Seasonal succession of zooplankton in the north basin of Lake Biwa

To examine the seasonal succession of the entire zooplankton community in Lake Biwa, zooplankton biomass (on an areal basis) and its distribution patterns among crustaceans, rotifers and ciliates were studied in the north basin from April 1997 to June 1998. Seasonal changes in phytoplankton and population dynamics of Daphnia galeata were also examined to assess food condition and predation pressure by fish. From March to November, crustaceans dominated zooplankton biomass, but rotifers and ciliates were dominant from December to February. Among crustaceans, Eodiaptomus japonicus was the most abundant species, followed by D. galeata. Zooplankton biomass increased from January to a peak in early April, just before the spring bloom of phytoplankton, then decreased in mid-April when mortality rate of D. galeata increased. From mid-June, zooplankton increased and maintained a high level until the beginning of November. During this period, both birth and mortality rates of D. galeata were relatively high and a number of rotifer and crustacean species were observed. However, their abundances were very limited except for E. japonicus which likely preys on ciliates and rotifers. In Lake Biwa, food sources other than phytoplankton, such as resuspended organic matter from the sediments, seems to play a crucial role in zooplankton succession from winter to early spring, while zooplankton community seems to be regulated mainly by fish predation from summer to fall.     

Predator-released compounds, ambient temperature and competitive exclusion among differently sized Daphnia species

1. We studied the effects of fish water and temperature on mechanisms of competitive exclusion among two Daphnia species in flow‐through microcosms. The large‐bodied D. pulicaria outcompeted the medium sized D. galeata × hyalina in fish water, but not in the control treatment. Daphnia galeata × hyalina was competitively displaced 36 days earlier at 18 °C than at 12 °C. 2. It is likely that the high phosphorus content of fish water increased the nutritional value of detrital seston particles by stimulating bacterial growth. Daphnia pulicaria was presumably better able to use these as food and hence showed a more rapid somatic growth than its competitor. This led to very high density of D. pulicaria in fish water, but not in the controls. The elevated D. pulicaria density coincided with high mortality and reduced fecundity in D. galeata × hyalina, resulting in competitive displacement of the hybrid. 3. It is clear that the daphnids competed for a limiting resource, as grazing caused a strong decrease in their seston food concentration. However, interference may also have played a role, as earlier studies have shown larger Daphnia species to be dominant in this respect. The high density of large‐bodied D. pulicaria in fish water may have had an allelopathic effect on the hybrid. Our data are inconclusive with respect to whether the reached seston concentration was below the threshold resource level (R*) of the hybrid, where population growth rate and mortality exactly balance, as it would be set in the absence of interference, or whether interference actually raised the hybrid's R* to a value above this equilibrium particle concentration. 4. Our results do clearly show that fish‐released compounds mediated competitive exclusion among zooplankton species and that such displacement occurred at a greatly enhanced rate at an elevated temperature. Fish may thus not only structure zooplankton communities directly through size‐selective predation, but also indirectly through the compounds they release.     

Effects of zooplankton on the sinking flux of organic carbon in Lake Biwa

To clarify the roles of zooplankton in the sedimentation of seston from the epilimnion, the sinking flux of particulate carbon was measured along with primary production rate and zooplankton biomass from July 1996 to October 1997 at a pelagic site in the north basin of Lake Biwa. During the study period, the flux varied seasonally from 66 to 510 mg C m⁻² day⁻¹ and was low in summer when zooplankton, composed mainly of Eodiaptomus japonicus and Daphnia galeata, were abundant. Simple correlation analysis revealed that the sinking flux correlated neither with the primary production rate nor with the amount of sestonic carbon above the sediment trap. However, the particle elimination rate, estimated as the difference between the primary production rate and the sinking flux, correlated positively with the zooplankton biomass. These results suggest that zooplankton play a substantial role in decreasing the sinking flux in Lake Biwa. Received: March 6, 2000 / Accepted: October 7, 2000     

