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.     

Benthic nematodes in acidified lakes: case of a neglected grazer

Abundance and composition of nematode fauna were examined in the benthic microbial mats and upper sediment layer of the littoral of acidified (pH 4.6 to 5.7) lakes. Nematodes constituted from 58% to 90% of all the invertebrates present (excluding protozoans and rotifers). In the examined material, the majority of nematodes was represented by 3 taxa of which Ironus was found specifically associated with the mats. It was concluded that the persistence of benthic mats may be linked to the metabolic activity of the associated nematode fauna.     

Movement of plankton through lake-stream systems

1. River plankton are often assumed to come from upstream lakes, but the factors controlling the movement of plankton between lakes and rivers into outflow streams are unclear. We tested the possibility that the physical structure of the littoral zone near the lake outlet (depth, presence of macrophytes) and diurnal differences in plankton composition at the lake surface influence the movement of plankton from the lake into the stream and determine their persistence downstream. 2. Zooplankton and phytoplankton biomass, community composition and mean body size were compared between two deep lakes without macrophytes at the lake edge and two shallow lakes with macrophytes at the lake edge. Samples were collected day and night on three dates, in the lake centre, in the littoral zone adjacent to the lake outlet, at the outlet and at two sites downstream in Algonquin Park, Ontario, Canada. 3. The morphology of lake edges clearly affects the movement of lake zooplankton into outlet streams. Outlets draining deeper littoral zones had higher zooplankton biomass than shallow littoral outlets (P < 0.0001), but these differences disappeared within 50 m downstream of the lake. There was no difference in mean zooplankton body size among lake outlets or between littoral and outlet samples. However, shallow littoral zones were dominated by cyclopoid copepods and deeper littoral zones were dominated by Bosmina longirostris. In contrast, phytoplankton biomass entering the outlet was similar to that found within the lake and did not vary with lake outlet morphology. These effects were consistent across several sampling weeks and were not affected by surface zooplankton biomass changes associated with diurnal vertical migration in the lake centre. 4. A comparison with published river zooplankton data suggests that zooplankton are rapidly eliminated from shallow outlet streams (≤1 m deep) but persist in most deeper outlet rivers (≥2 m deep). Because the depth of an outlet river determines downstream zooplankton community development, the contribution of lakes to river plankton communities may be influenced by the location of each lake within the drainage basin. These findings suggest that lake and outflow physical structure influences connection strength between spatially successive habitats.     

The effect of anthropogenic acidification on the hydrofauna of the lakes of the West Tatra Mountains (Slovakia)

Fourteen West Tatra lakes were studied, of which one could be considered to be recently anthropogenically acidified and eight others classified as acidification-endangered. In the anthropogenically acidified lake, the zooplankton assemblage has been substantially altered (three mountain-lake crustacean species have been eliminated). Several littoral macrobenthic species sensitive to acidification have either been eliminated from the acidified and acidification-endangered lakes or occur only sporadically. The effect of acidification has so far not been observed on the benthic fauna of the lakes medial which is probably due to the higher pH below the surface of sediment. In comparison with the High Tatra, acid depositions have had a less pronounced effect on the lakes of the West Tatra.     

Chironomid fauna of the lakes from the Pechora river basin (east of European part of Russian Arctic): Ecology and reconstruction of recent ecological changes in the region

