Contrasting ‘Top-Down’ Effects of Crustacean Zooplankton Grazing on Bacteria and Phytoflagellates

The combined effects of grazing and nutrient regeneration by Daphnia and Eudiaptomus on the growth of Rhodomonas and heterotrophic bacteria was assessed experimentally. The responses of Rhodomonas and bacteria to the grazers were measured as net specific growth rate over the entire experimental periods, as well as production and specific production at the end of the experiments. Both zooplankton species had a negative effect on Rhodomonas net specific growth rate due to grazing and a positive effect on specific primary production due to nutrient regeneration. Daphnia had no effect on bacterial net specific growth rate, bacterial production or specific bacterial production in one of two experiments. In the other experiment, however, both bacterial growth rate and production decreased as a result of grazing. Furthermore, Daphnia had a negative effect on specific bacterial production, but Eudiaptomus had a positive effect on all bacterial parameters due to nutrient regeneration, probably of phosphorus. Positive effects of copepods on bacterial growth has previously been attributed to trophic cascades via protozoa. However, the present experiments show that regeneration of nutrients, especially phosphorus, may account for a large part of the stimulation of bacterial growth.     

Cascading predation effects of Daphnia and copepods on microbial food web components

1. We performed a mesocosm experiment to investigate the structuring and cascading effects of two predominant crustacean mesozooplankton groups on microbial food web components. The natural summer plankton community of a mesotrophic lake was exposed to density gradients of Daphnia and copepods. Regression analysis was used to reveal top–down impacts of mesozooplankton on protists and bacteria after days 9 and 15. 2. Selective grazing by copepods caused a clear trophic cascade via ciliates to nanoplankton. Medium‐sized (20–40 μm) ciliates (mainly Oligotrichida) were particularly negatively affected by copepods whereas nanociliates (mainly Prostomatida) became more abundant. Phototrophic and heterotrophic nanoflagellates increased significantly with increasing copepod biomass, which we interpret as an indirect response to reduced grazing pressure from the medium‐sized ciliates. 3. In Daphnia‐treatments, ciliates of all size classes as well as nanoflagellates were reduced directly but the overall predation effect became most strongly visible after 15 days at higher Daphnia biomass. 4. The response of bacterioplankton involved only modest changes in bacterial biomass and cell‐size distribution along the zooplankton gradients. Increasing zooplankton biomass resulted either in a reduction (with Daphnia) or in an increase (with copepods) of bacterial biovolume, activity and production. Patterns of bacterial diversity, as measured by polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE), showed no distinct grouping after 9 days, whereas a clear treatment‐coupled similarity clustering occurred after 15 days. 5. The experiment demonstrated that zooplankton‐mediated predatory interactions cascade down to the bacterial level, but also revealed that changes occurred rather slowly in this summer plankton community and were most pronounced with respect to bacterial activity and composition.     

Zooplankton-mediated changes of bacterial community structure

Enclosure experiments in the mesotrophic Schöhsee in northern Germany were designed to study the impact of metazooplankton on components of the microbial food web (bacteria, flagellates, ciliates). Zooplankton was manipulated in 500-liter epilimnetic mesocosms so that either Daphnia or copepods were dominating, or metazooplankton was virtually absent. The bacterial community responded immediately to changes in zooplankton composition. Biomass, productivity, and especially the morphology of the bacteria changed drastically in the different treatments. Cascading predation effects on the bacterioplankton were transmitted mainly by phagotrophic protozoans which had changed in species composition and biomass. When Daphnia dominated, protozoans were largely suppressed and the original morphological structure of the bacteria (mainly small rods and cocci) remained throughout the experiment. Dominance of copepods or the absence of metazoan predators resulted in a mass appearance of bacterivorous protists (flagellates and ciliates). They promoted a fast decline of bacterial abundance and a shift to the predominance of morphologically inedible forms, mainly long filaments. After 3 days they formed 80–90% of the bacterial biomass. The results indicate that metazooplankton predation on phagotrophic protozoans is a key mechanism for the regulation of bacterioplankton density and community structure.Correspondence to: K. Jürgens.     

