A laboratory study of feeding and assimilation in Euchlanis dilatata lucksiana

Feeding of Euchlanis dilatata lucksiana, a brachionid rotifer isolated from the Loosdrecht Lakes (The Netherlands), was examined in the laboratory using ¹⁴C-labelled food. The gut-filling time at a food concentration of 9.6 µg C ml^–1 was about 15 minutes. Animals which were fed on 3 size fractions of the lake seston (< 7 µm, 7–15 µm, and 15–33 µm) showed a clear preference (Jacobs' selectivity index) for the largest size fraction. This fraction was composed predominantly of filamentous cyanobacteria. For animals weighing 0.37–0.49 µm C ind.^–1 the daily ration (daily food consumption per unit body weight × 100) ranged from 50 to 100% at food levels of 2 mg C l^–1 and below, but increased to 150–250% at food concentrations of 5 mg C l^–1 and above. The assimilation efficiency was 100% up to 5 mg C l^–1 of food, but decreased to about 80% at higher food levels.     

Structure and feeding activities of zooplankton community in Lake Zwemlust,in the two years after biomanipulation

The biomanipulation study in Lake Zwemlust (area 1.5 ha; mean depth 1.5 m) is among the series of such investigations initiated recently in The Netherlands. The effects of the lake's reffilling (after it was first completely drained empty) almost entirely the nutrient-rich seepage water from the River Vecht flowing nearby and of removal of the planktivorous bream (Abramis brama), on zooplankton community structure and feeding activities of crustaceans were monitored for two successive years (1987, 1988). In these years a classical pattern of succession occurred, with the rotifer spring maximum preceding the crustacean maximum by about 3 weeks. Among the fiveDaphnia species, which appeared in quick succession during May–July, two were large-bodied forms (D. magna; D. pulex). OnlyD. pulex persisted and was the important grazer species in the second year, especially in spring. In the first year the crustacean grazing, with several values >100%.d^–1, contributed significantly to the lake's improved water clarity, with Secchi-disc transparencies of 1.5 m and more almost throughout the summer, compared with 0.3 m before the biomanipulation. Even though the water clarity climate in the second year was quite similar to that in the first, the causal factor was high macrophytic vegetation, rather than zooplankton grazing. The lake developed a rich littoral flora and fauna in the second year in response to the optimal light and nutrient conditions. Apparently, the predation by the introduced planktivorous rudd (Scardinius erythrophthalmus) on zooplankton was an important factor in the changes in zooplankton structure, and in the reduced role of zooplankton in seston elimination during the second year. There is some evidence from bioassay work that, simultaneously with the littoral development, nitrogen limitation of the phytoplankton also contributed to the improved light situation in the second year.     

The relation between the feeding activity of wintering shovelers (Anas clypeata) and the horizontal distribution of zooplankton in Lake Teganuma,Japan

We investigated seasonal changes in the number and feeding activity of Shovelers wintering in Lake Teganuma in connection with temporal and horizontal fluctuations of zooplankton, and analyzed esophageal contents of a captured Shoveler. Zooplankton community was dominated by rotifers followed by copepods during the study period. Horizontal distribution of both invertebrate taxa differed; rotifers most abundant in the western and eastern parts of the lake in February–March, and cyclopoid copepodids and nauplii more common in the northeast area of the eastern portion of the lake. The number and feeding activity of Shovelers increased from January to March in the northeast area of Lake Teganuma. A Shoveler captured in the area had mostly consumed cyclopoid copepodids despite the fact that rotifers dominated in the zooplankton community at that location. These results suggest that the horizontal distribution and feeding activity of the Shovelers were correlated with the distribution of the cyclopoid copepodids.     