We investigated chironomid fauna of surface sediments and a short sediment core (Bol’shoy Kharbey Lake) from Pechora river basin, Northern Russia. Twenty three investigated lakes have thermokarst, glacial or floodplain origin and are characterised by low mineralization, mostly hydrocarbon-calcium type of water and low concentration of nutrients. Most of the lakes have circumneutral pH around ≤7 and only two lakes are slightly more acidic with pH ≤ 6. Ninety six chironomid taxa were identified in the surface sediments. Distribution of chironomids in the studied region is driven by continentality, mean T_July and рН. Chironomid communities from the core of the B. Kharbei Lake demonstrate the highest similarity with the fauna of the deeper lakes of the glacial origin. The glacial lakes have the highest indices of continentality and the lowest winter temperatures within the investigated data set. The chironomid fauna of the glacial lakes is composed of the profundal, oligotrophic and cold-stenotherm taxa. The fauna of the floodplain and thermokarst lakes is more closely related to T_July and is composed of littoral and phytophilic taxa of meso–or eutrophic waters and moderate temperature conditions. The fauna of the acidic thermokarst lakes considerably differs from the other lakes. Chironomid communities here are represented by tolerant to acidification taxa, and by the typically littoral and shallow water acid-tolerant taxa that apparently also can tolerate acidification. Studied sediment record covers ca last 200 years. The reconstructed T_July during the entire period remain slightly below the modern temperatures. From 1970 reconstructed T_July shows steady increase to the modern level. The reconstructed water depths (WDs) of the lake are higher than today till 1980. The highest WDs are reconstructed for ca 1970. After that the WDs gradually decrease to the modern level. Changes of the WDs are most probably related to changes in the precipitation rate.     

Littoral zone fish communities of two Florida lakes and a comparison with Michigan lakes

Synopsis The abundance and habitat distribution of littoral zone fishes in two small southern Florida lakes were quantified by underwater censuses. The bluegill (Lepomis macrochirus) and large-mouth bass (Micropterus salmoides) accounted for 75–80% of community biomass in both lakes; important coexisting species were predominantly benthic feeders in Lake Sirena and planktivores in Lake Annie. All species were largely confined to areas of macrovegetation which extended to a depth of 4 m in Lake Annie but only to 1.2 m in Lake Sirena. The differences in community structure were related to differences in habitat and also variation in water levels and benthic production.A comparison of community composition with that of small Michigan lakes indicated that similar numbers of species coexist in the littoral zones, despite a three-fold difference in the size of regional species pools. The majority of families and about 50% of the species were common to lakes in both regions; in addition a number of ecological analogues were noted. A major difference was that the small inshore species were members of the Cyprinodontiformes in Florida and Cyprinidae in Michigan. Together the largemouth bass and bluegill comprise similar community proportions in the two regions as do other major feeding groups. Lakes in the two regions that are similar in amount and distribution of vegetation exhibit greater similarity in fish communities than those within regions that differ in littoral vegetation.     

Acidification of a Small Watershed in Southern Norway and Some Characteristics of Acidic Aquatic Environments

Continuous monitoring of a small watershed in southern Norway shows that pH has decreased significantly by almost half a unit (from 5.9 to 5.4), conductivity increased from 23 to 29 μS/cm within the period from 1965–1975. Changes are also detected in sediment analyses where Daphnia and Chaoborus are only found deeper than 3–4 cm. Forest clear-cutting activities have probably accelerated the acidification processes of the already highly vulnerable freshwater localities. The acidic lakes show a solute content below 0.4 meq/1, and the bicarbonate buffer system may no longer work efficiently. Spring thaw and autumn rain episodes are accompanied by strong alternations in the chemical environment of the lakes; by considerable drop in pH and increasing conductivity and sulphate content. With increasing acidification the planktonic community changes from a fish predation governed system (brown trout and Eurasian perch as dominant species) to an invertebrate predation governed one with dominant predators Heterocope and pelagic corixids. Pelagic zooplankton are primarily composed of littoral species: Bosmina longispina, Diaphanosoma brachyurum, Eudiaptomus gracilis and Heterocope saliens. The littoral plant and invertebrate communities are described and discussed.     

The impact of water-level regulation on littoral macroinvertebrate assemblages in boreal lakes

Regulation of lake water level for power production and flood control is among the major anthropogenic disturbances in boreal aquatic ecosystems. In Finland, over 300 lakes, representing one third of the total inland water area of the country, are artificially regulated. To study the effects of regulation on lake littoral macroinvertebrate communities, samples were taken from upper stony littoral and from lower soft bottom littoral habitats of 11 lakes with different regulation amplitudes (wintertime fall in water level 1.19–6.75 m). Twelve unregulated (wintertime fall in water level 0.11–0.55 m) lakes with otherwise similar characteristics were used as a reference. Non-metric Multidimensional Scaling ordinations showed that the composition of macroinvertebrate assemblages was strongly associated with the amplitude of water level regulation. Taxon richness also decreased with increasing intensity of regulation. Freezing and flushing of sediments in late winter are probably the most important factors leading to the impoverished littoral macroinvertebrate fauna. Invertebrates with long life cycle seem to be particularly vulnerable to unnatural water level fluctuation. Our results show that regulation of water level has a major impact on functionally significant lake littoral macroinvertebrates.     

