Ammonium excretion by some freshwater zoobenthos from a eutrophic lake

Relationships between the ammonium excretion rate of zoobenthos and temperature were examined for two chironomids, Chironomus, plumosus and Tokunagayusurika akamusi, one chaoborid, Chaoborus flavicans, and two tubificids, Limnodrilus spp. and Branchiura sowerbyi, which often dominate in eutrophic lakes. The representative Q₁₀ value and excretion rate at 15° C for each species were as follows: C. plumosus, 1.67, 1.40, (µg N mg dry w.^–1 d^–1 ); T. akamusi, 1.02 (< 15° C), 0.33; C. flavicans, 1.49, 2.87; Limnodrilus spp., 2.98, 0.59; and B. sowerbyi, 3.15, 0.79.     

Effects of artificial destratification on zoobenthos and on the trophic status of a Polish reservoir

The macro-benthic fauna, especially the Chironomidae, and the sequence of faunal groups in a horizontal transect across Lake Mutek (1977–79) before and after artificial destratification are described. Before destratification (1977–78), Chaoborus flavicans was the dominant species. After destratification this form decreased in numbers rapidly. Chironomidae and Oligochaeta first appeared in the open water zone. A multiple increase in density of pelophilous forms (Chironomus plumosus and C. thummi) was also observed. The trophic status of the lake shifted from polytrophic to eutrophic.     

On the distribution patterns and population dynamics of sublittoral and profundal oligochaeta fauna from Lake Banyoles (Catalonia,NE Spain)

Oligochaeta assemblages from Lake Banyoles were composed of one species of Naididae and eight species of Tubificidae, Potamothrix heuscheri (8–1350 ind. m^–2) being the most abundant and widely distributed species, followed by Limnodrilus hoffmeisteri (25–858 ind. m^–2) and Psammoryctides barbatus (12–792 ind. m^–2). The remaining species were Potamothrix hammoniensis, Potamothrix bavaricus, Limnodrilus claparedeanus, Branchiura sowerbyi, Aulodrilus pigueti and Dero digitata, all of which occurred at lower densities and with restricted distributions.Regardless of sampling depth, maximum densities of oligochaetes were found in winter (up to 5,142 ind. m^–2 in December) and secondarily in June. Immature Tubificidae with hair setae and mature P. heuscheri were present and dominant throughout the year. Cestode parasites were recorded infesting L. hoffmeisteri and P. heuscheri populations.Oligochaeta densities increased with depth, but low oxygen concentrations (less than 1 mg l^–1) during a long period (4 months) in some basins of the lake acted as a key factor in reducing the density of worms and the species richness to one species, Potamothrix heuscheri.     

Macrozoobenthos of Lake Verevi

An overview on studies of macrozoobenthos in the small, hard-water, stratified and hypertrophic Lake Verevi (South-Eastern Estonia) is given. The list of macroinvertebrates comprises at least 105 taxa. In the open water habitats, the biomass and abundance of macrozoobenthos (except the phantom midge Chaoborus flavicans) was rather constant beginning from the epilimnion up to the upper hypolimnion (depth 2–4 m), but very low in the lower hypolimnion (depth 6 m), which was inhabited mainly by Chaoborus. Comparison with long-term reference data from other Estonian lakes, belonging to similar limnological types, indicated that the total biomass and abundance (without Chaoborus) in the profundal of Verevi were very low.     

Larval chaoborid mandibles in surface sediments of small shallow lakes in Finland: implications for palaeolimnology

The aim of this study was to assess the value of phantom midge Chaoborus (Diptera: Chaoboridae) larvae as a palaeoenvironmental proxy in northern Europe. The presence of Chaoborus was examined in 80 small shallow lakes across Finland (60–70°N), based on their subfossil mandibles preserved in lake sediments. Chaoborus flavicans was present in 33 lakes, but was the most abundant midge taxon in only one of the lakes. Chaoborus crystallinus/obscurripes mandibles were present in two lakes. Statistical analyses showed that the distribution of Chaoborus was significantly correlated with lake depth and mean July air temperature. Chaoborus was absent in the northern study lakes, whereas the results indicated that C. flavicans was frequently found in small shallow lakes in southern Finland. Chaoborus flavicans was particularly abundant in fish-free, macrophyte-rich, dystrophic lakes, but absent in oligotrophic clear-water lakes having fish. The results of this study indicate that the subfossil mandibles of Chaoborus can provide valuable information in multiproxy palaeolimnological studies, especially when investigating past changes in water level and temperature in small shallow lakes in northern Europe. Guest editors: K. Buczkó, J. Korponai, J. Padisák & S. W. Starratt Palaeolimnological Proxies as Tools of Environmental Reconstruction in Fresh Water     

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).     

