The chironomid‐temperature relationship: expression in nature and palaeoenvironmental implications

Fossils of chironomid larvae (non‐biting midges) preserved in lake sediments are well‐established palaeotemperature indicators which, with the aid of numerical chironomid‐based inference models (transfer functions), can provide quantitative estimates of past temperature change. This approach to temperature reconstruction relies on the strong relationship between air and lake surface water temperature and the distribution of individual chironomid taxa (species, species groups, genera) that has been observed in different climate regions (arctic, subarctic, temperate and tropical) in both the Northern and Southern hemisphere. A major complicating factor for the use of chironomids for palaeoclimate reconstruction which increases the uncertainty associated with chironomid‐based temperature estimates is that the exact nature of the mechanism responsible for the strong relationship between temperature and chironomid assemblages in lakes remains uncertain. While a number of authors have provided state of the art overviews of fossil chironomid palaeoecology and the use of chironomids for temperature reconstruction, few have focused on examining the ecological basis for this approach. Here, we review the nature of the relationship between chironomids and temperature based on the available ecological evidence. After discussing many of the surveys describing the distribution of chironomid taxa in lake surface sediments in relation to temperature, we also examine evidence from laboratory and field studies exploring the effects of temperature on chironomid physiology, life cycles and behaviour. We show that, even though a direct influence of water temperature on chironomid development, growth and survival is well described, chironomid palaeoclimatology is presently faced with the paradoxical situation that the relationship between chironomid distribution and temperature seems strongest in relatively deep, thermally stratified lakes in temperate and subarctic regions in which the benthic chironomid fauna lives largely decoupled from the direct influence of air and surface water temperature. This finding suggests that indirect effects of temperature on physical and chemical characteristics of lakes play an important role in determining the distribution of lake‐living chironomid larvae. However, we also demonstrate that no single indirect mechanism has been identified that can explain the strong relationship between chironomid distribution and temperature in all regions and datasets presently available. This observation contrasts with the previously published hypothesis that climatic effects on lake nutrient status and productivity may be largely responsible for the apparent correlation between chironomid assemblage distribution and temperature. We conclude our review by summarizing the implications of our findings for chironomid‐based palaeoclimatology and by pointing towards further avenues of research necessary to improve our mechanistic understanding of the chironomid‐temperature relationship.     

Hatching in dabbling ducks and emergence in chironomids: a case of predator–prey synchrony?

It has been hypothesized that dabbling ducks (Anas spp.) time breeding to coincide with annual regional peaks in emerging dipterans, especially Chironomidae, which are important prey for newly hatched ducklings. However, this hypothesis has never been evaluated in a replicated lake-level study, including year effects in emergence patterns. We collected duck and invertebrate data from 12 lakes during the nesting seasons 1989–1994 in a watershed in southern Finland. The oligotrophic study lakes are typical of the boreal Holarctic, as are the three focal duck species: mallard Anas platyrhynchos L., widgeon Anas penelope L and teal Anas crecca L. Hatching of ducklings showed a clear peak in relation to ambient phenology (annual ice-out date of lakes), whereas chironomid emergence was more erratic and showed no clear peak at the lake level, although total watershed-level emergence was somewhat higher before and long after the duck hatching peak. Thus, we find no evidence that ducklings hatch in synchrony with abundance peaks of emerging chironomids. There was large within-year temporal variation in chironomid emergence among lakes, but this was not correlated with ambient temperature. The rank of individual lakes with respect to the abundance of emerging chironomids was consistent among as well as within years, a predictability that ought to make adaptive lake choice by ducks possible. On the lake level, there was a positive correlation between the total amount of emerging chironomids and brood use. We argue that emergence patterns of chironomids on typical boreal lakes are neither compressed nor predictable enough to be a major selective force on the timing of egg-laying and hatching in dabbling ducks. Despite spatial (among-lake) patterns of abundance of emerging chironomids being predictable within and among years, the observed pattern of brood use suggests that other factors, e.g. habitat structure, also affect lake choice.     

