Performance trade-off across a natural resource gradient

An important environmental factor determining both phytoplankton and zooplankton community composition is lake depth and thermal stratification. However, there is little information on how the interaction between zooplankton grazers and their phytoplankton food changes along an environmental gradient of lake depth. We contrasted resource availability for daphniid zooplankton populations living in two shallow, unstratified lakes and in two deep, stratified lakes using a novel growth bioassay. Stratified lakes had consistently lower resource richness than shallow unstratified lakes. To test whether resources were important in explaining differences in daphniid composition of shallow and deep lakes, we performed reciprocal transplant experiments. We raised daphniids typical of shallow (Ceriodaphnia reticulata) and deep (Daphnia dentifera) lakes in the resources from replicate shallow and deep lakes and monitored survival and reproduction. The two species exhibited a performance trade-off, measured by life table r and R ₀, across a gradient in natural resource richness. D. dentifera had higher relative fitness than C. reticulata when raised in the poorest resource environment from a deep lake. However, under richer resource conditions typical of shallow lakes, C. reticulata outperformed D. dentifera. We further created a gradient in natural resource quantity (by dilution) to test whether this trade-off in species relative fitness involved aspects of resource quality. No trade-off in species performance was evident across the dilution gradient, indicating that resource quality was important to the trade-off. We conclude that shifts in daphniid species composition along a gradient of lake depth involve an adaptive trade-off in ability to exploit rich versus poor resource quality. Received: 11 May 1998 / Accepted: 15 January 1999     

Feeding of fish in Lake Glubokoe

Lake Glubokoe is inhabited by 13 species of fish. The commonest are roach, bream, perch and ruff. Comparison of seine catches for 1950 and 1984 revealed a considerable decrease in the share of perch (from 31% to 2%) and ruff (from 18.5 to less than 0.1%) in 1984. The richest food resource in the lake is crustacean plankton. In summer, it is mostly consumed by roach. Estimation of the electivity shows that, unlike perch, roach selectively consumes small crustaceans (Bosmina and Polyphemus). Poor growth rate of all common species of fish points to their food limitation. In summer the diets of common species of fish are differentiated to a considerable extent. Of 36 pairs of combinations a high food overlap was found only in 2 cases: 1) O + perch and Leucaspius delineatus, 2) yearlings and adult roach. Low diet overlap in fish during intensive feeding in summer probably reflects a feedback between the volume and intensity of food competition. The populations of bream and ruff probably compete for chironomid larvae which inhabit the sublittoral, as perch, roach and Leucaspius delineatus probably do for large littoral insects. However, intraspecific competition appears to be more important than interspecific for the roach and bream populations (the two most numerous species in the lake).     

The ecology of the planktonic diatom Cyclotella and its implications for global environmental change studies

The fossil record of diatoms in lake sediments can be used to assess the effects of climate variability on lake ecosystems if ecological relationships between diatom community structure and environmental parameters are well understood. Cyclotella sensu lato taxa are a key group of diatoms that are frequently dominant members of phytoplankton communities in low‐ to moderate‐productivity lakes. Their relative abundances have fluctuated significantly in palaeolimnological records spanning over a century in arctic, alpine, boreal and temperate lakes. This suggests that these species are sensitive to environmental change and may serve as early indicators of ecosystem effects of global change. Yet patterns of change in Cyclotella species are not synchronous or unidirectional across, or even within, regions, raising the question of how to interpret these widespread changes in diatom community structure. We suggest that the path forward in resolving seemingly disparate records is to identify clearly the autecology of Cyclotella species, notably the role of nutrients, dissolved organic carbon and light, coupled with better consideration of both the mechanisms controlling lake thermal stratification processes and the resulting effects of changing lake thermal regimes on light and nutrients. Here we begin by reviewing the literature on the resource requirements of common Cyclotella taxa, illustrating that many studies reveal the importance of light, nitrogen, phosphorus, and interactions among these resources in controlling relative abundances. We then discuss how these resource requirements can be linked to shifts in limnological processes driven by environmental change, including climate‐driven change in lakewater temperature, thermal stratification and nutrient loading, as well as acidification‐driven shifts in nutrients and water clarity. We examine three case studies, each involving two lakes from the same region that have disparate trends in the relative abundances of the same species, and illustrate how the mechanisms by which these species abundances are changing can be deciphered. Ultimately, changes in resource availability and water clarity are key factors leading to shifts in Cyclotella abundances. Tighter integration of the autecology of this important group of diatoms with environmental change and subsequent alterations in limnological processes will improve interpretations of palaeolimnological records, and clarify the drivers of seemingly disparate patterns in fossil records showing widespread and rapid changes across the northern hemisphere.     