Contrasting migration behaviour of Daphnia pulicaria and D. galeata x hyalina, in avoidance of predation by 0+perch (Perca fluviatilis)

In spring and early summer, a small population of the large-bodied Daphnia pulicaria coexists with a much larger population of the medium-sized hybrid Daphnia galeata × hyalina in the epilimnion of Lake Maarsseveen (The Netherlands). When large shoals of juvenile perch (Perca fluviatilis) appear in the open water, both species start to migrate vertically. Since D. pulicaria has a larger body-size than D. galeata × hyalina, and is therefore competitive dominant over the hybrid, it is unlikely that both species interact via their common food resource, but they react both to fish predation. However, since they differ in size, and therefore in vulnerability for fish predation, both species adopt different strategies. The smaller bodied, and less vulnerable D. galeata × hyalina exhibits diel vertical migration ascending to the surface at dusk, and staying there during the night. In this way, benefiting from the higher temperatures of the surface layers. In contrast, the large-bodied, and more vulnerable D. pulicaria selects the deep cold hypolimnion water layers as refuge against fish predation. In this way it benefits from a safe habitat, free from fish predators, but on the other hand suffers from low water temperatures, which decrease its fitness. It is likely that the relatively higher temperature in the upper water layers serves as a proximate factor for the downward migration of D. pulicaria.     

Temperature and kairomone induced life history plasticity in coexisting Daphnia

We investigated the life history alterations of coexisting Daphnia species responding to environmental temperature and predator cues. In a laboratory experiment, we measured Daphnia life history plasticity under different predation risk and temperature treatments that simulate changing environmental conditions. Daphnia pulicaria abundance and size at first reproduction (SFR) declined, while ephippia (resting egg) formation increased at high temperatures. Daphnia mendotae abundance and clutch size increased with predation risk at high temperatures, but produced few ephippia. Thus, each species exhibited phenotypic plasticity, but responded in sharply different ways to the same environmental cues. In Glen Elder reservoir, Kansas USA, D. pulicaria dominance shifted to D. mendotae dominance as temperature and predation risk increased from March to June in both 1999 and 2000. Field estimates of life history shifts mirrored the laboratory experiment results, suggesting that similar phenotypic responses to seasonal cues contribute to seasonal Daphnia population trends. These results illustrate species-specific differences in life history plasticity among coexisting zooplankton taxa.     

Environmentally controlled Daphnia spring increase with implications for sockeye salmon fry in Lake Washington, USA

In Lake Washington, juvenile sockeye salmon (Oncorhynchus nerka)strongly prefer Daphnia over other prey, switching uniformlyto Daphnia when the threshold abundance of 0.4 Daphnia L^–1is achieved. Using long-term Lake Washington data (1978–2001)and fry trap data (1992–2001) from a major tributary,we examined the following: (i) factors that predict Daphniapulicaria and Daphnia thorata increase to this threshold "switching"abundance, (ii) trends in Daphnia dynamics that may affect sockeyeforaging and (iii) temporal correspondence of Daphnia increaseand fry arrival. The winter abundance of D. pulicaria, in combinationwith basic parameters of spring conditions, was an importantpredictor of the date of D. pulicaria spring increase, indicatinggreater reliance on pelagic population dynamics (versus diapausehatch) than D. thorata exhibited. In addition, D. pulicariawas a more consistent prey than D. thorata, the latter exhibitinglarger population fluctuations. Thus, recently increasing D.thorata prominence could decrease diet consistency for sockeyefry. Additionally, the timing of sockeye arrival to Lake Washingtonand Daphnia’s increase to the switching threshold hasbecome less concordant, so that fry in recent years have hadto rely upon less profitable prey for longer periods. Long-termtrends and species-specific differences in Daphnia phenologymay affect fry through altering diet composition, with additionalimplications for other zooplankton withstanding greater predationpressure in Daphnia’s absence. Recent decades of warmingin Lake Washington are consistent with the warming of lakesworldwide, and complex phenological responses such as thosereported here may be common as the climate continues to change.     

