The extant Chaoborus assemblage can be assessed using subfossil mandibles

1. Assemblages of Chaoborus were examined in 80 thermally stratified southern central Canadian Shield lakes to explore whether subfossil mandibles could be useful in assemblage‐level studies of Chaoborus. 2. Chaoborus (Sayomyia) (probably Chaoborus punctipennis in this study region) and Chaoborus flavicans were the most common taxa recorded, while Chaoborus trivittatus was rarer. Chaoborus americanus was not recorded in subfossil assemblages, because no fishless lakes were included in this study. Chaoborus flavicans had higher relative abundances (%) in lakes with higher dissolved organic carbon (DOC), probably because of reduced fish predation in less transparent water. 3. Results from logistic regression indicate that patterns of presence/absence for Chaoborus mandibles in the study lakes were influenced primarily by hypolimnetic oxygen concentration, probably because of the presence or extent of a hypolimnetic refugium from fish predation. 4. Chaoborus species richness in lakes, derived from subfossil assemblages, did not differ significantly from species richness estimates derived from plankton sampling with a net. Patterns of dominance and coexistence [e.g. the widespread co‐occurrence of C. flavicans and C. (Sayomyia)] determined from subfossil assemblages agreed with previous studies of the contemporary living assemblage. 5. These results suggest that subfossil assemblages may be used as an alternative to nocturnal plankton sampling to carry out research on the community ecology of Chaoborus. 6. We propose a hierarchical conceptual model of assemblage‐level patterns of Chaoborus in temperate lakes. Chaoborus americanus dominates in fishless lakes, whereas in lakes with fish Chaoborus is typically absent where there is no anoxic hyplimnion. In lakes with anoxic strata, C. trivittatus tends to dominate in lakes with few fish; in the remaining lakes, C. flavicans and C. (Sayomyia) dominate, although C. flavicans is more relatively abundant in lakes with lower water clarity (higher DOC).     

Fish predation,lake acidity and the composition of aquatic insect assemblages

Aquatic insect assemblages were sampled in 2 sets of 18 small lakes in 2 regions of northeastern Ontario. Both sets included lakes with and without fish. In the set near Sudbury, fishless lakes were acidic. Using a standardized sweep net procedure, fishless lakes in both areas were found to have a greater abundance and richness of insects than lakes with fish. Irrespective of pH, fishless lakes supported a similar aquatic insect assemblage which was characterized by an abundance of nekton, especially Notonectidae, Corixidae, Graphoderus liberus (Dytiscidae) and Chaoborus americanus (Chaoboridae). Those taxa were typically absent from lakes with fish, which often had a marked abundance of Gerridae. It is concluded that fish predation is the most immediate factor structuring such aquatic insect assemblages, and is responsible for their change coincident with lake acidification.     

Predator driven changes in community structure

Summary The zooplankton community of a small pond changed markedly with temporal variation in predation pressure. Long term changes in zooplankton community structure occurred following the replacement of planktivorous fish by phantom midge (Chaoborus americanus) larvae as the predominant predator of zooplankton. The interannual changes following the establishment of Chaoborus included the apparent or near extinction, of species ill adapted to the new predation pressure and the successful colonization of well adapted species. Seasonal changes in the species composition and size distribution of the zooplankton community correlate with temporal variation in predation intensity associated with temperature-activity patterns of the predator or changes in the stage structure of the predator population.     

Crustacean zooplankton species richness and associations in reservoirs and ponds of Aguascalientes State, Mexico

We report the species of crustacean zooplankton found in 19 resevoirs and ponds that were sampled at on at least two occasions in Aguascalientes State, Mexico, at a latitude of about 21°30 North Latitude. We collected a total of 33 cladoceran taxa, 15 calanoid and cyclopoid copepods, an Ergasilus copepod, 2 anostracans, one notostracan species and an amphipod. All these taxa had previously been reported in Mexico. As in more temperate areas, there was a significant correlation between the species richness and lake size. The species-area relationship for Aguascalientes was indistinguishable from that of more northerly lakes. The two most common associations of zooplankton species were (1) those small species that occurred with fish: Daphnia parvula, Diaphanosoma birgei, Leptodiaptomus siciloides, Mastigodiaptomus albuquerquensis, and Thermocyclops inversus, and (2) the large Daphnia schodleri that occurred with the predator Notonecta, in the absence of fish. With minor variations, these two associations also occur throughout temperate North America.     

