Winter ecology of Arctic charr (<Emphasis Type="Italic">Salvelinus alpinus</Emphasis>) and brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>) in a subarctic lake,Norway

We studied habitat choice, diet, food consumption and somatic growth of Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) during the ice-covered winter period of a subarctic lake in northern Norway. Both Arctic charr and brown trout predominantly used the littoral zone during winter time. Despite very cold winter conditions (water temperature <1°C) and poor light conditions, both fish species fed continuously during the ice-covered period, although at a much lower rate than during the summer season. No somatic growth could be detected during the ice-covered winter period and the condition factor of both species significantly declined, suggesting that the winter feeding rates were similar to or below the maintenance requirements. Also, the species richness and diversity of ingested prey largely decreased from summer to winter for both fish species. The winter diet of Arctic charr <20 cm was dominated by benthic insect larvae, chironomids in particular, and Gammarus lacustris, but zooplankton was also important in December. G. lacustris was the dominant prey of charr >20 cm. The winter diet of brown trout <20 cm was dominated by insect larvae, whereas large-sized trout mainly was piscivorous, feeding on juvenile Arctic charr. Piscivorous feeding behaviour of trout was in contrast rarely seen during the summer months when their encounter with potential fish prey was rare as the small-sized charr mainly inhabited the profundal. The study demonstrated large differences in the ecology and interactions of Arctic charr and brown trout between the winter and summer seasons.     

Carbon sources for lake food webs in the Canadian High Arctic and other regions of Arctic North America

We investigated the role of autochthonous and terrestrial carbon in supporting aquatic food webs in the Canadian High Arctic by determining the diet of the dominant primary consumer, aquatic chironomids. These organisms were studied in fresh waters on 3 islands of the Arctic Archipelago (~74–76°N) including barren polar desert watersheds and a polar oasis with lush meadows. Stomach content analysis of 578 larvae indicated that chironomids primarily ingested diatoms and sediment detritus with little variation among most genera. Carbon and nitrogen stable isotope mixing models applied to 2 lakes indicated that benthic algae contributed 68–95% to chironomid diet at a polar desert site and 70–78% at a polar oasis site. Detritus, originating from either phytoplankton or terrestrial sources, also contributed minor amounts to chironomid diet (0–32%). Radiocarbon measurements for the 2 lakes showed that old terrestrial carbon did not support chironomid production. Carbon stable isotope ratios of chironomids in other High Arctic lakes provided further dietary evidence that was consistent with mixing model results. These findings indicate that, in the Canadian High Arctic, chironomids (and fish that consume them) are supported primarily by benthic algae in both polar desert and oasis lakes. In contrast, our review of carbon flow studies for lakes in other Arctic regions of North America shows that terrestrial carbon and phytoplankton can be important energy sources for consumers. This study provides a baseline to detect future climate-related impacts on carbon pathways in High Arctic lakes.     

Arctic charr in Lake Myvatn: The centennial catch record in the light of recent stock estimates

In Lake Myvatn, Arctic charr (Salvelinus alpinus) is economically the most important fish species. It is fast growing fish and its size at maturity is 35–50 cm at the age of 4 to 5 years. Catch statistics that are available for the whole 20th century show considerable long-term variation with the highest catch in the 1920s. The catches after 1970 are about 40% lower than the average from 1930–1969 and the annual catches for the last decade are the lowest. Stock size during the winter fishing season fluctuated considerably between years, with average annual fishing mortality of 83.9%. The Arctic charr population has been monitored annually since 1986, using standard series of gill nets of different mesh sizes. In 1988 one to three-year-old fish were heavily reduced in numbers during the summer months. There are indications of a similar event in 1997. In both years the charr changed its main diet from Cladocera and chironomid midges to three-spined sticklebacks Gasterosteus aculeatus and snails. At the same time its condition deteriorated. The catch in the monitoring fishery in Lake Myvatn correlates with the stock size in the beginning of the following winter fishing season deduced from Leslie's method and can give prospects for the fishery in the successive fishing season. An index of abundance of young charr also correlates with the number of chironomids and cladocerans and also with ducklings that feed on the same food as the charr. Large fluctuations in the Arctic charr population in Lake Myvatn seem to be related to changes in the main food species. The catch records available from Lake Myvatn can to a large extent be used as a measure of changes of the Arctic charr population in the lake for the past century.     