Relationship between feeding of blueback herring and the zooplankton community of a Texas reservoir

Following the introduction of blueback herring (Alosa aestivalis), we examined herring food habits and the crustacean zooplankton community in Lake Theo, a 30-hectare reservoir located on a tributary of the Red River in north Texas. Prior to the introduction in spring, 1982, the reservoir contained an established fish community dominated by centrarchids. Blueback herring stomachs and zooplankton were sampled quarterly from summer, 1982 through winter, 1985. Cladocerans accounted for 89.4% of the zooplankton consumed by herring. Median lengths of cladocerans and copepods in the herring diet were significantly greater than those in the reservoir. No appreciable changes in lengths of cladocerans and copepods in the reservoir were observed after blueback herring introduction, but the zooplankton community shifted from cladoceran to copepod domination.     

Impact of whitefish on an enclosure ecosystem in a shallow eutrophic lake: selective feeding of fish and predation effects on the zooplankton communities

Bag-type enclosures (75 m³) with bottom sheets and tube-type enclosures (105 m³) open to the bottom sediment were stocked with exotic whitefish (Coregonus lavaretus maraena) to study their predation effects on the plankton community. The fish fed mainly on adult chironomids during the period of their emergence (earlier part of the experimental period). Thereafter, the food preference was shifted to larvae of chironomids and crustacean zooplankters. The predation effects on the plankton community were not evident in the bag-type enclosures where zooplankton densities were consistently low. The fish reduced the crustacean populations composed ofBosmina fatalis, B. longirostris andCyclops vicinus in the tube-type enclosures where the prey density was high (above ca. 50 individuals 1⁻¹). The results suggested that the intensity of predation depended on the prey density. Rotifers increased in the fish enclosure, probably becauseCoregonus reduced the predation pressure byCyclops vicinus on rotifers and allowed the latter to increase. In the fish enclosures, no marked changes in species composition were observed. Zooplankton predated by the fish seemed to be distributed near the walls of the enclosures. Problems of enclosure experiments for examining the effects of fish predation on pelagic zooplankton communities are discussed.     

P-load,phytoplankton, zooplankton and fish stock in Loosdrecht Lake and Tjeukemeer: confounding effects of predation and food availability

The fish community in the Loosdrecht lakes is dominated by bream, pikeperch and smelt and is characteristic of shallow eutrophic lakes in The Netherlands. The biomasses of the respective fish species amount to ca. 250, 25 and 10 kg ha^–1 and correspond to those in Tjeukemeer, another lake in The Netherlands. The average size of bream, however, is much smaller in the Loosdrecht lakes as a consequence of poorer feeding conditions. The zooplankton community in the Loosdrecht lakes is predominantly composed of relatively small species such as Daphnia cucullata, Bosmina coregoni and cyclopoid copepods, whereas in Tjeukemeer, Daphnia hyalina is permanently present in relatively high densities and the other species show a larger mean length. In the Loosdrecht lakes, the absence of D. hyalina and the smaller sizes of the other zooplankton species could be the consequence of a higher predation pressure, in combination with unfavourable feeding conditions for the zooplankton including the low density of green algae and the high density of filamentous cyanobacteria. A biomanipulation experiment in Lake Breukeleveen, one of the Loosdrecht lakes, indicated that feeding conditions were too unfavourable for large zooplankton to develop in spring, when the reduced fish biomass was not yet supplemented by natural recruitment and immigration.     

Response of the microbial community to water table variation and nutrient addition and its implications for in situ preservation of organic archaeological remains in wetland soils

Wetland environments can preserve organic archaeological remains because of their anaerobic nature. The ongoing discovery of archaeological sites in wetlands is associated with a lack of funds for excavation and preservation. This situation has led to the consideration of preservation in situ the preferred option for dealing with the majority of waterlogged archaeological remains in England. To expand our understanding of the burial environment, we studied changes in environmental variables along with counts of total bacteria and microbial ¹⁴C-leucine assimilation down the soil profile at two wetlands in the North of England. Soil cores were sampled at five depth intervals between 10 and 100 cm. To test whether the addition of nutrients induces bacterial activity in the soil, inorganic phosphate and combined nitrogen were added to soil samples and the rate of ¹⁴C-leucine assimilation was recorded. Redox potential readings were positive above the water table and negative below. The total number of bacteria and the ¹⁴C-leucine assimilation rates differed among sites, but always decreased with increasing soil depth. Nutrient availability was limiting for the microbial communities in the upper soil horizons, but did not appear to limit those in the lower soil. These results allow a better understanding of the physico-chemical and microbiological conditions that potentially favour or inhibit the decomposition of organic archaeological remains at the studied wetlands.     