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.     

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.     

Assessing lake eutrophication using chironomids: understanding the nature of community response in different lake types

1. Total phosphorus (TP) and chlorophyll a (Chl a) chironomid inference models ( Brodersen & Lindegaard, 1999 ; Brooks, Bennion & Birks, 2001 ) were used in an attempt to reconstruct changes in nutrients from three very different lake types. Both training sets were expanded, particularly at the low end of the nutrient gradient, using contemporary chironomid assemblages and environmental parameters from 12 British lakes, although this had little improvement on the model performances. 2. Dissimilarity analyses showed that the historic chironomid assemblages did not have good analogues in the original calibration or extended datasets. However, since the transfer functions are based on weighted averages of the trophic optima for the taxa present and not on community similarities, reasonable downcore inferences were produced. Ordination analyses also showed that the lakes retain their ‘identity’ over time, as the sample dissimilarities within lakes were less than the dissimilarities between lakes. 3. Analysis of the three historic lake profiles showed a range of chironomid community responses to lake development. Chironomids from a shallow lake, Slapton Ley, responded indirectly to nutrient enrichment (TP), probably through altered substrate, macrophyte and fish conditions, rather than directly to primary productivity (Chl a). A stratified lake, Old Mill Reservoir, showed a loss of the profundal chironomid fauna due to increasing primary productivity (Chl a) coupled with increasing hypoxia. A response to nutrients (TP or total nitrogen (TN)) at this site is also indirect, and the TP reconstruction therefore cannot be reliably interpreted. The third lake, March Ghyll Reservoir has little change in historic chironomid communities, suggesting that this well mixed, relatively unproductive lake has changed less than the other lakes. 4. Using chironomids to reconstruct nutrient histories does not follow a simple scheme. The response to changes in nutrients may be direct, but mediated through other ecosystem components. As alternative stable states are possible at a given level of TP it is also likely that alternative chironomid communities exist under similar nutrient conditions. Changes in biological communities can thus occur over thresholds, and it is only biological proxies that can reflect such ecosystem switches within palaeoenvironmental investigations.     

Relationship between δ13C of chironomid remains and methane flux in Swedish lakes

1. Methanogenic carbon can be incorporated by methane‐oxidising bacteria, leading to a ¹³C‐depleted stable carbon isotopic composition (δ¹³C) of chironomids that feed on these microorganisms. This has been shown for the chironomid tribe Chironomini, but very little information is available about the δ¹³C of other abundant chironomid groups and the relationship between chironomid δ¹³C and methane production in lakes. 2. Methane flux was measured at the water surface of seven lakes in Sweden. Furthermore, fluxes from the sediments to the water column were measured in transects in two of the lakes. Methane fluxes were then compared with δ¹³C of chitinous chironomid remains isolated from the lake surface sediments. Several different chironomid groups were examined (Chironomini, Orthocladiinae, Tanypodinae and Tanytarsini). 3. Remains of Orthocladiinae in the seven study lakes had the highest δ¹³C values (?31.3 to ?27.0‰), most likely reflecting δ¹³C of algae and other plant‐derived organic matter. Remains of Chironomini and Tanypodinae had lower δ¹³C values (?33.2 to ?27.6‰ and ?33.6 to ?28.0‰, respectively). A significant negative correlation was observed between methane fluxes at the lake surface and δ¹³C of Chironomini (r = ?0.90, P = 0.006). Methane release from the sediments was also negatively correlated with δ¹³C of Chironomini (r = ?0.67, P = 0.025) in the transect samples obtained from two of the lakes. The remains of other chironomid taxa were only weakly or not correlated with methane fluxes measured in our study lakes (P > 0.05). 4. Selective incorporation of methane‐derived carbon can explain the observed correlations between methane fluxes and δ¹³C values of Chironomini. Remains of this group might therefore have the potential to provide information about past changes in methane availability in lakes using sediment records. However, differences in productivity, algal δ¹³C composition and the importance of allochthonous organic matter input between the studied lakes may also have influenced Chironomini δ¹³C. More detailed studies with a higher number of analysed samples and detailed measurement of δ¹³C of different ecosystem components (e.g. methane, dissolved inorganic carbon) will be necessary to further resolve the relative contribution of different carbon sources to δ¹³C of chironomid remains.     