Influence of different sized Daphnia species on chlorophyll concentration and summer phytoplankton community structure in eutrophic Wisconsin lakes

We conducted field observations on 13 eutrophic Wisconsin lakesdominated by either the larger bodied Daphnia pulicaria or thesmaller bodied Daphnia galeata mendotae. While Daphnia numericaldensities were not significantly different between groups oflakes, pulicaria lakes had much higher Daphnia biomasses andfiltration potentials than galeata lakes. Although we foundsignificant differences in chlorophyll (Chl) a between bothgroups of lakes during June, on a seasonal basis populationsof different sized Daphnia were not associated with significantdifferences in Chl a. Filtration potential per se was the majordeterminant of Chl a, regardless of which Daphnia species dominated.However, in pulicaria lakes, the clear-water phase started earlier,lasted longer, and was usually characterized by greater Secchidisc readings than in galeata lakes. For large blue-green algaesuch as Aphanizomenon, D.pulicaria appeared to delay bloom conditions,but ultimately did not prevent the alga from growing. Our resultssuggest that high densities of large-sized Daphnia are a desirablegoal of biomanipulation because they can attain filtration potentialshigh enough to increase summer water clarity in eutrophic lakes.     

Effects of stoichiometric dietary mixing on<Emphasis Type="Italic"> Daphnia</Emphasis> growth and reproduction

Herbivores often encounter nutritional deficiencies in their diets because of low nutrient content of plant biomass. Consumption of various diet items with different nutrient contents can potentially alleviate these nutritional deficiencies. However, most laboratory studies and modeling of herbivorous animals have been done with diets in which all food has uniform nutrient content. It is not clear whether heterogeneous versus uniform food of equal overall nutrient content is of equivalent nutritional value. We tested the effects of dietary mixing on performance of a model organism, Daphnia. We fed two species of Daphnia ( D. galeata, D. pulicaria) with diets of equivalent bulk stoichiometric food quality (C:P) and studied whether they would produce equivalent performance when C:P was uniform among cells or when the diet involved a mixture of high C:P and low C:P cells. Daphnia were fed saturating and limiting concentrations of a uniform food of moderate C:P (UNI) or mixtures (MIX) of high C:P (LOP) and low C:P (HIP) algae prepared to match C:P in UNI. Daphnia were also fed HIP and LOP algae separately. Juvenile growth rate and adult fecundity were measured. D. galeata performance in UNI and MIX treatments did not differ, indicating that partitioning of C and P among particles did not affect dietary quality. Similarly, D. pulicarias performance was similar in the MIX and UNI treatments but only at low food abundance. In the high food treatment, both growth and reproduction were higher in the MIX treatment, indicating some benefit of a more heterogeneous diet. The mechanisms for this improvement are unclear. Also, food quality affected growth and reproduction even at low food levels for both D. pulicaria and D. galeata. Our results indicate that some species of zooplankton can benefit from stoichiometric heterogeneity on diet.     

Planktivory by bluegill (Lepomis macrochirus) on Leptodora kindti in a small North American lake

Leptodora kindti (Crustacea: Cladocera) is a large species of zooplankton (2–18 mm length) that is exceptionally transparent. This transparency is believed to be a means by which it successfully coexists in lakes with planktivorous fishes. We investigated the gut remains of bluegill (Lepomis macrochirus) that had been feeding on L. kindti and Daphnia (D. galeata and D. retrocurva) in the wild (Lake Zurich, Illinois) and found that bluegill readily preyed on L. kindti as small as 3–5 mm length, and strongly selected L. kindti over Daphnia galeata and Daphnia retrocurva. The large compound eye of L. kindti is one half to one complete order of magnitude larger than Daphnia's eye, consistent with the hypothesis that eye area is an important visual cue for fishes. Moreover, the slope of the relationship between eye area and body length is an order of magnitude shallower in L. kindti than Daphnia, suggesting that eye area has been under stronger negative selection in L. kindti. Results suggest that L. kindti's large and dark eye compromises the transparent nature of its body.     