The food and feeding habits of Eutropius niloticus (Ruppell), family Schilbeidae (Osteichthyes: Siluriformes) in Lake Kainji,Nigeria

Studies on the food and feeding habits of Eutropius niloticus in Lake Kainji, Nigeria revealed that the fish is omnivorous but fed more on insects than on any other food. The most important insects were Povilla adusta Navas nymphs and imagos and adults of the common grasshoppers. Fish, particularly the clupeids Sierrathrissa leonensis Thys Van den Audernaerde and Pellonula afzeliusi Johnels, also formed an important part of the diet. There was seasonal variation of organisms selected for food and there were variations in the dietary items selected by different age groups. Accumulation of fat was found to correlate with seasonal feeding intensity. E. niloticus serves as food for other fishes.     

Composition,density and feeding of crustacean zooplankton community in a shallow,temperate lake (Lake Balaton,Hungary)

Composition density and filtering rates of crustacean zooplankton were studied in the open water and among the macrophytes of the oligo-mesotrophic part of Lake Balaton from 1981 to 1983. From the individual filtering rates of the different populations and the densities community grazing rates were derived.Copepoda made up 79–90% of crustacean plankton community in the open lake and 95–97% of it in littoral zone. Among them the nauplii dominated. At the end of summer 1982, when Anabaenopsis was in bloom, the filter-feeding species (Eudiaptomus, Daphnia) practically disappeared, being replaced by cyclopoids. Daphnia had the highest filtering rates followed by those of Eudiaptomus and Diaphanosoma. Among copepods, the filtering rates in decreasing order were: ovigerous > all adults > copepodites > nauplii. > . The filtering rates of the different species varied both seasonally as well as from year to year. In 1983, when the concentration of organic seston decreased, filtering rate increased compared with those in the earlier years. During the water bloom in 1982, the rates decreased by 70% on the average.The community grazing rate was very low (3% per day) in the open lake and among macrophytes, both in 1981 and 1982; also the share of crustacean zooplankton in grazing was very low. In 1983, together with the improving of water quality, the community grazing rate increased 4-fold. In 1981 and 1983 the rates were influenced by water temperature but in 1982 by seston concentration.     

Evidence for predatory flatworms as organizers of zooplankton and mosquito community structure in rice fields

I present evidence from two field studies and one laboratory experiment that the predatory flatworm, tentatively indentified as Mesostoma nr. lingua, can be important in organizing invertebrate communities in rice fields. In the first field study, I assessed the importance of various predators including two fishes, in explaining among-plot variation in mosquito abundance during a five week interval. Mosquitofish (Gambusia affinis) abundance was not significantly associated with densities of either mosquito, Culex tarsalis or Anopheles freeborni, except for the final week. Associations between green sunfish (Lepomis cyanellus) juveniles and mosquitoes were demonstrated for only A. freeborni — a positive association. Densities of Mesostoma were negatively associated with C. tarsalis and A. freeborni densities. C. tarsalis, protected from flatworms in small-mesh cages, had a much greater survival rate than those that were exposed in large-mesh cages. C. tarsalis survival, when left unprotected in large-mesh cages, was negatively correlated with field densities of flatworms and with the number of flatworms found inside the cages. Survival was also positively correlated with field densities of mosquitoes.In the second field study, Mesostoma densities were negatively correlated with C. tarsalis, A. freeborni, Cladocera and Ostracoda in rice field enclosures early in the season. Cyclopoid copepod densities were not significantly associated with this predator.In a laboratory experiment, predation by Mesostoma on C. tarsalis larvae was reduced when presented with an alternative prey, Moina micrura (a cladoceran).These results suggest that flatworms, via predation, can have large effects on the structure of mosquito and zooplankton communities in rice fields.     