Plankton community of a polyhumic lake with and without Daphnia longispina (Cladocera)

The impact of Daphnia longispina (Cladocera) on the plankton food web was studied in a polyhumic lake where this species comprised almost all zooplankton biomass. Plastic enclosures (volume 7 m³) were inserted into the lake retaining the initial water stratification except that in one enclosure zooplankton was removed. After the removal of Daphniaa rotifer, Keratella cochlearis, ciliates and heterotrophic nanoflagellates increased markedly and the density and biomass of bacteria decreased. Edible algal species, Cryptomonas rostratiformisand three small chrysophytes,Ochromonas, Pedinella and Spinifermonas, took advantage of the removal of Daphnia, while more grazing-resistant species declined. In spite of the changes in the species composition of phytoplankton, the removal of Daphnia did not affect the biomass, primary production or respiration of plankton. The results implied that the density of heterotrophic flagellates and ciliates was controlled by Daphnia, but in its absence the former took its role as the bacterial grazers.     

The consequences of a drastic fish stock reduction in the large and shallow Lake Wolderwijd,The Netherlands. Can we understand what happened?

In 1990 an experiment started in the large and shallow lake Wolderwijd (2700 ha, mean depth 1.5 m) to improve the water quality. About 75% of the fish stock was removed (425 000 kg fish). The fish was mainly composed of bream and roach. In May 600000 young pikes (3–4 cm) were introduced.In May 1991 the water became very clear (Secchi depth 1.8 m) during a spring bloom of large Daphnia. Then the grazing by zooplankton was eight times higher than the primary production of algae and the total suspended matter concentration became very low. Compared to the situation before the fish reduction, the grazing had increased only slightly, while the primary production had decreased significantly in early spring. The fish stock reduction might have contributed to the reduction in primary production by a reduced internal nutrient load. The clear water period lasted six weeks. Daphnia disappeared in July due to food limitation, the algal biomass increased and the Secchi depth became 50 cm. Daphnia did not recover during summer, due to predation that was not caused by 0 + fish but by the mysid shrimp Neomysis integer. Neomysis could develop abundantly, because of the reduced biomass of the predator perch. The production of young fish had been low because of the cold spring weather. The cold weather was probably also responsible for the slow increase in density of macrophytes. After 1991, perch probably can control Neomysis. Due to lack of spawning places and shelter for 0 + pike, pike was probably not able to control the production of 0 + fish. In a lake of this scale, it will not be easy to get more than 50% coverage of macrophytes, which seems necessary to keep the algal biomass low by nutrient competition. Therefore, we expect also in the future a decrease in transparency in the summer. Locally, especially near Characeae, the water might stay clear.     

Priority effects among young‐of‐the‐year fish: reduced growth of bluegill sunfish (Lepomis macrochirus) caused by yellow perch (Perca flavescens)?

1. When available, Daphnia spp. are often preferred by age‐0 yellow perch and bluegill sunfish because of energetic profitability. We hypothesised that predation by age‐0 yellow perch could lead to a midsummer decline (MSD) of Daphnia spp. and that priority effects may favour yellow perch because they hatch before bluegill, allowing them to capitalise on Daphnia spp. prior to bluegill emergence. 2. Data were collected from 2004 to 2010 in Pelican Lake, Nebraska, U.S.A. The lake experienced a prolonged MSD in all but 1 year (2005), generally occurring within the first 2 weeks of June except in 2008 and 2010 when it occurred at the end of June. MSD timing is not solely related to seasonal patterns of age‐0 yellow perch consumption. Nevertheless, when Daphnia spp. biomass was low during 2004 and 2006–2010 (<4 mg wet weight L^?1), predation by age‐0 yellow perch seems to have suppressed Daphnia spp. biomass (i.e. <1.0 mg wet weight L^?1). The exception was 2005 when age‐0 yellow perch were absent. 3. Growth of age‐0 bluegill was significantly faster in 2005, when Daphnia spp. were available in greater densities (>4 mg wet weight L^?1) compared with the other years (<0.2 mg wet weight L^?1). 4. We conclude that age‐0 yellow perch are capable of reducing Daphnia biomass prior to the arrival of age‐0 bluegill, ultimately slowing bluegill growth. Thus, priority effects favour age‐0 yellow perch when competing with age‐0 bluegill for Daphnia. However, these effects may be minimised if there is a shorter time between hatching of the two species, higher Daphnia spp. densities or lower age‐0 yellow perch densities.     