Community resistance and change to nutrient enrichment and fish manipulation in a vegetated lake littoral

1. High biomass of macrophytes is considered important in the maintenance of a clear‐water state in shallow eutrophic lakes. Therefore, rehabilitation and protection of aquatic vegetation is crucial to the management of shallow lakes. 2. We conducted field mesocosm experiments in 1998 and 1999 to study community responses in the plant‐dominated littoral zone of a lake to nutrient enrichment at different fish densities. We aimed to find the threshold fish biomass for the different nutrient enrichment levels below which large herbivorous zooplankton escapes control by fish. The experiments took place in the littoral of Lake Vesijärvi in southern Finland and were part of a series of parallel studies carried out jointly at six sites across Europe. 3. In 1998, when macrophyte growth was poor, a clear‐water state with low phytoplankton biomass occurred only in unenriched mesocosms without fish or with low fish biomass (4 g fresh mass m^?2). Both nutrient enrichment and high fish biomass (20 g fresh mass m^?2) provoked a turbid water state with high planktonic and periphytic algal biomass. The zooplankton community was dominated by rotifers and failed to control the biomass of algae in nutrient enriched mesocosms. The littoral community thus had low buffer capacity against nutrient enrichment. 4. In 1999, macrophytes, especially free‐floating Lemna trisulca L., grew well and the zooplankton community was dominated by filter‐feeding cladocerans. The buffer capacity of the littoral community against nutrient enrichment was high; a clear‐water state with low phytoplankton biomass prevailed even under the highest nutrient enrichment. High grazing rates by cladocerans, together with reduced light penetration into the water caused by L. trisulca, were apparently the main mechanisms behind the low algal biomass. 5. Effects of fish manipulations were less pronounced than effects of nutrient enrichment. In 1999, clearance rates of cladocerans were similar in fish‐free and low‐fish treatments but decreased in the high‐fish treatment. This suggests that the threshold fish biomass was between the low‐ and high‐fish treatments. In 1998, such a threshold was found only between fish‐free and low‐fish treatments. 6. The pronounced difference in the observed responses to nutrient enrichment and fish additions in two successive years suggests that under similar nutrient conditions and fish feeding pressure either clear or turbid water may result depending on the initial community structure and on weather.     

Food supply and the effects of recreational disturbance on the abundance and distribution of wintering Pochard on a gravel pit complex in southern Britain

The Cotswold Water Park, an area of over 100 flooded gravel pits in south central Britain, supports Nationally Important numbers of wintering Pochard Aythya ferina associated with abundant Stoneworts Chara spp. Based on extensive presence/absence data and intensive biomass sampling, the submerged macrophyte communities were surveyed. Charophytes were most frequent and developed highest biomass at water depths below 3 m. Counts of wintering Pochard showed that water-based recreational activity displaced birds from lakes. Analysis of bird density showed significantly higher use of reserves with restricted bankside access than lakes where angling, walking or other bank-side activities were permitted. These in turn supported higher Pochard densities than lakes with water-based recreation.     

