic metabolism. The percentage of surface irradiance reaching the thermocline was lower in high DOC lakes (0.3%) than in low DOC lakes (6%), reducing hypolimnetic photosynthesis (as measured by the depth and magnitude of the deep dissolved oxygen maxima) in the high DOC lakes. In June, the hypolimnia of lakes with < 400 mol L^–1 DOC had high concentrations of dissolved oxygen and no CH₄, while the hypolimnia of lakes with DOC > 800 mol L^–1 were completely anoxic and often had high CH₄ concentrations. Thus, DOC affects hypolimnetic metabolism via multiple pathways: DOC was significant in supporting hypolimnetic metabolism; and at high concentrations depressed photosynthesis (and therefore oxygen production and DIC consumption) in the hypolimnion.     

Trade-offs in the vertical distribution of zooplankton: ideal free distribution with costs?

Zooplankton vertical migratory patterns are a classic example of optimal habitat choice. We hypothesize that zooplankton distribute themselves vertically in the water column according to an ideal free distribution (IFD) with costs such as to optimize their fitness. In lakes with a deep-water chlorophyll maximum, zooplankton are faced with a trade-off, either experiencing high food (high reproductive potential) but low temperature (slow development) in the hypolimnion or high temperature and low food in the epilimnion. Thus, in the absence of fish predation (e.g. at night) they should allocate the time spent in the different habitats according to fitness gain dependent on the temperature gradient and distribution of food. We tested this hypothesis with a Daphnia hyalina x galeata clone in large indoor columns (Plön Plankton Towers) and with a dynamic energy budget model. In the tower experiments, we simulated a deep-water algal maximum below the thermocline with epilimnetic/hypolimnetic temperature differences of 2, 5 and 10 degrees C. Experimental data supported the model. We found a significantly larger proportion of daphniids in the hypolimnion when the temperature difference was smaller. Our results are consistent with the concept of IFD with costs originally developed for stream fishes. This concept can be applied to predict the vertical distribution of zooplankton in habitats where fish predation is of minor importance.