DISTRIBUTION OF THE GENUS CYCLOTELLA IN RELATION TO pH IN PRECAMBRIAN SHIELD LAKES: IMPLICATIONS FOR DIATOM-INFERRED pH1

Recent research on relationships between diatoms and pH suggests that the genus Cyclotella exhibits a strong relationship with lake acidity, being almost totally absent below pH 5.5. This decline has been used as an indicator of lake acidification in paleolimnological studies. In this study C. stelligera V.H. and C. kützingiana Thwaites were abundant in Precambrian Shield lakes with pH as low as 4.5. Cyclotella comta (Ehr.) Kütz. was found in lakes of pH < 5.5, but maximum abundance was observed in lakes of pH > 5.5. Cyclotella michiganiana Skv. was found in lakes of pH < 6.0. These results indicate that the use of C. stelligera, C. kützingiana, and possibly C. comta, in paleolimnological investigations of lake acidification, should be approached with caution. These taxa may exhibit a decline in abundance with decreasing lakewater pH, but this is partially a morphometric effect not necessarily related to anthropogenic acidification.     

Fish predation,lake acidity and the composition of aquatic insect assemblages

Aquatic insect assemblages were sampled in 2 sets of 18 small lakes in 2 regions of northeastern Ontario. Both sets included lakes with and without fish. In the set near Sudbury, fishless lakes were acidic. Using a standardized sweep net procedure, fishless lakes in both areas were found to have a greater abundance and richness of insects than lakes with fish. Irrespective of pH, fishless lakes supported a similar aquatic insect assemblage which was characterized by an abundance of nekton, especially Notonectidae, Corixidae, Graphoderus liberus (Dytiscidae) and Chaoborus americanus (Chaoboridae). Those taxa were typically absent from lakes with fish, which often had a marked abundance of Gerridae. It is concluded that fish predation is the most immediate factor structuring such aquatic insect assemblages, and is responsible for their change coincident with lake acidification.     

Macroinvertebrates as indicators of fish absence in naturally fishless lakes

1. Little is known about native communities in naturally fishless lakes in eastern North America, a region where fish stocking has led to a decline in these habitats. 2. Our study objectives were to: (i) characterise and compare macroinvertebrate communities in fishless lakes found in two biophysical regions of Maine (U.S.A.): kettle lakes in the eastern lowlands and foothills and headwater lakes in the central and western mountains; (ii) identify unique attributes of fishless lake macroinvertebrate communities compared to lakes with fish and (iii) develop a method to efficiently identify fishless lakes when thorough fish surveys are not possible. 3. We quantified macroinvertebrate community structure in the two physiographic fishless lake types (n = 8 kettle lakes; n = 8 headwater lakes) with submerged light traps and sweep nets. We also compared fishless lake macroinvertebrate communities to those in fish‐containing lakes (n = 18) of similar size, location and maximum depth. We used non‐metric multidimensional scaling to assess differences in community structure and t‐tests for taxon‐specific comparisons between lakes. 4. Few differences in macroinvertebrate communities between the two physiographic fishless lake types were apparent. Fishless and fish‐containing lakes had numerous differences in macroinvertebrate community structure, abundance, taxonomic composition and species richness. Fish presence or absence was a stronger determinant of community structure in our study than differences in physical conditions relating to lake origin and physiography. 5. Communities in fishless lakes were more speciose and abundant than in fish‐containing lakes, especially taxa that are large, active and free‐swimming. Families differing in abundance and taxonomic composition included Notonectidae, Corixidae, Gyrinidae, Dytiscidae, Aeshnidae, Libellulidae and Chaoboridae. 6. We identified six taxa unique to fishless lakes that are robust indicators of fish absence: Graphoderus liberus, Hesperocorixa spp., Dineutus spp., Chaoborus americanus, Notonecta insulata and Callicorixa spp. These taxa are collected most effectively with submerged light traps. 7. Naturally fishless lakes warrant conservation, because they provide habitat for a unique suite of organisms that thrive in the absence of fish predation.     

