Population dynamics and production of five crustacean zooplankters in a subtropical reservoir during years of contrasting turbidity

SUMMARY 1. The temporal dynamics and demography of Meta-diaptomus meridianus (Van Douwe), Lovenula excellens Kiefer. Daphnia gibba Methuen, D. barbata Weltner and Moina brachiata Jurine were studied for 2 years in a small bay of Lake le Roux (Orange River, South Africa). Total zooplankton biomass and population density were 1.4–3 times higher during the less turbid conditions of 1982/83 (Secchi depth transparency around 35 cm) than they were at around 25 cm Secchi depth during 1981/82, when D. barbata was absent.
2. On average, instantaneous birth rates, rates of population change and death rates varied only slightly between years. Birth and death rates were considerably higher above 15°C than below 15°C. These rates correlated with one another and with zooplankton abundance both inter- and intra-specifically suggesting that competitive interactions were important in population regulation. Mortality rates varied more strongly and consistently in a density-dependent direction than did birth rates. In addition to depressed fecundity, the inferred survival of young was poor and population growth low, possibly because food shortage caused high post-natal mortality.
3. Estimates of annual production derived from finite birth rate values varied consistently with annual differences in biomass, and amounted to between 6 and 10 g m⁻² y⁻¹ dry wt. Annual P/B values varied from around 20 for the daphnids to 55 for the copepods and 75 for Moina. Apart from the latter, whose annual P/B ratio virtually doubled from 45 to 75 following reductions in turbidity, annual differences in P/B ratio were slight.     

Zooplankton abundance, community structure and dynamics in relation to inorganic turbidity, and their implications for a potential fishery in subtropical Lake le Roux, South Africa

SUMMARY. 1. The abundance and composition of entomostracan zooplankton were studied between 1977 and 1984 in relation to abiotic and biotic conditions in Lake le Roux (LLR), a large silt-laden reservoir on the Orange River formed in 1976. The community consisted of Metadiaptomus meridianus, Lovenula excellens (Calanoida), Daphnia gibba, D. barbata, D. longispina and Moina brachiata (Cladocera), and various Cyclopoida. 2. Zooplankton biomass varied seasonally from winter lows below 50 mg m^?2 dry wt at temperatures of 9–10°C to summer peaks generally above 1000 mg m^?2 at 21–23°C. It correlated strongly and positively with prevailing water temperature and transparency but only weakly with chlorophyll concentration. 3. Mean annual zooplankton biomass, which varied four-fold (97–408 mg m^?2) in 7 years, increased with annual mean transparency (23–75 cm Secchi depth) and especially with summer heat content (33–230 degree-days above 20°C) which was itself positively correlated with transparency. 4. Daphnid species showed most annual variation in abundance. Large forms(D. gibba and D. barbata) were scarce or absent in two years of very low transparency and low planktivore abundance. The smaller D. longispina developed once during clearer conditions when more fish were present. Copepod biomass also varied inversely with turbidity, but was not as severely reduced at high turbidities. 5. Smallmouth yellowfish(Barbus aeneus: Cyprinidae) is the principal planktivore and candidate fishery species in LLR. It feeds selectively on Lovenula and large daphnids. Catches and growth rates of yellowfish varied directly with the abundance of zooplankton, particularly large food forms, and with water transparency. High turbidity and associated food limitation seems to reduce standing stocks especially of the daphnid zooplankton more than the effects of fish predation. 6. Spawning of yellowfish depends upon the release of water from an upstream reservoir which concurrently reduces transparency and thus zooplankton availability in LLR. Transparency values above 30–35cm SD appear necessary for the development of sufficient and suitable zooplankton to benefit the fishery.     

Macrophytes as refuge or risky area for zooplankton: a balance set by littoral predacious macroinvertebrates