Zooplankton abundance and composition in the hypertrophic Rietvlei Dam,South Africa,negate prospects for its remedial ‘top-down’ biomanipulation

Crustacean zooplankton abundance and composition were determined at one offshore and three nearshore sites in the hypertrophic Rietvlei Dam on 19 dates between July 2009 and December 2011. Total biomass fluctuated seasonally, generally declining from spring to winter through the annual cycle, but also appeared to decline progressively through the study. On overall average, total biomass was high (0.51 mg l^–1 or 2.39 g m^–2 DW), with Daphnia accounting for ～40%. Total volumetric biomass was invariably higher in shallow nearshore than offshore locations (average time-paired ratio = 8.1), with comparably large-bodied Daphnia (geometric mean ～1.2 mm, largest individuals ～1.75 mm) in both habitats, contra-indicating substantive opportunistic zooplanktivory by juvenile fish. Zooplankton was dominated numerically (ind. l^–1, overall average values) by cyclopoid copepods, mostly Thermocyclops (47.1 nauplii and 86.5 copepodites), plausibly favoured by their selective raptorial feeding mode. Cladoceran densities were lower — Daphnia (25.0), Bosmina (7.1), Ceriodaphnia (2.9) and sporadically occurring Moina and Chydorus (<0.5). Seasonal replacement of Daphnia by small-bodied cladocerans during late summer and into autumn was evident in near-monthly samples between July 2009 and June 2010. The findings reflect negligible zooplanktivory in Rietvlei, ruling out top-down biomanipulation prospects for its remedial management.     

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Although both nutrient inputs and zooplankton grazing are importantto phytoplankton and bacteria in lakes, controversy surroundsthe relative importance of grazing pressure for these two groupsof organisms. For phytoplankton, the controversy revolves aroundwhether zooplankton grazers, especially large cladocerans likeDaphnia, can effectively reduce phytoplankton populations regardlessof nutrient conditions. For bacteria, little is known aboutthe balance between possible direct and indirect effects ofboth nutrients and zooplankton grazing. However, there is evidencethat bacteria may affect phytoplankton responses to nutrientsor zooplankton grazing through direct or apparent competition.We performed a mesocosm experiment to evaluate the relativeimportance of the effects of nutrients and zooplankton grazingfor phytoplankton and bacteria, and to determine whether bacteriamediate phytoplankton responses to these factors. The factorialdesign crossed two zooplankton treatments (unsieved and sieved)with four nutrient treatments (0, 0.5, 1.0 and 2.0 µgphosphorus (P) l^–1 day^–1 together with nitrogen(N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300µm mesh reduced the average size of crustacean zooplanktonin the mesocosms, decreased the numbers and biomass of Daphnia,and increased the biomass of adult copepods. Nutrient enrichmentcaused significant increases in phytoplankton chlorophyll a(4–5x), bacterial abundance and production (1.3x and 1.6x,respectively), Daphnia (3x) and total zooplankton biomass (2x).Although both total phytoplankton chlorophyll a and chlorophylla in the <35 µm size fraction were significantly lowerin unsieved mesocosms than in sieved mesocosms, sieving hadno significant effect on bacterial abundance or production.There was no statistical interaction between nutrient and zooplanktontreatments for total phytoplankton biomass or bacterial abundance,although there were marginally significant interactions forphytoplankton biomass <35 µm and bacterial production.Our results do not support the hypothesis that large cladoceransbecome less effective grazers with enrichment; rather, the differencebetween phytoplankton biomass in sieved versus unsieved zooplanktontreatments increased across the gradient of nutrient additions.Furthermore, there was no evidence that bacteria buffered phytoplanktonresponses to enrichment by either sequestering P or affectingthe growth of zooplankton.     

Studies on the Effects of Silver Carp (Hypophthalmichthys molitrix) on the Phytoplankton in a Shallow Hypereutrophic Lake through an Enclosure Experiment

The responses of nutrients, water transparency, zooplankton and phytoplankton to a gradient of silver carp biomass were assessed using enclosure methods. The gradient of four silver carp biomass levels was set as follows: 0, 116, 176 and 316 g m^—2. Nutrients did not show any statistically significant differences among the treatments. An outburst of Daphnia only occurred in fishless enclosures where phytoplankton biomass was the lowest and water clarity significantly increased. While among fish enclosures, the small‐sized Moina micrura dominated throughout the experiment and both zooplankton and phytoplankton biomasses decreased with increased fish biomass. No large colonial cyanobacterial blooms occurred in the fishless enclosures as predicted. This might be due to low water temperature, short experiment time and the occurrence of large bodied Daphnia in our experiment. Cryptophyta was the most dominant group in most of the enclosures and the lake water throughout the experiment. The fishless enclosure had much lower proportion of Cyanophyta but higher proportion of Trachelomonas sp.     

