Population dynamic aspects of chironomid larvae of the littoral zone of Lake Maarsseveen I

Population dynamics of chironomid larvae in the littoral zone of Lake Maarsseveen I were investigated to a depth of 6 m. No essential differences in the composition and density between various sampling stations in the shallow sand flats were found. Two species,Cladotanytarsus mancus andStictochironomus sticticus, were found to be the main components of the chironomid population to a depth of 4 m. The density ofCladotanytarsus, which occurred predominantly to a depth of 1 m, increased from June to a maximum in December. In May and June, the entireCladotanytarsus population emerged.Stictochironomus had its peak of abundance in June. From this cohort, a part, differing in size for each of the various years, emerged in August. The remainder of this population, together with the offspring of the August adults, emerged in May. The size of the August midges, and the number of its eggs was smaller than that of the May midges. At depths greater than 4 m,Tanytarsus bathophillus was found to be the dominant chironomid.In summer the abundance of the small (second and third instars)Stictochironomus was primarily affected by water mite predation. In autumn and winter,Cladotanytarsus was the main food source for water mites. In autumn, whenStictochironomus larvae have molted into the fourth instar, predation by bream decreased the density of these larvae. In winter, the impact of this fish is negligible.     

Phenology and bathymetric distribution of the profundal chironomid fauna in Starnberger See (F.R. Germany)

Results of the first long-term investigation of the profundal chironomid fauna of Starnberger See, a prealpine lake are reported. Data are presented on the qualitative and quantitative composition of the chironomid fauna in the profundal of Starnberger See, on the spatial variation, and on the phenology of the dominant species, based on pupal occurrence and the seasonal distribution of larval instars. Twenty-five distinct chironomid taxa could be recorded in the profundal zone. Typical inhabitants of the profundal zone of Starnberger See are the larvae of the genus Micropsectra. The maximum abundance of Micropsectra larvae were found at 60 m with 5644 Ind.m⁻². Abundance, distribution and phenology of the dominant chironomid species are discussed in comparison with other lakes.     

Disappearance of deep profundal zoobenthos in Lake Ikeda, southern Kyushu, Japan, with relation to recent environmental changes in the lake

A benthological survey in a deep caldera, Lake Ikeda, southern Kyushu, Japan, in 1998 revealed that no zoobenthos were found in the deep profundal, although two tubificid oligochaetes, Tubifex tubifex and Limnodrilus hoffmeisteri, and a chironomid, Procladius sp., were distributed in the upper profundal zone. This is the first record of oligochaete composition in the lake. Lake Ikeda had been typically oligotrophic until the 1940s, and zoobenthic assemblages were recorded throughout the profundal bottom in the 1920s and 1970s. Recent disappearance of the deep profundal zoobenthos could be caused by the stagnation of anoxic waters in the hypolimnion, in connection with eutrophication triggered by nutrient loading, as well as change in the thermal circulation system presumably caused by global warming.     

Seasonal dynamics and spatial distribution of chydorid cladocerans in relation to chironomid larvae in the sandy littoral zone of an oligo-mesotrophic lake

