Life history of Daphnia galeata in a hypertrophic reservoir and consequences of non-consumptive mortality for the initiation of a midsummer decline

SUMMARY 1. Field and laboratory investigations were combined in a 2‐year study on the initiation of a midsummer decline of Daphnia galeata Sars in a hypertrophic reservoir. Quantitative field samples were taken twice a week, and, adult and juvenile mortality rates were calculated. Patterns of reproduction and survival of daphnids born during spring and early summer under fluctuating food conditions were determined in life‐table experiments. 2. The abundance of Daphnia increased strongly in early May and declined in June 1998 (midsummer decline). In 1999, Daphnia density increased only slowly in spring and remained constantly high throughout the summer. 3. Food conditions (concentrations of POC_{<30 μm}) for daphnids deteriorated in both years in response to increasing Daphnia densities, resulting in a clear‐water phase of about 4 weeks. When Daphnia abundance declined in 1998, POC_{<30 μm} concentrations increased greatly, whereas in 1999 food conditions improved only slightly and Secchi depth remained high. 4. Survival of daphnids in life‐table experiments decreased greatly after food became rare and was strongly reduced in those animals born during the clear‐water phase compared with those born later. In addition, age at first reproduction was retarded during the clear‐water phase, resulting in very low population growth rates. Survivorship patterns in life‐table experiments suggest a strong impact of non‐consumptive mortality on Daphnia population dynamics. 5. Field data of mortality point to differences in mortality patterns between years, probably resulting from different predation impacts of juvenile fish. In both years, however, adult mortality contributed substantially to overall mortality at the end of the clear‐water phase. As bottom‐up effects on D. galeata were very similar in both years, the significance of non‐consumptive mortality on the initiation of midsummer declines appears to depend largely on recruitment patterns before the clear‐water phase. A high impact can be expected when Daphnia populations are dominated by a peak cohort of nearly identical age during the clear‐water phase.     

Phytoplankton–zooplankton seasonal timing and the 'clear-water phase' in some English lakes

SUMMARY 1. An examination is made of the relative seasonal timing of the postwinter increase of phytoplankton and zooplankton populations in four English lake basins. It centres upon weekly sampling over 20 years and rough counts of larger Crustacea, as copepods and cladocerans, from filtered samples that were used for chlorophyll a (Chl) estimation. 2. Typically, a spring maximum of phytoplankton, dominated by diatoms and earlier in the shallower lakes, is accompanied or followed by a maximum of copepods and then one of cladocerans dominated by the Daphnia hyalina–galeata complex. Regarding timing, the maximum of copepods has no apparent relation with phytoplankton abundance (Chl). The maximum of cladocerans appears to be largely independent of variation in the phytoplankton maximum, but is generally associated with a minimum in Chl. Evidence for some direct causality in this inverse correlation after the spring phytoplankton maximum is best displayed by the shallow Esthwaite Water in which the peaks of Chl and cladocerans are separated further than in the deep Windermere basins where phytoplankton growth is delayed. In Esthwaite Water, and possibly often in Windermere, a principal minimum in Chl is ascribable to grazing by Daphnia. 3. The typical inverse relationship of Chl and cladocerans is lost in some years when relatively inedible large phytoplankters (e.g. colonial chrysomonads, filamentous cyanophytes) are abundant and Chl minima are less pronounced, although maxima of cladocerans still occur. Conversely, available edible phytoplankters include various small forms grouped as μ‐algae and Cryptomonas spp.; their probable depletions by Daphnia appear to be sequential and may limit the latter's maxima, whose inception is temperature‐dependent. 4. The spring–summer maxima of cladocerans and minima of Chl are generally coincident with a main seasonal maximum of Secchi disc transparency and light penetration – to which removal of non‐phytoplankton particles by filtering cladocerans may contribute.     