Spatial variability in the abundance of pelagic invertebrate predators in relation to depth and turbidity

The abundance of pelagic invertebrate predators in relation to turbidity and depth gradients in Lake Hiidenvesi (southern Finland) were studied. In the shallow (<5 m) and the most turbid (up to 75 NTU) part of the lake, the community of invertebrate predators consisted of cyclopoid copepods (max biomass >500 μg dw l⁻¹) and Leptodora kindtiii (Focke) (17 μg dw l⁻¹), while in the less turbid (10–40 NTU) stratifying area Chaoborus flavicans (Meigen) dominated (max 146 μg dw l⁻¹). In the temporarily stratifying and moderately turbid basin Chaoborus and small-bodied invertebrate predators co-existed. Mysis relicta (Lovén) occurred only in the stratifying area (max 15 μg dw l⁻¹). The results suggested that both water depth and turbidity contributed to the community structure of Chaoborus flavicans. Depth great enough for stratification was of special importance and its effect was amplified by elevated turbidity, while high turbidity alone could not maintain chaoborid populations. Mysis relicta also requires a hypolimnetic refuge but is more sensitive to low oxygen concentrations and may therefore be forced to the epilimnion where it is vulnerable to fish predation. Cyclopoids as rapid swimmers can take advantage at elevated turbidity levels and coexist in high biomass with fish even in shallow water. Leptodora kindtii can form high biomass despite planktivorous fish providing that turbidity exceeds 20 NTU. The results demonstrated that depth and water turbidity can strongly regulate the abundance and species composition of invertebrate predators. These factors must thus be taken into account when applying food web management, which aims to reduce phytoplankton biomass by depressing planktivorous fish.     

On the use of split moving window analysis for boundary detection in ordered dataseries from benthic communities

The capacity of Split Moving Window (SMW) boundary analysis was tested to detect the depth of the transition of a littoral to a profundal invertebrate community in two datasets that were ordered along a depth gradient. The littoral community was dominated byPolypedilum nubeculosum agg.,Potamopyrgus jenkinsi andGlyptotendipes cf.pallens. The species richness of the profundal community was much lower. Its fauna was dominated byChironomus cf.plumosus andChaoborus flavicans. The transition could be detected with small window sizes. The discontinuities are located at the same depth as the oxygen stratification. We conclude that SMW can be used to study artificial transects. Noisy datasets should be transformed to reduce the variation. The scale dependence of SMW is an advantage. Information about the nature of a discontinuity (transition, trend or local discontinuity) can be obtained by increasing the window size. The location of discontinuities that are detected at the edge of a series, with large windows, should be interpreted carefully due to the existence of a blind zone.     

The chemical environment of tubificid and lumbriculid worms according to the pollution level of the sediment

In Lake Geneva (Switzerland), 14 tubificid and 2 lumbriculid worm species were collected in a total of 170 samples of sediment. The sediment was analysed through ten chemical variables: organic carbon, total phosphorus, Cd, Zn, Sn, Pb, Hg, Cu, Cr, Mn. The chemical environment characterizing the presence of every worm species was defined by the mean value of each of these ten variables in all samples where the species was found. The multivariate comparison of the chemical environment typical of every species enabled six groups of species to be distinguished, each characterized by the high value of one chemical variable: (1) Peloscolex ferox, Potamothrix hammoniensis, Limnodrilus claparedeanus, and Cd. (2) Psammoryctides barbatus, and Zn. (3) Limnodrilus hofmeisteri, L. udekemianus, L. profundicola, and total P. (4) Potamothrix heuscheri, Aulodriluspluriseta, A. limnobius, Tubifex tubifex, Ilyodrilus templetoni, Stylodrilus heringianus, and organic C. (5) Potamothrixvejdovskyi, and, Hg. (6) Peloscolex velutinus, Stylodrilus lemani, and Mn. Pollution level of the sediment decreased from group 1 to 6, so each of these groups may be used to define a different level of pollution. The pooled coefficient of variation of the ten chemical variables used to define the chemical environment of each species diminished from group 1 to 6: the most opportunistic species colonized the most polluted areas.     

