Profundal Chironomid Research Articles

Multi-trophic level responses to environmental stressors over the past ~150 years: Insights from a lake-rich region of the world

Multiple stressors affect water quality and biodiversity in lakes worldwide. However, our understanding of which combinations of stressors are of greatest impact and how lakes have shifted from their pre-industrial baselines is fragmented. Questions remain regarding how multiple trophic groups are affected by global change stressors and whether region-specific differences need to be considered. Here, we apply multiple factor analyses together with latent variable modeling to quantify potential interactions within and among taxa that span multiple trophic levels (i.e. diatoms, cladocerans and chironomids) to improve our understanding of paleo-environmental dynamics from a suite of Canadian lakes spanning four ecozones: the Boreal Shield, the Mixedwood Plains, the Atlantic Highlands, and the Atlantic Maritimes. Across all ecozones, multi-trophic assemblages were distinguished in a multiple factor analysis along a land-use gradient, with mesotrophic/eutrophic diatoms and profundal chironomids tolerant of low bottom-water oxygen concentrations recorded in more disturbed sites. Functional units across three indicator groups formed distinct networks of co-responses to key environmental and land-use gradients, although cladocerans seemed to be driven by additional (i.e., residual) gradients. Applying a temporal beta-diversity approach between modern and pre-industrial assemblages, we detected significantly greater turnover of diatom functional groups in high versus low human impact sites (p = 0.03, n = 57), with similar but non-significant trends apparent with the chironomid and cladoceran groups. Ecozonal differences in temporal turnover were also evident in diatom (p = 0.003, n = 57) and chironomid functional groups (p = 0.04, n = 41). The ecozonal differences may be partially due to differential sensitivities of the pre-industrial assemblages and may also be driven by historical disturbances. Our work highlights how the magnitude and direction of ecological change vary across ecozones and can modify responses to human impacts, with a general trend of higher species turnover in sites with greater human disturbance.

Incorporation of wastes by native species during and after an experimental aquaculture operation

Freshwater aquaculture increases dissolved and particulate nutrients near fish cages, but the degree to which they are incorporated into tissues of native animals is uncertain. At the Experimental Lakes Area in northwestern Ontario, Canada, a cage culture of Rainbow Trout was operated seasonally for 5 y, and invertebrates and fishes were collected before, during, and after culturing from the experimental lake and a reference lake to assess changes in stable isotopes of C and N in their tissues. The feed contained marine fishmeal and was higher in C and N isotope values (δ13C and δ15N) than lake biota by ≥4‰ (all taxa) and 3‰ (all invertebrates), respectively. During the aquaculture operation, δ15N of littoral and pelagic invertebrates, profundal chironomids, minnows, and Lake Trout (Salvelinus namaycush) increased by 2 to 5‰ relative to before aquaculture values. In the 1st and 2nd years after aquaculture, δ15N of several invertebrate taxa and all fishes continued to increase 1 to 2‰/y. In contrast, during aquaculture, only minnows and trout had significant increases (up to 3‰) in δ13C. In the period after aquaculture, the δ13C of fishes, plankton, and profundal chironomids declined to below values measured before or during aquaculture. Isotopic analysis of native biota can be used to monitor assimilation of cage culture wastes in freshwater ecosystems.

Consensus among multiple trophic levels during high- and low-water stands over the last two millennia in a northwest Ontario lake

We investigated the modern distribution of fossil midges within a dimictic lake and explored downcore patterns of inferred lake depths over the last 2000years from previously published proxies. Modern midge distribution within Gall Lake showed a consistent and predictable pattern related to the lake-depth gradient with recognizable assemblages characteristic of shallow-water, mid-depth and profundal environments. Interpretations of downcore changes in midge assemblages, in conjunction with quantitative lake-depth inferences across a priori defined (based on diatom data) ~500-yr wet and dry periods, demonstrated that both invertebrate and algal assemblages exhibited similar timing and nature of ecological responses. Midges were quantified by their relative abundance, concentrations and an index of Chaoborus to chironomids, and all showed the greatest differences between the wet and dry periods. During the low lake-level period of the Medieval Climate Anomaly (MCA: AD 900 to 1400), profundal chironomids declined, shallow-water and mid-depth chironomids increased, chironomid-inferred lake level declined and the Chaoborus-to-chironomid index decreased. The coherence between multiple trophic levels provides strong evidence of lower lake levels in Gall Lake during the MCA.