Ecological influences on larval chironomid communities in shallow lakes: implications for palaeolimnological interpretations

1. To correctly interpret chironomid faunas for palaeoenvironmental reconstruction, it is essential that we improve our understanding of the relative influence of ecosystem variables, biotic as well as physicochemical, on chironomid larvae. To address this, we analysed the surface sediments from 39 shallow lakes (29 Norfolk, U.K., 10 Denmark) for chironomid head capsules, and 70 chironomid taxa (including Chaoborus) were identified. 2. The shallow lakes were selected over large environmental gradients of aquatic macrophytes, total phosphorus (TP) and fish communities. Redundancy analysis (RDA) identified two significant variables that explained chironomid distribution: macrophyte species richness (P < 0.001) and TP (P < 0.005). Generalised linear models (GLM) identified specific taxa that had significant relationships with both these variables. Macrophyte percentage volume infested (PVI) and species richness were significant in classifying the lake types based on chironomid communities under twinspan analysis, although other factors, notably nutrient concentrations and fish communities, were also important, illustrating the complexities of classifying shallow lake ecosystems. Lakes with plant species richness >10 all had relatively diverse (Hill’s N2) chironomid assemblages, and lakes with Hill’s N2 >10 all had TP <250 μg L⁻¹ and total fish densities <2 fish per m². 3. Plant density (PVI), and perhaps more importantly species richness, were primary controls on the distribution of chironomid communities within these lakes. This clearly has implications for palaeoenvironmental reconstructions using zoobenthos remains (i.e. chironomids) and suggests that they could be used to track changes in benthic/pelagic production and could be used as indicators of changing macrophyte habitat. 4. Measuring key biological gradients, in addition to physicochemical gradients, allowed the major controls on chironomid distribution to be assessed more directly, in terms of plant substrate, food availability, competition and predation pressure, rather than implying indirect mechanisms through relationships with nutrients. Many of these variables, notably macrophyte abundance and species richness, are not routinely measured in such studies, despite their importance in determining zoobenthos in temperate shallow lakes. 5. When physical, chemical and ecological gradients are considered, as is often the case with palaeo‐reconstructions rather than training sets chosen to maximise one gradient, complex relationships exist, and attempting to reconstruct a single trophic variable quantitatively may not be appropriate or reliable.     

Temperature change as a driver of spatial patterns and long‐term trends in chironomid (Insecta: Diptera) diversity

Anthropogenic activities have led to a global decline in biodiversity, and monitoring studies indicate that both insect communities and wetland ecosystems are particularly affected. However, there is a need for long‐term data (over centennial or millennial timescales) to better understand natural community dynamics and the processes that govern the observed trends. Chironomids (Insecta: Diptera: Chironomidae) are often the most abundant insects in lake ecosystems, sensitive to environmental change, and, because their larval exoskeleton head capsules preserve well in lake sediments, they provide a unique record of insect community dynamics through time. Here, we provide the results of a metadata analysis of chironomid diversity across a range of spatial and temporal scales. First, we analyse spatial trends in chironomid diversity using Northern Hemispheric data sets overall consisting of 837 lakes. Our results indicate that in most of our data sets, summer temperature (T_jul) is strongly associated with spatial trends in modern‐day chironomid diversity. We observe a strong increase in chironomid alpha diversity with increasing T_jul in regions with present‐day T_jul between 2.5 and 14°C. In some areas with T_jul > 14°C, chironomid diversity stabilizes or declines. Second, we demonstrate that the direction and amplitude of change in alpha diversity in a compilation of subfossil chironomid records spanning the last glacial–interglacial transition (~15,000–11,000 years ago) are similar to those observed in our modern data. A compilation of Holocene records shows that during phases when the amplitude of temperature change was small, site‐specific factors had a greater influence on the chironomid fauna obscuring the chironomid diversity–temperature relationship. Our results imply expected overall chironomid diversity increases in colder regions such as the Arctic under sustained global warming, but with complex and not necessarily predictable responses for individual sites.     