1. Zooplankton use macrophytes as day-time refuge areas when trying to escape from pelagic predators. But macrophytes can also host a diverse and abundant macroinvertebrate assemblage and zooplankton are also likely to face predacious macroinvertebrates once they enter the littoral zone. This study aimed to elucidate the role of macroinvertebrates in determining the refuge capacity of macrophytes.
2. We conducted a field enclosure experiment using plastic bags and complementary laboratory feeding trials to test how macroinvertebrates counteract the benefits to zooplankton of the macrophyte refuge. The field experiment consisted of three treatments with different macroinvertebrate assemblages: without predators (WP), low abundance and diversity (LAD) and high abundance and diversity of predators (HAD – which represents lake conditions).
3. Populations of Diaphanosoma brachyurum , Bosmina huaronensis and Moina micrura (Cladocera) and of both male and female Notodiaptomus incompositus (Copepoda, Calanoida) declined (by nearly 80%) in the presence of HAD in comparison to WP and LAD treatments.
4. Feeding trials revealed that Buenoa sp. (backswimmer), adults of Palaemonetes argentinus (grass shrimp) and Cyanallagma interruptum (damselfly) had a significant negative impact on cladocerans ( D. brachyurum, B. huaronensis ) and the calanoid copepod population (males, females and copepodites). These predators showed a strong predation effect ranging from 75% to 100% reductions of zooplankton populations.
5. The refuge effect offered by macrophytes to zooplankton depends on and is balanced by the predacious macroinvertebrate assemblage that plants host. The risk of confronting littoral predators is high and macroinvertebrate presence can turn the macrophytes into risky areas for zooplankton.     

Day/night differences in the grazing impact of marine copepods

Day/night differences in the removal rate of phytoplankton can occur as a result of increased copepod grazing rates at certain times of the day and diel vertical migration of animals. We conducted shipboard grazing experiments and fine-scale vertical zooplankton sampling to resolve these behaviors. Day/night feeding differences were compared in the center of several warm-core Gulf Stream rings, under conditions of no lateral water mass exchange, in the mesohaline portion of Chesapeake Bay and when following drogues in the Chesapeake Bay plume. Day/night variations in copepod biomass in the surface mixed layer were greater in neritic waters as compared to the open ocean stations. Day/night differences in weight-specific copepod filtration rates varied less than biomass. At the neritic stations copepod grazing was often higher at night, whereas at the oceanic stations day/night grazing rates were similar or daytime grazing rates were highest. The night/day ratio of zooplankton grazing impact on the phytoplankton community (the product of zooplankton biomass and their weight-specific grazing rate) averaged 4.8 in the Chesapeake Bay plume and 1.6 in warm-core Gulf Stream rings. Our results suggest that at lower food levels, there often are less day/night differences in the removal rate of phytoplankton by the copepod community.     

Periphyton as alternative food source for the filter-feeding cladoceran Daphnia magna

1.  Daphnia magna , a well-studied primary consumer, is mainly known as a filter feeder. In this study, we investigated the ability of D. magna to use periphyton as an alternative food source to phytoplankton. We examined the development of laboratory populations fed with different food sources ( Desmodesmus subspicatus and/or periphyton or neither) over a period of 42 days, and observed the behaviour of the daphnids.
2.  The addition of periphyton to phytoplankton food led to an increase of daphnid population biomass. When fed with periphyton as the only food source, a small but stable D. magna population developed.
3.  The behaviour of daphnids fed with both food sources revealed a preference for feeding on D. subspicatus . Only below a concentration of D. subspicatus of approximately 0.05 mg C L⁻¹ (0.4 × 10⁷ cells L⁻¹) did D. magna use periphyton as an alternative food source.
4.  Periphyton showed distinct reactions to grazing by D. magna . The thickness of the periphyton layer was reduced from about 4 to 1 mm and we observed a change in species composition due to grazing.
5.  The ability of D. magna to graze on periphyton could serve to stabilize its population density and reinforce its competitive advantage over other cladocerans. By switching between food sources, D. magna can act as a coupler between pelagic and benthic habitats and food webs.     

Predation on the Invasive Copepod,Pseudodiaptomus forbesi,and Native Zooplankton in the Lower Columbia River: An Experimental Approach to Quantify Differences in Prey-Specific Feeding Rates

Invasive planktonic crustaceans have become a prominent feature of aquatic communities worldwide, yet their effects on food webs are not well known. The Asian calanoid copepod, Pseudodiaptomus forbesi, introduced to the Columbia River Estuary approximately 15 years ago, now dominates the late-summer zooplankton community, but its use by native aquatic predators is unknown. We investigated whether three species of planktivorous fishes (chinook salmon, three-spined stickleback, and northern pikeminnow) and one species of mysid exhibited higher feeding rates on native copepods and cladocerans relative to P. forbesi by conducting `single-prey’ feeding experiments and, additionally, examined selectivity for prey types with `two-prey’ feeding experiments. In single-prey experiments individual predator species showed no difference in feeding rates on native cyclopoid copepods (Cyclopidae spp.) relative to invasive P. forbesi, though wild-collected predators exhibited higher feeding rates on cyclopoids when considered in aggregate. In two-prey experiments, chinook salmon and northern pikeminnow both strongly selected native cladocerans (Daphnia retrocurva) over P. forbesi, and moreover, northern pikeminnow selected native Cyclopidae spp. over P. forbesi. On the other hand, in two-prey experiments, chinook salmon, three-spined stickleback and mysids were non- selective with respect to feeding on native cyclopoid copepods versus P. forbesi. Our results indicate that all four native predators in the Columbia River Estuary can consume the invasive copepod, P. forbesi, but that some predators select for native zooplankton over P. forbesi, most likely due to one (or both) of two possible underlying casual mechanisms: 1) differential taxon-specific prey motility and escape responses (calanoids > cyclopoids > daphnids) or 2) the invasive status of the zooplankton prey resulting in naivety, and thus lower feeding rates, of native predators feeding on invasive prey.     