Ecological distributions of <Emphasis Type="Italic">Chaoborus</Emphasis> species in small,shallow lakes from the Canadian Boreal Shield ecozone

Small, shallow lakes and ponds are often the dominant landscape features in many regions, but are comparably less studied than larger lakes. Shallow lakes are more likely to lack fish populations; however, it is often difficult to ascertain whether these sites were naturally fishless or lost their fish populations due to anthropogenic or natural stressors. We examined the distributional abundances of four Chaoborus species by identifying and enumerating their larval mandibles in the surface sediments of 146 randomly selected lakes from northwestern Ontario, Canada, to determine the key environmental gradients influencing chaoborid distributions. Chaoborus mandibles were encountered at 110 lakes and, in 65% of those lakes, total counts were ≥10. Direct gradient analyses were then used to show that lakewater total aluminum concentrations (negatively correlated with pH), lakewater sodium concentrations, lake surface area, and maximum water depth were significant predictors of the distributional abundances of Chaoborus. Generalized linear models indicated that Chaoborus species varied in their responses to significant environmental factors. C. (Sayomyia) was not significantly associated with any environmental variable and the abundances of larger chaoborids may be an important biotic factor affecting this taxon. Chaoborus americanus, an indicator of fishless lakes, was significantly correlated with all five key variables and demonstrated a clear threshold of occurrence in relatively small lakes (i.e., <10 ha in surface area). Furthermore, based on the occurrence and abundance of C. americanus, we estimated that 20% of the lakes we surveyed are currently fishless. These lakes significantly differ in several geomorphic and water-chemistry measures compared to the other study lakes.     

Zooplankton Migration in Three Lakes of Western New York

We examined the diel vertical migration (DVM) of zooplankton in three lakes of western New York; Lakes Conesus, Lime, and McCargo. In all three lakes, the dipteran predator Chaoborus was a more dramatic migrator than any of the cladocerans or copepods. In contrast, another invertebrate predator, Leptodora, seemed to have the least vertical dispersal. In Conesus Lake, following the evening ascent and upper-water night-positioning of Chaoborus flavicans, Daphnia pulicaria expanded its own vertical range of dispersal thereby decreasing its degree of overlap with Chaoborus. The vertical distribution of Diaptomus sicilis was mostly below that of Daphnia pulicaria, thus reducing possible competition for food resources. Poor oxygen conditions in the lower waters of Lakes Lime and McCargo restricted all zooplankton (except Chaoborus) to an epilimnetic zone where the regions of niche overlap and predation were narrowed. Such annually-induced restrictions on vertical dispersal are probably common features of numerous stratified lakes with hypolimnetic reducing conditions. No fixed pattern of vertical dispersal or migration is likely in lakes where stratification and non-stratification follow each other seasonally. Vertical migration does provide some flexibility in niche separation, but the potential for chaotic behavior in interacting predators and prey may confound simple explanations of DVM.     

Functional shifts in lake zooplankton communities with hypereutrophication

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Does the impact of nutrients on the biological structure and function of brackish and freshwater lakes differ?