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

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

Chironomids of Italian rice fields

Studies on chironomids of Italian rice fields began in 1914, but a comprehensive faunistic picture of Italian rice field chironomids was provided only by COCCHI in 1966. In 1988 and 1989 field research south of Verona (northern Italy) permitted us to confirm the results of COCCHI regardingC. sylvestris andChironomus spp., and to increase the number of known species of Italian rice field chironomids. The list of species which are injurious to rice is similar to that of other European countries, with the exception ofHarnischia falcata andTanytarsus sp. The type and period of damage is different for certain species (e.g. Cricotopus sylvestris) in Italy in comparison to other countries. A few agrotechnical steps are recommended to control pest species. The finding of the parthenogenetic speciesParatanytarsus grimmii in lab. aquaria used for rearing purposes is documented.     

Gamasid mites (Parasitiformes,Mesostigmata) in nests of passerine birds on the Arctic Seven Islands Archipelago,the Barents Sea

Examination of 31 nests of 6 common passerine bird species collected on the Seven Islands Archipelago (the Barents Sea) revealed 25 species of gamasid mites. Most of them belonged to the widespread free-living species commonly occurring in the neighboring taiga and southern tundra areas. There were practically no true Arctic forms among the gamasids found; by contrast, the local flora consisted almost by half of Arctic and Hypoarctic species. The findings of only one species, Neoseiulus cf. tobon, appear to be restricted to the Arctic, while the range of the rare parthenogenetic Iphidinychus gaieri may be restricted to Hypoarctic and mountainous regions of Eurasia. No bird parasites were recorded. Host specificity of the mite assemblages of the nests was not observed.     

Feeding chronology and habits of Alosa spp. (Clupeidae) juveniles from the lower Hudson River estuary, New York

Synopsis Feeding habits of juvenile alewife, A. pseudoharengus, blueback herring, A. aestivalis, and American shad, Alosa sapidissima, were examined over a 24 h period at an inshore location on the lower Hudson River. Feeding periodicity differed by species: alewives showed no diel differences, whereas shad fed least intensively near dawn and herring least intensively at night. Alewives fed primarily on chironomids and the amphipod Corophium lacustre, shad on Formicidae and larval chironomids, and herring on chironomids and copepods. Diel variation in prey selection was evident. Dietary overlap was generally greatest between alewife and herring and least between shad and herring.     

Stratigraphy of Cladocera and Chironomidae in a core from a shallow North German lake

In the subfossil assemblages of chironomids and cladocerans significant changes were found during the late-glacial and postglacial development of a small, shallow North German lake. The successions observed were obviously not related to eutrophication but to factors such as climatic conditions and lake siltation. The absence of species of the subgenus Eubosmina during postglacial times as well as the predominance of Alonella species among the chydorids was due to the small dimensions of the water body.     

Frenulate siboglinids at high Arctic methane seeps and insight into high latitude frenulate distribution

Frenulate species were identified from a high Arctic methane seep area on Vestnesa Ridge, western Svalbard margin (79°N, Fram Strait) based on mitochondrial cytochrome oxidase subunit I (mtCOI). Two species were found: Oligobrachia haakonmosbiensis, and a new, distinct, and undescribed Oligobrachia species. The new species adds to the cryptic Oligobrachia species complex found at high latitude methane seeps in the north Atlantic and the Arctic. However, this species displays a curled tube morphology and light brown coloration that could serve to distinguish it from other members of the complex. A number of single tentacle individuals were recovered which were initially thought to be members of the only unitentaculate genus, Siboglinum. However, sequencing revealed them to be the new species and the single tentacle morphology, in addition to thin, colorless, and ringless tubes indicate that they are juveniles. This is the first known report of juveniles of northern Oligobrachia. Since the juveniles all appeared to be at about the same developmental stage, it is possible that reproduction is either synchronized within the species, or that despite continuous reproduction, settlement, and growth in the sediment only takes place at specific periods. The new find of the well‐known species O. haakonmosbiensis extends its range from the Norwegian Sea to high latitudes of the Arctic in the Fram Strait. We suggest bottom currents serve as the main distribution mechanism for high latitude Oligobrachia species and that water depth constitutes a major dispersal barrier. This explains the lack of overlap between the distributions of northern Oligobrachia species despite exposure to similar current regimes. Our results point toward a single speciation event within the Oligobrachia clade, and we suggest that this occurred in the late Neogene, when topographical changes occurred and exchanges between Arctic and North Atlantic water masses and subsequent thermohaline circulation intensified.     