Depth‐specific responses of a chironomid assemblage to contrasting anthropogenic pressures: a palaeolimnological perspective from the last 150 years

Summary The specific effects of three types of anthropogenic forcing (nutrient concentrations, climate change and fisheries management practices) on the benthic invertebrate community over the last 150 years in a re‐oligotrophicated large, deep subalpine lake were investigated using chironomid remains. The structural changes in the chironomid assemblages in different habitats were assessed based on sub‐fossil remains retrieved from sediment cores sampled along a depth gradient (30, 56 and 65 m) and analysed at a high temporal resolution (2–5 years). Until the 1940s, the chironomid assemblages were strongly depth specific and were characterised by oxyphilous taxa at all studied depths. Subsequently, the assemblages drifted towards domination by common hypoxia‐tolerant taxa. The scores of the first axis of principal component analyses for each studied depth were used as a proxy for community structure, and the influence of anthropogenic forcings on axis scores was assessed using general additive models. The temporal variability of the contribution of each covariate to our model estimates allowed their specific effects to be clarified. The sensitivity of the chironomid assemblages to the considered forcings was depth dependent. Profundal chironomid assemblages (65 m) responded mainly to nutrient enrichment, whereas assemblages at shallower depths (56 and 30 m) were mainly affected by top‐down effects of the fish community on the pelagic food web, triggering increasing losses of organic matter from the pelagic to the benthic zone. Since the late 1980s, increased air temperatures have probably been responsible for changes in assemblages at all sampled depths, potentially through their effect on the strength and duration of thermal stratification, possibly coupled with changes in mixing efficiency during winter. Our results highlight linkages between the pelagic and benthic food webs and identify possible temporal substitutions among anthropogenic forcings that successively drive the chironomid assemblages. These findings provide additional evidence of the multiple environmental controls structuring chironomid assemblages and highlight the associated difficulties in predicting chironomid assemblage trajectories following mitigation efforts.

Decadal changes in benthic invertebrate biomass and community structure in Lake Simcoe

Total benthic invertebrate biomass in shallow offshore waters (depths 10–15 m) increased 17-fold on average following the invasion of dreissenid mussels and implementation of P controls in Lake Simcoe, Ontario, despite a 50% overall decrease in total invertebrate density during the same time period. The increase in total invertebrate biomass at 10- and 15-m depths was primarily dreissenid biomass. Patterns of biomass with depth and dreissenid invasion for individual taxa were typically similar to those for density. Biomass of chironomid and nondreissenid bivalves declined at shallow (10–15 m) sites but increased at the deepest (20–30 m) sites, whereas biomass of oligochaetes declined at all sites. Density and biomass of Amphipoda, Isopoda, and Gastropoda increased at depths ≥10 m, and these taxa were found more frequently in deeper sites after dreissenid invasion than before. Increased habitation of deeper sites by these taxa may be mediated by increased habitat complexity caused by deposition of dreissenid shells, nutrient enrichment of substrate occupied by dreissenids, and improvements in hypolimnetic dissolved O2 and water clarity observed during dreissenid invasion. Increased length and biomass of profundal chironomids after dreissenid invasion may be the result of taxonomic changes in the chironomid community, which in turn appear to be closely associated with improvements in deep-water O2 concentrations. Changes in the benthic community described here probably have important consequences for the degree of coupling between nearshore and offshore habitats in Lake Simcoe.