The effects of nutrients on the biological structure of brackish and freshwater lakes were compared. Quantitative analysis of late summer fish, zooplankton, mysid and macrophyte populations was undertaken in 20–36 shallow brackish lakes of various trophic states and the findings compared with a similar analysis of shallow freshwater lakes based on either sampling (fish) or existing data (zooplankton, mysids and macrophytes). Special emphasis was placed on differences in pelagic top-down control. Whereas the fish biomass (CPUE, multiple mesh-size gill nets) rose with increasing P-concentration in freshwater lakes, that of brackish lakes was markedly reduced at P-concentrations above ca. 0.4 mg P l^-1 and there was a concomitant shift to exclusive dominance by the small sticklebacks (Gasterosteus aculeatus and Pungitius pungitius); as a result, fish density remained relatively high. Mysids (Neomysis integer) were found at a salinity greater than 0.5 and increased substantially with increasing P-concentration, reaching levels as high as 13 ind. l^-1. This is in contrast to the carnivorous zooplankton of freshwater lakes, which are most abundant at intermediate P levels. The efficient algal controller, Daphnia was only found at a salinity below 2 and N. integer in lakes with a salinity above 0.5. Above 2 the filter-feeding zooplankton were usually dominated by the less efficient algal controllers Eurytemora and Acartia. In contrast to freshwater lakes, no shift to a clearwater state was found in eutrophic brackish lakes when submerged macrophytes became abundant. We conclude that predation pressure on zooplankton is higher and algal grazing capacity lower in brackish eutrophic-hypertrophic lakes than in comparable freshwater lakes, and that the differences in trophic structure of brackish and freshwater lakes have major implications for the measures available to reduce the recovery period following a reduction in nutrient loading. From the point of view of top-down control, the salinity threshold dividing freshwater and brackish lakes is much lower than the conventionally defined 5.     

Macroinvertebrates as indicators of fish absence in naturally fishless lakes

1. Little is known about native communities in naturally fishless lakes in eastern North America, a region where fish stocking has led to a decline in these habitats. 2. Our study objectives were to: (i) characterise and compare macroinvertebrate communities in fishless lakes found in two biophysical regions of Maine (U.S.A.): kettle lakes in the eastern lowlands and foothills and headwater lakes in the central and western mountains; (ii) identify unique attributes of fishless lake macroinvertebrate communities compared to lakes with fish and (iii) develop a method to efficiently identify fishless lakes when thorough fish surveys are not possible. 3. We quantified macroinvertebrate community structure in the two physiographic fishless lake types (n = 8 kettle lakes; n = 8 headwater lakes) with submerged light traps and sweep nets. We also compared fishless lake macroinvertebrate communities to those in fish‐containing lakes (n = 18) of similar size, location and maximum depth. We used non‐metric multidimensional scaling to assess differences in community structure and t‐tests for taxon‐specific comparisons between lakes. 4. Few differences in macroinvertebrate communities between the two physiographic fishless lake types were apparent. Fishless and fish‐containing lakes had numerous differences in macroinvertebrate community structure, abundance, taxonomic composition and species richness. Fish presence or absence was a stronger determinant of community structure in our study than differences in physical conditions relating to lake origin and physiography. 5. Communities in fishless lakes were more speciose and abundant than in fish‐containing lakes, especially taxa that are large, active and free‐swimming. Families differing in abundance and taxonomic composition included Notonectidae, Corixidae, Gyrinidae, Dytiscidae, Aeshnidae, Libellulidae and Chaoboridae. 6. We identified six taxa unique to fishless lakes that are robust indicators of fish absence: Graphoderus liberus, Hesperocorixa spp., Dineutus spp., Chaoborus americanus, Notonecta insulata and Callicorixa spp. These taxa are collected most effectively with submerged light traps. 7. Naturally fishless lakes warrant conservation, because they provide habitat for a unique suite of organisms that thrive in the absence of fish predation.     

Rediscovery of Leptestheria dahalacensis and Eoleptestheria ticinensis (Crustacea: Branchiopoda: Spinicaudata): an overview on presence and conservation of clam shrimps in Austria

According to recent literature, five of the six known Austrian conchostracan species are extinct. However, interim results of a current study on large freshwater branchiopods in Austria show that five species still occur at a restricted number of sites in the Pannonian region of Lower Austria. The clam shrimps Leptestheria dahalacensis and Eoleptestheria ticinensis were rediscovered in May 1994 in the flood plains of the river Morava near Marchegg. Imnadia yeyetta and Cyzicus tetracerus have been known to the authors in the same region since 1981, and 1992, respectively. Limnadia lenticularis occurs in the flood plains of the rivers Morava and Danube. Lynceus brachyurus, the only Austrian representative of the Laevicaudata, was not found and most probably got extinct. All Austrian clam shrimp species are considered to be endangered. Main threats are agricultural activities and artificial changes of the hydrologic conditions. Conservational measures are discussed for their effectivity.     