Physical forces constrain the depth distribution of the abundant native mussel Elliptio complanata in lakes

1. Unionid mussels often account for a large portion of benthic biomass and contribute to nutrient cycling and sediment processes, but are thought to be limited to shallow areas (<2–3 m). 2. The depth distribution and body size of Elliptio complanata were compared in seven Canadian Shield lake basins of different sizes to test what factors determine the upper and lower limit of their depth range. Specifically, I tested whether (i) the upper range of their distribution is limited by exposure to winds and wave action and (ii) the lower range of their distribution is limited by the depth of the thermocline or by the boundary of mud deposition. 3. The average depth distribution of E. complanata shifted to greater depths in larger lake basins. When comparing individual transects, maximum mussel density was found deeper at more exposed sites. Mussel size decreased with increasing depth and was larger, on average, in larger lake basins. These results suggest that physical forces limit the upper range of mussel distribution in lakes. 4. The maximum depth at which mussels were found in different lakes was closely related to thermocline depth. However, mussels were commonly observed below the predicted depth of the mud deposition boundary. The thermocline limits the lower range of mussel distribution in lakes, probably by limiting food availability and by determining water temperature. Substratum type does not limit the lower distribution of mussels. 5. These results suggest that unionid mussels are present in the deeper parts of the littoral zone, especially in large lakes. Therefore, comparisons of mussel populations between sites and between lakes would be biased unless the full depth distribution of these mussels is considered. These results also suggest that long‐term changes in the thermal structure of lakes could affect the range of unionid mussel populations and their functional role in littoral ecosystems.     

Diel horizontal migration of zooplankton: costs and benefits of inhabiting the littoral

1. In some shallow lakes, Daphnia and other important pelagic consumers of phytoplankton undergo diel horizontal migration (DHM) into macrophytes or other structures in the littoral zone. Some authors have suggested that DHM reduces predation by fishes on Daphnia and other cladocerans, resulting in a lower phytoplankton biomass in shallow lakes than would occur without DHM. The costs and benefits of DHM, and its potential implications in biomanipulation, are relatively unknown, however. 2. In this review, we compare studies on diel vertical migration (DVM) to assess factors potentially influencing DHM (e.g. predators, food, light, temperature, dissolved oxygen, pH). We first provide examples of DHM and examine avoidance by Daphnia of both planktivorous (PL) fishes and predacious invertebrates. 3. We argue that DHM should be favoured when the abundance of macrophytes is high (which reduces planktivory) and the abundance of piscivores in the littoral is sufficient to reduce planktivores. Food in the littoral zone may favour DHM by daphnids, but the quality of these resources relative to pelagic phytoplankton is largely unknown. 4. We suggest that abiotic conditions, such as light, temperature, dissolved oxygen and pH, are less likely to influence DHM than DVM because weaker gradients of these conditions occur horizontally in shallow lakes relative to vertical gradients in deep lakes. 5. Because our understanding of DHM is rudimentary, we highlight potentially important research areas: studying a variety of systems, comparing temporal and spatial scales of DHM in relation to DVM, quantifying positive and negative influences of macrophytes, focusing on the role of invertebrate predation, testing the performance of cladocerans on littoral versus pelagic foods (quantity and quality), investigating the potential influence of temperature, and constructing comprehensive models that can predict the likelihood of DHM. Our ability to biomanipulate shallow lakes to create or maintain the desired clear water state will increase as we learn more about the factors initiating and influencing DHM.     

Seasonal fluctuations in macrophyte cover and water transparency of four brown-water lakes: implications for crustacean zooplankton in littoral and pelagic habitats

The dynamics of crustacean zooplankton in the littoral and pelagic zones of four forest lakes having variable water qualities (colour range 130–340 mg Pt l⁻¹, Secchi depth 70–160 cm) were studied. The biomass of zooplankton was higher in the littoral zone than in the pelagic zone only in the lake having the highest transparency. In the three other lakes, biomass was significantly higher in the pelagic zone than in the littoral zone. In the two lakes with highest transparency, the littoral biomass of cladocerans significantly followed the development of macrophyte vegetation, and cladoceran biomass reached the maximum value at the time of highest macrophyte coverage. In lakes with lowest transparency, littoral zooplankton biomass developed independently of macrophyte density and decreased when macrophyte beds were densest. The seasonal development of the littoral copepod biomass did not follow the development of macrophytes in any of the lakes. The mean size of cladocerans in the pelagic zone decreased with increasing Secchi depth of the lake, whereas in the littoral zone no such phenomenon was detected. Seasonally, when water transparency increased temporarily in two of the lakes, the mean size of cladocerans in the pelagic zone decreased steeply. For copepods, no relationship between water transparency and body size was observed. The results suggested that in humic lakes the importance of the littoral zone as a refuge decreases with decreasing transparency of the water and that low water transparency protects cladocerans from fish predation. All the observed between-lake differences could not be explained by fish predation, but were probably attributed to the presence of chaoborid larvae with variable densities. Feeding efficiency of chaoborids is not affected by visibility and thus they can obscure the relationship between water quality, fish density, and the structure of crustacean zooplankton assemblages. Handling editor: S. I. Dodson     