Long‐term impacts of invasive species on a native top predator in a large lake system

1. Declining abundances of forage fish and the introduction and establishment of non‐indigenous species have the potential to substantially alter resource and habitat exploitation by top predators in large lakes. 2. We measured stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) in field‐collected and archived samples of Lake Ontario lake trout (Salvelinus namaycush) and five species of prey fish and compared current trophic relationships of this top predator with historical samples. 3. Relationships between δ¹⁵N and lake trout age were temporally consistent throughout Lake Ontario and confirmed the role of lake trout as a top predator in this food web. However, δ¹³C values for age classes of lake trout collected in 2008 ranged from 1.0 to 3.9‰ higher than those reported for the population sampled in 1992. 4. Isotope mixing models predicted that these changes in resource assimilation were owing to the replacement of rainbow smelt (Osmerus mordax) by round goby (Neogobius melanostomus) in lake trout diet and increased reliance on carbon resources derived from nearshore production. This contrasts with the historical situation in Lake Ontario where δ¹³C values of the lake trout population were dominated by a reliance on offshore carbon production. 5. These results indicate a reduced capacity of the Lake Ontario offshore food web to support the energetic requirements of lake trout and that this top predator has become increasingly reliant on prey resources that are derived from nearshore carbon pathways.     

Use of cover habitat by bull trout, <Emphasis Type="Italic">Salvelinus confluentus</Emphasis>, and lake trout, <Emphasis Type="Italic">Salvelinus namaycush</Emphasis>, in a laboratory environment

Lacustrine-adfluvial bull trout, Salvelinus confluentus, migrate from spawning and rearing streams to lacustrine environments as early as age 0. Within lacustrine environments, cover habitat provides refuge from potential predators and is a resource that is competed for if limiting. Competitive interactions between bull trout and other species could result in bull trout being displaced from cover habitat, and bull trout may lack evolutionary adaptations to compete with introduced species, such as lake trout, Salvelinus namaycush. A laboratory experiment was performed to examine habitat use and interactions for cover by juvenile (i.e., <80 mm total length) bull trout and lake trout. Differences were observed between bull trout and lake trout in the proportion of time using cover (F _1,22.6 = 20.08, P < 0.001) and bottom (F _1,23.7 = 37.01, P < 0.001) habitat, with bull trout using cover and bottom habitats more than lake trout. Habitat selection ratios indicated that bull trout avoided water column habitat in the presence of lake trout and that lake trout avoided bottom habitat. Intraspecific and interspecific agonistic interactions were infrequent, but approximately 10 times greater for intraspecific interactions between lake trout. Results from this study provide little evidence that juvenile bull trout and lake trout compete for cover, and that species-specific differences in habitat use and selection likely result in habitat partitioning between these species.     

Habitat coupling in a large lake system: delivery of an energy subsidy by an offshore planktivore to the nearshore zone of Lake Superior

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Genomic signatures of divergent selection and speciation patterns in a ‘natural experiment’, the young parallel radiations of Nicaraguan crater lake cichlid fishes

Divergent selection is the main driving force in sympatric ecological speciation and may also play a strong role in divergence between allopatric populations. Characterizing the genome‐wide impact of divergent selection often constitutes a first step in unravelling the genetic bases underlying adaptation and ecological speciation. The Midas cichlid fish (Amphilophus citrinellus) species complex in Nicaragua is a powerful system for studying evolutionary processes. Independent colonizations of isolated young crater lakes by Midas cichlid populations from the older and great lakes of Nicaragua resulted in the repeated evolution of adaptive radiations by intralacustrine sympatric speciation. In this study we performed genome scans on two repeated radiations of crater lake species and their great lake source populations (1030 polymorphic AFLPs, n ～ 30 individuals per species). We detected regions under divergent selection (0.3% in the crater lake Xiloá flock and 1.7% in the older crater lake Apoyo radiation) that might be responsible for the sympatric diversifications. We find no evidence that the same genomic regions have been involved in the repeated evolution of parallel adaptations across crater lake flocks. However, there is some genetic parallelism apparent (seven out of 51 crater lake to great lake outlier loci are shared; 13.7%) that is associated with the allopatric divergence of both crater lake flocks. Interestingly, our results suggest that the number of outlier loci involved in sympatric and allopatric divergence increases over time. A phylogeny based on the AFLP data clearly supports the monophyly of both crater lake species flocks and indicates a parallel branching order with a primary split along the limnetic‐benthic axis in both radiations.     