Fatty‐acid profiles of juvenile lake trout reflect experimental diets consisting of natural prey

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Matching thirty years of ecosystem monitoring with a high resolution microfossil record

1. Monitoring of the ecosystem of Lake Mývatn, Iceland, since 1975 has revealed extreme fluctuations in important food web components, such as chironomids and cladocerans, with amplitudes of several orders of magnitude and a period of 5–8 years. This study uses sediment cores from the lake to examine if the food web fluctuations appear in the microfossil record of the sediment. 2. Dating was achieved by means of a combination of ¹³⁷Cs and volcanic tephra and was fine‐tuned by wiggle‐matching of chironomid microfossil and monitoring data. 3. Cladocera exuviae and chironomid egg capsules in the uppermost 34 cm of sediment were compared with the monitoring record that consisted of 30 years of window trap catches of flying chironomids and a 16‐year record of chydorid Cladocera caught in activity traps. 4. The observed chironomid and cladoceran population fluctuations were reflected in the sediment record of chironomid eggs and of the exuviae of three of seven cladocerans: Alonella nana, Alona rectangula and Eurycercus lamellatus, which also had the most extreme fluctuations in the monitoring data (3–4 orders of magnitude). Chydorus sphaericus, and to some extent Alona quadrangularis and Acroperus harpae, showed regular fluctuations in the core that the monitoring did not reveal. Density of subfossil chironomid eggs correlated positively with that of larval head capsules but not with other microfossils. 5. This study shows a reasonably good correspondence between the fossil records of chironomids and cladocerans on the one hand and biomonitoring data on the other. The results pave the way for an extension of the food web history to much earlier time intervals of the ecosystem, allowing the study of long‐term variation in the food web dynamics, including the impact of climatic variation and other external forcing. The results also indicate the usefulness of chironomid egg capsules in palaeolimnological studies.     

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.     

Predation and facilitation determine chironomid emergence in a bromeliad‐insect food web

1. Ecological theory has focused on negative interactions, such as competition and predation, to explain species' effects on one another. This study demonstrates the importance of considering both positive and negative interactions in explaining how species influence abundances at the local scale. 2. Two experiments were conducted using the aquatic insect food web in Costa Rican bromeliad phytotelmata. Manipulations contrasted the strength of predation between trophic levels versus facilitation within a trophic level on the emergence of detritivore chironomids. 3. Predation had a strong negative effect on chironomids, reducing emergences by 81% overall. Most predation was as a result of the top predator, the odonate Mecistogaster modesta; the intermediate predator, a tanypodine chironomid, had little effect. In the absence of predators, shredder and scraper detritivores (tipulid and scirtid larvae) increased the emergence rate of chironomid larvae by 86%. The mechanism of facilitation was likely the processing, by tipulids and scirtids, of intact detritus into fine particles that the detritivore chironomids consume or use to build protective cases. 4. This study is among the first demonstrations of a processing chain in a multi‐species context, and in bromeliad‐insect food webs. Our finding that top‐down effects are of similar magnitude to facilitative effects suggests that the relative importance of processing chains in nature will depend on food web context.     