Benthic–pelagic coupling in lake ecosystems: the key role of chironomid pupae as prey of pelagic fish

Studies of lake ecosystems have often treated benthic and pelagic habitats as discrete food chains with parallel, separated compartments. A limited number of studies on the importance of fish feeding for benthic‐pelagic coupling have been conducted with a food‐web perspective of lakes. We hypothesize that pelagic fish couple benthic and pelagic food webs by feeding on chironomid pupae ascending from the sediment to the surface. Over a two‐year period, an analysis of the stomach contents of Eurasian perch (Perca fluviatilis L.) was combined with simultaneous catches of chironomids in near‐bottom pupal traps and floating emergence traps exposed in the littoral and profundal region of a deep temperate reservoir. Chironomid larvae rarely served as prey for perch. While chironomid pupae from the littoral region were of minor importance as perch prey, we determined a substantial vertical trophic biomass transfer from the profundal to the pelagic food web for the pupation peak of profundal chironomids from May through June. During this period, chironomid pupae represented approximately 20–48% of the ingested biomass of all size classes of perch (70–340 mm). The pupae of only three large profundal species out of 84 chironomid taxa detected in the reservoir represented approximately 80% of the chironomids consumed by perch, indicating a key role for these taxa in benthic‐pelagic coupling. The strong influences of species, habitat and size on the consumption of chironomid pupae by the pelagic perch provide the importance of detecting key benthic species that substantially contribute to the pelagic food web. The phenology of profundal species results in a seasonal shift in the feeding behavior of pelagic fish, which thereby influences the trophic dynamics of lake ecosystems. Therefore, we argue for a more integrated view of lake ecosystems that recognizes not only the littoral but also the complementarity of profundal and pelagic production pathways crucial for ecosystem functioning.

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

Summary 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 Paijanne (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.

Biogenic methane in freshwater food webs

1. It has long been known that substantial amounts of methane are produced in anoxic lake sediments, and the components of the methane cycle in lakes have been well described. At oxic–anoxic interfaces, methane-oxidising bacteria (MOB) convert methane to microbial biomass and can be highly productive. However, only recently has methane been recognised as a potentially important carbon and energy source for lake food webs, and some instances have also been reported of methane contribution to river food webs. Stable isotope analysis (SIA) has provided compelling evidence in this respect and has been supplemented by other lines of evidence. 2. In the benthic food webs of lakes, profundal chironomid larvae appear to be the main conduits for trophic transfer of biogenic methane via grazing on MOB. The mode of feeding of these larvae and the microhabitats they generate both promote larval ability to exploit MOB production. Support to chironomid larvae from methane is rather widespread, but its degree is highly variable; estimates suggest that in some lakes methane-carbon might contribute more than 60% of chironomid carbon biomass. 3. Evidence of crustacean zooplankton in lakes deriving part of their carbon from methane is currently more limited. Reports from some lakes have indicated Daphnia with a substantial (>50%) contribution of methane-carbon in their biomass. However, for this to happen, an oxic–anoxic interface where sufficient MOB production can occur needs to be within the range of vertical migrations by zooplankton, which may only rarely be the case. Hence, a significant methane subsidy of pelagic food webs in lakes is probably much less widespread than for benthic food webs. 4. There is also recent and currently very limited evidence that some stream benthos derives biomass carbon (reported values up to 30%) from methane. This can occur in stagnant backwater pools where conditions can be analogous to those in lake sediments. However, groundwater aquifers can also supply water supersaturated with methane to some rivers, providing a basis for a microbially-mediated transfer of methane-carbon to river benthos. 5. Evidence for significant transfer of methane-derived carbon to higher trophic levels is still very limited. Within some lakes, those fish species that feed extensively on chironomid larvae can derive a substantial part (perhaps up to 20%) of their carbon biomass from methane. It is also likely that methane-carbon produced in lakes or rivers is exported to riparian ecosystems when emerging chironomids or other insects are eaten by invertebrate or avian predators. 6. We argue that conceptual models of freshwater food webs, and especially those for lakes, need to be modified to enable incorporation of biogenic methane as a carbon and energy source. For some types of lakes, carbon and energy budgets certainly need to take account of the production and utilisation of biogenic methane, and the accumulating evidence indicates that this is a more widespread phenomenon that has generally been acknowledged hitherto.