Temperature impact on the midsummer decline of Daphnia galeata: an analysis of long-term data from the biomanipulated Bautzen Reservoir (Germany)

1. The influence of water temperature on occurrence and duration of a midsummer decline (MSD) of Daphnia galeata was studied in the biomanipulated Bautzen Reservoir in Germany. The proportion of piscivores in the fish community of the reservoir has been enhanced experimentally since 1981. As a consequence, Daphnia galeata has dominated the zooplankton. Over 18 years of study (1981–1998), a long‐lasting MSD (longer than 30 days) occurred in 7 years, whereas a short MSD (shorter than 30 days) was observed in 6 years. During the remaining 5 years, an MSD was not observed.
2. Two hypotheses were examined to explain the observed patterns. First, we postulated that high water temperature during winter and early spring (January–April) leads to an MSD after an early and high spring peak of daphnids. On the other hand, low temperature during winter and early spring should not cause an MSD owing to a slower increase of the population, resulting in a later peak of daphnids. Second, we hypothesized that the mean water temperature during early summer (May and June) influences the occurrence of an MSD (by controlling young‐of‐the‐year (YOY) fish predation on daphnids).
3. The water temperature during winter and early spring explains 83%, and the early summer water temperature 55%, of interannual variation in the occurrence of an MSD.
4. The interannual variation in duration of an MSD was neither explained by temperature during winter and early spring nor by early summer temperature alone, but in 14 of the 18 years (78%) by a combination of both.
5. We conclude that water temperature during winter and early spring had a strong impact on Daphnia mortality by influencing height and timing of the spring peak which, in turn, influenced the extent of overexploitation of their food resources. By contrast, the water temperature during early summer probably influenced the mortality of daphnids caused by predation of YOY fish. The relative timing of both sources of mortality, which depends on the temperature regime during the first 6 months of the year, is the key process in controlling the occurrence and duration of an MSD. A long‐lasting MSD, therefore, is likely in Bautzen Reservoir only if temperatures are high during winter and early spring, as well as during early summer.
6. As a consequence of climate warming, recent climate records reveal warming during winter, spring and early summer in middle Europe, rather than an increase in mean annual temperatures. If our findings and conclusions are related to this regional and temporal pattern of climate warming, an increasing frequency of years with a long‐lasting MSD and, consequently, a decreasing efficiency of biomanipulation can be predicted.     

Nutrient-phytoplankton relationships in a tropical meromictic soda lake

Seasonal variation through one year in total nitrogen (TN), total phosphorus (TP), phytoplankton biomass, phytoplankton species composition and other environmental factors were examined in Lake Sonachi, a tropical meromictic soda lake. Mean concentrations of TN and TP were 11 000 µg N l^-1 and 100 µg P l^-1, respectively. Maximum concentrations of TN and TP occurred in the monimolimnion. Phytoplankton biomass ranged from 350 to 1260 mg m^-3. Synechococcus bacillaris, a small coccoid cyanophyte, dominated the phytoplankton. The mean chlorophyll a concentration of 37 mg · m^-3 was a modest value when compared with those of other tropical soda lakes. High TN:TP ratios indicated phosphorus limitation in the lake.     

Taxonomy and ecology of the phytoplankton of Lake Fidler and Sulphide Pool,meromictic Tasmanian lakes

A floristic list of 89 freshwater phytoplanktonic algae occurring in two neighbouring, dystrophic, meromictic Tasmanian lakes is given. In both lakes the preponderance of desmids and phytoflagellates, especially chrysophytes, is a characteristic in keeping with their dystrophic nature. All the alga must be adapted to low levels of red light and some habitually inhabit crepuscular depths rather than executing diel vertical migrations such as happens in many dystrophic waters. Floristic differences between the two lakes are related to morphometric differences and the degree of entrainment of tychoplankton. The photosynthetic biomass of both lakes is predominantly monimolimnetic, made up of few species. The floristically-rich mixolimnion contributes little to biomass. The ecology of the dystrophic flora is discussed in relation to the special circumstances of meromixis.     

δ15N of zooplankton species in subarctic lakes in northern Sweden: effects of diet and trophic fractionation

1. To assess the use of stable nitrogen isotopes (δ¹⁵N) for reconstructing trophic relationships in planktonic food webs, crustacean zooplankton species and pelagic dissolved and particulate matter were analysed in 14 subarctic lakes in northern Sweden. The lakes are situated along an altitudinal gradient and show a substantial variation in nutrient content and energy mobilization by bacterioplankton and phytoplankton. 2. The δ¹⁵N of dissolved and particulate matter was comparatively low, suggesting efficient N recycling and low losses of depleted N from the pelagic zone of these unproductive lakes. 3. Copepods had a systematically higher δ¹⁵N than cladocerans, with an average difference of 3.1–4.9‰ within lakes, implying different trophic positions of the two groups. Comparisons of nitrogen pools and energy fluxes suggest that the low cladoceran δ¹⁵N was a result of feeding on bacteria. 4. The difference in δ¹⁵N between copepods and cladocerans declined with decreasing bacterioplankton production among lakes, due either to increasing trophic isotope fractionation or decreasing relative importance of bacteria in the diet of cladocerans.     