Microhabitat influence on chironomid community structure and stable isotope signatures in West Greenland lakes

Most functional feeding types are represented within the species rich group of aquatic chironomids. Thus, we hypothesized that different lake types and microhabitats within lakes would (1) host specific chironomid communities and (2) that the individual communities would show specific δ ¹³C stable isotope signatures reflecting the prevailing origin of food source. To test our hypotheses, five lakes in southwest Greenland were investigated at a high taxonomic resolution and with detailed information on δ ¹³C signature of the chironomids and of individual microhabitats (macrophytes, sediment, stones, and profundal). We found that there was a significant difference in δ ¹³C between the chironomid assemblages of freshwater lakes and oligosaline lakes, while assemblages of the littoral microhabitats did not differ significantly. The δ ¹³C of chironomids reflected the wide variety of habitat signals, particularly in the freshwater lakes. Our results indicate that many chironomid taxa are ubiquitous and are found in several microhabitats, suggesting that they can adjust their feeding strategy according to the habitat. The implication is that chironomid assemblage composition has only limited use as indicator of littoral microhabitats in the Arctic. On the other hand, the δ ¹³C signature of fossil chironomids might have a potential as indicator of microhabitats in freshwater lakes.     

Factors governing the spatial and temporal distribution of Chironomid larvae in the Maarsseveen lakes with special emphasis on the role of oxygen conditions

Distribution patterns of the larvae of Chironomidae are compared in three water systems in The Netherlands, which vary in trophic state and oxygen regimes. The life cycles and flying periods of some dominant chironomid species in two of the investigated lakes, Lakes Maarsseveen I and II, are determined by comparing data on the seasonal variations in larval densities with existing literature on Chironomidae in the Maarsseveen lakes. In the oligo-mesotrophic Lake Maarsseveen I (LM I), hypoxic or anoxic conditions in the hypolimnion are observed only at the end of the stratification period. A clear zonation of the chironomid fauna is present in this lake. The littoral zone is dominated byCladotanytarsus gr.mancus andStictochironomus sticticus, the littoriprofundal zone byTanytarsus bathophilus, and the profundal zone byChironomus anthracinus. In comparison with the other species in LM I,T. bathophilus larvae show the most variable distribution patterns over time. Larvae are found in all depths from July to September, but disappear from the hypolimnion as soon as oxygen conditions deteriorate. In the eutrophic Lake Maarsseveen II (LM II), oxygen depletion of the hypolimnion starts immediately after the onset of the thermal stratification in June, and continues until autumnal turnover in November. In this lake, the chironomid community consists primarily ofS. sticticus andCl. gr.mancus, and is confined to the narrow littoral zone. No chironomid larvae are found in the deeper parts of the lake. The eutrophic Lake Gijster in the Brabantse Biesbosch is a deep, man-made reservoir, that is artificially destratified during the summer. In this lakeTanytarsus bathophilus is found in the profundal sediments, whereas almost noChironomus is found in this zone. It is concluded that oxygen conditions existing in the deeper regions of the investigated lakes in large part determine the occurrence and distribution of chironomid species. The distribution ofT. bathophilus is limited by unfavorable oxygen conditions and not by the trophic state of the lake. These findings are part of a thesis (HEINIS, 1993).     

Changes in fossil chironomid remains along a depth gradient: evidence for common faunal thresholds within lakes

Many environmental variables that are important for the development of chironomid larvae (such as water temperature, oxygen availability, and food quantity) are related to water depth, and a statistically strong relationship between chironomid distribution and water depth is therefore expected. This study focuses on the distribution of fossil chironomids in seven shallow lakes and one deep lake from the Plymouth Aquifer (Massachusetts, USA) and aims to assess the influence of water depth on chironomid assemblages within a lake. Multiple samples were taken per lake in order to study the distribution of fossil chironomid head capsules within a lake. Within each lake, the chironomid assemblages are diverse and the changes that are seen in the assemblages are strongly related to changes in water depth. Several thresholds (i.e., where species turnover abruptly changes) are identified in the assemblages, and most lakes show abrupt changes at about 1–2 and 5–7 m water depth. In the deep lake, changes also occur at 9.6 and 15 m depth. The distribution of many individual taxa is significantly correlated to water depth, and we show that the identification of different taxa within the genus Tanytarsus is important because different morphotypes show different responses to water depth. We conclude that the chironomid fauna is sensitive to changes in lake level, indicating that fossil chironomid assemblages can be used as a tool for quantitative reconstruction of lake level changes.     

