Identification of the maternal source of young‐of‐the‐year Arctic charr in Lake Hazen,Canada

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Stable isotope analysis reveals ecological segregation in a bimodal size polymorphism in Arctic charr from Loch Tay, Scotland

The serendipitous discovery of a body-size dimorphism amongst the sexually mature Arctic charr Salvelinus alpinus of Loch Tay is described. Sexually mature Arctic charr, collected by gill netting on spawning areas, showed a clear and distinct bimodal size distribution with no overlap in fork length distributions. The upper (19–29 cm L _F) and lower modes (8–16 cm) were not solely the result of sex or age differences. Analysis of stable isotope ratios of C and N in muscle showed highly significant differences in mean δ¹³C and δ¹⁵N between populations, demonstrating a difference in trophic ecology between the two body-size morphs. Overlap in the range of δ¹³C and δ¹⁵N values for the two morphs, however, suggested that they occasionally shared a common diet. Data from other studies strongly indicated that the proximate and ultimate mechanisms that control body-size dimorphisms in Arctic charr differed between sites. Clear differences in trophic ecology in the Loch Tay Arctic charr suggested that the available feeding opportunity may differ for the two morphs. The most likely proximate mechanism resulting in this dimorphism is growth rate differences resulting from differences in food availability for the two subgroups occupying alternative foraging niches in Loch Tay.     

A stable isotope analysis of trophic polymorphism among Arctic charr from Loch Ericht, Scotland

Analysis of nitrogen stable isotope ratios (δ¹⁵N) in white muscle and liver shows temporal stability in the trophic levels at which the two sympatric morphs of Arctic charr Salvelinus alpinus from Loch Ericht feed. The results confirm an ontogenetic dietary switch to a more piscivorous diet, occurred in all individuals of one of the morphs, above a size threshold of 16·5 cm fork length, indicating that this occurs at a high frequency in this population and that once made, the return to feeding to a lower trophic level is infrequent.     

The Eocene Arctic Azolla bloom: environmental conditions, productivity and carbon drawdown

Enormous quantities of the free-floating freshwater fern Azolla grew and reproduced in situ in the Arctic Ocean during the middle Eocene, as was demonstrated by microscopic analysis of microlaminated sediments recovered from the Lomonosov Ridge during Integrated Ocean Drilling Program (IODP) Expedition 302. The timing of the Azolla phase (~48.5 Ma) coincides with the earliest signs of onset of the transition from a greenhouse towards the modern icehouse Earth. The sustained growth of Azolla , currently ranking among the fastest growing plants on Earth, in a major anoxic oceanic basin may have contributed to decreasing atmospheric p CO₂ levels via burial of Azolla -derived organic matter. The consequences of these enormous Azolla blooms for regional and global nutrient and carbon cycles are still largely unknown. Cultivation experiments have been set up to investigate the influence of elevated p CO₂ on Azolla growth, showing a marked increase in Azolla productivity under elevated (760 and 1910 ppm) p CO₂ conditions. The combined results of organic carbon, sulphur, nitrogen content and ¹⁵N and ¹³C measurements of sediments from the Azolla interval illustrate the potential contribution of nitrogen fixation in a euxinic stratified Eocene Arctic. Flux calculations were used to quantitatively reconstruct the potential storage of carbon (0.9–3.5 10¹⁸ gC) in the Arctic during the Azolla interval. It is estimated that storing 0.9 10¹⁸ to 3.5 10¹⁸ g carbon would result in a 55 to 470 ppm drawdown of p CO₂ under Eocene conditions, indicating that the Arctic Azolla blooms may have had a significant effect on global atmospheric p CO₂ levels through enhanced burial of organic matter.     

Habitat use and trophic position determine mercury concentration in the straight fin barb Barbus paludinosus, a small fish species in Lake Awassa, Ethiopia

The diet, habitat use and mercury concentration of the small fish species, the straight fin barb Barbus paludinosus , were studied in Lake Awassa, Ethiopia, for a period of 1 year from February 2003 to January 2004. Stable isotope signatures of nitrogen and carbon in different total length ( L _T) classes were used to determine trophic positions and organic carbon sources, respectively. Barbus paludinosus mainly occupied the protected benthic habitats (littoral and profundal) of the lake. The δ¹³C values were in the range from −24 to −19‰, indicating that the carbon source for B. paludinosus was benthic, as well. Small individuals (≤ 60 mm L _T) mainly preyed upon ostracods, intermediate sizes (60–100 mm) on aquatic insects and gastropods, while a tiny cyprinodont fish Aplocheilichthys antinorii dominated the diet of large individuals (100–160 mm). The progressively increase in δ¹⁵N with increasing L _T also indicated a diet shift towards piscivory in larger individuals. The mercury concentration ranging from 0·02 to 0·74 mg kg⁻¹ wet mass (wm), was unexpectedly high in this small species, and was significantly positively related to L _T, as well as to δ¹⁵N. Some large individuals had mercury concentrations < 0·1 mg kg⁻¹ wm, and low δ¹⁵N, indicating substantial variations in diet between individuals of same size. The study suggests that other piscivorous species which include B. paludinosus in their diet may have a high mercury intake risk.     