Prey utilization by Chaoborus punctipennis Say in a small,eutrophic reservoir

Prey selection by 2nd-, 3rd-, and 4th-instar larvae of Chaoborus punctipennis was evaluated by comparing larval crop contents to the species structure of the zooplankton community in a small, eutrophic lake. The rotifers, Brachionus caudatus and Trichocerca similis, were positively selected by and comprised most of the diet of 2nd- and 3rd-instar larvae. Rotifers comprised 67% of all prey items identified in the crops of even 4th-instar larvae. Whereas 2nd-and 3rd-instar larvae selected the two rotifers with equalfrequency, 4th-instar larvae selected Trichocerca significantly more frequently than they did Brachionus. Although copepods comprised only 8% of all prey identified in 3rd-instar crops, they comprised 33% of the crop contents of 4th-instar larvae. Both 3rd and 4th instars exhibited a positive electivity for Cyclops vernalis significantly more frequently than for Diaptomus pallidus. We conclude that the larvae of Chaoborus punctipennis can, and do, select between Trichocerca and Brachionus and between Cyclops and Diaptomus.The paper by Brooks and Dodson (1965) triggered a flurry of studies on aquatic predators and their ability to discriminate between available prey species. Although the original work dealt with vertebrate predators, it has become increasingly apparent that invertebrate predators may also exhibit considerable selectivity in choosing between prey categories. The larval stages of the various species of Chaoborus are ubiquitous lentic predators, especially in mesotrophic or eutrophic systems. Although there have been a number of recent publications which deal with the food habits and preferences of several species of Chaoborus, only a few studies have included analyses of larval crop contents and none of these have included Chaoborus punctipennis. This species is of particular interest because it is one of the smaller species in the genus, and perhaps for this reason, because it is frequently present in lakes which support large populations of planktivorous fish (von Ende 1979). The purpose of our research was to characterize the food habits and preferences of Chaoborus punctipennis in a small, eutrophic reservoir which supports a rather depauperate number of species of zooplankters.     

Mesozooplankton of the Kosi Bay lakes,South Africa

The Kosi coastal lake system, a chain of four interconnected basins, is located in the subtropical north-eastern corner of South Africa. Little information is available on zooplankton of the system and the main aim of this study is to report on zooplankton samples collected during 2002 and 2003. The set of samples consists of seasonal, subsurface mesozooplankton samples that were collected during nighttime in each of the lakes. A well-developed salinity gradient was evident along the interconnected lakes in the subsurface water during all seasons, ranging from freshwater in the upper lake Amanzamnyama to a maximum of 22 recorded in Lake Makhawulani. The zooplankton community structures of the lakes reflected the salinity gradient of the system, with some coastal marine taxa recorded in the lakes closer to the mouth and only freshwater taxa recorded in Lake Amanzamnyama. Mesozooplankton diversity and abundance were relatively low compared to other estuarine systems along the eastern coast of South Africa. The dominant taxa were calanoid copepods Acartiella natalensis and Pseudodiaptomus stuhlmanni and the mysid Mesopodopsis africana in the lower lakes, whereas cyclopoids Mesocyclops sp. and Thermocyclops sp. dominated the freshwater lake Amanzamnyama.     