Species abundance models and patterns in dragonfly communities: effects of fish predators

We investigated if dragonfly larvae community composition and species abundance curves are sensitive to variation in predation intensity, and whether the fit to a particular niche partitioning model could be used to make inferences about mechanisms structuring communities. The approach taken was to compare communities in lakes either having or lacking fish predation. Dragonfly species classified as active, strongly dominated the dragonfly communities in fishless lakes, and low active species dominated fishless lakes. As activity level is known to correlate with susceptibility to fish predation this indicates that these communities are structured by fish predation. Fitting relative abundance data to five niche partitioning models showed that the same model fitted data from both types of habitats (fish/no fish). This means that the observed differences in relative abundances were substitutive, i.e. the relative abundance of a rank stayed constant, even though the identity of the species having this rank changed. The best fit to data from both types of lakes was found for the random assortment model, which is usually interpreted as an indication that the community is not structured by within-guild interactions. This interpretation for fishless lakes did not seem to agree with other community measures (i.e. lowered diversity and evenness and no relationship between species richness and dragonfly biomass), which indicate that the community is structured by within-guild interactions. Moreover, a detail in the fitting procedure, the number of species included in the analysis, affected which model that fitted data best. Thus, we question if fitting niche partitioning models to data can provide mechanistic understanding of how resources are partitioned in natural communities.     

Benthic fauna in a series of lakes displaying a gradient of pH

The benthic fauna was examined in a series of four isolated headwater lakes, displaying a pH gradient of 5.4–7.0. A slight reduction in number of taxa present occurred below pH 6, with the fauna below 3 m dominated by the Diptera. Although epibenthic gastropods were rare, in contrast with European studies, Amphipoda, Ephemeroptera and Pisidium were common in the least buffered lake, which experiences spring pH values as low as 4.7. There were little relation between the pH or alkalinity, and the abundance, and biomass of the fauna at depths greater than 3 m. However, biomass of the littoral fauna increased significantly in the lakes with lower pH, as a result of an increase in large littoral species normally susceptible to fish predation.     

Cladocera assemblages from the surface sediments of south-central Ontario (Canada) lakes and their relationships to measured environmental variables

Fossil cladoceran remains preserved in surface sediment samples from 44 oligotrophic lakes in south-central Ontario were examined to evaluate the relationships between species assemblages and measured environmental variables. Differences in cladoceran assemblages were related to physical and chemical variables using multivariate techniques. Redundancy Analysis (RDA) identified five environmental variables as significantly influencing assemblage composition: sulphate (SO₄²⁻), calcium (Ca²⁺), pH, maximum lake depth (Z _max) and dissolved organic carbon (DOC). There was a distinct separation of lakes and taxa along the ion gradient based on SO₄, Ca and pH. Additionally, cladoceran communities in coloured, shallow lakes had relatively higher abundances of littoral chydorid species and the pelagic taxa Holopedium spp., and the Daphnia pulex complex. Deep, clear lakes had relatively higher abundances of other pelagic taxa. Predation by fish (measured as presence–absence) and Chaoborus (measured as density) were less significant than some of the physico-chemical variables in influencing cladoceran assemblage structure. However, this could be due to the limited resolution of the predation data that was available at the time of this study. The distribution of cladocerans in the surface sediment, and their relation to these important environmental variables, suggests that there is considerable potential for the use of sedimentary cladoceran remains as environmental indicators in south-central Ontario lakes. Handling editor: J. Saros     

Weedbeds and big bugs: the importance of scale in detecting the influence of nutrients and predation on macroinvertebrates in plant‐dominated shallow lakes

