A change in the pattern of vertical migration of Chaoborus flavicans after the introduction of trout

The pattern of diel vertical migration exhibited by Chaoborusflavicans in Lake Lenore, Washington has changed over the pastdecade. Formerly Chaoborus larvae and pupae were present inthe water column during the day. Since 1982, a strong patternof diel vertical migration has been evident in this population.Third and fourth instar larvae and pupae reside in the sedimentsduring the day and then ascend to the surface waters at night.The change in migratory activity of C. flavicans was coincidentwith the introduction of cutthroat trout (Salmo clarki henshawi)into Lake Lenore in 1979. Predation by trout on fourth instarlarvae and pupae of C. flavicans was probably responsible forthe observed changes in migratory behavior.     

Linking lakes? The population genetic structure of Chaoborus flavicans

Larvae of Chaoborus flavicans (Diptera: Chaoboridae) are commonin various lakes and are important members of plankton communities.To assess gene flow between lakes, we sampled several populationsthroughout Europe. To explicitly test whether gene flow is higherwithin regions than between regions, we examined four regionseach containing several populations. For a detailed analysisof regional gene flow, 12 populations within a region in NorthGermany were analysed. Allozymes and mitochondrial restrictionfragment length polymorphism (RFLP) were used to estimate therelative amounts of gene flow. Results indicate that most ofthe dispersal in C. flavicans takes place between lakes withinregions. Therefore, the population genetic structure of C. flavicanssets this species apart from other planktonic organisms, whichare mainly passive dispersers and display low gene flow betweenlakes within regions. Consequently, these data are the firstevidence that C. flavicans may form an important link betweenlakes within regions. Local adaptation within lakes may be decreasedbecause of these processes, and the role of C. flavicans asa biomonitor of local lake conditions needs to be carefullyreinvestigated.     

Density- and Size-Dependent Winter Mortality and Growth of Late Chaoborus flavicans Larvae

Winter processes such as overwinter survival and growth of individuals can have wide-ranging consequences for population dynamics and communities within and across seasons. In freshwater organisms winter processes have been mainly studied in fish despite that invertebrates also have substantial impacts on lake and pond food webs. One of the major invertebrate consumers in lake and ponds is the planktonic larvae of the dipteran insect Chaoborus spec. However, while much is known about Chaoborus feeding ecology, behaviour and structuring role in food webs, its winter ecology and how it affects its populations are poorly understood. Here size- and density-dependent winter mortality and body growth of late Chaoborus flavicans larvae were quantified over naturally occurring size and density ranges in autumn and under natural winter conditions using two field enclosure experiments. Winter mortality increased with autumn density but decreased with autumn body size while winter growth rates decreased with autumn density and body sizes. There was also a density- and size-independent background mortality component. The proportion of pupae found in spring decreased strongly and exponentially with autumn density. These results may explain the commonly observed univoltine life cycle and multi-annual density fluctuations in northern Chaoborus populations. They further demonstrate the relevance of winter processes and conditions for freshwater invertebrates and ecosystems.     

Tracking cod Gadus morhua L. in a Scottish sea loch

Equipment has been developed and used to follow the movements of free-swimming individual fish in the sea. A pulsed ultrasonic transmitter inserted into the stomach is tracked by measuring differences in the time of arrival of the acoustic pulses at an array of omnidirectional hydrophones. The method was used to plot the movements of cod in an enclosed sea-loch. After an initial phase of active movement the cod became less mobile and adopted a nocturnal cycle of activity.     

Morphological responses of Daphnia ambigua to different concentrations of a chemical extract from Chaoborus flavicans

1. Daphnia ambigua were reared individually with eight different concentrations of a chemical extract from larvae of Chaohorus flavicans, and the frequency of appearance of helmeted individuals and the helmet sizes were determined in the instars 1–6. 2. The animals developed most marked helmets in the second instar in response to the extract. The frequency of the helmeted individuals and helmet size in the second instar increased with increasing chemical concentration, although their dose-response curves differed between them. 3. The results may explain the cyclomorphosis of Daphnia, which develop the highest helmets in summer, when the predators are abundant and active, and the concentrations of the predator-released chemicals would presumably be high. 4. The extract became toxic to Daphnia at the highest concentration prepared.     

Planktonic diatoms and some diatom-silica relations in a shallow eutrophic Scottish loch

The species composition of planktonic diatom crops in Loch Leven, Kinross, Scotland varied considerably over the years studied (1968–71) with centric forms predominating. Dense populations were often produced and rates of increase and decrease were high. Relations with dissolved silica did not appear always to be simple; instances occurred during which both diatoms and silica were increasing or decreasing. Moreover, certain rates of increase in dissolved and frustule silica exceeded the rates at which silica flows into the loch even when the inflows appeared to be the only available dissolved silica source.     

