The influence of flatworm predation on zooplankton inhabiting small ponds

Experiments were performed in 1977 to determine which large zooplankton in a series of high altitude ponds can be consumed by the predatory flatworm Mesostoma ehrenbergii. This predator consumes Daphnia at a high rate and the fairy shrimp Branchinecta at a low rate, but does not consume Diaptomus. Experiments were performed in 1978 and 1979 to determined the rate of predation on Daphnia in 30 liter tubs and to determine if predation rate is correlated with surface to volume ratio of experimental containers. There is a clear correlation between surface to volume ratio and predation rate. Determinations of Mesostoma and Daphnia densities were made in a series of eight high altitude ponds, and pond surface to volume ratios were determined. Examination of these parameters lends credence to the argument that Mesostoma predation affects Daphnia dynamics in some circumstances. The results suggest that benthic invertebrate predators may affect zooplankton dynamics, especially in shallow ponds.     

Environmental effects of introducing whitefish,Coregonus lavaretus (L.), in Lake Ring

The impact of whitefish (Coregonus lavaretus (L.)) on the trophic structure of eutrophic lakes was studied in Lake Ring, a small eutrophic Danish lake (22.5 ha, mean depth 2.9 m) in which the natural fish fauna is dominated by pike (Esox lucius L.), perch (Perca fluviatilis L.), and eel (Anguilla anguilla (L.)), roach (Rutilus rutilus (L.)) and burbot (Lota lota (L.)) being the only other fish species present. A total of 10993 0+ whitefish were stocked in the lake from October 1989 to July 1990 and the structure of the fish, zooplankton and benthic invertebrate communities studied during the period 1989–1991. Stomach contents analysis revealed that the whitefish mainly ate Daphnia and copepods in 1990–1991, the proportion of copepods decreasing with increasing size of the fish and Daphnia being the overall most important food source. The density of Daphnia in the lake decreased from 72 ind. 1^-1 in 1989 to 9 ind. 1^-1 in 1991; concomitantly the large species Daphnia magna and D. pulex almost disappeared and the density of cyclopoid copepods increased from 72 to 101 ind. 1^-1, presumably because of improved food conditions, while that of calanoid copepods remained virtually unchanged. As a result chl-a increased from 19 to 47 µg 1^-1 and Secchi depth decreased from 2.4 m to 1.7 m, despite there being no change in total P and total N (0.6 mg P 1^-1 and 1.3 mg N 1^-1, respectively). Changes were also observed in the benthic invertebrates; Chaoborus, oligochaetes, and chironomids all decreased, whereas Pisidium increased. It is concluded that the stocking of whitefish in eutrophic lakes for commercial purposes may delay their recovery, or even lead to enhanced eutrophication.     

Influence of benthic and interstitial processes on nutrient changes along a regulated reach of a large river (Rhône River,France)

An enclosure study was conducted in Ranger Lake in south-central Ontario, Canada from 4 July to 5 August 1997 to determine predation effects of the larvae of the phantom midge fly Chaoboruson the zooplankton community. Zooplankton assemblages were established in 12 enclosures (2 m in diameter, 7.5 m deep). Three densities of fourth-instar Chaoborus trivittatus (0 l^–1, 0.1 l^–1 and 0.5 l^–1) were introduced as predator treatments to the enclosures. Temperature, dissolved oxygen and zooplankton community composition were monitored for six weeks. To determine if the zooplankton community composition changed, a repeated measures multivariate analysis was performed on percent biomass of Bosmina and calanoid copepods. There were no significant differences in mean taxon percent biomass among predator treatments. There were significant differences in mean taxon percent biomass between water layers (epilimnion and metalimnion). There were also significant differences in lengths of Bosmina and calanoid copepods among predator treatments at the end of the experiment. Crop content analysis of C. trivittatusshowed that Bosmina constituted 88–98% of the prey items found in the crops. These results demonstrate that the use of deep enclosures, a Chaoborus species which vertically migrates, and lower natural densities of Chaoborus may provide prey with an important natural refuge from predation and so allow a more accurate determination of the predation impact of Chaoborus trivittatusin temperate lakes where fish control Chaoborus densities.     