1. The scale of investigations influences the interpretation of results. Here, we investigate the influence of fish and nutrients on biotic communities in shallow lakes, using studies at two different scales: (i) within‐lake experimental manipulation and (ii) comparative, among‐lake relationships. 2. At both scales, fish predation had an overriding influence on macroinvertebrates; fish reduced macroinvertebrate biomass and altered community composition. Prey selection appeared to be size based. Fish influenced zooplankton abundance and light penetration through the water column also, but there was no indication that fish caused increased resuspension of sediment. 3. There were effects of nutrients at both scales, but these effects differed with the scale of the investigation. Nutrients increased phytoplankton and periphyton at the within‐lake scale, and were associated with increased periphyton at the among‐lake scale. No significant effect of nutrients on macroinvertebrates was observed at the within‐lake scale. However, at the among‐lake scale, nutrients positively influenced the biomass and density of macroinvertebrates, and ameliorated the effect of fish on macroinvertebrates. 4. Increased prey availability at higher nutrient concentrations would be expected to cause changes in the fish community. However, at the among‐lake scale, differences were not apparent in fish biomass among lakes with different nutrient conditions, suggesting that stochastic events influence the fish community in these small and relatively isolated shallow lakes. 5. The intensity of predation by fish significantly influences macroinvertebrate community structure of shallow lakes, but nutrients also play a role. The scale of investigation influences the ability to detect the influence of nutrients on the different components of shallow lake communities, particularly for longer lived organisms such as macroinvertebrates, where the response takes longer to manifest.     

Environmental factors influencing zooplankton species composition of lakes in north-central Ontario,Canada

To assess the relative importance of lake chemistry, morphometry and zoogeography on limnetic zooplankton, we collected zooplankton, water, and morphometric data from 132 headwater Canadian Shield lakes in 6 regions across north-central Ontario. A subset of these lakes (n = 52) were fished with gill nets. We clustered lakes based on their zooplankton species composition (presence/absence). Discriminant analysis was employed to determine how well lake characteristics could predict zooplankton community types. Correct classification of zooplankton communities for three models ranged from 72 to 91%. Lake size, lake location, and buffering capacity were ranked as the most important factors separating lake groups. Fish abundance (CPUE) was not significant in distinguishing between zooplankton communities. Though the range of lake sizes was limited (1–110 ha), larger lakes tended to support more species. Lake location (zoogeography) also influenced species composition patterns. Although Algoma lakes tended to be larger (\-x = 18.0 ha, other lakes \-x = 2.5 ha), they supported relatively depauperate zooplankton communities. Buffering capacity was ranked third in the discriminant analysis models, but pH and alkalinity were not significantly different between lake groups.     

Duckling response to changes in the trophic web of acidified lakes

We reared American Black Duck (Anas rubripes Brewster) and Common Goldeneye (Bucephala clangula Linnaeus) ducklings on two Quebec laurentian lakes in which we manipulated brook trout populations (Salvelinus fontinalis Mitchill), lake acidity and lake productivity to relate waterfowl foraging to trophic status of lakes. We developed a preliminary model to assess the effects of lake acidity and productivity, fish predation and interspecific fish/duck competition in relation to available food (aquatic invertebrates). We then validated the model using a factorial analysis of the relationships between the variables pertaining to the diet of the fish and ducklings, and the environmental characteristics of the lakes (acidity, biological production and fish predation).The first factorial axis can be interpreted in terms of biological productivity, while the second axis illustrates the effect that fish have on the quantity and type of food available to ducklings. Two different trends appear to occur depending on whether the carrying capacity of the lake is reduced by acidification of the water or increase through liming or fertilization. In the first case, fish predation appears to have a marked effect on available food, whereas in the second case, interspecific fish/duck competition is apparently to blame for changes in the diet of ducklings. In both instances, but to a lesser extent, fish compete increasingly (exploitation and/or interference) with the ducklings, forcing them to feed to a greater extent in riparian sites that are less accessible to fish.     