Feeding efficiency of Chaoborus flavicans (Insecta, Diptera) under turbulent conditions

Turbulence can affect predator–prey interactions. The effect of turbulence on the feeding efficiency of an ambush predator was tested with laboratory experiments. The experiments were conducted in 100-L aquaria in which ten individuals of fourth instar Chaoborus flavicans larvae were placed as predators. Two prey densities (3 and 10 ind. of Daphnia pulex L^?1) and two durations (30 and 120 min) were tested in a nonturbulent treatment and five different turbulence levels [average root-mean-square (RMS) velocities ranging from 0 to 7.3 cm s^?1, corresponding dissipation rates from 7.2 × 10^?7 to 1.3 × 10^?3 m² s^?3]. We hypothesized that the feeding rate of C. flavicans would be enhanced by turbulence due to increasing encounter rates up to a turbulence level above which a disturbance in post-encounter processes would lead to reduced feeding efficiency. However, the results showed no significant increase in the feeding rate of C. flavicans at intermediate turbulence. At high turbulence we found the expected significant negative response in the feeding rate of Chaoborus larvae. The feeding rate declined below the rates at nonturbulent and intermediate turbulence conditions as the average RMS velocity exceeded 3.1 cm s^?1 (dissipation rate 9.9 × 10^?5 m² s^?3, respectively).     

The feeding relationships of the larger fish species in a Scottish sea loch

Patterns within a fish assemblage have been examined from catches obtained over a period of 17 months using fixed nets and rod and line sampling. A total of 19 species were caught with summer samples dominated by dab and saithe and winter samples by whiting. Cod were caught regularly throughout the year. Catches at the study site of dab and whiting occurred mainly at night but for cod and saithe catches occurred throughout the day. There was no indication ofa crepuscular or tidal rhythm for any species. Differences between our own observations and those from other published studies lead us to conclude that no species-specific generalizations about feeding periodicities can be made. Feeding patterns are more likely to be determined by site-specific features of particular habitats and their associated prey and predator assemblages.
Examination of stomach contents data using a variety of methods indicates that conclusions are highly dependent on the form of the resource matrix used. However, our analyses indicate that during summer cod and dab feed on a much narrower range of prey than saithe and that there is significant overlap in their diets if a resource matrix based on percentage occurrence of prey species is used. Nevertheless, most diet overlap values for the species examined were low (between 0.1 and 0.2) suggesting that competition for food is unlikely. To date, no experiments have been performed to test the hypothesis that cod and dab compete for food.     

Effects of predation by Chaoborus flavicans on crustacean zooplankton of Lake Lenore, Washington

1. After Chaoborus flavicans became established in 1974, the species composition of zooplankton in Lake Lenore changed. Diaptomus nevadensis disappeared and densities of Diaptomus sicilis were reduced.
2. A transplant experiment showed the D. nevadensis could survive in Lake Lenore water in the absence of Chaoborus . The disappearance of D. nevadensis was probably a consequence of Chaoborus predation.
3. Although densities of Daphnia pulicaria † were similar before and after the colonization of C. flavicans , juvenile D. pulicaria began producing neck-teeth during the first summer Chaoborus was present.
4. The seasonal pattern of neck-teeth production by juvenile D. pilicaria did not correlate with density of Chaoborus but rather with a combination of water temperature and Chaoborus density, suggesting that metabolic processes are a component of induction of neck-teeth in this species.     

Isolation of Aeromonas salmonicida in association with purple-pigmented bacteria in sediment from a Scottish loch

Aeromonas salmonicida was recovered in close association with an unidentified purple-pigmented organism, which was isolated from sediment in a Scottish loch during November (1997) and February (1998). However, there has not been any evidence of A. salmonicida infections, specifically furunculosis, associated with the fish in this loch.     

The Feeding of Chaoborus flavicans Meigen (Diptera,Chaoboridae). and its Predation on Lake Zooplankton

In a study of Chaoborus feeding in a eutrophic lake, selectivity was found to be positive with Crustacea (especially copepodit stages). and negative with Rotatoria. Daily food rations were about 20% for most of the feeding period, but higher (106%). during the month of intensive growth after hatching. Feeding intensity correlated positively with amount of food an temperature, and negatively with Chaoborus concentration. Elimination of Crustacea (in the epilimnion of the central zone of the lake). equalled about 30–40% of Crustacea production in June and September and slightly exeeded the August production (it was almost zero in the remaining months because Chaoborus larvae stayed at the bottom). This applies, however, only in the central zone – about 50% of the lake volume. Chaoborus probably influences both the density of zooplankton and the quantitative relations between zooplankton species.     