Why on the snow? Winter emergence strategies of snow‐active Chironomidae (Diptera) in Poland

A long‐term study of adult non‐biting midges (Chironomidae) active in winter on the snow in mountain areas and lowlands in Poland yielded 35 species. The lowland and mountain communities differed significantly in their specific composition. The mountain assemblage was found to be more diverse and abundant, with a substantial contribution from the subfamily Diamesinae, whereas Orthocladiinae predominated in the lowlands. Orthocladius wetterensis Brundin was the most characteristic and superdominant species in the winter‐active chironomid communities in both areas. Only a few specimens and species of snow‐active chironomids were recorded in late autumn and early winter. The abundance of chironomids peaked in late February in the mountain and lowland areas with an additional peak in the mountain areas in early April. However, this second peak of activity consisted mainly of Orthocladiinae, as Diamesinae emerged earliest in the season. Most snow‐active species emerged in mid‐ and late winter, but their seasonal patterns differed between the 2 regions as a result of the different species composition and the duration of snow cover in these regions. Spearman's rank correlation coefficient tests yielded positive results between each season and the number of chironomid individuals recorded in the mountain area. A positive correlation between air temperature, rising to +3.5 °C, and the number of specimens recorded on the snow in the mountain community was statistically significant. The winter emergence and mate‐searching strategies of chironomids are discussed in the light of global warming, and a brief compilation of most important published data on the phenomena studied is provided.     

Chironomid assemblages from seabird-affected High Arctic ponds

Seabirds can shunt nutrients and contaminants from marine to terrestrial ecosystems by forming dense breeding colonies and releasing wastes to these sites. A large colony of seabirds at Cape Vera (Devon Island, High Arctic Canada) has resulted in eutrophic conditions and potentially toxic concentrations of sedimentary metals in several freshwater ponds that drain their nesting sites. Here, we investigated the effects of elevated nutrient and sedimentary metal concentrations on the distribution of subfossil chironomids in surface sediments from 21 ponds that span a gradient of seabird influence. Although many ponds registered high nutrient concentrations (e.g., mean TP = 45 μg l ⁻¹), eutrophic taxa typical of temperate waters were not common, with most assemblages being dominated by morphotypes of Psectrocladius and Tanytarsina, as well as Corynoneura arctica-type, and Metriocnemus hygropetricus-type. Although the ponds within and outside the area influenced by seabirds contained largely similar taxa, variations did exist in the relative abundances of the different species. Lakewater pH was the only measured environmental variable that explained statistically significant amounts of variation in the chironomid assemblages. Although direct effects of pH on chironomids cannot be ruled out, pH is likely tracking production-related changes driven by limnetic dissolved inorganic carbon dynamics. Sediment cores collected from seabird-affected and seabird-free ponds showed a greater number of chironomid taxa and higher head capsule abundances in the pond receiving seabird inputs. Chironomid assemblages in both cores recorded increased abundances in recent decades, likely in response to warmer conditions and lengthened growing seasons.     

Molecular phylogeography of Dryas integrifolia: glacial refugia and postglacial recolonization

Chloroplast DNA variation in the Arctic plant species Dryas integrifolia (Rosaceae) was analysed in relation to both the present-day geographical distribution of populations and to Pleistocene fossil records of this species. The phylogeographical structure was weak but the analysis of haplotype diversity revealed several groups of haplotypes having present-day geographical ranges that overlap locations postulated from geographical and fossil evidence to have been glacial refugia. Based on this information we infer that two important refugial sources of Arctic recolonization by this species were Beringia and the High Arctic. Two other putative refugia, located southeast of the ice sheet and along coastal regions of the eastern Arctic may have served as sources for recolonization of smaller portions of the Arctic. The genetic substructure in the species is mostly due to variation among populations regardless of the ecogeographical region in which they are found. Spatial autocorrelation at the regional scale was also detected. High levels of diversity both within populations and ecogeographical regions are probably indicative of population establishment from several sources possibly combined with recent gene flow.     