Field evidence for stoichiometric relationships between zooplankton and N and P availability in a shallow calcareous lake

1. According to stoichiometric theory, zooplankters have a species‐specific elemental composition. Daphniids have a relatively high phosphorus concentration in their tissues and copepods high nitrogen. Daphniids should, therefore, be more sensitive to phosphorus limitation and copepods more sensitive to nitrogen. A 2‐year study of a shallow marl lake in the west of Ireland investigated whether population fluctuations of the two dominant taxa, Daphnia spp. and the calanoid Eudiaptomus gracilis, were associated with the availability of phosphorus and nitrogen. 2. In accordance with stoichiometric predictions, Daphnia and Eudiaptomus reproduction had contrasting relationships with dietary phosphorus and nitrogen availability. Egg production by Daphnia was negatively associated with the ratio of dissolved inorganic nitrogen (DIN) : total phosphorus (TP) and the ratio of light to TP which was used as an indirect index for seston carbon (C) : phosphorus (P). Conversely calanoid egg production had a positive relationship with the DIN : TP ratio and was unrelated to the estimated C : P (light : TP) ratio. 3. Daphnia biomass was not, however, correlated with phosphorus availability, and neither was calanoid biomass correlated with nitrogen. The high ratio of DIN : TP when Daphnia dominated the zooplankton biomass and the low ratio when calanoids dominated, is consistent with Daphnia acting as a sink for phosphorus and calanoids as a sink for nitrogen and suggests consumer‐driven nutrient recycling.     

Distribution and abundance of zooplankton populations in Crater Lake,Oregon

The zooplankton assemblages in Crater Lake exhibited consistency in species richness and general taxonomic composition, but varied in density and biomass during the period between 1988 and 2000. Collectively, the assemblages included 2 cladoceran taxa and 10 rotifer taxa (excluding rare taxa). Vertical habitat partitioning of the water column to a depth of 200 m was observed for most species with similar food habits and/or feeding mechanisms. No congeneric replacement was observed. The dominant species in the assemblages were variable, switching primarily between periods of dominance of Polyarthra-Keratella cochlearis and Daphnia. The unexpected occurrence and dominance of Asplanchna in 1991 and 1992 resulted in a major change in this typical temporal shift between Polyarthra-K. cochlearis and Daphnia. Following a collapse of the zooplankton biomass in 1993 that was probably caused by predation from Asplanchna, Kellicottia dominated the zooplankton assemblage biomass between 1994 and 1997. The decline in biomass of Kellicottia by 1998 coincided with a dramatic increase in Daphnia biomass. When Daphnia biomass declined by 2000, Keratella biomass increased again. Thus, by 1998 the assemblage returned to the typical shift between Keratella-Polyarthra and Daphnia. Although these observations provided considerable insight about the interannual variability of the zooplankton assemblages in Crater Lake, little was discovered about mechanisms behind the variability. When abundant, kokanee salmon may have played an important role in the disappearance of Daphnia in 1990 and 2000 either through predation, inducing diapause, or both. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Relationships between zooplankton community structure and phytoplankton in two lime-treated eutrophic hardwater lakes

1. North Halfmoon Lake and Lofty Lake (Alberta, Canada) were chosen for whole-lake liming experiments as a new restoration technology to enhance calcite precipitation and reduce eutrophication. During a 3-year study (1991–93) the relationships between zooplankton and phytoplankton were assessed, together with the effects of lime additions. 2. Zooplankton communities were numerically dominated by rotifers, while the major contribution to biomass was due to large filter-feeding Daphnia during the first half of the summer season. In Lofty Lake, cladocerans made up to 93% of biomass, whereas in North Halfmoon Lake both cladocerans and calanoids were strongly represented. 3. Total zooplankton and cladoceran biomasses were inversely correlated with chlorophyll a (chl a). The same relationship was found between large Daphnia (≥ 1 mm) and chl a. These relationships suggest that the decline in Daphnia may have been caused by an increase in cyanobacteria biomass during bloom events. 4. There were minor changes in rotifer populations after liming; however, these changes have been caused by natural year-to-year variation rather than liming. In general, cladocerans showed an increase in body size and population biomass when pre and post-treatment data were compared by means of ANCOVA. Statistical analysis showed that there were more cladocerans per unit of chl a after liming; however, further research is needed to relate these patterns unambiguously to the application of lime as a restoration technology.     