RELATIONSHIPS OF SEDIMENTED DIATOM SPECIES (BACILLARIOPHYCEAE) TO ENVIRONMENTAL GRADIENTS IN DILUTE NORTHERN NEW ENGLAND LAKES1

Limnological gradients of small, oligotrophic, and low conductance lakes in northern New England were defined by principal components analysis; relationships of sedimented diatom species to the gradients were investigated by correlation analysis. Diatom distributions were most strongly related to the gradient of pH and alkalinity and the covarying variables, conductance, Mg, Ca, total Al, and exchangeable Al. Weaker relationships to lake morphology, dissolved organic carbon and water color, altitude and marine aerosol inputs, and the distinctive water chemistry of some New Hampshire lakes were also present. Results for 16 taxa of importance in our studies of lake acidity are given in detail and are compared to results from other regions of eastern North America. Planktonic taxa were absent below pH 5.5, with the exception of the long form of Asterionella ralfsii var. americana Korn. The two forms of this taxon differed ecologically: the long form (>45μm) had an abundance weighted mean (AWM) pH 4.90 and occurred mostly in lakes that were deep relative to transparency; the short form (<45μm)had an AWM pH and occurred on lakes that were shallow relative to transparency. The ecological advantage of a “splitter” approach to diatom taxonomy was demonstrated by examination of other taxa as well, including Tabellaria flocculosa (Roth) Kütz. These results have important implications for paleolimnological interpretations.     

Ecological impact of introduced fish in high altitude lakes: a case of study from the European Alps

Fish introduction is a major threat to alpine lake biota leading to the loss of native species and to the degeneration of natural food-webs. This study provides an extensive investigation on the impact of the introduced fish Salvelinus fontinalis on the native communities of alpine lakes in the Gran Paradiso National Park. We compared the macroinvertebrate and zooplankton communities of six stocked and nine fishless lakes with a repeated sampling approach during the summers 2006–2009. The impact of fish presence on alpine lake fauna is often mediated by the strong seasonality governing these ecosystems, and it dramatically affects the faunal assemblage of littoral macroinvertebrates and the size, structure, and composition of the pelagic zooplankton community with a strong selective predation of the more visible taxa. Direct ecological impacts include a decrease or extinction of non-burrower macroinvertebrates and of large zooplankton species, while small zooplankton species and burrower macroinvertebrates were indirectly advantaged by fish presence. Due to the existence of a compensation between rotifers and crustaceans, fish presence does not affect total zooplankton biomass and diversity even if fish are a factor of ecological exclusion for large crustaceans. These compensatory mechanisms are a key process surrounding the impact of introduced fish in alpine lakes.     

Applicability of planktonic biomanipulation for managing eutrophication in the subtropics

Although large-bodied cladocerans such asDaphnia can reduce algal biomass significantly in temperate lakes if freed from fish predation, the applicability of such biomanipulation techniques for eutrophication management in the subtropics and tropics has been examined only recently. Subtropical cladoceran assemblages differ from those of temperate lakes by their low species richness, early summer gameogenesis, and greatly reduced body size. Eutrophic Florida lakes are dominated by pump-filter feeding fish rather than by size selective planktivores as a temperate lakes. Cladocerans in Florida lakes can increase in abundance significantly if freed from fish but fail to have an impact on algal biomass or composition. The greatest potential for using biomanipulation to manage phytoplankton-dominated lakes in the subtropics and tropics lies with phytophagous fish. Future research should concentrate on defining the role of individual fish taxa on phytoplankton composition and community structure, nutrient cycling, and planktonic productivity before embarking on whole lake manipulation projects.     

Effects of habitat complexity on community structure and predator avoidance behaviour of littoral zooplankton in temperate versus subtropical shallow lakes