Trophic transfer and trophic modification of fatty acids in high Arctic lakes

1. A comparative study of fatty acid (FA) profiles in particulate matter (seston) and the key grazer Daphnia was performed in six high Arctic ponds (79°N, Svalbard). The ponds were all small and shallow, but followed a strong gradient with respect to nutrient content and optical properties. 2. A distinct locality‐specific pattern was detected by principal component analysis of FA profiles, where samples from each locality clustered both with regard to seston and Daphnia. Linear discriminant analysis using nine sestonic fatty acids as discriminant functions gave on average a correct prediction of the Daphnia lake membership in 47% of cases, with very high predictability in some lakes but poor predictability in others. 3. No significant correlation was detected between FA and nutrient concentration or levels of dissolved organic carbon. The major determinant of FA profiles as judged from a redundancy analysis was the taxonomic composition of phytoplankton communities, notably the biomass of Chlorophyceae. 4. The FA profiles of Daphnia were for some FAs strongly enriched relative to the seston, while diluted for others. Among the polyunsaturated fatty acids (PUFAs), a pronounced magnification of eicosapentaenoic acid (EPA, 20 : 5 n‐3), and to some extent 18 : 3 n‐3 and 20 : 4 n‐6 was found, while docosahexaenoic acid (DHA, 22 : 6 n‐3) contributed in general less to FAs in Daphnia than in seston and was hardly detectable in Daphnia from most localities. 5. The overall content of PUFAs in Daphnia was consistently high, close to 40% of total FA in all investigated localities, despite major differences in seston PUFA content. Daphnia thus acts as a regulator with regard to overall PUFAs, reflecting its physiological constraints and relatively fixed demands for PUFAs in general. The distinct locality‐specific profiles in Daphnia strongly suggest a kind of FA‐fingerprint, but our data do not allow strict statements on the use of specific FAs as trophic markers.     

Contrasting taxonomic stratification of microbial communities in two hypersaline meromictic lakes

Hypersaline meromictic lakes are extreme environments in which water stratification is associated with powerful physicochemical gradients and high salt concentrations. Furthermore, their physical stability coupled with vertical water column partitioning makes them important research model systems in microbial niche differentiation and biogeochemical cycling. Here, we compare the prokaryotic assemblages from Ursu and Fara Fund hypersaline meromictic lakes (Transylvanian Basin, Romania) in relation to their limnological factors and infer their role in elemental cycling by matching taxa to known taxon-specific biogeochemical functions. To assess the composition and structure of prokaryotic communities and the environmental factors that structure them, deep-coverage small subunit (SSU) ribosomal RNA (rDNA) amplicon sequencing, community domain-specific quantitative PCR and physicochemical analyses were performed on samples collected along depth profiles. The analyses showed that the lakes harbored multiple and diverse prokaryotic communities whose distribution mirrored the water stratification patterns. Ursu Lake was found to be dominated by Bacteria and to have a greater prokaryotic diversity than Fara Fund Lake that harbored an increased cell density and was populated mostly by Archaea within oxic strata. In spite of their contrasting diversity, the microbial populations indigenous to each lake pointed to similar physiological functions within carbon degradation and sulfate reduction. Furthermore, the taxonomy results coupled with methane detection and its stable C isotope composition indicated the presence of a yet-undescribed methanogenic group in the lakes'' hypersaline monimolimnion. In addition, ultrasmall uncultivated archaeal lineages were detected in the chemocline of Fara Fund Lake, where the recently proposed Nanohaloarchaeota phylum was found to thrive.     