Comparative and experimental approaches to top-down and bottom-up regulation of bacteria

The regulation of bacterial community biomass and productivity by resources and predators is a central concern in the study of microbial food webs. Resource or bottom-up regulation refers to the limitation of bacteria by carbon and nutrients derived from allocthonous inputs, primary production, and heterotrophic production. Predatory or top-down regulation refers to the limitation of bacteria below levels supportable by resources alone. Large scale comparative studies demonstrate strong correlations between bacterial productivity and biomass, suggesting significant resource regulation. Comparisons of the abundances of heterotrophic flagellates and bacteria, however, imply that in some cases there may be top-down regulation of bacteria in eutrophic environments. Experimental studies in lakes support the importance of resource regulation and reveal little top-down control from protozoans. Increases in bacterial abundance and production with nutrient enrichment were limited in enclosure experiments with high abundances of the cladoceran, Daphnia. Regulation of bacteria by Daphnia may occur in many lakes seasonally and prevail in some lakes throughout the year where these animals sustain dense populations. In most situations, however, bacteria appear to be limited primarily by resources.     

The effects of Daphnia on nutrient stoichiometry and filamentous cyanobacteria: a mesocosm experiment in a eutrophic lake

1. Stoichiometric theory predicts that the nitrogen : phosphorus (N : P) ratio of recycled nutrients should increase when P‐rich zooplankton such as Daphnia become dominant. We used an enclosure study to test the hypothesis that an increased biomass of Daphnia will increase the relative availability of N versus P sufficiently to decrease the abundance of filamentous cyanobacteria. The experiment was conducted in artificially enriched Lake 227 (L227) in the Experimental Lakes Area (ELA), north‐western Ontario, Canada. Previous studies in L227 have shown that the dominance of filamentous, N‐fixing cyanobacteria is strongly affected by changes in the relative loading rates of N and P. 2. We used a 2 × 2 factorial design with the addition or absence of D. pulicaria and high or low relative loading rates of N and P (+NH₄, –NH₄) in small enclosures as treatment variables. If Daphnia can strongly affect filamentous cyanobacteria by altering N and P availability, these impacts should be greatest with low external N : P loading rates. The phytoplankton community of L227 was predominantly composed of filamentous Aphanizomenon spp. at the start of the experiment. 3. Daphnia strongly reduced filamentous cyanobacterial density in all enclosures to which they were added. The addition of NH₄ had only a small impact on algal community composition. Hence, we conclude that Daphnia did not cause reductions in cyanobacteria by altering the N : P ratio of available nutrients. 4. Despite the lack of evidence that Daphnia affected filamentous cyanobacteria by altering the relative availability of N and P, we found changes in nutrient cycling consistent with other aspects of stoichiometric theory. In the presence of Daphnia, total P in the water column decreased because of an increase in P sedimentation. In contrast to P, a decrease in suspended particulate N was offset by an increase in dissolved N (especially NH₄). Hence, dissolved and total N : P ratios in the water column increased with Daphnia as a result of differences in the fate of suspended particulate N versus P. There was minimal accumulation and storage of P in Daphnia biomass in the enclosures. 5. Our experiment demonstrated that Daphnia can strongly limit filamentous cyanobacterial abundance and affect the biogeochemical cycling of nutrients. In our study, changes in nutrient cycling were apparently insufficient to cause the changes in phytoplankton community composition that we observed. Daphnia therefore limited filamentous cyanobacteria by other mechanisms.     