Intraspecific stoichiometric variability and the ratio of nitrogen to phosphorus resupplied by zooplankton

1. An in situ experiment was carried out in a high-mountain oligotrophic lake (La Caldera) to quantify the specific release rates of ammonium and phosphorus in different phases of the seasonal succession of plankton. The zooplankton community was dominated by the calanoid copepod Mixodiaptomus laciniatus .
2. The rates of release of N and P (from non-detectable to 3.58 μg N mg^–1 DW h^–1 and from 0.26 to 2.29 μg P mg^–1 DW h^–1, respectively) and the N : P released ratio (not detectable to 4.77) fell within the range typical of oligotrophic lakes dominated by copepodite assemblages.
3. The values of the zooplankton N : P ratio (5 : 1 to 10 : 1 by weight) were lower than those established for other species of calanoids, and followed a well-established pattern of seasonal variation from year to year with higher values after ice thaw and lower values as individual size increased (ontogenetic development).
4. The elemental composition of the zooplankton depended on individual size, while the released N : P ratio was inversely related to the N : P ratio of the food. This pattern conforms to the predictions of the homeostasis theory of Sterner (1990).
5. A feedback regulation was established between the stoichiometric composition of the zooplankton, their food and the released N : P ratio that can help explain changes in pelagic community structure during the ice-free period.     

Intraspecific stoichiometric variability and the ratio of nitrogen to phosphorus resupplied by zooplankton

1. An in situ experiment was carried out in a high-mountain oligotrophic lake (La Caldera) to quantify the specific release rates of ammonium and phosphorus in different phases of the seasonal succession of plankton. The zooplankton community was dominated by the calanoid copepod Mixodiaptomus laciniatus .
2. The rates of release of N and P (from non-detectable to 3.58 μg N mg^–1 DW h^–1 and from 0.26 to 2.29 μg P mg^–1 DW h^–1, respectively) and the N : P released ratio (not detectable to 4.77) fell within the range typical of oligotrophic lakes dominated by copepodite assemblages.
3. The values of the zooplankton N : P ratio (5 : 1 to 10 : 1 by weight) were lower than those established for other species of calanoids, and followed a well-established pattern of seasonal variation from year to year with higher values after ice thaw and lower values as individual size increased (ontogenetic development).
4. The elemental composition of the zooplankton depended on individual size, while the released N : P ratio was inversely related to the N : P ratio of the food. This pattern conforms to the predictions of the homeostasis theory of Sterner (1990).
5. A feedback regulation was established between the stoichiometric composition of the zooplankton, their food and the released N : P ratio that can help explain changes in pelagic community structure during the ice-free period.     

The impact of planktivorous flsh on the structure of a plankton community

SUMMARY. 1. The abundance of pianktivorous juvenile yellow perch, Perca flavescens , was manipulated in three 750 m³ enclosures in a eutrophic lake.
2. There was a significant negative relationship between fish and zoopiankton biomasses. At high fish densities the zooplankton community was dominated by small filter-feeding cladocera. primarily bosmi- nids. At low fish densities the zooplankton community was dominated by large filter-feeding cladocera, primarily daphnids.
3. There was no significant relationship between zooplankton and phytoplankton biomasses when considered over the whole experiment but there was a trend towards lower phytoplankton biomass in the enclosure dominated by daphnids during mid-summer.
4. We conclude that although planktivorous fish have a strong negative impact on zooplankton community biomass and size structure, the relationship at the next lower trophic level, zooplankton and phytoplankton, is much weaker. Therefore, the biomanipulation of planktivorous fish populations as a management technique to control phytoplankton abundance is largely ineffective.     