A comparison between predation effects on zooplankton communities by Neomysis and Chaoborus

Community level effects of predation by two invertebrate predators, the opossum shrimp (Neomysis intermedia), and the larva of the phantom midge (Chaoborus flavicans) were studied and compared. N. intermedia appeared abundantly in the shallow eutrophic Lake Kasumigaura and had a significant impact on the zooplankton community. The predation pressure by Neomysis was highest on cladocerans, followed by rotifers, and finally copepods. At high densities (maximum nearly 20 000 individuals m^–2), Neomysis excluded almost all cladocerans, rotifers and copepods from the lake.Zooplankton communities were established in experimental ponds, into which C. flavicans was introduced. The predator's density was around 1 individual l^–1, and was probably controled by cannibalism. Although Chaoborus larvae feed on various zooplankton species, their predation impact on zooplankton populations was markedly selective. They eliminated medium- and small-sized cladocerans and calanoid copepods from the ponds, but rotifers increased.Although the feeding selectivities of Neomysis and Chaoborus individuals were similar, the predation effects on zooplankton communities by the two predators were different.     

Predation on and by pelagic Turbellaria in some lakes in Brazil

Planktonic Turbellaria are of common occurrence in both natural and man-made lakes in Brazil. Experiments were performed in 1987 and 1989 to determine which zooplankton species are consumed by predatory Mesostoma sp. from three natural lakes in the Rio Doce Valley. Experiments were also performed in 1989 with a yet unidentified flatworm from Barra Bonita reservoir. Both predators consumed Daphnia and Ceriodaphnia at a high rate: 4 individuals per day in the case of Mesostoma sp., a large species and 1.5 ind day^–1 in the case of the smaller species from Barra Bonita reservoir. Consumption of copepod nauplii, copepodids and adults was much lower, and Ostracoda were not consumed at all. Experiments on food selectivity showed a clear preference by the flatworms for cladocerans.In the lakes studied, flatworms are heavily preyed upon by larvae of Chaoborus and by Mesocyclops species.Turbellaria densities in the natural lakes were around 300 individuals per cubic meter, whereas in Barra Bonita reservoir, 1000 individuals per cubic meter was a mean value in a fifteen days study.Federal University of São Carlos - Laboratory of LimnologyUniversity of São Paulo - Canter of Water Resource and Applied EcologyPost-graduate student - University of São Paulo     

Zooplankton feeding in the fish Trichogaster pectoralis Regan

The traditional culture method of Trichogaster pectoralis, using zooplankton produced from fermenting aquatic weeds as a source of food for the fry, leads to productions of ca 1 000 kg ha^–1 a^–1. If chicken manure is applied at a rate of ca 450 kg ha^–1 month^–1 instead, much more zooplankton is produced more rapidly, and fish production increases to ca 2 000 kg ha^–1 a^–1. This results from a higher survival rate of the fry, which feeds on rotifers, ciliates, copepods and cladocera until a size of ca 15.0 mm, when plant material also becomes important in the diet.     

The spring-time abundance and feeding ofEurytemora affinis (Poppe) in Volkerak-Zoommeer,a newly-created freshwater lake system in the Rhine delta (The Netherlands)

Eurytemora affinis, a calanoid copepod, has been encountered in Volkerak-Zoommeer (Rhine delta region, S.W. Netherlands) both before this lake system was isolated in 1987 from the estuarine influence, and after. It was the main particle-feeding crustacean at all the 3 sampling stations in March–April 1990 when it reached densities of up to 215 ind.l^–1. Its decline from mid April onwards, and low densities through summer, coincided with increase in cladocerans, especiallyDaphnia spp. (D. pulex andD. galeata), a decrease in seston (<33m) and chlorophyll concentrations and in primary production rates. The clearance rates (CR) ofEurytemora measured in the spring period varied enormously (0.6–24 ml.ind^–1.d^–1) depending mainly on size (0.44–1.06 mm), food concentration (0.8–2.2 mg C.l^–1), and the water temperature which varied only narrowly (8.0–9.0°C). Mean ingestion rates of the animals measuring 0.68±0.02 mm during the study was 6.7±3.2 gC.ind^–1.d^–1; and assimilation efficiency varied between 27 and 53% (mean: 41±9%). The weight specific CR (SCR) varied between 0.96 and 6.4 litre.mg^–1 body C.d^–1. Pooled regression of SCR on the animal's body weight at the 3 study stations revealed a significant inverse relationship. Also daily ration and specific assimilation ofE. affinis varied greatly and inversely with the body weight. This calanoid contributed from about 50 to 100% to the zooplankton community grazing rates and assimilation rates, the former often exceeding the phytoplankton primary production.     