Autotroph:herbivore biomass ratios; carbon deficits judged from plankton data

A survey on phytoplankton:zooplankton biomass ratios was performed in 342 Norwegian lakes, covering a wide range in lake size and productivity (total phosphorus: 3–246 g l^–1), but with most localities being oligo- to mesotrophic. Mean phytoplankton biomass was 88 g C l^–1, yet with the majority below 50 g C l^–1and a median of 25 g C l^–1. Total zooplankton biomass displayed a mean and median of 37 and 26 g C l^–1, respectively. Cladocerans were by far the dominant group, making up a median of almost 60% of total zooplankton biomass. Total zooplankton biomass as well as that of major aggregated metazoan taxa (cladocerans, calanoid copepods, cyclopoid copepods and rotifers) all showed a positive, but weak correlation with total phytoplankton biomass. These weak correlations suggest that algal biomass per se is a poor predictor of zooplankton biomass. An average phyto-:zooplankton biomass ratio (C:C) of 2.8 (SD±4.7) was found. 30% of the lakes had a phyto-:zooplankton biomass ratio below unity. While there was no correlation between the phyto-:zooplankton biomass ratio with increasing productivity in terms of P concentration, there was a higher biomass ratio in lakes with high fish predation pressure. The low ratio of phyto-:zooplankton biomass suggest major requirements from non-algal sources of C in the zooplankton diet. The need for dietary subsidizing is also supported by the fact that more than 75% of the lakes had algal biomass less than the estimated threshold for net positive growth of zooplankton, although it should be kept in mind that a high share of picoplankton would imply an underestimation of autotroph biomass in these lakes. Since the C-deficiency apparently is most pronounced in oligotrophic systems, it contradicts the view that the detritus pathways plays a predominant role in highly productive systems only, but while the source of detritus probably is mostly of autochthonous origin in eutrophic lakes, allochthonous detritus will be more important in oligotrophic systems.     

Chaoborus americanus predation on various zooplankters; Functional response and behavioral observations

Summary Predation rates for Chaoborus americanus on different types of noncyclomorphic zooplankton prey were determined in the laboratory as a function of both prey species and density. The sequence of events leading to consumption of prey was also observed, and probabilities determined for the various components.Predation rates generally reached an upper limit as the density of prey increased to high levels. Larger prey were always less vulnerable to Chaoborus predation. Among the species of zooplankton prey examined Holopedium gibberum suffered the lowest rates of predation, while those of the other plankters (Daphnia pulex, D. rosea, Moina hutchinsoni, Diaptomus birgei, D. leptopus, Cyclops vernalis) were approximately equivalent for any given size category.Observation of the predatory sequence indicates that for all the plankters except H. gibberum there were relatively high rates of evasion of attack by C. americanus and low rates of escape after capture. For H. gibberum the situation was reversed, and escape after capture was particularly important in determining the predation rate.The gelatinous envelope of Holopedium gibberum seems to operate as an effective anti-predator device which reduces the likelyhood of retention by Chaoborus after contact has been made.     

Fish-eating bird abundance in oligotrophic lakes in Kejimkujik National Park,Nova Scotia,Canada

Aquatic bird population data in 40 oligotrophic lakes and ponds in Nova Scotia, Canada indicates that only lakes > 20 ha support territorial pairs of Common Loon (Gavia immer) and only lakes 40 ha produce their young. Broods of Common Merganser (Mergus merganser americanus) occurred in lakes > 25 ha and loon and merganser broods together occurred only in lakes 80 ha. The fish production in the lakes was estimated from the total phosphorus vs fish yield relationship obtained in similar, oligotrophic lakes in Ontario. Considering the fish consumption and mergansers until fledging along with the maintenance of the adults during the same period (about 200 and 130 kg fish respectively) and the estimated fish production in these lakes, there is a close balance between the size of water body and its fish production to the occupancy and production of piscivorous birds.     