Mechanisms of food selection in Daphnia

A conceptual behavioural and mechanistic Holling-type model of food selection in Daphnia pulicaria is derived from SEM observations with animals feeding on mixtures of spherical-cylindrical diatoms, oblongate green algae, and filamentous cyanobacteria, as well as ultrafine particles. The algae used were Stephanodiscus hantzschii (<- 6 µm length), Monoraphidium setiforme ( 20 µm), and Oscillatoria aghardii (strands, >- 80 µm). Cell (strand) selection can occur at any or all of three stages: (i) interception from the feeding currents, (ii) collection and channeling to the food groove, and (iii) compaction and transport to the mouth. During each stage, given equal initial cell densities, elongate cells are more likely to escape collection than spherical cells and are more likely to be rejected. In addition, filaments require increased handling time at stages (ii) and (iii) and promote entanglement with limb 5 and the postabdominal claw. Food is collected primarily with the aid of limbs 3 (and 4), but limbs 1 and 2 also intervene. Neither the leaky sieve hypothesis alone nor any other single-process hypothesis explains the observations on examined in corpore positions, morphology, and derived movements of the feeding limbs. Attachment and mucus appear to be important for the ingestion of bacteria and ultrafine particles.The model is consistent with many experimental results of differential feeding by Daphnia pulicaria on mixtures of variously shaped algae and other observations on Daphnia feeding behaviour. The paradigm of invariate, nonselective feeding by Daphnia is rejected.     

Food quality for Daphnia in humic and clear water lakes

1. Growth and reproduction of Daphnia fed lake seston were measured in two categories of meso‐ to eutrophic lakes differing with respect to terrestrial organic matter influence (humic and clear water lakes). The content of highly unsaturated fatty acids (HUFA), P and N, as well as the taxonomical composition of seston were analysed. 2. Seston HUFA and C : P ratios were similar between lake categories, whereas C : N ratios were lower in the clear water lakes in both spring and summer. Despite the similarity in HUFA and P content of seston, Daphnia growth rate, clutch size and the proportion of gravid females were, respectively, about 1.5, 3 and 6 times higher in the clear water lakes. 3. Differences in growth and reproduction were related to a combination of higher N content and good fatty acid quality of the seston in the clear water lakes. Relatively high biomass of edible algae, such as Rhodomonas sp. and Cryptomonas sp., in the clear water lakes, and differences in water pH likely contributed to the observed differences in Daphnia growth and reproduction between lake categories. Additionally, it is possible that Daphnia was energy limited in the humic lakes despite high particulate organic carbon (POC) concentrations, as the contribution of non‐algal and detrital C to the POC pool was high. 4. Our results suggest that dietary HUFA content has the potential to improve herbivore growth and reproduction if N and P are not limiting. N merits more attention in studies of zooplankton nutrition.     

Nutrient limitation of phytoplankton production in Alaskan Arctic foothill lakes

We used 54 enrichment bioassays to assess nutrient limitation (N, P) of ¹⁴C uptake by natural phytoplankton assemblages in 39 lakes and ponds in the Arctic Foothills region of Alaska. Our purpose was to categorize phytoplankton nutrient status in this under-represented region of North America and to improve our ability to predict the response of primary production to anticipated anthropogenically mediated increases in nutrient loading. Experiments were performed across several watersheds and included assays on terminal lakes and lakes occupying various positions in chains (lakes in series within a watershed and connected by streams). In total, 89% (48 of 54) of the bioassays showed significant stimulation of ¹⁴C primary production by some form of nutrient addition relative to unamended controls. A significant response was observed following enrichment with N and P, N alone and P alone in 83, 35 and 22% of the bioassays, respectively. In experiments where N and P proved stimulatory, the influence of N alone was significantly greater than the influence of P alone. Overall, the data point to a greater importance for N than P in regulating phytoplankton production in this region. The degree of response to N and P enrichment declined as the summer progressed and showed no relationship to irradiance or water temperature, suggesting secondary limitation by some micronutrient such as iron as the summer advanced. Phytoplankton nutrient status was often consistent across lakes within a watershed, suggesting that watershed characteristics influence nutrient availability. Lakes in this region will clearly show increased phytoplankton production in response to anthropogenic activities and anticipated changes in climate that will increase nutrient loading.     

Zooplankton (Cladocera) species turnover and long-term decline of Daphnia in two high mountain lakes in the Austrian Alps

We investigated long-term succession of sedimentary cladoceran assemblages in two morphologically divergent mountain lakes by utilizing sediment traps and previously available sediment data. We aimed to detect lake-specific changes in pelagic communities potentially attributable to climate warming under the presumption that lakes and biotic communities may respond individually to the same regional climatic forcing. Both lakes showed a similar community turnover, as Daphnia was first replaced by Chydorus cf. sphaericus and during the twentieth century by the latest colonizer Bosmina. The community succession was similar among the lakes and consistent with the regional temperature increase, although the timing of community shift, the magnitude of change, and taxa in question differed. Decline of Daphnia mismatched with historical fish stockings, but the eventual extirpation of Daphnia in one of the lakes corresponded to the start of fish introductions. We propose that the observed shifts were mainly governed by increasing temperatures and its limnoecological consequences (e.g., deeper mixing). We suggest that Bosmina distribution may be extending to lakes at higher altitudes as a response to climate warming, and that it may replace Daphnia as the key component of pelagic alpine food webs by coping in interspecific resource competition under changed limnological regimes.     