The seasonal variation of the principal macro- and meiobenthic taxa in the sandy littoral zone of the oligomesotrophic Dutch Lake Maarsseveen I was studied during two years. Population peaks of the different taxa were clearly separated in time. In early spring there were chironomid density peaks of Stictochironomus sticticus, in one year followed by a Polypedilum maximum. From June–July chydorid cladocerans dominated, with a peak of Monospilus dispar followed by a peak of Rhynchotalona falcata. In autumn the chironomid Cladotanytarsus mancus became dominant, remaining so throughout the winter.To study the spatial heterogeneity of the major macro- and meiofaunal taxa, samples were taken in a grid of 2.5 × 10 m. Distributions of all but one taxon were significantly different from random, with Morisita indices varying from 1.23 (chironomids) to 2.10 (the chydorid Monospilus dispar). Wind-induced disturbance presumably had strong and species-specific effects on littoral macro- and meiobenthic taxa. Chydorid heterogeneity increased immediately following the first autumn storm of the season; this may be explained by the tendency of these organisms to remain attached to detritus particles.Temporal and spatial correlation coefficients between macro- and meiobenthic taxa were generally low, suggesting that interactions between these groups are weak, with distribution patterns that are independent of each other. Among the meiofaunal taxa, positive spatial and temporal correlation coefficients were found; apparently, the similarity in both seasonal dynamics and spatial distribution is larger among the meiofaunal taxa than between macro- and meiofaunal taxa. Also among most of the chironomid species significant positive temporal correlations were found, except for Stictochironomus sticticus and Polypedilum sp.; these species have similar life-cycles, but were during one year temporally separated by several alternating settlement peaks. In the next year, Polypedilum disappeared completely following an extremely strong Stictochironomus settlement peak.     

Thermal stratification and benthic foraging patterns of white sucker

Abundances of white sucker, 100–500 mm FL, were not significantly different among the epilimnia, metalimnia and portions of the hypolimnia shallower than 20 m in each of two lakes. However, small suckers < 200 mm were captured most frequently in the epilimnion and no white suckers were captured in the deepest region, 20–38 m, of the two lakes. White suckers consumed prey from all three temperature zones in each lake. Prominent food items were Hyalella azteca and the chironomid larvae Heterotrissocladius, Djalmabatista and Procladius. Despite differences in relative densities of benthic invertebrates among thermal zones of the two lakes, suckers in neither lake foraged exclusively on prey of epilimnetic origin. Suckers captured in the metalimnia foraged on invertebrates that were common to all three thermal zones. And, only 0–4% of the suckers captured in the hypolimnia of the two lakes contained prey that were unique to the epilimnia. Suckers caught in the hypolimnia mainly consumed deep water invertebrates; 83% of the suckers foraged in the metalimnion and hypolimnion of Islets Lake and 45% foraged in the hypolimnion in Burnt Island Lake. Consequently there was little evidence of a massive inshore feeding migration followed by a post-feeding return to the hypolimnion. Northern pike and lake trout rarely fed on white suckers in these lakes and thus piscivory was an unlikely factor in the observed distribution of suckers.     

Subfossil chironomid assemblages in deep,stratified European lakes: relationships with temperature,trophic state and oxygen

1. The distributions of subfossil remains of chironomid larvae in 28 large, deep and stratified lakes in Europe were examined in surface sediments along a latitudinal transect ranging from northern Sweden to southern Italy. 2. Canonical correspondence analysis (CCA) showed that summer surface water and July air temperature, as well as total phosphorus (TP) concentrations, hypolimnetic oxygen availability and conductivity were statistically significant (P < 0.05) explanatory variables explaining between 11 and 14% of the variance in the chironomid data. 3. Owing to the spatial scale covered by our study, many environmental variables were covarying. Temperature, TP concentration and oxygen availability were positively or negatively correlated with the first axis of a detrended correspondence analysis (DCA) of chironomid assemblages, suggesting that climatic and trophic conditions influenced profundal chironomid assemblages either in a direct (food and oxygen) or in an indirect (temperature) way. Parameters related to local environmental conditions, lake morphology and bedrock geology, such as organic matter content of the sediment, maximum lake depth, Secchi depth and pH, were not significant in explaining the distribution of chironomid assemblages in our study lakes. 4. The strong relationship between chironomid assemblages and summer temperature may be related to the covariation of temperature with parameters, such as nutrient and oxygen availability, known to affect chironomid assemblages in deep, stratified lakes. However, summer temperature explained a statistically significant proportion of the variance in the chironomid assemblages even when effects of oxygen availability and TP concentrations were partialled out. This suggests that summer temperature has an effect on chironomid assemblages in deep lakes, which is not related to its covariation with trophic state. 5. The potential of fossil chironomid analysis for quantitatively reconstructing past nutrient conditions in deep, stratified lakes was examined by calculating the Benthic Quality Index (BQI) based on subfossil chironomids and by comparing BQI values with observed TP concentrations. BQI was linearly related to log‐transformed TP. Applying this relationship to fossil chironomid assemblages from Lake Päijänne (Finland) produced a TP reconstruction in agreement with measured TP during the period 1970–1990, demonstrating that this approach can provide quantitative estimates of past nutrient concentrations in deep, stratified lakes.     