Relative impacts of Daphnia grazing and direct stimulation by fish on phytoplankton abundance in mesocosm communities

• 1 Planktivorous fish were hypothesised to influence the abundance of algal biomass in lakes by changing zooplankton grazing, affecting zooplankton nutrient recycling and by direct recycling of nutrients to phytoplankton. The relative roles of direct fish effects vs. zooplankton grazing were tested in mesocosm experiments by adding to natural communities large grazing zooplankton (Daphnia carinata) and small planktivorous fish (mosquitofish or juveniles of Australian golden perch).
• 2 The addition of Daphnia to natural communities reduced the numbers of all phytoplankton less than 30 µm in size, but did not affect total biomass of phytoplankton as large Volvox colonies predominated.
• 3 The addition of Daphnia also reduced the abundance of some small (Moina, Bosmina, Keratella) and large (adult Boeckella) zooplankton, suggesting competitive interactions within zooplankton.
• 4 The addition of mosquitofish to communities containing Daphnia further reduced the abundance of some small zooplankton (Moina, Keratella), but increased the numbers of Daphnia and adult Boeckella. In spite of the likely increase in grazing due to Daphnia, the abundance of total phytoplankton and dominant alga Volvox did not decline in the presence of mosquitofish but was maintained at a significantly higher level than in control.
• 5 The addition of juveniles of golden perch to communities containing Daphnia reduced the abundance of small zooplankton (Moina), increased the abundance of large zooplankton (adult Boeckella) but had no significant effect on Daphnia and total phytoplankton abundance.
• 6 The results of the present study suggest that some planktivorous fish can promote the growth of phytoplankton in a direct way, probably by recycling nutrients, and even in the presence of large grazers. However, the manifestation of the direct effect of fish can vary with fish species.

     

Eutrophication mediates rapid clonal evolution in Daphnia pulicaria

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Impacts of a submerged plant (Elodea canadensis) on interactions between roach (Rutilus rutilus) and its invertebrate prey communities in a lake littoral zone

1. Using 5‐m² field enclosures, we examined the effects of Elodea canadensis on zooplankton communities and on the trophic cascade caused by 4–5 year old (approximately 16 cm) roach. We also tested the hypothesis that roach in Elodea beds use variable food resources as their diet, mainly benthic and epiphytic macroinvertebrates, and feed less efficiently on zooplankton. Switching of the prey preference stabilises the zooplankton community and, in turn, also the fluctuation of algal biomass. The factorial design of the experiment included three levels of Elodea (no‐, sparse‐ and dense‐Elodea) and two levels of fish (present and absent). 2. During the 4‐week experiment, the total biomass of euplanktonic zooplankton, especially that of the dominant cladoceran Daphnia longispina, decreased with increase in Elodea density. The Daphnia biomass was also reduced by roach in all the Elodea treatments. Thus, Elodea provided neither a favourable habitat nor a good refuge for Daphnia against predation by roach. 3. The electivity of roach for cladocerans was high in all the Elodea treatments. Roach were able to prey on cladocerans in Elodea beds, even when the abundance and size of these prey animals were low. In addition to cladocerans, the diet of roach consisted of macroinvertebrates and detrital/plant material. Although the biomass of macroinvertebrates increased during the experiment in all Elodea treatments, they were relatively unimportant in roach diets regardless of the density of Elodea beds. 4. Euplanktonic zooplankton species other than Daphnia were not affected by Elodea or fish and the treatments had no effects on the total clearance rate of euplanktonic zooplankton. However, the chlorophyll a concentration increased with fish in all the Elodea treatments, suggesting that fish enhanced algal growth through regeneration of nutrients. Thus, our results did not unequivocally show that Elodea hampered the trophic cascade of fish via lowered predation on grazing zooplankton. 5. In treatments with dense Elodea beds (750 g FW m^?2), chlorophyll a concentration was always low suggesting that phytoplankton production was controlled by Elodea. Apparently, the top‐down control of phytoplankton biomass by zooplankton was facilitated by the macrophytes and operated simultaneously with control of phytoplankton production by Elodea.     

Effects of zooplankton dynamics on epilimnetic phosphorus loss

1. Our aim was to analyse the impact of zooplankton dynamics on the relative importance of two mechanisms contributing to the loss of phosphorus (P) from the epilimnion of stratified lakes: net population incorporation into zooplankton biomass and P sedimentation. 2. We established enclosures without Daphnia (control), with a growing Daphnia population (treatment D) and with a high, stable Daphnia population (treatment D+). The P incorporated in zooplankton biomass and sedimented was measured at short intervals over a period of 17 days. 3. In both Daphnia treatments, sedimentation increased and the P content of sedimented matter was higher than in the control and highest in D+. The P loss by sedimentation between day 3 and 17 was generally high (>25%, >1.8% day^?1) with particularly high values in D+ (～60%, 4.3% day^?1). Phosphorus sedimentation was higher in the zooplankton treatments, although the contribution of exuviae and dead Daphnia was minor. Faecal material was probably a major component of sedimentation. 4. By contrast, the amount of P in zooplankton (mainly Daphnia) biomass increased in D but remained constant in D+. Phosphorus loss owing to net population incorporation was generally low and ranged up to 6.5% (0.5% day^?1) in treatment D. A positive relationship between Daphnia dry mass and P sedimentation, as well as P incorporation, was found. 5. Sedimentation is evidently an important cause of P loss from the epilimnion where Daphnia is abundant. By contrast, P incorporation into Daphnia biomass may only become an important loss factor when the population is growing.     