Ciliate community structure and interactions within the planktonic food web in two alpine lakes of contrasting transparency

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Bird abundance and species richness on Florida lakes: influence of trophic status,lake morphology,and aquatic macrophytes

Data from 46 Florida lakes were used to examine relationships between bird abundance (numbers and biomass) and species richness, and lake trophic status, lake morphology and aquatic macrophyte abundance. Average annual bird numbers ranged from 7 to 800 birds km^–2 and bird biomass ranged from 1 to 465 kg km^–2. Total species richness ranged from 1 to 30 species per lake. Annual average bird numbers and biomass were positively correlated to lake trophic status as assessed by total phosphorus (r = 0.61), total nitrogen (r = 0.60) and chlorophyll a (r = 0.56) concentrations. Species richness was positively correlated to lake area (r = 0.86) and trophic status (r = 0.64 for total phosphorus concentrations). The percentage of the total annual phosphorus load contributed to 14 Florida lakes by bird populations was low averaging 2.4%. Bird populations using Florida lakes, therefore, do not significantly impact the trophic status of the lakes under natural situations, but lake trophic status is a major factor influencing bird abundance and species richness on lakes. Bird abundance and species richness were not significantly correlated to other lake morphology or aquatic macrophyte parameters after the effects of lake area and trophic status were accounted for using stepwise multiple regression. The lack of significant relations between annual average bird abundance and species richness and macrophyte abundance seems to be related to changes in bird species composition. Bird abundance and species richness remain relatively stable as macrophyte abundance increases, but birds that use open-water habitats (e.g., double-crested cormorant, Phalacrocorax auritus) are replaced by species that use macrophyte communities (e.g., ring-necked duck, Aythya collaris).     

Resource polymorphism and diversity of Arctic charr Salvelinus alpinus in a series of isolated lakes

Morphological, dietary and life‐history variation in Arctic charr Salvelinus alpinus were characterized from three geographically proximate, but isolated lakes and one large lake into which they drain in south‐western Alaska. Polymorphism was predicted to occur in the first three lakes because S. alpinus tend to become polymorphic in deep, isolated lakes with few co‐occurring species. Only one morph was evident in the large lake and two of the three isolated lakes. In the third isolated lake, Lower Tazimina Lake, small and large morphs were found, the latter including two forms differing in growth rate. The small morph additionally differed from the two large forms by having more gill rakers and a deeper body than same‐sized individuals of the large morph, consuming more limnetic and fewer benthic resources, having a greater gonado‐somatic index and maturing at a smaller size. The two large forms consumed only slightly different foods (more terrestrial insects were consumed by the medium‐growth form; more snails by the high‐growth form). Trends in consumption of resources with body shape also differed between lakes. Variability in life history of S. alpinus in these Alaskan lakes was as broad as that found elsewhere. This variability is important for understanding lake ecosystems of remote regions where this species is commonly dominant.     

Summer diet of juvenile lake sturgeon reintroduced into the Genesee and St. Regis Rivers,New York USA

The restoration of threatened species involves understanding multiple aspects of the life history and ecology of the target species. One important consideration in the restoration of threatened species is feeding ecology. We examined the summer diet of reintroduced juvenile lake sturgeon (Acipenser fulvescens) in the Genesee (n = 119, ages 1 and 2) and the St. Regis rivers (n = 40, ages 1, 2, 5, 6, and 7) of New York State, USA, in the summer of 2005. The most common taxa consumed were Diptera (88% of prey biomass) in the Genesee River and Ephemeroptera (35% of prey biomass) and Diptera (29%) in the St. Regis River. Diptera: (Chironomidae) was the major prey taxon in both benthic communities. Selectivity analysis showed a positive selection of Diptera by Genesee River lake sturgeon and an avoidance of Diptera by St. Regis River lake sturgeon. The St. Regis River lake sturgeon showed avoidance of Coleoptera, but positive selection of Ephemeroptera, Trichoptera, and Oligochaeta. Results indicate that the reintroduced juvenile lake sturgeon are growing and successfully finding prey in these two rivers and appear to actively select specific prey types.     