Reconstruction of the recent history of a large deep prealpine lake (Lake Bourget, France) using subfossil chironomids, diatoms, and organic matter analysis: towards the definition of a lake-specific reference state

This paper presents the recent history of a large prealpine lake (Lake Bourget) using chironomids, diatoms and organic matter analysis, and deals with the ability of paleolimnological approach to define an ecological reference state for the lake in the sense of the European Framework Directive. The study at low resolution of subfossil chironomids in a 4-m-long core shows the remarkable stability over the last 2.5 kyrs of the profundal community dominated by a Micropsectra-association until the beginning of the twentieth century, when oxyphilous taxa disappeared. Focusing on this key recent period, a high resolution and multiproxy study of two short cores reveals a progressive evolution of the lake’s ecological state. Until AD 1880, Lake Bourget showed low organic matter content in the deep sediments (TOC less than 1%) and a well-oxygenated hypolimnion that allowed the development of a profundal oxyphilous chironomid fauna (Micropsectra-association). Diatom communities were characteristic of oligotrophic conditions. Around AD 1880, a slight increase in the TOC was the first sign of changes in lake conditions. This was followed by a first limited decline in oligotrophic diatom taxa and the disappearance of two oxyphilous chironomid taxa at the beginning of the twentieth century. The 1940s were a major turning point in recent lake history. Diatom assemblages and accumulation of well preserved planktonic organic matter in the sediment provide evidence of strong eutrophication. The absence of profundal chironomid communities reveals permanent hypolimnetic anoxia. From AD 1995 to 2006, the diatom assemblages suggest a reduction in nutrients, and a return to mesotrophic conditions, a result of improved wastewater management. However, no change in hypolimnion benthic conditions has been shown by either the organic matter or the subfossil chironomid profundal community. Our results emphasize the relevance of the paleolimnological approach for the assessment of reference conditions for modern lakes. Before AD 1900, the profundal Micropsectra-association and the Cyclotella dominated diatom community can be considered as the Lake Bourget reference community, which reflects the reference ecological state of the lake.

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.

Assimilation of freshwater salmonid aquaculture waste by native aquatic biotaThis paper is part of the series “Forty Years of Aquatic Research at the Experimental Lakes Area”.

An experimental finfish aquaculture farm was operated in a small lake at the Experimental Lakes Area in northwestern Ontario, Canada. In this study, we used the distinct and enriched carbon and nitrogen isotope ratios (δ13C and δ15N) of rainbow trout ( Oncorhynchus mykiss ) feed and waste to determine whether the operation provided a novel source of energy for native biota. For 1 year prior to and for 3 years during the cage culture, we collected littoral, pelagic, and profundal invertebrates and minnows from the experimental and reference lakes. In both the second and third years of aquaculture, there was a significant increase in δ15N of all organisms sampled in the experimental lake; mean δ15N values of littoral, pelagic, and profundal invertebrates and minnows shifted towards the signature of the fish feed by up to 4.2‰. Significant increases in δ13C of up to 2.6‰ were observed in Mysis , profundal chironomids, and minnows but not in littoral invertebrates or zooplankton. Aquaculture waste became a progressively more important component of minnow diets over the course of this study. The dissolved and (or) particulate wastes of the cage operation became a novel source of energy for the native biota in this experimental lake.

WIDESPREAD CONTRIBUTION OF METHANE-CYCLE BACTERIA TO THE DIETS OF LAKE PROFUNDAL CHIRONOMID LARVAE

Reports of unexpectedly 13C-depleted chironomid larvae in lakes have led to an hypothesis that significant transfer of detrital organic matter to chironomid larvae may occur via methane-cycle bacteria. However, to date little is known of how such transfer might vary across species and lakes. We gathered data from 87 lakes to determine how widespread this phenomenon might be and to define boundaries for its likely magnitude. Carbon stable isotope values of chironomid larvae varied greatly between taxa. Very marked 13C-depletion was evident only in certain taxa, especially Chironomus plumosus, C. anthracinus, and C. tenuistylus, all characteristic of eutrophic or dystrophic lakes and known to be tolerant of low oxygen conditions. Furthermore, marked 13C-depletion was only found in larvae from lakes in which late-summer hypolimnetic oxygen depletion near the sediment surface was below an apparent threshold concentration of 2-4 mg O2/L. Similarly, application of a two-source mixing model suggested that methanotrophic bacteria made the greatest contribution to profundal chironomid growth (0-70% of larval carbon) when the late-summer oxygen concentration dropped below approximately 2 mg O2/L. Our study demonstrates that methane-derived carbon is an important, but often neglected, contribution to the flux of carbon through the food webs of many productive or dystrophic lakes.