The zooplankton community in bays of Toulon area (northwest Mediterranean Sea,France)

The abundance and the diversity of the zooplankton community were studied during an annual cycle in three shallow bays differently affected by anthropogenic inputs of Toulon area in the northwest Mediterranean Sea (France). Environmental variables of water quality were also considered. Our results discriminated Little Bay from the La Garonne and Niel Bay. Little Bay, heavely perturbed by anthropogenic inputs, had high chlorophyll a levels, abundant zooplankton, a low diversity index, a high dominance Index and a single dominant species (Oithona nana). La Garonne Bay and Niel Bay, much less polluted, had low chlorophyll a levels, low zooplankton abundance, low dominance Index and high diversity index and eveness. In addition, O. nana occurred rarely in the latter bays. Zooplankton successions were also different between Little Bay and the two other bays.     

The effect of temperature,irradiance and animal size on incorporation rates of Simocephalus vetulus

Carbon incorporation rates of Simocephalus vetulus were measured to study the effects of the physical state of the animals, size of the animal, varying temperature and light conditions. Physical state of the animal showed little effect on incorporation rates after the first hour. Incorporation rates increased in proportion to the third power of animal size. Experimental animals collected at temperatures of 12, 20 or 25°C fed maximally at 10, 15 and 25°C respectively, when subjected to a feeding temperature range of 5 to 30°C. We have interpreted this as an indication that S. vetulus is able to acclimate and incorporate maximally at various temperatures after prolonged exposure to that temperature. When fed over an irradiation range of 0 to 4.68 × 10^–3 cal cm^–2 s^–1 incorporation rates were indirectly proportional to irradiance. This suggests a response to decreased irradiance in the weedy, littoral habitat of these animals.     

Feeding behavior of the cichlid,Sarotherodon galilaeum: selective predation on Lake Kinneret zooplankton

In laboratory feeding trials, we analyzed the feeding behavior and selectivity of the cichlid, Sarotherodon galilaeum, for zooplankton prey from Lake Kinneret, Israel. The feeding behavior was dependent on fish size. Fish less than 20 mm SL fed on zooplankton as obligate particulate feeders. Fish from 20 to 42 mm SL fed either as particulate feeders or as filter feeders. Fish larger than 62 mm SL fed as obligate filter feeders. Particulate-feeding fish were size selective and had highest feeding electivities for large-sized zooplankton species. Filter-feeding fish had highest feeding electivities for zooplankton species with poor escape ability. In general, S. galilaeum predation pressure would be greatest on Ceriodaphnia reticulata, a large-bodied and easily captured species which is selected by both particulate-feeding and filter-feeding fish.     

Secondary production and energetics of the shrimp Caridina nilotica in Lake Victoria, East Africa: model development and application

Measurements of body mass, carbon content, respiration, growth, and egestion are combined in a model of secondary production by the tropical freshwater shrimp Caridina. The model is developed to permit its direct application to empirical data for abundances and size frequency distributions of field populations. Model calculations combined with population data for offshore Lake Victoria over a period of two years indicate that Caridina consume the equivalent of 2.2% of annual lake primary production. Present net annual secondary production by the shrimp is an order of magnitude greater than the present fishery yield of the lake. Detritus-fed experimental organisms evidently had assimilation efficiencies as low as 10% by model calculation.     

Content and turnover rates of phosphorus in Daphnia pulex: Effect of food quality

The content of phosphorus in individual Daphnia pulex and the rates at which the element turns over are functions of the P contents of algal cells fed to the zooplankton. Chlamydomonas and Ankistrodesmus were grown in semi-continuous cultures containing 2 µM and 10 µM PO₄ to produce differences in cellular P of the algae. Animals reared on the cultures high in P contained 60 percent more P than did animals of equal size that were fed low P cells. Daphnia with high body contents of P turned over that P faster than did the animals fed low P algae. Measured turnover rates imply that large differences exists between rates of gross P assimilation and P excretion for animals fed high and low P sources.     