Food-web structure in two shallow salt lakes in Los Monegros (NE Spain): energetic vs dynamic constraints

Energetic and dynamic constraints have been proposed as rival factors in determining food-web structure. Food-web length might be controlled either by the amount of energy entering the web (energetic constraints) or by time span between consecutive disturbances relative to time needed to build up a population (dynamic constraints). Dynamic constraints are identified with processes functioning at a regional scale such as climate, lithology and hydrogeology. Energetic constraints are related with processes operating both at a regional and a local scale. We studied the contribution of energetic constraints to food-web organization in two temporary saline lakes with similar dynamic constraints. Lakes were sampled fortnightly during two hydroperiods (1994/1995 and 1995/1996). Differences in energetic constraints between lakes result in divergent assemblages of primary producers. Consumer assemblages in both lakes, however, are similar in species composition although differ in total biomass and species abundances. Food-webs are short with a high proportion of omnivores. To simulate an increase in the energy input entering to these systems, an addition of nutrients (to a final concentration of 100 gl^–1 P-PO₄ ^3-) was done in mesocosms placed within the lakes in order to obtain an increase in the phytoplankton biomass. No significant response to nutrient enrichment was found in food-web structure (composition, density or biomass).     

Duckling response to changes in the trophic web of acidified lakes

We reared American Black Duck (Anas rubripes Brewster) and Common Goldeneye (Bucephala clangula Linnaeus) ducklings on two Quebec laurentian lakes in which we manipulated brook trout populations (Salvelinus fontinalis Mitchill), lake acidity and lake productivity to relate waterfowl foraging to trophic status of lakes. We developed a preliminary model to assess the effects of lake acidity and productivity, fish predation and interspecific fish/duck competition in relation to available food (aquatic invertebrates). We then validated the model using a factorial analysis of the relationships between the variables pertaining to the diet of the fish and ducklings, and the environmental characteristics of the lakes (acidity, biological production and fish predation).The first factorial axis can be interpreted in terms of biological productivity, while the second axis illustrates the effect that fish have on the quantity and type of food available to ducklings. Two different trends appear to occur depending on whether the carrying capacity of the lake is reduced by acidification of the water or increase through liming or fertilization. In the first case, fish predation appears to have a marked effect on available food, whereas in the second case, interspecific fish/duck competition is apparently to blame for changes in the diet of ducklings. In both instances, but to a lesser extent, fish compete increasingly (exploitation and/or interference) with the ducklings, forcing them to feed to a greater extent in riparian sites that are less accessible to fish.     

Effects of mixing on the pelagic food web in shallow lakes

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Influence on phytoplankton biomass in lakes of different trophy by phosphorus in lake water and its regeneration by zooplankton

In 49 unpolluted lakes of north-eastern Poland the biomass of algae in summer is significantly related to the concentration of total phosphorus and to the rate of phosphorus regeneration by zooplankton. Using a model with equations describing these relationships, the biomass of blue-green algae and other phytoplankton groups was predicted for 14 polluted lakes. A good approximation of actual values was obtained only for the biomass of blue-green algae calculated from the estimated rate of P regeneration by zooplankton in these lakes. It is hypothesized that more-or-less edible algae of other classes did not show dependence on the rate of input of regenerated P because their biomass was heavily reduced by grazing of zooplankton.     

Food web manipulation in Lake Zwemlust: Positive and negative effects during the first two years