Stoichiometry in an ecological context: testing for links between <Emphasis Type="Italic">Daphnia</Emphasis> P-content,growth rate and habitat preference

We used laboratory experiments with ten Daphnia taxa to test for links between Daphnia P-content, growth rate and habitat preference. The taxa represent a wide range of body sizes and most show distinct preferences for one of three habitats: shallow lakes, deep, stratified lakes or fishless ponds. Previous studies show that taxa from shallow lakes and fishless ponds experience high predation risk and rich food resources, whereas taxa from deep lakes experience low predation risk, strong food limitation and potentially P-deficient resources. Thus, we predicted higher P-content and higher maximal growth rates in taxa from ponds and shallow lakes and lower P-content, lower maximal growth but reduced sensitivity to P-limitation in taxa preferring stratified lakes. In each of 25 experiments, a clonal Daphnia cohort was cultured for 4 days on a P-sufficient (molar C:P ratio 70) or a P-deficient (C:P 1,000) diet of a green alga at a high concentration (1 mg C l^–1). The P-content of adult Daphnia fed the P-sufficient diet ranged from 1.52 to 1.22% mass. Small-bodied taxa from shallow lakes had higher P-content than larger-bodied taxa from deep lakes or fishless ponds. However, we found a nonsignificant negative correlation between P-content and growth on the P-sufficient diet, rather than the positive relationship predicted by the growth rate hypothesis. The P-deficient diet resulted in declines in both growth rate and P-content compared with the P-sufficient controls and the extent of the declines differed between taxa. Taxa from ponds showed a marginally greater decline in growth with the P-deficient diet compared with taxa from shallow or deep lakes. However, contrary to stoichiometric theory, no relationship was found between a species P-content and growth depression on the P-deficient diet. Although we found evidence for habitat adaptations, our results show that factors other than Daphnia P-content are important in determining differences between Daphnia species in both maximal growth rate and sensitivity to P-limited growth.     

Variation in horizontal zooplankton abundance in mountain lakes: shore avoidance or fish predation?

Hypothesizing that fish predation, active shore avoidance and outlet stream avoidance may be separately affecting horizontal zooplankton distribution, the effects of fish presence, sampling location (midlake, outlet and non-outlet shore) and time (day or night) on zooplankton abundance and body size were tested. Statistically significant horizontal zooplankton abundance gradients occurred in both fish-present and fish-absent lakes. Fish may strengthen zooplankton spatial patterns common to both fish-present and fish-absent lakes, as abundance differences among locations were often greater in fish-present systems compared to fish-absent systems. Horizontal zooplankton abundance gradients differed through a diel cycle, but were species specific with some species exhibiting gradients only during the day, while others exhibit gradients only during the night. Avoidance of the outlet over and above active shore avoidance appeared to take place in Daphnia sp. Other taxa provided equivocal support of active outlet avoidance with most showing no significant difference between shore and outlet abundance (seven of nine), one taxa showing a significant decrease and one a significant increase in outlet compared to shore abundance. No gradients in zooplankton body size were found.      

Effects of humic stress on the zooplankton from clear and DOC‐rich lakes

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Acoustic observations of zooplankton in lakes using a Doppler current profiler

1. In this paper we test the usefulness of acoustic backscatter measurements from a 614 kHz Acoustic Doppler Current Profiler (ADCP) for the qualitative and quantitative characterisation of zooplankton distributions in lakes. ADCP‐based backscatter estimates were compared with frequent net hauls obtained during a calibration experiment in which the acoustic backscatter was strongly dominated by vertical migrating Chaoborus flavicans larvae. 2. The correlation between backscatter estimates and the C. flavicans concentration was very good. Vertical swimming speed of larvae, measured directly by the ADCP, was up to a maximum of 5 mm s^?1 and agreed very well with the observed vertical movement of the backscatter contour lines. Although the strong backscatter from C. flavicans overwhelmed the signal from the remaining zooplankton, a good correlation between backscatter strength and the total remaining zooplankton concentration, dominated by Cyclops spp., was found for the depth and time intervals where no C. flavicans were present. 3. In addition to the calibration experiment, longer‐term ADCP measurements from different lakes revealed a strong temporal correlation between the onset of the up‐ and downward migration of zooplankton and the local sunset and sunrise. 4. We conclude that ADCPs can be used to monitor plankton distributions both temporally and spatially. It also seems possible to estimate plankton densities after appropriate calibration.