Grazing on filamentous algae by herbivorous zooplankton

SUMMARY. 1. Feeding experiments were conducted to examine filtering rates and selectivity of a variety of zooplankton taxa (including cladocerans, copepods and a rotifer) for filamentous diatoms, green and blue-green algae.
2. Most herbivores were capable of consuming some filamentous algae at rates similar to or higher than those on unicellular algae. Only feeding of Diaphanosoma brachyurum Liéven and Moina micrura Kurz seemed to be primarily limited by the filamentous morphology.
3. Filtering rates and selectivities of most herbivores were much higher for the diatom Melosira granulata angustissima Müller than for similarly sized blue-green algal filaments, indicating that chemical factors strongly influence consumption of filamentous algae.
4. The toxic blue-green algal filament Anabaena flos-aquae De Brébisson NRC 44–1 had a much strong inhibitory effect on zooplankton feeding than other filaments. The only herbivores that were not inhibited by this strain have been shown to be resistant to blue-green algal toxins, or strongly avoided consuming the blue-green alga. These results indicate that the inhibitory effect of filamentous algae is due more to toxic or noxious chemicals than to the filamentous morphology.
5. Selectivities of zooplankton for filamentous algae were largely independent of herbivore body size. The small-bodied cladoceran Bosmina longirostris Müller had the highest selectivities for filamentous algae.     

Effects of eutrophication on seasonal patterns of mesozooplankton in a tropical reservoir: a 4‐year study in Pampulha Lake, Brazil

1. Pampulha Lake has undergone rapid eutrophication. Annual maxima of total phosphorus and conductivity increased steadily from 1993 to 1996. Nitrogen quickly decreased after macrophyte removal in 1994, but increased again in the following years
2. The zooplankton was dominated by Daphnia gessneri , D. laevis , Diaphanosoma birgei and Thermocyclops decipiens . The biomass of major zooplankton organisms, including Daphnia and rotifers, increased during the period covered in this study.
3. Autocorrelation coefficients have confirmed the existence of recurrent seasonal patterns for both chemical and biological variables.
4. Daphnia , calanoid copepods and rotifers showed clear and recurrent seasonal patterns. Cyclopoid copepods and Diaphanosoma had temporal patterns more affected by long‐term trophic changes. Other organisms such as Moina, Ceriodaphnia and Bosmina exhibited high temporal variability with no recurrent patterns.
5. There was a strong and positive correlation between total phosphorus and Daphnia biomass. Seasonal patterns of this cladoceran were also inversely associated with sestonic C:P ratios. Thus, Daphnia may be limited primarily by phosphorus and not by total food availability as expressed by particulate carbon.
6. This study suggests that increasing eutrophication has changed the structure as well as seasonal patterns of the zooplankton community in Pampulha Reservoir.     

The estimation of crop evacuation rates in Chaoborus larvae (Diptera: Chaoboridae) using natural prey

SUMMARY. 1. Crop evacuation rates were estimated for the first time in Chaoborus larvae, using natural prey.
2. Fourth instar C. americanus Matheson digested copepods (Diaptomus leptopus S.A. Forbes) 48.7% faster than daphnids (Daphnia rosea Leydig) of similar size; meal size did not significantly affect the instantaneous rate of digestion (IRD) within each prey type. Prey specific IRD has not been reported before for zooplankton.
3. Prey specific differences in IRD require the use of natural prey when digestion experiments are to be used to estimate natural rates of food consumption of animals.     

Effects of food type on the life history of Daphnia clones from lakes differing in trophic state. II. Daphnia cucullata feeding on mixed diets*

1. The effects of feeding on suboptimal foods were investigated in Daphnia cucullata a zooplankton common in many types of lakes. Eleven clones of D. cucullata were collected from four lakes of varying trophic levels and fed a high (1 mg C l^–1) concentration of one of two diets: (i) a 1 : 9 mixture of the cryptophyte Cryptomonas pyrenoidifera with the green alga Scenedesmus obliquus ; and (ii) a 1 : 9 mixture of the cryptophyte C. pyrenoidifera with the filamentous cyanobacterium Oscillatoria limnetica.
2. As expected, the diet dominated by Oscillatoria was found to be significantly poorer compared with the diet dominated by Scenedesmus. All D. cucullata clones were, however, able to achieve positive population growth rates on both diets, supporting the view that daphnids can grow and reproduce on a diet dominated by filamentous cyanobacteria.
3. Clones originating from different types of lakes differed in their responses to the two diets. On the Scenedesmus -dominated diet, clones originating from moderately eutrophic lakes had lower population growth rates than clones from hypertrophic lakes. In contrast, on the Oscillatoria -dominated diet, the clones from moderately eutrophic lakes had higher population growth rates than clones from hypertrophic lakes. Contrary to expectation, clones originating from lakes dominated by filamentous cyanobacteria had more difficulty utilizing filamentous cyanobacteria as food than clones from less eutrophic lakes which contain less filamentous cyanobacteria.
4. As the reactions of clones originating from same type of lake resembled each other, it is hypothesized that the life histories of D. cucullata populations are locally adapted to environmental factors other than abundance of filamentous cyanobacteria, which are correlated with trophic levels.     