Growth of dominant zooplankton species feeding on plankton microflora in Lake Shira

Batch cultures and continuous flow cultures were used to study the growth rates of zooplankton species from Shira lake, the rotifer Brachionus plicatilis Muller and calanoid copepod Arctodiaptomus salinus Daday, which were fed on phytoplankton and bacterioplankton from the lake. Analyses of the birth and survival rates were used to demonstrate that the lake phytoplankton, consisting mostly of cyanobacteria and diatomaceous algae, is inadequotes for optimal realisation of the reproductive potential of B. plicatilis when compared with the bacterial diet. The study revealed that the kinetic growth characteristics of the two zooplankters were similar: B. plicatilis r _max, 0.120 d^–1; S ₀, 0.253; and K _s, 0.114 mg dry mass l^–1; and for A. salinus r _max, 0.129 d^–1; S ₀, 0.240; and K _s, 0.171 mg dry mass l^–1. Fluctuations in natural food concentration reduced the growth rate of both species. Even though the threshold concentration of food for B. plicatilis and A. salinus were quite similar, the copepods were less sensitive to food limitation.     

Zooplankton as a compound mineralising and synthesizing system: phosphorus excretion

Data on phosphate excretion rates of zooplankton are based on measurements using the pelagic crustacean zooplankton of Lake Vechten and laboratory-cultured Daphnia galeata. In case of Daphnia sp we measured the effects of feeding on P-rich algae and P-poor algae (Scenedesmus) as food on the P-excretion rates at 20°C. The excretion rates of the natural zooplankton community, irrespective of the influence of the factors mentioned, varied by an order of magnitude: 0.025–0.275µg PO₄-Pmg^–1C in zooplankton (C _zp ) h^–1. The temperature accounted for about half the observed variation in excretion rates. The mean excretion rates in the lake, computed for 20°C, varied between 0.141 and 0.260 µg Pmg^–1C _zp h^–1. Based on data of zooplankton biomass in the lake the P-regeneration rates by zooplankton covered between 22 and 239% of the P-demand of phytoplankton during the different months of the study period.In D. galeata, whereas the C/P ratios of the Scenedesmus used as food differed by a factor 5 in the experiments, the excretion rates differed by factor 3 only. Despite the higher P-excretion rates (0.258± 0.022 µg PO₄-P mg^–1 C h^–1) of the daphnids fed with P-rich food than those fed with P-poor food (0.105 ± 0.047 µg PO₄-P mg^–1 C h^p–1), both the categories of the animals were apparently conserving P. A survey of the literature on zooplankton excretion shows that in Daphnia the excretion rates vary by a factor 30, irrespective of the species and size of animals and method of estimation and temperature used.About two-thirds of this variation can be explained by size and temperature. A major problem of comparability of studies on P-regeneration by zooplankton relates to the existing techniques of P determination, which necessitates concentrating the animals several times above the in situ concentration (crowding) and prolonged experimental duration (starving), both of which manifest in marked changes that probably lead to underestimation of the real rates.     

Beneficial and detrimental interactive effects of dissolved organic matter and ultraviolet radiation on Zooplankton in a transparent lake

While changes in dissolved organic matter (DOM) concentrations are expected to affect zooplankton species through attenuation of potentially damaging ultraviolet (UV) radiation, generation of potentially beneficial or harmful photoproducts, pH alteration, and microbial food web stimulation, the combined effects of such changes on zooplankton community structure have not been studied previously. Our purpose was to determine how an increase in allochthonous DOM and associated changes in pH in an initially transparent lake may affect zooplankton community structure, and how exposure to solar UV may alter these DOM and pH effects. We ran microcosm experiments manipulating UV, DOM, and pH near the surface of Lake Giles in northeastern Pennsylvania. We found that when DOM was added in the presence of ambient UV, Daphnia and copepod UV-mortality was reduced by approximately three and two times compared to UV exposure without extra DOM. When DOM was added in the absence of UV, adult Daphnia and copepods were reduced compared to no DOM addition in the absence of UV. Daphnia and cyclopoid egg production and rotifer abundance were generally higher in the presence of DOM, regardless of UV treatment. The lower abundance yet high egg production in the presence of DOM and absence of UV may be explained by higher abundance of egg-bearing adults compared to non-egg-bearers. We conclude that allochthonous DOM benefits some zooplankton in a high-UV environment, but may be detrimental under low-UV conditions. Overall, Daphnia abundance and egg production were higher than that of calanoid copepods in the DOM additions, indicating that in some lakes an increase in allochthonous DOM may lead to a zooplankton community shift favoring Daphnia over calanoid copepods.     