The effects of global warming on Daphnia spp. population dynamics: a review

Various species of Daphnia usually play a key role in the food web of temperate freshwater systems. There is much evidence to show that climate change may influence Daphnia population dynamics, consequently altering both predator–prey interactions and the efficiency of algal biomass control in these ecosystems. This review will analyse and discuss the current knowledge on Daphnia responses to climate warming based on an analysis of selected papers. The presented results indicate that warming may have important direct and indirect effects on Daphnia biology and ecology via its influence on their life-history processes (metabolism, growth, reproduction) and the properties of their habitats. The plasticity of daphnids in terms of adaptive responses is generally high and includes phenotypic adaptations and changes in genotypes, although it also depends upon the strength of selection and the available genetic variation. The seasonal timing and magnitude of temperature increases are important for seasonal biomass fluctuations of Daphnia and similarly influence the potential synchrony of daphnids and phytoplankton succession (the timing hypothesis). In light of the most recent studies on this topic, even a minor warming during short but critical seasonal periods can cause factors that disturb Daphnia population dynamics to coincide, which may destabilize lake food webs by decoupling trophic interactions. Both winter and spring are important critical periods for determining future seasonal fluxes of Daphnia spp. and, consequently, the time of the clear-water phase and the occurrence and duration of Daphnia midsummer decline. Winter conditions may also affect the impact of fish predation on daphnids during summer months. However, the effects of global warming on Daphnia population dynamics and on ecosystem functioning are often difficult to predict due to their complexity and the presence of both antagonistic and synergistic drivers. Thus, the diverse responses of daphnids to climate anomalies depend on both biotic (predator abundance and seasonal phytoplankton succession) and abiotic factors (e.g. hydrodynamics, intensity and duration of thermal stratification, trophic state or geomorphology) of lakes, which are directly influenced by weather changes. The analysis and quantification of such complex interactions require the involvement of different kinds of specialists and the development of accurate research approaches, such as molecular genetic methods or mathematical modelling.     

Elevated methylmercury in High Arctic Daphnia and the role of productivity in controlling their distribution

Mercury is a contaminant of concern in polar regions due to long‐range atmospheric transport of this metal from southern latitudes followed by intense deposition on snow. We surveyed zooplankton in 16 lakes and ponds in the Canadian Arctic Archipelago (74–76°N) to determine methylmercury (MeHg) content and the role of environmental characteristics and taxonomic composition on accumulation processes. Zooplankton communities containing Daphnia (mainly D. middendorffiana) had on average five times the MeHg content of copepod‐dominated communities. The percent biomass of Daphnia best explained MeHg variation in bulk zooplankton compared with water chemistry and morphometric variables. Water‐column concentrations of MeHg were low at most study sites (mainly ≤0.07 ng L⁻¹), and Daphnia strongly bioaccumulated mercury through species‐specific processes. As we observed Daphnia in more productive water bodies (i.e., ponds, a eutrophied lake), we then tested the role of productivity in determining the distribution of this keystone herbivore using a broad‐scale literature dataset of 47 High Arctic lakes (65–77°N). Daphnia density was positively related to the amount of organic carbon in the water column in both dissolved and particulate fractions [dissolved organic carbon (DOC) partial , P < 0.001; particulate organic carbon (POC) partial , P=0.032]. The strong influence of DOC suggests that bacterial production is an important energy source for Arctic Daphnia. Our findings indicate that productivity influences the MeHg content of zooplankton communities through its control of species composition; specifically, low productivity limits the presence of mercury‐rich Daphnia in many copepod‐dominated lakes of the High Arctic. Aquatic productivity is expected to increase with climate warming, and we present a conceptual model that predicts how environmental drivers could extend the distribution of Daphnia in lakes and alter the movement of mercury in food webs of the Canadian High Arctic.