Effects of sediment loading on food perception and ingestion by freshwater copepods

Seasonal influx of suspended sediments to lakes is common in many aquatic systems, particularly during periods of high flow. Suspended particles not only affect visibility and net primary productivity, as light penetration is reduced, but may also affect the ability of herbivorous zooplankton to locate and ingest food. In Swan Lake, an oligotrophic lake in northwestern Montana, the dose association between zooplankton population development and seasonal maxima in turbidity levels suggests that the community present during peak turbidity is minimally impacted by suspended sediments. This report presents the results of laboratory investigations into the effect of suspended sediments on feeding by the copepod Diaptomus ashlandii, the dominant zoo‐plankter in Swan Lake. Ingestion rate for copepods feeding on ³²P labeled algae was significantly reduced in the presence of suspended particles at all turbidity levels tested (5 to 200 NTU's) compared to ingestion rate in the absence of suspended particles (0 NTU's).     

Environmental constraint of intraguild predation: Inorganic turbidity modulates omnivory in fairy shrimps

1. Omnivory is widespread in food webs, with an important stabilising effect. The strength of omnivorous trophic interactions may change considerably with changes in the local environment.
2. Shallow temporary waters are often characterised by high levels of inorganic turbidity that may directly limit the food uptake of filter-feeding organisms, but there is little evidence on how it might affect omnivorous species. Anostracans are key species of temporary waters and recent evidence suggests that these organisms are omnivorous consumers of both phyto- and zooplankton.
3. Using Branchinecta orientalis as a model species, our aim was to test how turbidity affects the feeding of an omnivorous anostracan. To do this, we used short-term feeding experiments and stable isotope analyses, with animals collected from soda pans in eastern Austria. In the feeding experiments, algae and zooplankton were offered as food either separately or in combination. The prey type treatments were crossed with turbidity levels in a factorial design.
4. There was a pronounced decrease in the ingested algal biomass with increasing turbidity. Conversely, ingestion rates on zooplankton were less affected by turbidity. Stable isotope analyses from field material supported our experimental results by showing a positive relationship of the trophic position of anostracans and the trophic niche of the communities with turbidity.
5. Our results show that turbidity modulates the intraguild trophic relationship between anostracans and their prey by shifting the diet of anostracans from more herbivorous in transparent to more carnivorous in turbid waters. Thus, inorganic turbidity might also have a community-shaping role in plankton communities of temporary waters through altering trophic relationships.

     

Acute inhibition of protease and suppression of growth in zooplankter,Moina macrocopa,by Microcystis blooms collected in Central India

Cyanobacterial blooms consisting of Microcystis spp., collected from 14 water-bodies in Central India, and an adapted culture, were studied for likely impact on zooplankton community. When fed with single cells of Microcystis from several locations, in mixtures with Chlorella, population growth of the cladoceran Moina macrocopa was suppressed. Microcystis alone was unsuitable as food. In three cases, bloom extracts enhanced mortality of starved zooplankton. Extracts from several sources inhibited protease activity when trypsin or a crude extract from zooplankton served as enzyme source. Upon fractionation by solid-phase extraction, the C-18 passed extract contained the anti-protease and toxic substances for zooplankton, whereas a methanol eluted fraction retained the trypsin inhibitory substance. The study suggests that production of protease inhibitors by cyanobacteria is a factor responsible for feeding inhibition and mortality in zooplankton, which in turn could regulate the community structure of grazers.     