Secondary production of the brackish copepod communities and their contribution to the carbon fluxes in the Westerschelde estuary (The Netherlands)

The zooplankton community of the brackish part of the Westerschelde estuary (November 1989–October 1990) was dominated by two calanoid copepods, Eurytemora affinis and Acartia tonsa. Eurytemora was present during a longer period of the year and was much more important in terms of total abundances and biomasses than Acartia.The secondary production of these species was estimated by means of the growth rate method, using weight-specific growth rates obtained from laboratory cultures (Eurytemora) or from the literature (Acartia).Due to the substantially higher growth rates of Acartia compared to Eurytemora, total yearly productions of both communities were comparable, notwithstanding the large discrepancies in biomass. They amounted to about 5 and 6 g C m^–2 y^–1 by Acartia and Eurytemora respectively.The food needed to realise this production was estimated to be about 14 and 17 g C m^–2 y^–1 by Acartia and Eurytemora respectively. Provided that the copepods are able to selectively ingest the phytoplankton, in situ net production provides sufficient carbon for zooplankton demands for a short period of the year only. As phytoplankton standing stock is very low and net phytoplankton productivity is negative from late fall to early spring, nutritional demands of the copepods have to be fulfilled by other than algal food at least during this period of the year.Although the copepods in the brackish part can have an important impact on some food items, their contribution to total carbon fluxes in the brackish zone is negligible: each year some 6% of all consumed carbon in the brackish part of the estuary passes through the copepod food web.     

Spatial and seasonal variation of the zooplankton in the coastal zone and main khors of Lake Nasser (Egypt)

A prominent feature of Lake Nasser is the presence of khors (dendritic side extensions). We studied the zooplankton of the larger khors and coastal zone of the main lake in 1996 and 1997, and found an assemblage of rotifers, cladocerans and copepods that was partly tropical, partly temperate, at relatively high biomasses. Spatial differences were weak, but the upstream khors (Toushka and Korosko) were consistently richer than the downstream khors (El-Ramla and Kalabsha), with a rather sudden transition around km 150 at El Madiq. Summer standing crops were higher than those in winter by a factor 2–3. The zooplankton of the littoral of the main channel showed the same spatial pattern as that in the khors, being more abundant in spring (average 82300 ind m^–3) than in autumn (average 72700 ind m^–3). Zooplankton dry weight increased from 4 g m^–2 at khor El-Ramla to 7 g m^–2 at khor Toushka. These rather high values had low variation. The number of species, diversity and evenness all showed a high degree of similarity among the khors and in the littoral of the main lake. The lake fish fauna is poor, lacking a pelagic planktivore. The predominance of medium-sized Copepoda (one calanoid, two cyclopoids) in the zooplankton suggests that fish predation is moderate. This is confirmed by the persistence of two Daphnia species at low abundance, although rather strong variations in time suggest that Cladocera suffer from summer predation by invertebrates (copepods) as well as vertebrates (mostly larval fish). Because the zooplankton is underutilised by higher trophic levels, we suggest to assess the benefits of introducing a pelagic zooplanktivorous fish.     

Predatory — prey relationships in subtropical zooplankton: water mite against cladocerans in an Argentine lake

Summary In a small lake in northern Argentina pelagic water mite Piona sp. had the maximum of population density in January following with a five-day delay after the peak of zooplankton dominant — Daphnia laevis. The mite density was highly predicted by the previous variations of Daphnia density during 4 months of observation (December–March). Daphnia density was a negative delayed function of the predator density but only in December–January when Piona was abundant. During that period Daphnia death rate, d was also correlated with the mite density (r ²=0.80, P<0.005). In laboratory experiments water mites killed 1–7 Daphnia · predator^-1 · hour^-1 in a broad range of prey density. Another zooplankton component, Diaphanosoma birgei, was consumed at the same rates. The mite hardly consumed any copepods. In the pelagium during 24 hours the mite was more associated with Daphnia, than with Diaphanosoma, probably, because of the coincidence in photoreactions with Daphnia. Piona contribution to the death rates of its prey estimated by using the data on functional and numerical responses as well as by means of Edmondson-Paloheimo model, could reach 53% for Daphnia and 40% for Diaphanosoma. A computer experiment on the reconstruction of prey dynamics after subtraction of predator influence showed that the mite could have caused a depression in Daphnia numbers observed in the lake, but the declines in Diaphanosoma population were caused by other factors. After the removal of mite pressure model Daphnia population increased its average density 10-fold. Experiments on Piona feeding revealed a strong effect of interference among predators. This was eliminated by putting one mite per experimental vessel, which led to a 20-fold increase in predation rate. The effect explains the low feeding rates of Piona obtained by the previous authors who ignored the possibility of interference.     

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.     

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.     