Taxonomy and diversity of Afroalpine Chironomidae (Insecta: Diptera) on Mount Kenya and the Rwenzori Mountains, East Africa

Aim Anthropogenic climate change is expected to result in the complete loss of glaciers from the high mountains of tropical Africa, with profound impacts on the hydrology and ecology of unique tropical cold‐water lakes located downstream from them. This study examines the biodiversity of Chironomidae (Insecta: Diptera) communities in these scarce Afroalpine lake systems, in order to determine their uniqueness in relation to lowland African lakes and alpine lakes in temperate regions, and to evaluate the potential of Afroalpine Chironomidae as biological indicators to monitor future changes in the ecological integrity of their habitat. Location Mount Kenya (Kenya) and Rwenzori Mountains (Uganda). Methods The species composition of Afroalpine chironomid communities was assessed using recent larval death assemblages extracted from the surface sediments of 11 high‐mountain lakes between 2900 and 4575 m. Results were compared with similar faunal data from 68 East African lakes at low and middle elevation (750–2760 m), and with literature records of Chironomidae species distribution in sub‐Saharan Africa, the Palaearctic region and elsewhere. All recovered taxa were fully described and illustrated. Results The 11‐lake analysis yielded 1744 subfossil chironomid larvae belonging to 16 distinct taxa of full‐grown larvae, and three taxa of less differentiated juveniles. Eleven of these 16 are not known to occur in African lakes at lower elevation, and eight taxa (or 50% of total species richness) appear restricted to the specific habitat of cold lakes above 3900 m, where night‐time freezing is frequent year‐round. The faunal transition zone coincides broadly with the Ericaceous zone of terrestrial vegetation (c. 3000–4000 m). Snowline depression during the Quaternary ice ages must have facilitated dispersion of cold‐stenothermous species among the high mountains of equatorial East Africa, but less so from or to the Palaearctic region via the Ethiopian highlands. Main conclusions Chironomid communities in glacier‐fed lakes on Africa's highest mountains are highly distinct from those of lowland African lakes, and potentially unique on a continental scale. By virtue of excellent preservation and their spatial and temporal integration of local community dynamics, chironomid larval death assemblages extracted from surface sediments are powerful biological indicators for monitoring the hydrological and ecological changes associated with the current retreat and loss of Africa's glaciers.     

Preliminary study of the development of the subfossil chironomid fauna (Diptera) of Lake Taylor,South Island,New Zealand,during the younger Holocene

The composition and succession of the subfossil chironomid fauna has been studied in a sediment core from Lake Taylor. The sediments of the core are all within the Nothofagus phase of the last 3000 years within the Aranuian. In the older sediments of the core the chironomid fauna was dominated by Tanytarsini and Orthocladiinae whereas Chironomini dominated in the more recent sediments. The species diversity is very high compared with the results of other studies of the recent chironomid fauna of lakes in the South Island. The most abundant head capsules belong to the taxa Chironomus zealandicus, Tanytarsus vespertinus, Corynocera sp., and Eukiefferiella sp. Several additional taxa which have not yet been described also occurred in large numbers. It is not possible to correlate any definite changes in the composition of the subfossil chironomid fauna with a change in climatic conditions or the trophic status of the lake.     