Reproduction strategies of <Emphasis Type="Italic">Daphnia pulicaria</Emphasis> population in a high mountain lake of Southern Spain

For three consecutive years, a population study of Daphnia pulicaria was undertaken in Río Seco Lake, a small high-mountain lake, in order to elucidate the reproductive strategies adopted by Daphnia in this system. Daphnia appears to colonize this lake every spring by hatching from ephippia and reproduce by means of subitaneous (non-diapausing) and ephippial (diapausing) eggs. D. pulicaria in this lake is an obligate parthenogenetic population. There is a short time period for subitaneous egg production and a much longer period for ephippial egg production. The contribution of subitaneous eggs to Daphnia population density and structure appears to be low. Diapause onset showed a high temporal synchronization in the three studied years in Río Seco Lake, and day-length emerged as the main cue triggering diapause onset and the main explanatory factor for the proportion of ephippial females observed. The development and reproduction of D. pulicaria in Río Seco Lake involves taking a gamble on resting forms to guarantee inter-annual Daphnia persistence in the lake, giving priority to investment in future generations. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Effect of Body Size on Toxicity of Zinc in Neonates of Four Differently Sized <Emphasis Type="Italic">Daphnia</Emphasis> Species

The sensitivity of neonates of four Daphnia species to zinc was tested in relation to their mean body size. These mean sizes of these four Daphnia spp were: D. magna, 0.813 ± 0.055 mm, D.␣pulicaria, 0.745 ± 0.063 mm, D. pulex, 0.645 ± 0.044 mm and D. galeata, 0.611 ± 0.058 mm. A positive relationship between EC₅₀ (24, 48) values and neonates size was found. The smaller the size of the daphnid the higher was the sensitivity to heavy metal toxicity. For all tested species did the EC₅₀ values decrease with time; the decrease was most marked for D. magna and the least for D. galeata. The EC₅₀ values of D. magna were higher than would be expected on basis of its body size.     

The effect of suspended sediments on Lake Texoma Daphnia: field distributions and in situ incubations

1. We measured the abundance and eggs per female of four Daphnia species in turbid and relatively clear regions of Lake Texoma (Oklahoma‐Texas, U.S.A.) on 12 dates over the course of 5 years. 2. Two species, Daphnia lumholtzi and Daphnia parvula, occurred and reproduced in turbid locations, but two other species, Daphnia mendotae and Daphnia pulicaria, occurred almost exclusively in relatively clear conditions. 3. To test the hypothesis that interference with foraging excludes clear‐water Daphnia species from turbid locations, we incubated adult D. mendotae at both a clear and a turbid site. In three successive experiments D. mendotae individuals incubated at the turbid site carried as many or more eggs than individuals incubated at the clear site.     

Trophic interactions and habitat segregation between competing Daphnia species

Summary Two commonly coexisting species of Daphnia segregate by habitat in many stratified lakes. Daphnia pulicaria is mostly found in the hypolimnion whereas D. galeata mendotae undergoes diel vertical migration between the hypolimnion and the epilimnion. I examined how habitat segregation between these two potentially competing species might be affected by trophic interactions with their resources and predators by performing a field experiment in deep enclosures in which I manipulated fish predation, nutrient levels, and the density of epilimnetic Daphnia. The results of the experiment indicate that habitat use by D. pulicaria can be jointly regulated by competition for food from epilimnetic Daphnia and predation by fishes. Patterns of habitat segregation between the two Daphnia species were determined by predation by fish but not by nutrient levels: The removal of epilimnetic fish predators resulted in higher zooplankton and lower epilimnetic phytoplankton densities and allowed D. pulicaria to expand its habitat distribution into the epilimnion. In contrast, increased resource productivity resulted in higher densities of both Daphnia species but did not affect phytoplankton levels or habitat use by Daphnia. The two species exhibit a trade-off in their ability to exploit resources and their susceptibility to predation by fish. D. g. mendotae (the less susceptible species) may thus restrict D. pulicaria (the better resource exploiter) from the epilimnion when fish are common due to lower minimum resource requirements than those needed by D. pulicaria to offset the higher mortality rate imposed by selective epilimnetic fish predators. D. g. mendotae does not appear to have this effect in the absence of fish.     