Benthic macroinvertebrate biomass and wildfires: evidence for enrichment of boreal subarctic lakes

1 We quantified the effects of forest fire on littoral benthic macroinvertebrate biomass on a boreal subarctic plateau in Alberta, Canada. Water chemistry and benthos were collected from six lakes, 1 and 2 years following a 1995 fire which burned about 91% of their catchments (i.e. recently burnt lakes), and from four other lakes whose catchments burned between 1961 and 1985 (i.e. previously burnt lakes). Seven lakes whose catchments had not burned since at least 1957 served as reference systems.
2 Total benthic macroinvertebrate biomass and biomass of Chironomidae were 1.5‐ and 2‐fold (P<0.05) greater within recently burnt lakes than in reference systems, whereas the biomass of Oligochaeta (P=0.06) and Amphipoda (P=0.07) were marginally higher in burnt than reference lakes. Burnt lakes had greater colour (P<0.05) and marginally (P=0.06) higher concentrations of soluble reactive phosphorus than reference lakes.
3 Nutrient diffusing substrata deployed in a previously and a recently burnt lake indicated that littoral epilithic communities were co‐limited by the availability of phosphorus (P) and nitrogen (N), although co‐limitation was stronger in the previously burnt than the recently burnt lake. Epilithic chlorophyll a on natural stone surfaces in the recently burnt lake was also 3.5 times higher (P<0.05) than that from the previously burnt lake.
4 Among all 17 lakes, total benthic biomass and biomass of Chironomidae, Amphipoda and Nematoda, were significantly (P<0.05) or marginally (P=0.06) related to soluble reactive phosphorus (SRP) but not dissolved inorganic nitrogen or colour. These regressions explained between 11% and 64% of variation in benthic biomasses.
5 Linear regressions and second‐order polynomials explained 18% and 24% of the variation in concentrations of SRP and water colour with time since fire, respectively, and between 22% and 70% of variation in total biomass and biomass of the five dominant invertebrate groups. These analyses suggest that benthic biomasses continue to be elevated for about 15–20 years following fire before declining to pre‐disturbance levels.     

Littoral microcrustaceans (Cladocera and Copepoda) as indices of recovery of a limed water system

A 7 year study (1992–1998) of littoral microcrustaceans (Cladocera and Copepoda) in the watercourse of the River Rore, South Norway, illustrates that qualitative data on cladocerans and copepods are well suited to indicate the recovery of lakes following liming. Eight limed, two acid and two circum neutral reference lakes, were sampled twice a year (June/July and September/October). In the limed lakes, species associated with neutral lakes have become more common, whereas apparently acid-tolerant species have become rarer. In Lake Rore and Lake Syndle, the two largest limed lakes which exhibited a gradual increase in pH, the changes in species composition indicated that these lakes were about to recover. Species composition in Lake Røynelandsvatn, which has reacidified after liming, first reflected improved water quality, then reverted to dominance by acid-tolerant species. In the remaining lakes, the species composition reflects a fauna which has recovered compared with the preliming situation. There is strong evidence, however, that temporary fluctuations in pH have a negative influence on the speed of recovery, confirming the importance of keeping pH stable.     