Larvae of Chironomids (Insecta,Diptera) Encountered in the Mantle Cavity of Zebra Mussels,Dreissena polymorpha (Bivalvia,Dreissenidae)

The paper includes data on species composition of chironomid larvae which were encountered in the mantle cavity of zebra mussels (Dreissena polymorpha) within 7 waterbodies in the Republic of Belarus. All were found to be free‐living species commonly present in periphyton and/or benthos. A long‐term study of the seasonal dynamics of these larvae in Dreissena did not reveal any typical pattern. Our data suppose that chironomids do not have an obligate association with zebra mussels and possibly enter their mantle cavity inadvertently. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Selection on Arctic charr generated by competition from brown trout

We experimentally explored population‐ and individual‐level effects on Arctic charr (Salvelinus alpinus) resulting from resource competition with its common European competitor, the brown trout (Salmo trutta). At the population level, we compared performance of the two species in their natural sympatric state with that of Arctic charr in allopatry. At the individual level, we established selection gradients for morphological traits of Arctic charr in allopatric and in sympatric conditions. We found evidence for interspecific competition likely by interference at the population level when comparing differences in average performance between treatments. The growth and feeding rates did not differ significantly between allopatric and sympatric Arctic charr despite lower charr densities (substitutive design) in sympatric enclosures indicating that inter‐ and intraspecific competition are of similar strength. The two species showed distinct niche segregation in sympatry, and brown trout grew faster than Arctic charr. Arctic charr did not expand their niche in allopatry, indicating that the two species compete to a limited degree for the same resources and that interference may suppress the growth of charr in sympatric enclosures. At the individual level, however, we found directional selection in sympatric enclosures against individual Arctic charr with large head and long fins and against individuals feeding on zoobenthos rather than zooplankton indicating competition for common resources (possibly exploitative) between trout and these charr individuals. In allopatric enclosures these relations were not significant. Diets were correlated to the morphology supporting selection against the benthic‐feeding type, i.e. individuals with morphology and feeding behaviour most similar to their competitor, the benthic feeding brown trout. Thus, this study lends support to the hypothesis that Arctic charr have evolved in competition with brown trout, and through ecological character displacement adapted to their present niche.     

Genetic differentiation between Arctic and Antarctic monothalamous foraminiferans

Monothalamous (single-chambered) foraminifers are a major component of the benthic meiofauna in high latitude regions. Several morphologically similar species are common in the Arctic and Antarctic. However, it is uncertain whether these morphospecies are genetically identical, or whether their accurate identification is compromised by a lack of distinctive morphological features. To determine the relationship between Arctic and Antarctic species, we have compared SSU rDNA sequences of specimens belonging to four morphotaxa: Micrometula, Psammophaga, Gloiogullmia, and one morphospecies Hippocrepinella hirudinea from western Svalbard (Arctic) and McMurdo Sound (Antarctic). Wherever possible, we include in our analyses representatives of these taxa from the deep Arctic and Southern Oceans, as well as from Northern European fjords. We found that in all cases, the bipolar populations were clearly distinct genetically. As expected, Arctic specimens were usually more closely related to those from Northern Europe than to their Antarctic representatives. The deep-sea specimens from Weddell Sea branched as a sister to the McMurdo Sound population, while those from the Arctic Ocean clustered with ones from Norwegian fjords. Our study has revealed a high number of cryptic species within each of the examined genera, and demonstrates the unexplored potential of monothalamous foraminifers for use as a tool to evaluate the origin and biogeography of polar meiofauna.     

Shifts in the composition and distribution of Pacific Arctic larval fish assemblages in response to rapid ecosystem change

The Pacific Arctic marine ecosystem has undergone rapid changes in recent years due to ocean warming, sea ice loss, and increased northward transport of Pacific-origin waters into the Arctic. These climate-mediated changes have been linked to range shifts of juvenile and adult subarctic (boreal) and Arctic fish populations, though it is unclear whether distributional changes are also occurring during the early life stages. We analyzed larval fish abundance and distribution data sampled in late summer from 2010 to 2019 in two interconnected Pacific Arctic ecosystems: the northern Bering Sea and Chukchi Sea, to determine whether recent warming and loss of sea ice has restricted habitat for Arctic species and altered larval fish assemblage composition from Arctic- to boreal-associated taxa. Multivariate analyses revealed the presence of three distinct multi-species assemblages across all years: (1) a boreal assemblage dominated by yellowfin sole (Limanda aspera), capelin (Mallotus catervarius), and walleye pollock (Gadus chalcogrammus); (2) an Arctic assemblage composed of Arctic cod (Boreogadus saida) and other common Arctic species; and (3) a mixed assemblage composed of the dominant species from the other two assemblages. We found that the wind- and current-driven northward advection of warmer, subarctic waters and the unprecedented low-ice conditions observed in the northern Bering and Chukchi seas beginning in 2017 and persisting into 2018 and 2019 have precipitated community-wide shifts, with the boreal larval fish assemblage expanding northward and offshore and the Arctic assemblage retreating poleward. We conclude that Arctic warming is most significantly driving changes in abundance at the leading and trailing edges of the Chukchi Sea larval fish community as boreal species increase in abundance and Arctic species decline. Our analyses document how quickly larval fish assemblages respond to environmental change and reveal that the impacts of Arctic borealization on fish community composition spans multiple life stages over large spatial scales.     