Emission rate of amino acids and ammonia and their role in olfactory preference behaviour of juvenile Arctic charr, Salvelinus alpinus (L.)

The behaviour of juvenile Arctic charr, Salvelinus alpinus (L.) , to an abrupt concentration step of L-amino acids, L-alanine and ammonium chloride was studied by fluviarium technique. The emission rates of these substances were studied. Juvenile Arctic charr emit 8.0 × 10⁻⁴ mol total ammonia-N kg⁻¹ h⁻¹ and 3.3 × I0⁻⁵ mol amino acids kg⁻¹ h⁻¹. In behaviour tests the charr avoided 5.6x 10⁻⁶and 5.6 × 10⁻⁷ M ammonium chloride. The 17 L-amino acid mixture, ranked as observed in the analysis of emission, was avoided at 4.6 × 10⁻⁷ M, while 100 times dilution of this value gave neither avoidance nor attraction. The charr avoided L-alanine tested alone at the concentration of 4.6 × 10 ⁻⁷ M. Anosmic charr showed neither avoidance nor attraction to the mixture of 17 amino acids tested at 4.6 × 10⁻⁷ M. The results indicate that ammonia as well as emitted amino acids are not responsible for the olfactory mediated attraction to conspecific odour shown earlier in Arctic charr. On the contrary, these substances may have a negative effect by reducing the strength of attraction.     

Evidence for Two Morphotypes of Lake Charr, Salvelinus Namaycush, from Great Bear Lake, Northwest Territories, Canada

The lake charr, Salvelinus namaycush, is generally characterized as a morphologically stable specialized deepwater piscivore. Empirical observations suggested the existence of two or more morphotypes of lake charr in Great Bear Lake, NWT. I gathered morphological data over two seasons from a total of 72 individuals and used principal components analysis to summarize variation in the dataset. UPGMA classification showed that there were two groups in the dataset. I classified individuals into one of two morphotypes using discriminant function analysis. The two forms differed primarily in head proportions. I developed a function, usable in the field, that classifies 80% of all specimens correctly. This study represents one of the first documentations of sympatric morphotypes of lake charr outside the Laurentian Great Lakes and provides the basis for further investigation into origins of and genetic, morphological and ecological differences between the forms.     

Ecological segregation within and among Arctic char morphotypesin Gander Lake,Newfoundland

Synopsis Stable isotopes of carbon and nitrogen were used to examine differences in the feeding ecology of sympatric morphotypes of Arctic char, Salvelinus alpinus, from Gander Lake, Newfoundland. Morphotypes were differentiated on the basis of colour (pale and dark) and meristic characteristics. Significant differences in length, age and habitat depth were also noted, with the pale-form being smaller, younger and caught at greater average depths than the dark-form. Pale and dark morphotypes possessed significantly different carbon and nitrogen stable isotope signatures, with the dark-form being more δ¹³C enriched than the pale-form. Within each morphotype grouping distinctive isotopic signature differences related to diet (dark-form) and capture depth (pale-form) were also observed. Differences in the isotopic signatures within and among habitat groupings were induced by differences in feeding opportunities arising from differences in habitat use and feeding strategies and generally corroborated by stomach content analysis and an index of dietary overlap. Study results provide evidence for the contention that sympatric forms of Arctic char in Gander Lake occupy significantly different foraging niches within the lake. Results further suggest ecologically relevant separations within morphotype groupings result in lower resource competition.     

Unique perspectives on the influence of size and age on consumer δ15N from a rainbow smelt complex

Analysis of covariance indicated size had a greater influence than age on δ¹⁵N in three rainbow smelt Osmerus mordax morphotypes of Lake Utopia, New Brunswick, Canada. The distinction in size between the three forms allowed a unique analysis and separation of size and age effects, which is not commonly possible because of the strong correlation between size and age. These results provide support for the continued use of δ¹⁵N as a trophic level indicator in food web studies. While a slight increase in δ¹⁵N with age ( P ≤ 0·05) in intermediate-sized smelt raises questions about a potential accumulation of ¹⁵N with age, the effect of size was far stronger in governing δ¹⁵N values. The possible confounding influence of age when interpreting trophodynamics of short-lived consumers appears to be small.     