Cyanobacterial chemical warfare affects zooplankton community composition

1. Toxic algal blooms widely affect our use of water resources both with respect to drinking water and recreation. However, it is not only humans, but also organisms living in freshwater and marine ecosystems that may be affected by algal toxins. 2. In order to assess if cyanobacterial toxins affect the composition of natural zooplankton communities, we quantified the temporal fluctuations in microcystin concentration and zooplankton community composition in six lakes. 3. Microcystin concentrations generally showed a bimodal pattern with peaks in early summer and in autumn, and total zooplankton biomass was negatively correlated with microcystin concentrations. Separating the zooplankton assemblages into finer taxonomic groups revealed that high microcystin concentrations were negatively correlated with Daphnia and calanoid copepods, but positively correlated with small, relatively inefficient phytoplankton feeders, such as cyclopoid copepods, Bosmina and rotifers. 4. In a complementary, mechanistic laboratory experiment using the natural phytoplankton communities from the six lakes, we showed that changes in in situ levels of microcystin were coupled with reduced adult size and diminished juvenile biomass in Daphnia. 5. We argue that in eutrophic lakes, large unselective herbivores, such as Daphnia, are ‘sandwiched’ between high fish predation and toxic food (cyanobacteria). In combination, these two mechanisms may explain why the zooplankton community in eutrophic lakes generally comprise small forms (e.g. rotifers and Bosmina) and selective raptorial feeders, such as cyclopoid copepods, whereas large, unselective herbivores, such as Daphnia, are rare. Hence, this cyanobacterial chemical warfare against herbivores may add to our knowledge on population and community dynamics among zooplankton in eutrophic systems.     

The effect of sewage-enriched river Ganga water on the biomass production of theAzolla-Anabaena complex

The biomass formation ofAzolla was greatly enhanced by water of the River Ganga and by prevailing environmental conditions. It increased gradually from January to April (first maximum 2.409 g.m^–2.day^–1), declined during June (1.185 g.m^–2.day^–1), and reached a second its maximum during September (2.629 g.m^–2.day^–1). The biomass formation was related to the nutrient availability in the medium in a particular season (measured were: nitrate-N, available phosphorus, total suspended solids, and conductivity). The average annual production of 6.73 ton.ha^–1.yr^–1 is equivalent to the average production of 0.025 ton.ha^–1.yr^–1 phosphorus, 0.252 ton.ha^–1.yr^–1 nitrogen, and 1.57 ton.ha^–1.yr^–1 crude protein.     

Zooplankton of Fishless Ponds of Northern Patagonia: Insights into Predation Effects of Mesostoma ehrenbergii

The turbellarian predator Mesostoma ehrenbergii, a common inhabitant of fishless ponds of northern Patagonia, can consume prey larger than 1 mm. Because the feeding strategy of M. ehrenbergii includes mucus trapping and external digestion, this predator may exploit a broad range of prey sizes. We hypothesize that M. ehrenbergii could exert a strong effect on zooplankton composition and body size spectra in Patagonian fishless ponds. We investigated this hypothesis by analyzing the crustacean zooplankton composition and size spectra in five fishless ponds of northern Patagonia, and we carried out experiments to assess predation rates of M. ehrenbergii on potential prey species from 0.8 mm to 6 mm. These ponds were colonized by macrophytes, which favored habitat heterogeneity, especially in the smaller ponds that had higher species richness. The surveyed ponds showed distinctive crustacean zooplankton assemblages and sizes, but all were dominated by calanoid copepods of the genus Boeckella. Our results indicated that M. ehrenbergii consumed the whole size range of offered prey, from ∼0.8 mm (Ceriodaphnia dubia) to ∼6 mm (Parabroteas sarsi). Predation rates were higher for intermediate‐bodied copepods (∼1.5 mm) and C. dubia (0.8 mm), but we did not find conclusive evidence that variations in size spectra of crustacean zooplankton are a result of M. ehrenbergii predation. We suggest that an interaction between prey body size and its evasion tactic might be important to determine the true effect Mesostoma on zooplankters. The structural complexity created by macrophytes in Patagonian fishless ponds may also help account for the lack of a strong predation effect of Mesostoma in the field survey. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Zooplankton and their relationships with water quality and fisheries

Zooplankton population structure in Rutland Water changed over the first five years of the reservoir's existence. Daphnia pulex, initially dominant was replaced by D. hyalina in late 1975 and since 1979 this latter cladoceran has coexisted for part of each year with Bosmina longirostris. Population fluctuations of all the main crustacean zooplankton — D. hyalina, Cyclops spp. and Diaptomus gracilis — were erratic in the first three years (1975–1977) but have since become more regular. The relationships of the zooplankton with their potential algal food supply and with their potential predators, the fish population, are discussed.