Effect of landscape factors on fish distribution in arctic Alaskan lakes

1. The distribution of species is affected by many factors operating at a variety of temporal and spatial scales in a heterogeneous landscape. In lakes, fish communities are dynamic, influenced by landscape‐level factors that control colonisation and extinction. 2. We used classification and regression tree (CART) analyses to quantify the importance of landscape‐level factors in determining the distribution of fish species in 168 arctic Alaskan lakes. Factors including lake size, depth, outflow gradient, distance to other lakes, lake order, altitude, river drainage and age of glacial surface were analysed. These factors could affect either access of fish to a lake (colonisation variables), or their survival in a lake that already had been colonised (extinction variables). 3. The presence of a species was predicted accurately in 78.4% ± 10.5% (mean ± SD) of cases, and absence in 75.0% ± 6.1% of cases. The relative importance of extinction versus colonisation variables varied with species. Extinction variables were most important for lake trout (Salvelinus namaycush) and slimy sculpin (Cottus cognatus), a mixture of extinction and colonisation variables was important for arctic char (Salvelinus alpinus), and colonisation variables were most important for arctic grayling (Thymallus arcticus) and round whitefish (Prosopium cylindraceum). 4. Ecological differences among species account for much of the difference in relative importance of colonisation versus extinction variables. In addition, stream piracy events have occurred over geologic time scales, which have resulted in lakes that are currently inaccessible but support relict fish populations. 5. Climate warming, currently occurring in the arctic, is likely to alter further the stream network, which could have dramatic effects on fish distributions by affecting access to isolated lakes or isolating lakes that are currently accessible.     

Resource division by perlid stoneflies (Plecoptera) in a lake outlet ecosystem

In a stream draining a productive lake, two abundant (250/m²) stoneflies Calineuria californica and Hesperoperla pacifica differed in population size structure, habitat use and food although overlap in resource use was extensive. Life cycles of two (Calineuria) and three years (Hesperoperla) were indicated. A multivariate analysis of habitat use demonstrated size dependent habitat selection in both species and between species habitat differences. Feeding habits differed although only the largest Hesperoperla had a food refuge. Diets of individual nymphs represented an interaction of species, size and habitat.     

Interactions between invading benthivorous fish and native whitefish in subarctic lakes

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Quantitative analysis of freshwater ostracode assemblages in southwestern Yukon Territory,Canada

Relations between modern ostracode assemblages and environmental variables from lakes in the southwest Yukon and northern British Columbia were explored. A total of 29 freshwater species representing 8 genera were identified from the sediments of 36 lakes, with the number of species ranging between 3 and 8 per lake. Species widespread throughout the study area include Cyclocypris ampla, Candona candida, Cypria turneri, Cypria ophtalmica, and Candona protzi. The Mg/Ca ratio is an important factor determining the ostracode species composition of a lake. Species richness is at a maximum when the lake water has intermediate values of conductivity. Lakes in which one species clearly dominates the assemblage (‰>‰70% relative abundance) have water saturated with respect to CaCO₃. Mg/Ca ratio, depth and Sr are the environmental factors that are most highly correlated with species distributions in this region.     

Trophic consequences of non‐native species for commercial fish in a backwater bay of the Three Gorges Reservoir,Southwest China

Trophic niche overlap in native and alien fish species can lead to competitive interactions whereby non‐native fishes outcompete indigenous individuals and eventually affect the viability of natural populations. The species Erythroculter mongolicus and Erythroculter ilishaeformis (belonging to the Culterinae), which are two commercially important fish species in the backwater bay of the Pengxi River in the Three Gorges Reservoir (TGR), were threatened by competition from the non‐native Coilia ectenes (lake anchovy). The latter is an alien species introduced into the lower reaches of the Yangtze River in China and now widespread in the TGR. The trophic consequences of non‐native lake anchovy invasion for E. mongolicus and E. ilishaeformis were assessed using stable isotope analysis (δ¹³C and δ¹⁵N) and associated metrics including the isotopic niche, measured as the standard ellipse area. The trophic niche of native E. mongolicus had little overlap (<15%) with the alien fish species and was significantly reduced in size after invasion by lake anchovy. This suggests that E. mongolicus shifted to a more specialized diet after invasion by lake anchovy. In contrast, the trophic niche overlap of native fish E. ilishaeformis with the alien fish species was larger (>50%) and the niche was obviously increased, implying that fish in this species exploited a wider dietary base to maintain their energetic requirements. Thus, marked changes for the native E. mongolicus and E. ilishaeformis were detected as the trophic consequences of invasion of non‐native lake anchovy.     