Temporal and spatial distribution of chironomid larvae and oligochaetes in two Dutch storage reservoirs

Long-term changes in distribution and taxonomic composition of chironomid larvae and oligochaetes in two water storage reservoirs in The Netherlands were studied. A succession among the chironomid species was observed. In the last 10–15 years chironomid densities varied. Compared with other lakes chironomid densities were high. Densities of chironomids were higher in the littoral zone than in the profundal zone. The opposite was found for oligochaetes. Densities and composition of the chironomid fauna in the two reservoirs were similar.Procladius, Tanytarsus andHarnischia dominated at all depths. However, pupal exuviae samples showed some differences in chironomid taxonomic composition between the two reservoirs. Orthocladiinae, rarely found in bottom samples, abounded in pupal exuviae samples.     

Chironomid-based inference models for estimating end-of-summer hypolimnetic oxygen from south-central Ontario shield lakes

1. Subfossil chironomid head capsules were sampled from surficial sediments from 86 boreal shield lakes in south‐central Ontario, Canada. Lake characteristics ranged from shallow to very deep (> 80 m), ultraoligotrophic to mesotrophic, and with end‐of‐summer hypolimnetic oxygen conditions ranging from near‐saturation to anoxic. 2. Subfossil chironomid assemblages, comprising 44 taxa from 59 lakes, were analysed using multivariate ordination techniques such as redundancy analysis (RDA) and canonical correspondence analysis (CCA). Forward selection in RDA and CCA both showed that measures of oxygen, such as end‐of‐summer volume‐weighted hypolimnetic oxygen concentration (VWHO) and bottom oxygen concentration (botO₂), were the strongest explanatory variables for the chironomid data. Maximum depth and major ion chemistry were also important explanatory variables. 3. Oxygen inference models were developed using partial‐least‐squares regression (PLS), weighted‐averaging partial‐least‐squares regression (WA‐PLS), and weighted averaging regression (WA). Models were developed using both the full 44 taxa assemblage (which included littoral taxa) and using only 15 profundal‐type taxa. 4. Cross‐validated models (jackknifing) using full‐assemblage or profundal‐only taxa had similar statistical power (similar root mean squared error of prediction, RMSEP). The best models had moderate predictive power, with an r²_jack as high as 0.56, and an RMSEP as low as 2.15 mg L^–1 for [VWHO], and an r²_jack of 0.49 and an RMSEP of 0.24 for log([botO₂] + 1). 5. Reconstruction of [VWHO] and [botO₂] using a previously published chironomid profile that showed strong lake response to land‐clearance and logging suggests that oxygen inference models are reliable and accurate, reflecting the qualitative changes occurring in subfossil assemblages. However, the profundal‐only models may be misleading in situations where the ratio of littoral‐to‐profundal subfossils changes drastically in response to lake disturbance.     

Habitat, diet and food assimilation of Arctic charr under the winter ice in two subarctic lakes

The Arctic charr Salvelinus alpinus populations of the subarctic lakes Takvatn and Fjellfrøsvatn, north Norway, concentrated in the littoral zones (0–15 m) of the lakes during the entire winter (December to May) despite very low temperatures (0·2 and 0·7° C). High prey availability, low predation and competition and comparatively better light under snow and ice in shallow compared with deep water are probable reasons. At ice break in June, all Arctic charr moved to the profundal zone for a brief period, probably in response to the sudden light increase and a profundal resource peak of chironomid pupae. In the summer, the Arctic charr are found in the pelagic, profundal and littoral zones of the lakes. These populations therefore perform regular habitat shifts between the littoral zone in the winter, the profundal zone at ice break and the whole lake in the summer and autumn. The fish fed continuously during winter despite the cold water and the poor light. Amphipods and chironomid larvae dominated the diet. Catch per unit effort, numbers of stomachs with food and food intake rates varied with the subarctic light cycle but were lowest after the winter solstice. The winter assimilation of energy was about equal to the standard metabolism in Takvatn but was higher in Fjellfrøsvatn. The assimilation increased in both lakes under the spring ice in May. The habitat choice, diet and energy assimilation indicate that the Arctic charr is well adapted to the extreme winter conditions of subarctic lakes.     