Recent ecological change in a remote Scottish mountain loch: An evaluation of a Cladocera-based temperature transfer-function

Recent ecological changes in a small Scottish mountain loch, Loch Coire Fionnaraich (LCFR), were inferred using Cladocera analysis and an application of a Cladocera-based temperature transfer-function approach. Modern assemblages of Cladocera from 68 mountain lakes in Scotland and Norway were used to derive the transfer function. Among 21 measured environmental variables, mean summer lake surface-water temperature (LSWT) accounted for the highest (8%) variation in a redundancy analysis of the modern data. A quantitative inference model (r² = 0.72, RMSEP = 1.81 °C) for summer LSWT was developed using linear partial least squares regression and calibration. The resulting reconstructed summer LSWT at LCFR was compared with local instrumental air-temperature records over the last 20 yr and the Central England Temperature (CET) series over the last 110 yr. The reconstruction showed a broadly similar pattern to the local instrumental temperature records. However, the relationship between the longer CET series and reconstructed summer LSWT was poor suggesting that the site was relatively insensitive to temperature changes prior to the recent warming of the last few decades. Clear changes in Cladoceran species diversity and relative abundance of littoral taxa coincident with the most recent period of climate warming recorded both locally and in the CET suggest that Cladocera do respond to climate change. However, their response to temperature is complex and indirect. The underlying mechanism leading to change in Cladocera assemblages may involve changes in trophic status and habitat availability of the system. It is concluded that understanding other factors such as atmospheric pollution, controls of community dynamics of Cladocera and the overall ecological complexity of mountain lakes is required before any reliance can be placed on a Cladocera-temperature transfer-function for reconstructing climatic change.     

Factors contributing to coexistence of Chaoborus flavicans and C. punctipennis (Diptera,Chaoboridae) in a small meromictic lake

In Crawford Lake, a small meromictic water body in southern Ontario, Canada, the life cycles of planktonic Chaoborus flavicans and C. punctipennis were out of phase by about two months, the former pupating from mid May through mid June and the latter from July through September. C. flavicans possibly produced a second annual generation. Fourth instars of both species were strong diel migrators and occupied similar strata at most times. C. punctipennis fourth instar diet consisted almost entirely of rotifers. C. flavicans ate rotifers but fed heavily also on daphnids in May and August. C. punctipennis fourth instars showed little growth until early spring, possibly owing to a sparseness of rotifers, then grew rapidly until pupation. C. flavicans had a slow, but relatively constant growth rate at all times during the open water season, presumably because its greater mouth gape allowed it a wider range of food items.     

Impact of intensive cage fish farming on the phytoplankton and periphyton of a Scottish freshwater loch

Nutrients, phytoplankton and periphyton were monitored in a 71 ha shallow, unstratified lake used for intensive cage culture of rainbow trout. Inorganic nitrogen, ortho-phosphate and suspended solids were significantly higher near the cages and the bottom and, although declining during summer, nutrients did not reach levels which limit phytoplankton growth. Microcystis aeruginosa dominated the phytoplankton, with surface chlorophyll a reaching 189 µg l^–1 in August, but with no subsequent bloom collapse or deoxygenation. A sub-dominant community of vernal diatoms and Pediastrum spp. persisted. Periphyton was dominated by Melosira italica-subarctica. Algal species and water quality showed the lake to be highly eutrophic. Chlorophyll values predicted from a phosphorus-dependent eutrophication model agreed with observations but light limitation by self-shading and suspended farm wastes, aided by wind-induced turbulence, is believed to control algal growth rates and biomass. Implications for environmental management of intensive freshwater cage farms are discussed.     