Predation,sediment stability and food availability as determinants of the benthic invertebrate fauna in two shallow lakes

The sedimentary benthos of a series of shallow, eutrophicated lakes, the Norfolk Broads is, in general, low not only in number of species but unexpectedly in number of individuals. In two of the lakes, Hudsons Bay and Hoveton Great Broad, chironomids and oligochaetes dominated the fauna. Hudsons Bay has an extensive stand of water lilies (Nuphar lutea); Hoveton Great Broad does not. There were significant relationships between number of chironomids and of Potamothrix hammoniensis with organic content of the sediments, but these were due not to food availability but to the structure imparted to the otherwise fluid sediment by the organic matter. Sediment stabilised in plastic bowls developed much larger populations of oligochaetes than found in the unrestricted sediment. Protection of the community from fish predation resulted in a further major increase in numbers. Sediment stability and predation rather than food supply were the major determinants of these benthic populations.     

Feeding rates,assimilation efficiencies and growth of two amphipod species on biodeposited material from zebra mussels

1. Accumulation of organic material by the zebra mussel Dreissena polymorpha is assumed to be the source of a biodeposition‐based food web. However, only little is known about the importance of the biodeposited material as a food source and its contribution to increased abundances of macroinvertebrates in the presence of D. polymorpha. 2. Feeding, assimilation and growth of the amphipods Gammarus roeselii and Dikerogammarus villosus on food sources directly and indirectly associated with D. polymorpha (biodeposited material and chironomids) and on conditioned alder leaves were measured. The stoichiometry of carbon, nitrogen and phosphorus of the diets was measured as an important determining factor of food quality. 3. Chironomids had the highest nitrogen and phosphorus contents, alder leaves were depleted in nitrogen and phosphorus, and the stoichiometry of biodeposited material was intermediate. 4. Both amphipod species had highest feeding rates and assimilation efficiencies on chironomids. Gammarus roeselii fed more on biodeposited material than on alder leaves, but assimilation efficiencies were similar; D. villosus also had similar feeding rates and assimilation efficiencies on the two diets. 5. Both amphipod species had highest growth rates on chironomids and lowest growth rates on alder leaves. Both grew at intermediate rates on biodeposited material of D. polymorpha. The growth rates of the amphipod species were related to food stoichiometry. Overall, the invasive D. villosus grew faster than the indigenous G. roeselii. 6. Food resources directly and indirectly associated with D. polymorpha are potential diets for amphipods, providing further evidence for a D. polymorpha biodeposition‐based food web.     

Relationships between Na+, K+-ATPase, sialic acid content and fluidity in rainbow trout density-separated erythrocytes

Smolt of anadromous Arctic charr Sahelinus alpinus (L.) migrating from Storvatn, northern Norway, had a similar feeding intensity as resident Arctic charr from the benthic- and pelagic zone in early summer in fresh water. While smolts fed mostly on surface insects, however, resident Arctic charr had a more broader diet consisting mostly of chironomids, Bosmina sp., benthic prey and surface insects. This indicates that Arctic charr smolts have adopted a surface oriented feeding behaviour prior to their sea migration.     

Selective predation by four darter (Percidae) species on larval chironomids (Diptera) from a Mississippi stream

We determined seasonal foraging modes of four sympatric darter species, Etheostoma lynceum, E. stigmaeum, E. swaini and Percina nigrofasciata, from Beaverdam Creek, Mississippi (USA) at two scales of taxonomic resolution: (1) chironomid prey identified to family and (2) chironomid prey identified to genus/species. When chironomids were identified to family, high proportional similarity (PS) and low niche breadth (NB) values suggested the darters fed opportunistically on a relatively small number of available prey taxa. In contrast, when chironomids were identified to genus/species, concordant low PS and NB values suggested the darters fed like classic specialists, selecting a small number of prey taxa relative to prey availability in the resource base. The darters selected just one to four chironomid taxa from 52 available taxa across seasons. Our study shows that the scale of taxonomic resolution used to identify darter prey may influence the characterization of darter foraging modes.