Oxygen demand and long term changes of profundal zoobenthos

The paper attempts to combine the low oxygen content of the hypolimnion during stratification and the oxygen uptake of zoobenthos. Data of declining oxygen content in the hypolimnion and critical limits of respiration are combined for Chironomus anthracinus, Potamothrix hammoniensis and three species of Pisidium, P. casertanum, P. subtruncatum and P. henslowanum. The respiratory adaptation to low oxygen content influences both growth and population dynamics of the different species. The results have important bearing on eutrophication of the Lake Esrom ecosystem and temperate eutrophic lakes in general as well as the composition of profundal zoobenthos and its population dynamics.Publication No. 389 from Freshwater-Biological Laboratory, University of Copenhagen.     

Subfossil chironomids (Diptera,Chironomidae) in three Tatra Mountain lakes (Slovakia) on an acidification gradient

Three lakes were studied in the High Tatra Mountains at altitudes from 2000 to 2157 m a.s.l., which represent three categories of acidity status recognised in the Tatra lakes in the 1980s: non-acidified, acidified, and strongly acidified. Subfossil chironomid remains from dated sediment cores covering ca. throughout 200 years were analysed. The chironomid thanatocoenoses of all cores layers reflected ultra-oligotrophic non-acidified conditions in L’adové pleso. Nevertheless, the finding of the acid-tolerant species Zalutschia tatrica in littoral samples in 1980s indicates that the littoral zone was more influenced by acidification than deeper areas of the lake. Three stages of lake developmental history can be distinguished in the acidified lake Vy?né Wahlenbergovo pleso based on the chironomid subfossil record: the pre-acidification stage before the 1920s, the anthropogenic acidification stage between the 1920–1980s, and the recovery of the lake from the end of the 1980s. The period of acidification was accompanied by a decline in chironomid numbers, while chironomid fauna composition was unchanged during the whole investigated history of the lake. None of the chironomid taxa present prior to the acidification period disappeared during the peak of acidification. The most apparent change in the sediment record taken from the strongly acidified lake Starolesnianske pleso was recognised in the layers corresponding to the period 1960–1980. It is characterised by the disappearance of the dominant acid-sensitive Tanytarsus lugens group and the dominance of acidtolerant Tanytarsus gregarius group. The most recent chironomid assemblage probably benefits from the amount of food resources as a result of increased lake productivity induced by acidification.     

Responses of insects,especially Chironomidae (Diptera), and mites to 130 years of acidification in a Scottish lake

About 130 years of anthropogenic acidification of Round Loch of Glenhead, SW Scotland, has resulted in successively decreased stability, diversity, productivity and survival rate of the non-biting midge (Diptera: Chironomidae) fauna. Similar trends have also been observed among mayflies, caddis-flies and water mites. The first effects of anthropogenic acidification on the insect and mite fauna, as evidenced by palaeolimnological analyses of²¹⁰Pb-dated sediment cores, occurred as early as around 1850, i.e. earlier than in any other lake hitherto studied. The drop in the lake's pH was first indicated by decreased stability and changes in species composition of chironomids and mayflies in the littoral zone. Major changes in the profundal chironomid fauna did not take place until about 1950, when mean lake pH dropped below 5.0. At the same time, the littoral insect fauna became even more unstable and the first significant elimination of species occurred. Comparison with insect fauna of other lakes suggests that a pH of less than 5 might be critical for the ecological conditions in many acidified lakes. None of the twelve most common chironomid species present prior to the acidification of the lake had disappeared after 120–130 years of considerable acidification, and they are all common in oligotrophic lakes with a pH of 6.5–7.0. This is in contrast to the effects of heavy acidification on other aquatic animal groups. Chironomids are probably more sensitive to lake trophic status than lake acidity.     