Studies on the biology of Chaoborus flavicans (Meigen) (Diptera: Chaoboridae) in a fish-free eutrophic pond,Japan

Population dynamics of Chaoborus flavicans larvae of various instars was studied from November 1986 to December 1987 in a eutrophic, fish-free pond, Japan. First and 2nd instar larvae were observed from late April to late October, indicating a reproductive period of about half a year. C. flavicans overwintered in the 4th instar larvae. In water column samples, total density of all instars was 680–23680 m^-2, and pupal density 0–2600 m^-2; larvae of the 1st, 2nd, and 3rd instars showed 5–6 density peaks in 1987, suggesting that 5–6 generations occur during a year (peaks of the 4th instar larvae were not clear, probably due to their longer development than those of younger instars). In sediment samples, no 1st and 2nd instar larvae were found, 3rd instar larvae were found occasionally but density of the 4th instar larvae was 280–18600 m^-2, and pupal density varied between 0–502 m^-2. Fouth instar larvae accumulated in sediment in the cold season and in the water column in the warm season; high temperature and low oxygen concentration were the most important factors limiting the distribution of larvae in the sediment in summer in the NIES pond. The dry weight of total C. flavicans larvae was 0.08–4.2 g m^-2 in sediment samples and 24–599 μg l^-1 (0.10–2.40 g m^-2) in water column samples. Comparisons of maximum densities in the NIES pond in different years and in waters of different trophic status show that density is generally higher in eutrophic than in oligotrophic habitats. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modern pollen from the catchment and surficial sediments of a Scottish sea loch (fjord)

Modern pollen was extracted both from moss polster samples collected from a range of sites across the land surface of the Loch Sunart catchment, north‐west Scotland and from a number of marine sediment‐water interface sites in the sea loch. Comparisons between the pollen results and the major vegetation types growing in the catchment area revealed that in general the moss polsters contained a localised picture of the vegetation whilst the sea loch sediments varied much less from sample to sample and better represented a regional picture of the vegetation. It was anticipated that the pollen in the sea loch samples would be in a much poorer state of preservation because of the many pathways through which it travels in order to become incorporated into the sediments of the loch. However, this proved not to be the case and the study demonstrates that marine sediments of the kind found in Loch Sunart have the potential to provide a new source of Holocene vegetation data which is as good as the freshwater lake sediments that have been preferentially sampled in the past.     

Spatial and temporal distribution of benthic stages of Cyclops vicinus and Chaoborus flavicans in relation to abiotic factors and benthic fauna

The horizontal distributions of the benthic stages of Chaoborus flavicans and Cyclops vicinus were studied in a eutrophic stratified lake in the Massif-Central (France) over one year, at 5 stations from the shore to the centre of the lake. Their distribution was investigated in relation to temperature, dissolved oxygen, sediment grain-size and other benthic organisms. The dominant taxa of the benthic fauna of Lake Aydat were dipterans, crustaceans and oligochaetes and their distributions were independent of the grain size. In contrast to chironomids which preferentially inhabited the sublittoral zone, chaoborids and crustaceans were more numerous in the profundal zone. The sediment-dwelling oligochaetes remained numerous in both zones, according to the season. The fourth copepodite stages of Cyclops and Tubifex are tolerant to low oxygen concentrations in contrast to the fourth instar larvae of Chaoborus whose distribution was positively correlated with oxygen. The guts of these dipteran larvae were found to be empty and we assumed that, in contrast to the chironomids and oligochaetes, the resting stages of Cyclops vicinus and the benthic stages of Chaoborus flavicans did not use benthic resources. The former are activated at the autumn overturn, while the latter escape from the bottom at the start of the spring oxygen depletion. This suggests that physical factors are largely responsible for their reactivation. Both animals suffered of the effects of starvation and probably lost weight. The reactivation of the copepod at the autumn overturn would be facilitated by fluid mechanical disturbance. In addition, after the spring overturn, a small increase in temperature near 4 °C would be a reliable environmental signal for the dipteran. Food limitation does not occur, invertebrate predation pressure seems to be negligible and the predation by fish on the macrobenthic fauna and by chance on the meiofauna, clearly remains limited in both space and time. This revised version was published online in July 2006 with corrections to the Cover Date.     

Where to stay by night and day: Size-specific and seasonal differences in horizontal and vertical distribution of Chaoborus flavicans larvae

1. Data on the distributions of pelagic and benthic Chaoborus flavicans larvae were gathered in 1994 and tested for their agreement with the predator avoidance hypotheses. The development of all Chaoborus life stages, as well as the horizontal and vertical distribution in the four larval instars, was followed from May until October. We expected the largest larvae to dwell deeper by day, thus avoiding predation by visually foraging fish.
2. In agreement with this prediction body size increased with daytime depth, and this was true both between and within instars. The migration amplitude consequently increased with larval instar.
3. There was also evidence for horizontal migration, mainly in the third but also in the fourth instar.
4. Along a horizontal transect with increasing depth, locations with many benthic larvae had fewer pelagic larvae. Oxygen concentration was a good predictor of maximum benthic larval depth for most of the season but failed to predict their distribution in autumn.