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

Summary1. Total phosphorus (TP) and chlorophyll a (Chl a) chironomid inference models (Brodersen &amp; Lindegaard, 1999; Brooks, Bennion &amp; 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.

Lake trophic status and bioaccumulation of polychlorinated biphenyls in benthic organisms: a comparison between littoral and profundal invertebrates

We investigated the influence of lake trophic status on accumulation of polychlorinated biphenyls (PCBs) in littoral and profundal invertebrates from 19 lakes in southern Sweden. The PCB concentration in profundal chironomids (Chironomus sp.) was negatively correlated with lake trophy but the concentration in littoral crayfish (Pacifasticus leniusculus) was unrelated to shifting nutrient regimes. PCB concentrations in chironomids were negatively correlated, while crayfish did not correlate with lake total phosphorus. Two findings indicate that different processes determine bioaccumulation in littoral and profundal invertebrates: (i) a lack of correlation between PCB concentrations in crayfish and chironomids within lakes and (ii) some differences in congener distribution in the two species. We suggest a possible explanation in that chironomids are directly exposed to settling material from the pelagic zone, while crayfish are omnivorous and feed mainly in the littoral zone. A intensive investigation on one pond population could not reveal any individual factors, such as lipid content, size, or sex, influencing bioaccumulation of PCBs in crayfish.

Importance of diffuse nutrient loading and lake level changes to the eutrophication of an originally oligotrophic boreal lake: a palaeolimnological diatom and chironomid analysis

The recent environmental history of Lake Lappajarvi in western Finland (63°00' N, 23°30' E, area 149 km 2 ), a humic, brown water lake with an average phosphorus content of ca. 20 µg l ‐1 , was studied from short core sediment samples taken from the two main basins of the lake. Based on the stratigraphy of diatoms and chironomids and the sediment quality it was possible to distinguish four developmental stages during the past century: (1) a pre-industrial stage covering the time up to about 1935; (2) a stage of increasing nutrient loading (ca. 1936‐1960); (3) a stage of pronounced erosion from lake level regulation and extensive ditching of the catchment area (ca. 1960‐ 1970); and (4) a meso-eutrophic stage from ca. 1970 onwards. Acidophilous Aulacoseira distans coll. and other species typical of dystrophic, nutrient-poor lakes characterized the diatom assemblages during the first stage, and the profundal zoobenthic assemblages, characterized by Heterotrissocladius subpilosus and Micropsectra, indicated good hypolimnetic oxygen conditions and a low sedimentation of organic matter (approximately less than 50 g m ‐2 a ‐1 ). The increased loading rapidly led to changes both in diatoms and chironomids (e.g., to an early extinction of H. subpilosus in the 1950s). The process finally led to eutrophication with a successive proliferation of diatom species such as Asterionella formosa followed by Aulacoseira ambigua, Fragilaria crotonensis, and finally Melosira varians. The relative proportion of alkaliphilous species reached a maximum in the final stage and the original profundal chironomid fauna was replaced by Chironomus anthracinus gr. and C. plumosus which are typical of profundal areas suffering from temporal oxygen deficit. It is notable that the considerable decrease in waste water loading from the point sources (80‐86%) during the past two decades has not led to a recovery in the lake. This highlights the importance of diffuse loading from agriculture, forestry and other human activities even to this comparatively large lake.

Bathymetric distributions of chironomid communities in ten French lakes: Implications on lake classification

Biological typology of lakes is commonly based on the analysis of profundal chironomid communities (THIENEMANN 1913, BRUNDIN 1949, SAETHER 1979, WIEDERHOLM 1980). In this study, ten French lakes were characterized using BRUNDIN and SAETHER's schemes of lake classification. The results showed that all these lakes were Chironomus anthracinus or eutrophic lakes, even though environmental factors (chiefly oxygen concentration and the amount of organic matter in the sediment) differed from lake to lake. We also compared the classification of these lakes as mentioned above with a new type of classification based on the analysis of bathymetric distribution of chironomid fauna. Taking the bathymetric distribution of chironomids into account seems to be more informative than relying only on an analysis of profundal communities. The lakes were classified according to bathymetric modifications of their communities related more to oxygen and organic matter than to chlorophyll-a concentrations. As these results were in disagreement with the trophic typology proposed by SAETHER (1979), we propose a hypothesis to explain the differences between the two types of classifications based upon the factors influencing chironomid communities.