Structure, biomass, production and depth distribution of periphyton on artificial substratum in shallow lakes with contrasting nutrient concentrations

1. To examine how the vertical distribution of periphytic biomass and primary production in the upper 0–1 m of the water column changes along an inter‐lake eutrophication gradient, artificial substrata (plastic strips) were introduced into the littoral zones of 13 lakes covering a total phosphorus (TP) summer mean range from 11 to 536 μg L^?1. Periphyton was measured in July (after 8 weeks) and September (after 15 weeks) at three water depths (0.1, 0.5 and 0.9 m). 2. Periphyton chlorophyll a concentration and dry weight generally increased with time and the communities became more heterotrophic. Mean periphytic biomass was unimodally related to TP, reaching a peak between 60 and 200 μg L^?1. 3. The proportion of diatoms in the periphyton decreased from July to September. A taxonomic shift occurred from dominance (by biovolume) of diatoms and cyanobacteria at low TP to dominance of chlorophytes at intermediate TP and of diatoms (Epithemia sp.) in the two most TP‐rich lakes. 4. The grazer community in most lakes was dominated by chironomid larvae and the total biomass of grazers increased with periphyton biomass. 5. Community respiration (R), maximum light‐saturated photosynthetic rate (P_max), primary production and the biomass of macrograzers associated with periphyton were more closely related to periphyton biomass than to TP. Biomass‐specific rates of R, P_max and production declined with increasing biomass. 6. Mean net periphyton production (24 h) was positive in most lakes in July and negative in all lakes in September. Net production was not related to the TP gradient in July, but decreased in September with increasing TP. 7. The results indicate that nutrient concentrations alone are poor predictors of the standing biomass and production of periphyton in shallow lakes. However, because periphyton biomass reaches a peak in the range of phosphorus concentration in which alternative states occur in shallow lakes, recolonisation by submerged macrophytes after nutrient reduction may potentially be suppressed by periphyton growth.     

Distribution of macroinvertebrates in relation to physical and biological variables in the littoral zone of nine New Zealand lakes

Macroinvertebrates play a key role in the littoral zone of lakes. Macroinvertebrate community composition is closely linked to habitat conditions. To date, there have been few attempts to relate macroinvertebrates to habitat factors in lakes. In this study, nine mainly oligotrophic lakes from throughout New Zealand were surveyed for macroinvertebrates. The lakes were selected to represent a range of suspended sediment loading and lake level regimes. Within each lake, several sites were selected to provide a range of exposure to wave action. A multiple regression approach was taken to relate macroinvertebrate community composition and habitat characteristics. The results of the analysis suggest that the littoral zone of the lakes we studied could be divided into four general habitats. The first is the wave wash zone characterised by coarse substrates and macroinvertebrate taxa usually associated with lotic environments, such as Ephemeroptera and Plecoptera. The second habitat is associated with macrophytes and is limited at the top by wave action and at depth by light attenuation. In this zone, the snail Potamopyrgus antipodarum is dominant, along with Trichoptera and Odonata. At the base of the macrophytes is the detrital habitat characterised by fine, organic rich sediments and dominated by chironomids, oligochaetes and Trichoptera. At depths below the macrophyte zone, fine sediments are found, and bivalves such as the freshwater mussel Hyridella menziesi are common. While macroinvertebrate abundance can be highly variable, some general predictions of community structure can be made based on a few key environmental factors. Abundance of snails Odonata and Trichoptera was positively related to macrophyte biomass. Some macroinvertebrate groups such as oligochaetes, chironomids, snails and bivalves were more common in fine substrates, while Ephemeroptera were characteristic of coarse substrates. Detrital biomass was important for most of the macroinvertebrate groups studied showing a positive relationship for oligochaetes and Trichoptera and a negative relationship for Ephemeroptera and Plecoptera.     