Effects of pH and predation by Chaoborus larvae on the plankton of a shallow and acidic forest lake

1. Eutrophic acid lakes are not common. Delamere Lake in Cheshire, U.K. is shallow and acid (mean pH 4.5) with a very high phytoplankton crop (mean 290 μg chlorophyll a L^?1), dominated by Dictyosphaerium pulchellum. Rotifers were dominant in the pelagic waters but small cladocerans (Alona guttata, Chydorus sphaericus and Scapholeberis mucronata) were occasional in the littoral waters. Chaoborus flavicans larvae were the top predators in this fishless lake. Two mesocosm experiments were carried out in which pH and Chaoborus populations were manipulated. 2. Progressively higher concentrations of D. pulchellum were maintained in the elevated pH treatments (pH 6 and 8; P < 0.001) with increased amounts of a Chlamydomonas species at the end of the experiment. Highest species richness was seen at ambient pH. Thus the low pH of Delamere Lake alone did not control the structure of the phytoplankton community. Keratella quadrata showed significantly higher abundance at pH 6 than in other pH treatments (P < 0.001). Species richness of rotifers was unaffected by pH. 3. Most Cladocera were C. sphaericus. Although never seen in the open lake, Daphnia pulex appeared in all the pH treatments. Low pH did not control small Cladocera abundance in Delamere Lake, but probably hampered reproduction in Daphnia. Negative correlations between chlorophyll a concentrations and Daphnia in the mesocosms (r² = 0.215, P < 0.05), however, indicated the potential of large‐bodied daphniids in controlling phytoplankton. 4. Neither different combinations of Chaoborus instars (none, instars 1 and 2 and instars 3–5) nor different densities of instars 3–5 (0.15, 0.5 and 1.0 L^?1) had a negative impact on Cladocera. Daphnia pulex remained unaffected in the experiment, perhaps because of its large size, and C. sphaericus because of its high reproductive rate compensating predatory losses. 5. Very low pH in Delamere Lake might suppress Daphnia by hampering its reproduction. Consequently, Daphnia may be vulnerable to invertebrate predation even at low predator density in the lake.     

The food, growth and abundance of five co-existing cyprinids in lake basins of different morphometry and water quality

The food, growth and abundance of five co-existing cyprinid fish species in the eutrophic Lake Hiidenvesi were studied. The diet overlaps within the cyprinid community in shallow lake basins were compared with those in a deep basin, where littoral resources are less available. Roach, bleak and white bream inhabited both the shallow and the deep parts of the lake. Their growth rate was slow, probably due to the low availability of animal food, indicated by the increasing proportion of detritus and plant material in the diets towards the end of the summer. In the deep basin, roach and bleak, contrary to white bream, did not forage on the very abundant invertebrate Chaoborus flavicans, explained by the migration behaviour of C. flavicans. Blue bream, unlike other cyprinids, utilized copepods and had a relatively fast growth rate, but was mostly restricted to the shallow areas. The condition of the bream stock was weak both in terms of growth and abundance. The availability of zoobenthos was low and bream was not able to compete for zooplankton with the more specialized planktivores.     

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.     

Vertical migration of Chaoborus flavicans in a Scottish loch

The pattern of diel vertical migration of Chaoborus flavicans larvae in a shallow Scottish loch varies according to the instar and, in third and fourth instars, according to the season. The planktonic phase of the migration is not exclusively concerned with predation, the larvae feeding at least equally actively in the benthos. Although there is no conclusive evidence as to the role of vertical migration in Chaoborus flavicans, it has some of the characteristics of an epideictic display. Upward and downward locomotion can be induced experimentally in a plankton wheel by manipulating light intensity alone. However, the responses are complex and there is a great deal of individual variation, suggesting that additional factors are involved.     

Reversal of eutrophication in Lake Geneva: evidence from the oligochaete communities

1. In Lake Geneva (Switzerland), total phosphorus concentrations have decreased from 90 mg m^?3 in 1979 to 55 mg m^?3 in 1990. 2. To assess the effects of this improvement, tubificid and lumbriculid communities were sampled in 1982 and in 1991 in the same areas, at a depth of 40 m. 3. Abundance of mesotrophic species (mostly Potamothrix vejdovskyi) and of eutrophic species (mostly Potamothrix hammoniensis) was lower in 1991 than in 1982; in contrast, oligotrophic species (mostly Peloscolex velutinus) were more abundant in 1991 than in 1982. 4. The changes recorded in 1991 were the same as those associated with a decrease of organic sedimentation. 5. Mean relative abundance of oligotrophic species increased from 17% in 1982 to 41% in 1991. According to these values, Lake Geneva was mesoeutrophic in 1982, but mesotrophic in 1991.     