Halomonas magadii sp. nov., a new member of the genus Halomonas, isolated from a soda lake of the East African Rift Valley

A number of novel alkaliphilic organotrophic bacteria have been isolated from several saline and alkaline East African soda lakes. The new isolates grow at pH values between 7.0 and 11.0, with pH optima for growth between 9.0 and 10.0. Growth occurs at total salts concentration between 0% and 20% (w/v) with optimum at 0%–7% (w/v). Phylogenetic analyses based on 16S rDNA sequence comparison indicate that these isolates are related (>96% similarity) to members of the Halomonadaceae within the γ-3 subdivision of the Proteobacteria. These analyses indicate that existing species within the Halomonadaceae fell within three main groups, one group comprising the type species of Halomonas, Halomonas elongata, and a number of other known species including one soda lake isolate. A second group constituting most of the remaining known species of Halomonas and related Chromohalobacter spp. includes 3 soda lake isolates with high DNA–DNA homologies. The third group included Halomonas halodenitrificans, Halomonas desiderata, Halomonas cupida, and 13 soda lake isolates. Phenotypic comparisons indicated that the majority of soda lake strains shared similar morphological, phenotypic, and chemotaxonomic properties to known strains of Halomonas but grew under alkaline conditions. The 3 soda lake isolates with high DNA–DNA homologies were, however, significantly different in antibiotic sensitivity pattern and in the utilization of several substrates, were unable to reduce nitrite, and showed low DNA–DNA homologies with known halomonads in the same group. We propose that these isolates comprise a new species of the genus Halomonas that we name Halomonas magadii sp. nov. The type strain is strain 21 MI (NCIMB 13595). Received: July 20, 1999 / Accepted: October 29, 1999     

Niche overlap between herbivorous cladocerans; the role of food quality and habitat homogeneity

In spite of resource limitation, five abundant species of herbivorous metazoan zooplankton in a humic lake exhibited extensive niche overlap both with regard to seasonal and spatial occurrence, time of reproductive maxima, juvenile release and food choice. Their coexistence could not be explained by modifying predation, environmental oscillations or recolonization.Laboratory bottle experiments indicated only weak interspecific interactions between the three tested species at low food levels, but negative interactions were induced at elevated food levels. Bosmina appeared as competitively inferior during enrichment with cultured algae, but as the superior species during starvation. At low nutrient levels, all species coexisted for several generations with low reproduction, in accordance with the lake situation. It was concluded that the observed niche overlaps would be promoted if; 1) Intraspecific competition is more important than interspecific competition. 2) All species are co-adapted to low nutrient availability, food is quantitatively in surplus, but qualitatively deficient (mainly recycled detritus). During such conditions, no species would be capable of obtaining a population increase until extinction of the other species. This situation may be typical of oligotrophic humic lakes, and of other localities with low predation pressure and high inputs of allochthonous particulate carbon.     

松嫩湖群鱼类群落相似性

根据2008年5月至2011年1月对松嫩湖群20个主要渔业湖泊的鱼类资源调查,分析了该湖群鱼类区系特征和群落相似性状况。松嫩湖群的鱼类区系由4目9科34属46种和亚种构成,其中土著鱼类3目8科27属39种和亚种,包括中国特有种3种,中国易危种1种,冷水种5种;由5个区系复合体构成,以东部江河平原区系复合体为主体;鲤形目31种,鲤科26种,分别占优势;鱼类区系具有南北方物种相互渗透、古北界与东洋界交汇过渡的混色类群特征。目前松嫩湖群鱼类群落种类组成的相似度总体较低,群落数量结构的相似度总体较高,鱼类群落相似性面临的主要问题是自然与人为因素导致湖泊生态环境的变化和放养、移殖与过度捕捞导致鱼类资源的减少与小型化,二者的叠加效应使鱼类群落长期处于受损状态,群落结构及其相似性处在动态变化中,群落内种间关系的协调性、种群结构的合理性和群落结构的稳定性均在下降。针对这些情况和群落相似性现状,提出未来松嫩湖群湖泊渔业的发展方向是优化调整群落结构,发展多种群湖泊渔业,合理利用土著鱼类资源。     

The effects ofTilapia introductions in Lake Luhondo,Rwanda

Synopsis Cyprinid fishes apparently constituted the major part of the original fish fauna in Lake Luhondo, Rwanda, at least until 1934. At that time only three species, all cyprinids, were known from the lake:Barbus neumayeri, a small barbel, very common in the lake (described from the lake asBarbus luhondo) and two larger cyprinid species:Barbus microbarbis andVaricorhinus ruandae. These two large species were probably not very common in the lake. Between 1935 and 1938 some youngTilapia were introduced into Lake Luhondo. Since then and certainly since 1952 the large cyprinids seem to have disappeared completely from the lake. The smallBarbus neumayeri has become extremely rare; at present it survives only in some small tributaries of the lake. In the lake itselfTilapia andHaplochromis species are now the dominant fauna. A survey of the available information is given.Africa-Museum, Steenweg op Leuven, B-1980 Tervuren, Belgium (mailing address)