Fluctuation of the zooplankton community in Lake Biwa during the 20th century: a paleolimnological analysis

Detailed zooplankton records from a 26-cm sediment core with a time resolution of approximately 3–10 years were obtained from Lake Biwa, Japan, to examine the historical variations in the zooplankton community during the 20th century. In the sediments, selected zooplankton remains have fluctuated over the years. Daphnia – large zooplankton herbivores – did not occur from 1900 to 1920, and formed a very minor component of the zooplankton community in the following 30 years, while Bosmina – small zooplankton herbivores – were common during this period. In the mid-1960s, however, when eutrophication was noticeable in this lake, Daphnia numbers increased dramatically and became the dominant zooplankton thereafter. In contrast, Difflugia brevicolla and D. biwae, two amoeboid protozoans that live in connection with the lake bottom environment, occurred abundantly until the late 1950s, but gradually decreased after the mid-1960s. In particular, D. biwae, a species peculiar to this lake, was not found in sediment dated after 1980, suggesting its extinction. These results indicate that the zooplankton community structure changed greatly in the 1960s, and suggest that the eutrophication occurring at this time altered the relative strength of top-down and bottom-up forces on the zooplankton community in Lake Biwa.     

Coupling of growth rate and body stoichiometry in Daphnia: a role for maintenance processes?

1. The growth rate hypothesis predicts positive relationships among growth rate (μ), body RNA (%RNA of dry mass) and body P (%P of dry mass) contents. 2. We tested this within‐ and across‐species by growing five species/clones of Daphnia (Daphnia magna, Daphnia pulex, Daphnia galeata and two isolates of Daphnia pulicaria) with different combinations of food quantity and stoichiometric food quality. 3. Within each species, positive correlations among μ, %RNA and %P were seen and across species there was a strong association between%RNA and %P, consistent with the growth rate hypothesis. However, coupling of growth to %RNA and to %P differed for different species. In particular, the %RNA–μ and %P–μ relationships had similar slopes but considerably different y‐intercepts (i.e.%P or %RNA at zero growth), with D. pulicaria and D. galeata having higher intercepts than D. magna and especially D. pulex. As a result of these displacements, the relative rankings of the species on the basis of %P and %RNA did not correspond to their rankings based on μ. 4. These findings suggest that within a narrow clade (e.g. the daphnids), interspecific differences in body P content may reflect not growth rate‐related RNA allocation but instead the amount of RNA required for support of maintenance processes.     

Filamentous cyanobacteria,temperature and <Emphasis Type="Italic">Daphnia</Emphasis> growth: the role of fluid mechanics

Viscosity increases significantly with a fall in water temperature, thus temperature change affects not only the metabolic rates of aquatic suspension feeders, but also the physical properties of the surrounding fluid. This mechanistic effect of water temperature change on growth was separated from the effect of metabolism by using culture media with modified viscosity, while the temperature was kept constant. The effect of water viscosity on growth rate and feeding of four Daphnia species (D. magna, D. pulicaria, D. hyalina, D. galeata) was investigated. Increased viscosity decreased the growth rate significantly for three species, with the exception of D. galeata. Changing viscosity also affects growth qualitatively: the filamentous blue-green Cylindrospermopsis raciborskii reduces the growth rate of D. pulicaria at low viscosity, but its negative effect disappears when viscosity is higher. The findings are consistent with the hypothesis that it is the Reynolds number of the filtering appendages that determines the qualitative features of Daphnia filtration. The edibility of C. raciborskii at high water viscosity is most probably caused by lack of interference with filtering combs, and explains the coexistence of D. pulicaria with filamentous blue-green species in the field, and also the observed temperature dependence of growth inhibition of filaments.Electronic Supplementary Material Supplementary material is available for this article at     