The feeding behaviour of Leptodora kindti and its impact on the zooplankton community of Neusiedler See (Austria)

Leptodora kindti is a very efficient invertebrate predator. Its searching mode of preying is tactile. The setae of the first thoracic limb act as mechanoreceptors, the other thoracic limbs, thorax and head together form the shape of an open basket in which after encounter the prey is pushed in by the aid of the first thoracic limbs and the furca. In Neusiedler See, small individuals of Diaphanosoma brachyurum (0.6–0.9 mm) are the preferred prey, rarely copepods are taken. The predation rate is influenced by temperature, prey density and predator size and varies between less than one and 12 prey items per predator per day. At high predator densities, Leptodora will have a substantial effect on the Diaphanosoma population of Neusiedler See.     

The structure of feeding behavior in the colorado potato beetle,Leptinotarsa decemlineata (Coleoptera: Chrysomelidae)

Feeding behavior, in an ad libitum situation on potato plants in the laboratory, was continuously observed for approximately 7 h/day on 2 successive days for 18 adult femaleLeptinotarsa decemlineata. Additional behaviors were also recorded including resting, walking, biting, local movements, grooming, defecating, and regurgitating. These data were used to calculate a time budget for the various behaviors. The feeding data were analyzed to describe the structure of feeding for young adult females on their normal host plant. The criterion for a meal (minimum intermeal interval) was determined to be 286 s. This criterion was used to distinguish between intra- and intermeal interruptions in feeding for all subsequent analyses. Meals taken from leaves that were young, medium aged, or old did not differ, but on average beetles took 60% of their meals from young leaves. Meal size and meal duration were equally good predictors of when a meal would end. Feeding from stems was a prominent feature for most beetles. The stem meals had much longer durations than leaf meals, but stem feeding did not affect subsequent leaf feeding. The structure of feeding by these beetles is compared with that found in other insects, especiallyLocusta migratoria.     

Differential feeding behaviour of the dominant cladocera as an explanation of zooplankton community structure in the Loosdrecht Lakes

The zooplankton of Loosdrecht Lakes is dominated byBosmina longirostris in the Spring period but is replaced by the largerB.coregoni in early summer at a time when phytoplankton populations, dominated by filamentous cyanophytes, are rapidly increasing. Throughout the summerB.coregoni coexists with the longer, but more slender,Daphnia cucullata.Different size-categories of¹⁴C labelled lake-phytoplankton were fed to natural assemblages of crustacean zooplankton to investigate effects of body length on feeding and the relative selectivities of theDaphnia andBosmina species on food particle-size.B.coregoni feeds more effectively, especially on larger sizes of food, thanB.longirostris and this may be a factor in the observed change of bosminid species dominance.B.coregoni is able to feed on a wide size range of particles and grazes and assimilates greater volumes of food thanD.cucullata. B.coregoni preferentially feeds on food-particles greater than 7 m whereasD.cucullata selects food below 7 m. The coexistence of these species appears to depend on a niche separation through different feeding behaviours and selectivities.     

The role of the external mycelial network of vesicular-arbuscular mycorrhizal fungi: a study of carbon transfer between plants interconnected by a common mycelium

The transfer of ¹⁴C from Lolium perenne (the donor) to Plantago lanceolata (the receiver), mediated by vesicular-arbuscular (VA) mycorrhizal fungi, was examined when the two species were grown together or separately. The VA mycorrhizal infection led to a significant increase, relative to that in uninfected plants, in the ¹⁴C transferred from donor to receiver plants, not only when the roots of the two plants were growing in intimate mixture, but also when they were separated by a root-free zone of 2.33 cm. The majority of isotope transfer between the two plant species was along the direct pathway via VA mycelium.     

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.