The hypertrophic Lake Zwemlust, a small water body used as a swimming pool, was characterized by algal blooms in summer, reducing the Secchi disk transparency to less than 0.3 m. Since in The Netherlands a Secchi disk transparency of 1 m is obligatory for swimming waters, corrective measures were called for to improve the light climate of the lake. In March, 1987, as an experiment, the lake was drained by pumping out the water to facilitate fish elimination. Planktivorous and benthivorous fish species, which were predominant, were removed by seine- and electro-fishing. After the lake had refilled by seepage it was restocked by a new simple fish community comprising pike (Esox lucius) and rudd (Scardinius erythrophthalmus) only. Stacks of willow twigs (Salix) and macrophytes (roots ofNuphar lutea and seedlings ofChara globularis) were introduced into the lake as spawning grounds and refuges for the pike against cannibalism and as shelter for the zooplankton. The effects of this food web manipulation on the light climate, phytoplankton, zooplankton, fish, macrophytes, macrofauna and on the nutrient concentrations were monitored during 1987 and 1988. In summer 1987, despite of high nutrient concentrations, the phytoplankton density was low, due to control by zooplankton, causing a Secchi disk transparency of 2.5 m, the maximum depth. Chlorophyll-a concentrations were low (<5 g Chl.l^–1), blooms of cyanobacteria did not occur and a shift from rotifers to cladocerans took place. In 1988, however, also some negative effects were noticed. Macrophytes and filamentous green algae reached a much higher biomass (50–60% cover of the lake bottom) than in 1987; some species, growing through the entire water column, interfered with the lake's recreational use. Associated with the macro-vegetation and possibly with the absence of larger cyprinids, the diet of which also comprises snails, a large scale development of the snail population, among themLymnaea peregra var.ovata took place. This species is known to act as an intermediate host of the bird-parasitizing trematodeTrichobilharzia ocellata, the cercariae of which cause an itching sensation at the spot of penetration of the human skin, accompanied by rash (schistosome dermatitis or swimmers' itch); in July, 1988, about 40% of the bathers complained about this itching. A positive effect of the macrophytes and filamentous green algae was the high uptake of nitrogen, resulting in a low nitrogen concentration in the lake and growth limitation of the phytoplankton population by nitrogen in the summer of 1988. In 1988 the cladocerans were abundant in April only; and unlike in 1987, in the summer of 1988 there was a shift from cladocerans to rotifers. Therefore, only in early spring (April) zooplankton grazing controlled phytoplankton growth and in summer nitrogen limitation was the major controlling factor, keeping chlorophyll-a concentrations low.     

Determination of available phosphorus for phytoplankton populations in lakes and rivers of southeastern Norway

A biotest method with diluted phytoplankton populations was used to determine external concentrations of available phosphorus in water samples with high concentrations of inorganic seston from River Rømua. RP (total molybdate reactive P measured on unfiltered samples) was approximately the P fraction available for Synedra cf. acus, Asterionella formosa and Oscillatoria agardhii. In filtered samples RP_F was the available concentration of P.The ratio RP:RP_F may give valuable information on the ratio between total available P (including available P in seston) and available P in filtered water. Different filter types may give different RP: RP_F ratios. The ratio RP:RP_F was often high during spring and autumn in River Rømua and the lakes studied. During the period June–September RP:RP_F 1 in most of the lakes and periodically in River Rømua.     

Eutrophication mediates rapid clonal evolution in Daphnia pulicaria

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Daphnia invasion: population dynamics of Daphnia assemblages in two eutrophic lakes with particular reference to the introduced alien Daphnia ambigua

The population dynamics of Daphnia species (D.cucullata, D.ambigua,D.galeata) in two small, productive lakes were studied and experimentswith field enclosures were carried out to elucidate the ecologicalniches of the introduced, alien species D.ambigua. Daphnia ambiguocoexisted temporally with the similarly sized D.cucullata, butlived in deeper water. Although both species had the same clutchsizes, similar proportions of ovigerous females, and death rates.D.cucullata was nurnerically dominant over D.ambigua. This suggeststhat the observed difference in abundance is simply a resultof the slower population growth rates of D.ambigua at lowertemperatures in greater depths. The experiments with field enclosuresshowed that when D.culcullata and D.ambigua were forced to livein the same layer of water, D.ambigua was numerically dominantover D.culullata both in the enclosures with and without fish.As the difference between the abundances of the two speciesdecreased in the enclosure with fish, D.cucullata seems to bebetter protected against fish predation than D.ambigua. In theenclosures without fish, D.ambigua and D.cucullata were numericallysurpassed by the larger species D.galeara. The present resultsimply that in some productive lakes with a high fish stock,D.mbigua invaded the niche of D.galeara and that the speciescan become an important component of the native zooplankton.The results also support the opinion that co-existing Daphniacan have very similar dynamics and niches. ¹ Present address: Zoologisches Institut, Christian-Albrechts-UniversitätKiel, Olshausenstrasse 40, D-24098 Kiel, FRG