Feeding efficiency of planktivores under disturbance,the effect of water colour,predation threat and shoal composition

The consumption of phantom midge Chaoborus flavicans larvae by Perca fluviatilis showed clear response to water colour, predation threat and shoal composition with the most significant negative effect for water colour. In the case of Rutilus rutilus, no similar combined response was observed and the total prey consumption was significantly negatively affected by predation threat of Esox lucius. The results suggest that differences in life‐history traits may result in disparity in species‐specific responses to disturbance.     

Distribution of Benthic Fauna in Relation to Environmental Conditions in an Inundated Opencast Sulphur Mine (Piaseczno Reservoir, Southern Poland)

Highly specific environmental factors such as the presence of strongly mineralized water, oxygenation of the water down to 10 m, a low trophic status of the water and low organic matter content in the bottom sediments, were found in the studied post-exploitation Lake Piaseczno. Of the 42 macroinvertebrate taxa found, Oligochaeta (Tubificidae) and Chironomidae predominated. Almost exclusively ubiquitous species were found, except for some chironomid taxa recorded for the first time in Poland. The most diversified faunal communities occurred at 1 m water depth whereas the highest densities were found at 5 m. At the deepest points studied only oligochaetes and Chaoborus flavicans were found. There was no correlation between organic matter content and macrofauna densities. Differences in inhabitation at the studied transects could result from various environmental factors such as the degree of macrophyte cover and protection from the wind. Undisturbed transects overgrown by macrophytes had a similar community composition and higher faunal densities than those which were less protected.     

Half a century of oligochaete research in Estonian running waters

About 51500 specimens from 1542 samples, collected over the years 1954–1975 and 1986–1999 in different running water bodies throughout Estonia, were identified. Tubificidae prevailed in the material, with Limnodrilus hoffmeisteri forming about 40%. This species was followed by the tubificids Tubifex tubifex, Potamothrix hammoniensis, Psammoryctides barbatus, L. udekemianus and Spirosperma ferox, the naidid Stylaria lacustris, and the lumbriculid Stylodrilus heringianus. Two main ecological assemblages were distinguished: the pelophilous assemblage, dominated by L. hoffmeisteri, and the psammophilous one, where usually P. barbatus was dominant. The relationships between different species and the chemical parameters of water were usually weak but in contrast, correlated well with sediment preferences. In organically enriched reaches, L. hoffmeisteri usually dominated. The fauna of the streams of the islands was poorer in species due to their small size rather than geographical isolation. Some recent antropochorous Ponto-Caspian invaders have only reached the lowermost reaches of the two largest rivers. Some brackish water species were found in the mouth of the Pärnu River. No essential differences were found between the comparable sets of oligochaete samples collected in 1954–1975 and 1987–1997 in the Estonian running waters.     

External structures used during attachment and sperm transfer in tubificids (Annelida,Oligochaeta)

The genital region of seven species of Tubificidae has been studied by SEM (Scanning Electron Microscopy). The form and the position of penial and spermathecal chaetae, male and spermathecal pores and other special structures have been examined. Peristodrilus montanus shows a special system to hold the partner: the penial chaetae anchor in an elaborated structure of the body wall formed between the spermathecal pores, the `anchorage bridge'. Protuberodrilus tourenqui has a long glandular porophore with the male pores at the tip, allowing contact with the spermathecal pores which are located in deep, close to the mid-ventral line of the body. The grooved and strongly curved penial chaetae of Rhyacodrilus falciformis seem to be used both for attachment and for sperm transfer, entering into the lateral spermathecal pores. The embrace of the partners, as suggested by observations on Psammoryctides barbatus, Potamothrix bavaricus, Potamothrix hammoniensis and Potamothrix heuscheri, seems to be another important mechanism to fix contact between male and spermathecal pores and allow sperm transfer. The spermathecal chaetae could be interpreted as piercing chaetae with a chemical or mechanical stimulating role. Sensitive cilia near the penial chaetae seem to be used by the three rhyacodrilines studied to find the correct anchorage place. There is a great variety of structures which appear to be used for attachment and sperm transfer in tubificids, and consequently their role in the evolution of the whole family may be profound.     

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.