Food and habitat of four Xenotilapia species (Teleostei, Cichlidae) in a sandy bay of northern Lake Tanganyika (Burundi)

The feeding ecology of four species of Xenotilapia (Teleostei, Cichlidae) from Lake Tanganyika was investigated by examining their intestine contents in three different habitats from two 24‐h catch cycles. The fish were caught in a sandy bay 4 km north‐west of Bujumbura (Burundi) during the dry season, from August until October 1993. Three different habitats were sampled: an area 600 m offshore at a depth of 15 m, a littoral zone near a reed bed (depth, 0–5 m), and an area near a sandy beach (depth, 5 m). The fish were most active in the early morning and at night, except for X. caudafasciata which was the most diurnal. X. ochrogenys was confined to the vicinity of the reeds, where few individuals of the other species were found, and prey comprised mainly ostracods, copepods and chironomid larvae (Tanytarsini). X. ornatipinnis and X. longispinis occurred in large numbers both at a depth of 15 m and near the sandy beach. X. ornatipinnis preyed mainly on copepods, oligochaetes, diatoms and chironomid larvae. X. longispinis had a wider range of temporal and spatial distribution compared with the other species and fed on copepods, ostracods and chironomid larvae (mainly Microchironomus spp.). X. caudafasciata (only caught at 15 m) fed mostly on profundal chironomid larvae (Tanypodinae and Microchironomus spp.). Trophic niche overlap between X. longispinis and X. caudafasciata appeared to be reduced through different activity patterns. Few intraspecific differences in food diet were found between sex and age groups.

Sediment Colonization by Littoral and Profundal Insects

The colonization rates of insects onto disturbed animal-free sediment were measured at profundal and littoral stations in a Canadian Shield lake. Sediments were exposed in containers to potential colonizers for up to one month in the early autumn. Most of the colonizing taxa at both stations were larvae of Chironomidae (Diptera). A simple model provided a reasonable fit to the colonization dynamics of many of these taxa. The colonization rates of profundal chironomids were very slow or negligible, and densities on the introduced sediments remained far below those on surrounding sediments at the end of the experiment. Larvae of the phantom midge Chaoborus punctipennis colonized profundal sediments rapidly as a consequence of their diel vertical migrations between the water column and the sediments. Littoral taxa varied widely in the rate at which they colonized the sediments; some taxa reached densities measured on surrounding, non-disturbed sediments within a few weeks. Species living on the sediment surface appeared to colonize the containers most rapidly. The colonization rate of a taxon was directly related to its abundance in the surroundings. Estimated apparent steady state densities of colonizing taxa were either equivalent to or higher than densities on surrounding sediments.

Mercury in Macroinvertebrates from Swedish Forest Lakes: Influence of Lake Type, Habitat, Life Cycle, and Food Quality

Mercury concentrations in lacustrine macroinvertebrates were concurrently studied in eight remote Swedish forest lakes of differing dystrophy, acidity, and eutrophy. The aim was to assess the influence of ecological factors (taxon, habitat, and feeding habits), chemical factors (characterizing different types of lakes), and regional and climatic factors (Hg deposition and mean temperature) on the accumulation of Hg. Concentrations varied from &lt;50 to &gt;5000 ng Hg∙g dry weight−1. A large part of this high variability could be ascribed to differences in water and sediment chemistry, ecological niches, and species-specific seasonalities. Both taxonomic composition and Hg concentrations were highly dependent on the type of lake. Concentrations were highest in acidic dystrophic lakes and lowest in oligotrophic lakes. Mean Hg concentrations in the examined taxa within a lake differed 100-fold. Contrary to widely held views on biomagnification, the lowest concentrations among profundal chironomids were found in predators whereas the highest concentrations occurred in detritivores. Seasonal variations were negligible in some taxa but considerable in others and appeared in some taxa to be related to the life cycle. We conclude that Hg accumulation in macroinvertebrates is largely determined by feeding behaviour and food quality.

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.