1. Structural complexity may stabilise predator–prey interactions and affect the outcome of trophic cascades by providing prey refuges. In deep lakes, vulnerable zooplankton move vertically to avoid fish predation. In contrast, submerged plants often provide a diel refuge against fish predation for large‐bodied zooplankton in shallow temperate lakes, with consequences for the whole ecosystem. 2. To test the extent to which macrophytes serve as refuges for zooplankton in temperate and subtropical lakes, we introduced artificial plant beds into the littoral area of five pairs of shallow lakes in Uruguay (30°–35°S) and Denmark (55°–57°N). We used plants of different architecture (submerged and free‐floating) along a gradient of turbidity over which the lakes were paired. 3. We found remarkable differences in the structure (taxon‐richness at the genus level, composition and density) of the zooplankton communities in the littoral area between climate zones. Richer communities of larger‐bodied taxa (frequently including Daphnia spp.) occurred in the temperate lakes, whereas small‐bodied taxa characterised the subtropical lakes. More genera and a higher density of benthic/plant‐associated cladocerans also occurred in the temperate lakes. The density of all crustaceans, except calanoid copepods, was significantly higher in the temperate lakes (c. 5.5‐fold higher). 4. Fish and shrimps (genus Palaemonetes) seemed to exert a stronger predation pressure on zooplankton in the plant beds in the subtropical lakes, while the pelagic invertebrate Chaoborus sp. was slightly more abundant than in the temperate lakes. In contrast, plant‐associated predatory macroinvertebrates were eight times more abundant in the temperate than in the subtropical lakes. 5. The artificial submerged plants hosted significantly more cladocerans than the free‐floating plants, which were particularly avoided in the subtropical lakes. Patterns indicating diel horizontal migration were frequently observed for both overall zooplankton density and individual taxa in the temperate, but not the subtropical, lakes. In contrast, patterns of diel vertical migration prevailed for both the overall zooplankton and for most individual taxa in the subtropics, irrespective of water turbidity. 6. Higher fish predation probably shapes the general structure and dynamics of cladoceran communities in the subtropical lakes. Our results support the hypothesis that horizontal migration is less prevalent in the subtropics than in temperate lakes, and that no predator‐avoidance behaviour effectively counteracts predation pressure in the subtropics. Positive effects of aquatic plants on water transparency, via their acting as a refuge for zooplankton, may be generally weak or rare in warm lakes.     

Effects of temperature, salinity and fish in structuring the macroinvertebrate community in shallow lakes: implications for effects of climate change

Climate warming may lead to changes in the trophic structure and diversity of shallow lakes as a combined effect of increased temperature and salinity and likely increased strength of trophic interactions. We investigated the potential effects of temperature, salinity and fish on the plant-associated macroinvertebrate community by introducing artificial plants in eight comparable shallow brackish lakes located in two climatic regions of contrasting temperature: cold-temperate and Mediterranean. In both regions, lakes covered a salinity gradient from freshwater to oligohaline waters. We undertook day and night-time sampling of macroinvertebrates associated with the artificial plants and fish and free-swimming macroinvertebrate predators within artificial plants and in pelagic areas. Our results showed marked differences in the trophic structure between cold and warm shallow lakes. Plant-associated macroinvertebrates and free-swimming macroinvertebrate predators were more abundant and the communities richer in species in the cold compared to the warm climate, most probably as a result of differences in fish predation pressure. Submerged plants in warm brackish lakes did not seem to counteract the effect of fish predation on macroinvertebrates to the same extent as in temperate freshwater lakes, since small fish were abundant and tended to aggregate within the macrophytes. The richness and abundance of most plant-associated macroinvertebrate taxa decreased with salinity. Despite the lower densities of plant-associated macroinvertebrates in the Mediterranean lakes, periphyton biomass was lower than in cold temperate systems, a fact that was mainly attributed to grazing and disturbance by fish. Our results suggest that, if the current process of warming entails higher chances of shallow lakes becoming warmer and more saline, climatic change may result in a decrease in macroinvertebrate species richness and abundance in shallow lakes.     