Foraging strategies and growth inhibition in five daphnids feeding on mixtures of a toxic cyanobacterium and a green alga

1. Laboratory experiments were used to study the feeding, growth and reproduction of five daphnids in mixtures of a toxic cyanobacterium, Microcystis aeruginosa, and a green alga, Scenedesmus acutus. The mixtures included 0%, 20%, 50%, 80% and 100% Microcystis with a total food concentration of 0.5 mg C L^??1 in each treatment. The feeding rate was measured after 1 and 24 h of acclimatization to the mixtures. 2. Toxic Microcystis inhibited feeding in all the species, but they exhibited an unexpected diversity and complexity in the pattern of feeding inhibition. Daphnia magna exhibited the strongest inhibition of feeding after 1 h of exposure to toxic food, but had substantially recovered after 24 h in the same mixtures. This pattern of inhibition and recovery may balance the benefits of reduced ingestion of toxin with the disadvantage of a reduced energy intake. 3. All five daphnids grew quickly in the Scenedesmus control, whereas growth and reproduction declined with an increasing proportion of the toxic alga in the diet. Daphnia pulicaria showed the least inhibition of growth and reproduction, D. pulex showed the strongest inhibition and the three remaining species exhibited intermediate sensitivity. 4. Estimates of gross growth efficiency (GGE; growth/ingestion) provided a means for discriminating between the effects of feeding inhibition and direct toxicity on zooplankton growth. Daphnia pulex exhibited a sharp decline in GGE, suggesting that growth inhibition was a result of both feeding inhibition and direct toxicity. In contrast, D. magna exhibited a nearly constant GGE, indicating that feeding inhibition accounted for its decline in growth. However, two Daphnia species (i.e. D. pulicaria and D. galeata) exhibited improbable increases in GGE with toxic cyanobacteria, suggesting that their feeding rates were underestimated. Growth assays with sensitive and resistant zooplankton species are proposed for testing the potential impacts toxic cyanobacteria in lakes.     

A comparison of the effects of prey and non-prey neighbours on foraging rates of Epischura nevadensis (Copepoda: Calanoida)

SUMMARY. 1. Adults of the calanoid copepod, Epischura nevadensis , aggregate in situ near the thermocline in Lake Tahoe, California-Nevada. together with adults of another species of calanoid copepod, Diaptomus tyrelli and juveniles of both species. With a series of laboratory predation and algal clearance trials, we show that foraging rates of adult E. nevadensis are determined not only by the density of co-occurring potential prey (small copepods), but also by the presence of co-occurring non-prey neighbours (large, adult copepods). These effects occur at densities and in zooplankton assemblages found naturally, emphasizing the ecological importance of neighbours other than prey on zooplankton feeding.
2. Neighbours are distinguished primarily by size. Although predation rates increase linearly with the densities of small copepods. both algal clearance and predation rates decrease in the presence of large copepods. We also show, with a field predation experiment using small enclosures, that adults are size selective within species and that Diaptomus are selected over conspecifics of the same size.
3. We hypothesize that by reducing foraging rates in the presence of large zooplankton. E. nevadensis avoids predators and reduces predation risk at the cost of reduced energy consumption.     

Trophic interactions in rockpool food webs: regulation of zooplankton and phytoplankton by Notonecta and Daphnia

We studied trophic interactions in experimental rockpools with three different food web structures: phytoplankton and small-bodied zooplankton; phytoplankton, small-bodied zooplankton and Daphnia ; and phytoplankton, small-bodied zooplankton, Daphnia and Notonecta . Nutrients, primary productivity, chlorophyll a and zooplankton species composition and biomass were measured over eight weeks.
2. Daphnia had a negative impact on other zooplankton and reduced the phytoplankton biomass and primary productivity. In the absence of Daphnia , small-bodied zooplankton species were abundant, in particular cyclopoid copepods. Concentrations of dissolved nutrients were lower and the standing crop of primary producers was higher when Daphnia was absent.
3. The presence of the invertebrate predator Notonecta produced a top-down effect which was similar to that reported for planktivorous fish, i.e. a selective reduction of daphnids followed by an increase of small-bodied zooplankton species and phytoplankton biomass.
4. The study showed that consumer regulation of Daphnia by Notonecta and of algae by Daphnia are important, but also demonstrated that trophic level biomasses were controlled by a combination of predation and resource limitation.     

A field technique for the determination of zooplankton grazing on natural bacterioplankton

SUMMARY. 1. A method for the determination of zooplankton community grazing rates on natural populations of bacteria is described.
2. Undisturbed zooplankton samples were incubated with ³H-thymidine-labelled bac teriioplankton, and animals and their food subsequently separated by size.
3. Prehandling of zooplankton by concentration before incubation reduce the grazing rates by a factor ranging from three to four.
4. Time series experiments showed that appropriate incubation periods ranged from 40 min at 14°C to 60 min at 10°C.
5. Spatial variability in the zooplankton was the main source of the overall standard error. This was considerably reduced by using composite samples.