Swimming Behaviour of <Emphasis Type="Italic">Chironomus acerbiphilus</Emphasis> Larvae in Lake Katanuma

We conducted a seasonal survey of the swimming behaviour of Chironomus acerbiphilus larvae in volcanic Lake Katanuma from April 1998 to December 2001. Swimming C. acerbiphilus density was much higher than other chironomid species in lakes. All C. acerbiphilus larvae (1st through 4th instars) swam, but the earlier instars (especially the 1st) had the greatest densities and fluctuations. First instars were never found in the benthic population. This result indicates that the 1st-instar larvae are planktonic. Low water temperature (below about 10 °C) resulted in the seasonal disappearance of swimming chironomid larvae. Chemical factors – oxygen depletion or presence of hydrogen sulfide – also restricted the distribution of swimming and benthic larvae. Larvae were distributed only in the oxygen-rich part of the lake bottom and swam only in the oxygen-rich layer of the water column. The density of older swimming C. acerbiphilus (3rd and 4th instars) tended to increase with increasing benthic larval densities. The chemical stress of oxygen depletion or presence of hydrogen sulfide during holomixis within and after the stratification period leads to conspicuous swimming behaviour of benthic C. acerbiphilus larvae. Almost all C. acerbiphilus larvae died on this occasion.     

Instar distribution and biomass of Chironomidae larvae in Lago El Junco, Isla San Cristobal, the Galápagos

The distribution, abundance and standing crop biomass of chironomid larvae were determined at one-meter depth intervals along three radial transects. Samples were collected by coring soft sediments while diving. Three genera were found in the lake: Chironomus sp. (collector-filtering larvae), Ablabesmyia sp. (predatory larvae) and Goeldichironomus sp. (collector-filtering larvae). Standing crop densities of chironomids, averaged over the entire lake, varied from 30,594 larvae/m² to 11,428 larvae/m² at different depths. No statistically significant zonation in density was found for the two most common taxa, Chironomus sp. (87.8% of specimens) and Ablabesmyia sp. (9.0%), however the deepest zones (>4 m) had the lowest estimated densities. Significant differences in standing crop biomass were detected, with the 6 m depth having greatest biomass. The increase in standing crop biomass was a function of (1) lower frequency of first instars of Chironomus sp. and Ablabesmyia sp. at 6 m (2) higher average larval biomass of both species at 6 m and (3) very significant increase in average biomass of fourth instars of Chironomus sp. at 6 m compared to fourth instars at shallower depths. These results indicate that the lentic chironomids of this isolated oceanic habitat consist of a small number of species that are ecological generalists and tolerant of low oxygen concentrations.     

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.     

Variability of mouth width in European eel,Anguilla anguilla,in relation to varying feeding conditions in three Dutch lakes

Synopsis Mouth widths of eels in three Dutch lakes were compared relative to feeding conditions over two years. Average mouth width varied by season and year and per lake. Of two groups of feeding specialists distinguished in each population chironomid feeders showed a smaller mouth width compared to fish feeders of the same body length. Proportions of these specialists differed between years, lakes, and seasons and changed in response to changing feeding conditions. The shift to fish feeding occurred when chironomid biomass decreased and young fish recruited. The shift was greatest in the year when chironomids collapsed completely and in the lakes with high eel densities. Mature males were narrow-headed and left the lakes at sizes between 30 and 40 cm. Larger eels were always broad-headed females. Implications for foraging efficiency and population structure are discussed.