The Diatoms,Productivity and Morphometry of 43 Lakes in New York State,U.S.A

Diatoms were examined in surface sediments collected from 43 lakes throughout New York State to determine the interrelationships among the abundances of various diatom taxa,' lake productivity and lake morphometry. Diatom ratios, including the A/C, C/P and five new quotients, were not well correlated with lake productivity, probably because of the diversity of the taxa which they employ. The abundance of individual genera or combinations of genera were better indicators of productivity. Cyclotella was the best single indicator of productivity being most abundant in low productivity lakes. Several fairly distinct diatom associations were identified by correlations between all possible pairs of diatom taxa. Finally, lake productivity was found to be significantly correlated with morphometry, being highest in small/shallow lakes.     

Predation and refugia: implications for Chaoborus abundance and species composition

1. Previous studies have suggested that the occurrence of larval Chaoborus in lakes may be affected by fish predation, pH, elevation, temperature, nutrient level, water transparency and interspecific competition, but so far, a detailed statistical evaluation of these findings has not been performed. 2. The aim of this study was to apply regression and ordination techniques to a large data set of 56 lakes in order to test which variables related to lake morphology, water chemistry, and fish predation determine (1) the abundance of individual Chaoborus species and (2) their species composition. 3. Individual Chaoborus species were influenced by very different sets of environmental factors. Nutrient levels positively affected the largest species, Chaoborus americanus, which was restricted to fishless lakes. Abundance of the smallest and most transparent species, C. punctipennis, seemed to be controlled more by the larger Chaoborus species than by fish. Larger chaoborids required low water clarity in order to co‐exist with fish, probably to increase refuge availability. Generally, small lakes (for C. flavicans/C. trivittatus) and shallow lakes (for C. punctipennis) supported higher abundances of Chaoborus.     

Can lake restoration by fish removal improve the status of profundal macroinvertebrate assemblages?

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Mechanisms regulating zooplankton populations in a high-mountain lake

SUMMARY 1. We studied the seasonal succession of phyto- and zooplankton and the potential impact of predation by salmonids on zooplankton population dynamics in a high-mountain Swiss lake.
2. A comparison of patterns in the abundance, body length, fecundity and age structure in the Daphnia galeata population strongly suggests that trout predation had little impact on the population and was not the cause for a decline in summer.
3. The dominance in the lake of adult trout that feed mainly on benthic prey may buffer the effect of predation on the larger zooplankton. Further, the relatively high amount of phytoplankton after spring thaw could be important for sustaining the Daphnia population under moderate fish predation.
4. Partial correlation analyses proved circumstantial evidence for both exploitative and interference competition between some zooplankton taxa. D. galeata depressed performance of other plankton species through exploitative competition.
5. Our study shows that the impact of fish on zooplankton in high-mountain lakes depends strongly on food web structure and trophic state of the lake. Where fish predation is weak, invertebrate predation combined with competition for food may be responsible for the dominance of large-bodied zooplankton species.     

Subtle top‐down control of a freshwater meiofaunal assemblage by juvenile fish

1. Top‐down control of prey assemblages by fish predators has been clearly demonstrated in lakes (for zooplankton prey) and rivers (for macroinvertebrate prey). Fish predation can have a significant impact on the body size of prey assemblages; often large‐bodied prey are reduced in abundance, and indirect facilitation of small‐bodied prey occurs potentially initiating a trophic cascade. 2. Benthic communities in aquatic ecosystems also include a numerous and functionally important meiofaunal‐sized component, but in freshwaters the impact of fish predation on meiofaunal assemblages is unknown. We used a laboratory microcosm study to explore the impact of juvenile fish predation on the abundance and size structure of a riverine meiofaunal assemblage. 3. The presence of fish in our microcosms had no significant effect on overall meiofaunal (temporary and permanent) abundance. However, for the Copepoda, we found the first evidence of top‐down control of freshwater meiofaunal assemblages; in microcosms with juvenile fish, the abundance of large‐bodied Copepoda was significantly reduced, whereas small‐bodied Copepoda were significantly more abundant suggesting indirect facilitation. 4. We conclude that predation by juvenile fish can alter the structure of freshwater meiofaunal assemblages, although we do not yet know whether these relatively subtle changes are overwhelmed by large‐scale events such as flow disturbances.