A trophic pathway from biogenic methane supports fish biomass in a temperate lake ecosystem

Although some primary consumers such as chironomid larvae are known to exploit methane‐derived carbon via microbial consortia within aquatic food webs, few studies have traced the onward transfer of such carbon to their predators. The ruffe Gymnocephalus cernuus is a widespread benthivorous fish which feeds predominantly on chironomid larvae and is well adapted for foraging at lower depths than other percids. Therefore, any transfer of methanogenic carbon to higher trophic levels might be particularly evident in ruffe. We sampled ruffe and chironomid larvae from the littoral, sub‐littoral and profundal areas of Jyväsjärvi, Finland, a lake which has previously been shown to contain chironomid larvae exhibiting the very low stable carbon isotope ratios indicative of methane exploitation. A combination of fish gut content examination and stable isotope analysis was used to determine trophic linkages between fish and their putative prey. Irrespective of the depth from which the ruffe were caught, their diet was dominated by chironomids and pupae although the proportions of taxa changed. Zooplankton made a negligible contribution to ruffe diet. A progressive decrease in δ¹³C and δ¹⁵N values with increasing water column depth was observed for both chironomid larvae and ruffe, but not for other species of benthivorous fish. Furthermore, ruffe feeding at greater depths were significantly larger than those feeding in the littoral, suggesting an ontogenetic shift in habitat use, rather than diet, as chironomids remained the predominant prey item. The outputs from isotope mixing models suggested that the incorporation of methane‐derived carbon to larval chironomid biomass through feeding on methanotrophic bacteria increased at greater depth, varying from 0% in the littoral to 28% in the profundal. Using these outputs and the proportions of littoral, sub‐littoral or profundal chironomids contributing to ruffe biomass, we estimated that 17% of ruffe biomass in this lake was ultimately derived from chemoautotrophic sources. Methanogenic carbon thus supports considerable production of higher trophic levels in lakes.     

Chironomid communities as water quality indicators

Recent mathematical indices summarizing biological communities of indicators are recapitulated. Improvements of these indices based on weighting according to width of trophic ranges of each species are suggested. Their principle deficiencies, however, are pointed out.
Revised lists of characteristic profundal as well as littoral and sublittoral chironomids in Nearctic and Palearctic lakes show that at least 15 characteristic chironomid species communities can be delineated, 6 in each of the oligotrophic and the eutrophic ranges and 3 in the mesotrophic range. It is proposed that these communities be lettered consecutively in the Greek alphabet from α (alpha) to o (omikron). A key to the 15 divisions based on the species associations in the profundal zone of harmonic lakes is put forward. There is very good correlation between the 15 divisions and the ratios of average total phosphorus to mean lake depth and average chlorophyll a to mean lake depth.
The ratio of chironomids to oligochaetes and the distribution patterns of single species have proven useful in pin-pointing localized areas of pollution. The primary mechanism governing the distribution of chironomid communities in oligotrophic and mesotrophic lakes appears to be the availability of food materials rather than the annual hypolimnetic oxygen concentration. In eutrophic lakes the relationships between organic matter accumulation and oxygen levels are so interdependent as to be inseparable.     

A characterization and description of the Maarsseveen Lake system

The Maarsseveen Lakes system, located near the City of Utrecht, The Netherlands, consists of two major bodies of water, a larger lake (Maarsseveen I) and a smaller lake (Maarsseveen II), interconnected by a maze of channels known as a ‘petgaten’ area. The hydrodynamics of the lake region coupled with this ‘petgaten’ area are responsible for the maintenance of the relatively oligomesotrophic character of Lake Maarsseveen I, as compared with the more eutrophic lake maarsseveen II. Both lakes are plagued by problems of summer anoxia, although this phenomenon is far more pronounced in the more eutrophic small lake. Atmospheric deposition is responsible for contributions of significant quantities of trace metals to both lakes, and the River Vecht acts as an additional source for Lake Maarsseveen II.     