Depth Distribution and Abundance of the Common Bully, Gobiomorphus cotidianus (Eleotridae), in Three Oligotrophic New Zealand Lakes, One of which is Turbid

The depth distribution of the common bully, Gobiomorphus cotidianus, a small benthic forage fish, was measured by trapping at set depths from 0–70m in three large oligotrophic lakes, including one where inorganic sediment from a glacially-fed river produces turbid conditions. Bullies occurred at all depths from 0.5–70m in the clear lakes, but none were present below 25m in the turbid lake. Two groups of bullies were present in the clear lakes; a high-density, littoral stock at depths of 0.5–25m, and a low-density, profundal stock at depths of 30–70m. These groups were further distinguished by differing buoyancy requirements and feeding habits. The swimbladders of littoral bullies contained gas, but those of the profundal bullies, which fed more than littoral bullies by both day and night, did not. The variation in mean CPUE with depth within the littoral zone was not related to water temperature, oxygen concentration, or conductivity. Nor was it related to a reduction in light levels or to reduced water transparency caused by increased turbidity. It may therefore be controlled by biotic factors. The absence of a profundal stock below the littoral zone in the turbid lake indicates that the settlement of fines from turbid inputs may affect bully abundance in deeper waters. As conventional measures of the abundance of benthic fish in lakes are often restricted to littoral habitats, and do not reflect changes in abundance with depth, an index of overall abundance based on depth distribution was developed to allow comparisons between lakes.     

Acidity versus habitat structure as regulators of littoral microcrustacean assemblages

1. Emergence traps were set overnight on the sediment surface to sample the littoral microcrustaceans of 22 Canadian Shield lakes that ranged in pH from 4.56 to 6.92. Traps were randomly allocated in quintuplicate in both wave‐washed sandy habitats where pipewort (Eriocaulon septangulare) was the dominant macrophyte (termed pipewort habitats), and more protected habitats dominated by floating‐leaved macrophytes (termed floating‐leaved habitats). 2. In total, 50 cladoceran and 22 copepod species were found, with 16–45 species in each lake. Lakes that had never acidified exhibited a more diverse fauna than lakes that had acidified. 3. There were only minor differences between the numbers of species found in floating‐leaved versus pipewort habitats except for one lake. Non‐chydorid cladoceran, chydorids and copepods constituted 45%, 26% and 29% of the total number of individuals, respectively. 4. Based on presence/absence, dominance scores and frequency of occurrences of species, the microcrustacean faunal composition was similar in the two habitats. A detrended correspondence analysis confirmed that there was no separation between the two main types of habitat, and that pH was the parameter most strongly correlated with the dominant microcrustacean compositional gradient among lakes.     

Stocked trout have minimal effects on littoral invertebrate assemblages of productive fish‐bearing lakes: a whole‐lake BACI study

1. Rainbow Trout (Oncorhynchus mykiss [Walbaum]) is commonly stocked as a sport fish throughout the world but can have serious negative effects on native species, especially in headwater systems. Productive fish‐bearing lakes represent a frequently stocked yet infrequently studied system, and effects of trout in these systems may differ from those in headwater lakes. 2. We used a Before‐After Control‐Impact (BACI) design to determine how stocked trout affected assemblage‐level and taxon‐level biomass, abundance and average length of littoral invertebrates in a stocked lake relative to three unstocked control lakes in the boreal foothills of Alberta, Canada. Lakes were studied 1 year before and for 2 years after stocking. Because characteristics of productive fish‐bearing lakes should buffer impacts of introduced fish, we predicted that trout would not affect assemblage‐level structure of littoral invertebrates but might reduce the abundance or average length of large‐bodied taxa frequently consumed by trout. 3. Relative to the unstocked control lakes, biomass, but not abundance, of the littoral invertebrate assemblage was affected indirectly by trout through increases of some taxa after trout stocking. At the individual taxon‐level, trout stocking did not affect most (23 of the 27) taxa, with four taxa increasing in abundance or biomass after stocking. Only one taxon, Chironomidae, showed evidence of size‐selective predation by trout, being consumed frequently by trout and decreasing significantly in average length after stocking. 4. Our results contrast with the strong negative effects of trout stocking on invertebrate assemblages commonly reported from headwater lakes. A combination of factors, including large and robust native populations of forage fish, the generalised diet of trout, overwinter aeration, relatively high productivity and dense macrophyte beds, likely works in concert to reduce potentially negative effects of stocked trout in these systems. As such, productive, fish‐bearing lakes may represent a suitable system for trout stocking, especially where native sport fish populations are lacking.