Bacterioplankton communities of Crater Lake,OR: dynamic changes with euphotic zone food web structure and stable deep water populations

The distribution of bacterial and archaeal species in Crater Lake plankton varies dramatically over depth and with time, as assessed by hybridization of group-specific oligonucleotides to RNA extracted from lakewater. Nonmetric, multidimensional scaling (MDS) analysis of relative bacterial phylotype densities revealed complex relationships among assemblages sampled from depth profiles in July, August and September of 1997 through 1999. CL500-11 green nonsulfur bacteria (Phylum Chloroflexi) and marine Group I crenarchaeota are consistently dominant groups in the oxygenated deep waters at 300 and 500 m. Other phylotypes found in the deep waters are similar to surface and mid-depth populations and vary with time. Euphotic zone assemblages are dominated either by β-proteobacteria or CL120-10 verrucomicrobia, and ACK4 actinomycetes. MDS analyses of euphotic zone populations in relation to environmental variables and phytoplankton and zooplankton population structures reveal apparent links between Daphnia pulicaria zooplankton population densities and microbial community structure. These patterns may reflect food web interactions that link kokanee salmon population densities to community structure of the bacterioplankton, via fish predation on Daphnia with cascading consequences to Daphnia bacterivory and predation on bacterivorous protists. These results demonstrate a stable bottom-water microbial community. They also extend previous observations of food web-driven changes in euphotic zone bacterioplankton community structure to an oligotrophic setting.     

Identifying century-old long-spined <Emphasis Type="Italic">Daphnia</Emphasis>: species replacement in a mountain lake characterised by paleogenetic methods

Mountain lakes often harbour morphologically or genetically unique populations of zooplankton species, including cladocerans. Daphnia lacustris Sars, predominantly found in Fennoscandia but also known from two Central European lakes in the Tatra Mountains, is one of such taxa. This Daphnia species often forms morphotypes with extremely long tailspines. Historical literature from a century ago documented similar morphs from another lake in the Tatra mountain range, presently inhabited by the phenotypically very different D. galeata. Using a paleogenetic approach (partial sequencing of the mitochondrial gene for 12S rRNA from preserved ephippial eggs in the lake sediment), we tested the hypothesis that Daphnia species composition changed in the lake due to anthropogenic disturbances, and that long-spined morphs were actually another relict population of currently extinct D. lacustris. Ephippia with extremely long spines were successfully retrieved from sediment cores. Despite being morphologically very well preserved, intact eggs were found in less than 2% of analysed ephippia. Genetic analyses, benefiting in most cases from amplification of short 12S fragments using internal primers, proved that long-spined ephippia belonged to D. longispina, which apparently coexisted with D. galeata in the mid-twentieth century. Our results confirm that paleogenetic methods are useful for studying the recent population structures of zooplankton species forming dormant egg banks but lacking reliably identifiable remains in sediments, and show that the extreme development of tailspines in mountain-lake Daphnia is associated with as-yet unclear environmental factors rather than taxonomic status.     

Postembryonal morphological variation of <Emphasis Type="Italic">Daphnia galeata</Emphasis> in water bodies of different types

Despite many ecological studies the population morphological variability within the freshwater crustacean genus Daphnia is poorly investigated, especially during postembryonic development. Unusual phenotypic plasticity of some Daphnia species results in tremendous difficulties in morphological species delineation. The ontogenetic morphological variation of this species was studied, revealing the general trends in body shape variation in different populations. The morphotypes of size and age groups of D. galeata turned out to be more variable at the mouth of the Kargat River (Lake Chany basin) than in Lake Todzha (Bol’ shoi Yenisei River basin); however, the growth of characters such as the helmet and tail spine was described by an allometric function and their absolute sizes decreased with age in Daphnia from both water basins. It is shown that the first mature size-and-age group of D. galeata is most suitable for investigating population morphological variation.