Patterns in Benthic Biodiversity Link Lake Trophic Status to Structure and Potential Function of Three Large,Deep Lakes

Relative to their scarcity, large, deep lakes support a large proportion of the world’s freshwater species. This biodiversity is threatened by human development and is in need of conservation. Direct comparison of biodiversity is the basis of biological monitoring for conservation but is difficult to conduct between large, insular ecosystems. The objective of our study was to conduct such a comparison of benthic biodiversity between three of the world’s largest lakes: Lake Tahoe, USA; Lake Hövsgöl, Mongolia; and Crater Lake, USA. We examined biodiversity of common benthic organism, the non-biting midges (Chironomidae) and determined lake trophic status using chironomid-based lake typology, tested whether community structure was similar between the three lakes despite geographic distance; and tested whether chironomid diversity would show significant variation within and between lakes. Typology analysis indicated that Lake Hövsgöl was ultra-oligotrophic, Crater Lake was oligotrophic, and Lake Tahoe was borderline oligotrophic/mesotrophic. These results were similar to traditional pelagic measures of lake trophic status for Lake Hövsgöl and Crater Lake but differed for Lake Tahoe, which has been designated as ultra-oligotrophic by traditional pelagic measures such as transparency found in the literature. Analysis of similarity showed that Lake Tahoe and Lake Hövsgöl chironomid communities were more similar to each other than either was to Crater Lake communities. Diversity varied between the three lakes and spatially within each lake. This research shows that chironomid communities from these large lakes were sensitive to trophic conditions. Chironomid communities were similar between the deep environments of Lake Hövsgöl and Lake Tahoe, indicating that chironomid communities from these lakes may be useful in comparing trophic state changes in large lakes. Spatial variation in Lake Tahoe’s diversity is indicative of differential response of chironomid communities to nutrient enrichment which may be an indication of changes in trophic state within and across habitats.     

First Record of Podonominae Larvae Living Phoretically on the Shells of the Water Snail Chilina dombeyana (Diptera: Chironomidae / Gastropoda: Lymnaeidae)

Larvae of Podonomus albinervis Edwards, 1931 were found phoretic on Gastropoda Chilina dombeyana (Bruigière, 1789) on the shores of Lake Nahuel Huapi, in Bariloche, Argentina. Larvae were attached with mucus and some had developed a tube made of detritus on the spire of the snail shell. Adults and pupae with associated larval exuviae were found, which helped to identify the species. Approximately 10% of the snail individuals examined on the shores of the lake had chironomid larvae on their shells. Laboratory observations showed that the midge larvae leave the snail shell when the snail leaves the water, thus demonstrating the phoretic relationship. The high density of both snails and Chironomidae in the littoral habitats of the lake may explain this phoretic relationship.     

The role of chironomids (Diptera) in the profundal macrozoobenthos in Finnish reservoirs

The proundal macrozoobenthos of 12 Finnish inland polyhumic reservoirs was studied by quantitative sampling in 1984 and 1986. Additionally, the succession of the macrozoobenthos in one young reservoir (Lake Kyrkösjärvi) was studied during its first six years. The zoobenthos material was analyzed using the DCA (detrended correspondence analysis) method, and the ordination of sites (i.e. reservoirs), was related to associated environmental data. The role of chironomids was, in general, important in the profundal of the reservoirs. Because of the coarse bottom quality at many sampling sites, many littoral taxa were found in the profundal zone. The genusChironomus dominated (>50% of the total zoobenthic biomass) especially in the acidic, upper-reach reservoirs.Chironomus larvae also dominated in the zoobenthos of Lake Kyrkösjärvi during its first three years. Some of the reservoirs showed similarities with the dystrophic lake type of the lake typology system (Brundin, 1956). Because of the initially eutrophic character of the bottom, many reservoirs belonged to eutrophic lake types. Finnish reservoirs appear to contain a zoobenthic fauna intermediate between natural lakes and bog pools. This theme was discussed from the ecological and evolutionary point of view ofChironomus species.