The effects of preservation on fish tissue stable isotope signatures

The effects of formalin and ethanol preservation on the δ¹³C and δ¹⁵N isotope signatures of Arctic charr Salvelinus alpinus muscle tissue were examined. The lipid content of the tissue samples studied ranged from 3·6 to 6·1% and was not correlated with the magnitude of observed isotopic shifts in preserved samples. Ethanol and formalin significantly depleted and enriched, respectively, the δ¹³C isotope signatures of preserved tissues when compared to control samples. Ethanol did not significantly enrich δ¹⁵N signatures in comparison to controls, whereas formalin did. A meta-analysis of multiple species effects further demonstrated significant preservation effects in fish tissue. Statistical analysis of data obtained by correcting preserved tissue isotope signatures with literature, bootstrapped or meta-analysis derived correction factors demonstrated significant differences between corrected and control sample isotope signatures or failure to produce a unity slope when the data sets were regressed against one another. Species-specific, bootstrapped linear correction models resulted in no such errors. Results suggest that species-specific correction methods should be used for fishes because of the known wide variation in fish tissue lipid content and composition. Accordingly, the use of pilot studies will be required to develop correction factors that properly adjust for preservation effects when interpreting temporal patterns in historic analyses of food webs.     

ISOTOPIC ANALYSIS OF δ13C, δ15N, AND δ34S "A FEEDING TALE" IN TEETH OF THE LONGBEAKED COMMON DOLPHIN, DELPHINUS CAPENSIS

Stable isotopic analyses of carbon, nitrogen, and sulfur were performed on teeth of different ages and sexes of the longbeaked common dolphin, Delphinus capensis, from the Gulf of California. Similarities in diet are suggested between the sexes, with no significant differences in isotopic compositions being observed. Differences in the δ¹³C, δ¹⁵N, and δ³⁴S signatures were found among the age groups (nursing calf, juvenile, subadult, and adult). These data suggest that this species is generally a coastal feeder, and that it changes its feeding habits with increasing age, drawing more nutrition from higher trophic level organisms later in life.     

Latitudinal variation in fecundity among Arctic charr populations in eastern North America

Variation in fecundity was examined from 32 populations of Arctic charr Salvelinus alpinus in eastern North America covering a range of 37° latitude and extending from Maine, U.S.A., to northern Ellesmere Island in the Canadian Arctic. Populations were classed as dwarf, normal or anadromous and covered a suite of different habitat and climatic regimes. Fecundity varied with fork length ( L _F), with L _F adjusted fecundity differing significantly among populations within each of the morphotypes implying that fecundity was a continuously responsive trait influenced by local environmental factors. Latitudinal variation in fecundity was also evident among morphotypes when the simultaneous effects of both latitude and L _F were controlled. There was a significant trade‐off between fecundity and egg size in two of five populations of anadromous Arctic charr, but no evidence in limited data from either normal or dwarf populations. In contrast with some other studies of fecundity in salmonids, there was no evidence for a latitudinal cline in egg size.     

Effects of Partially Anadromous Arctic Charr (Salvelinus alpinus) Populations on Ecology of Coastal Arctic Lakes

Little research has been conducted on effects of iteroparous anadromous fishes on Arctic lakes. We investigated trophic ecology, fish growth, and food web structure in six lakes located in Nunavut, Canada; three lakes contained anadromous Arctic charr (Salvelinus alpinus) whereas three lakes did not contain Arctic charr. All lakes contained forage fishes and lake trout (Salvelinus namaycush; top predator). Isotope ratios (δ¹³C, δ¹⁵N) of fishes and invertebrates did not differ between lakes with and without anadromous Arctic charr; if anadromous Arctic charr deliver marine-derived nutrients and/or organic matter to freshwater lakes, these inputs could not be detected with δ¹³C and/or δ¹⁵N. Lake trout carbon (C):nitrogen (N) and condition were significantly higher in lakes with Arctic charr (C:N = 3.42, K = 1.1) than in lakes without Arctic charr (C:N = 3.17, K = 0.99), however, and ninespine stickleback (Pungitius pungitius) condition was significantly lower in lakes with Arctic charr (K = 0.58) than in lakes without Arctic charr (K = 0.64). Isotope data indicated that pre-smolt and resident Arctic charr may be prey for lake trout and compete with ninespine stickleback. Linear distance metrics applied to isotope data showed that food webs were more compact and isotopically redundant in lakes where Arctic charr were present. Despite this, lake trout populations in lakes with Arctic charr occupied a larger isotope space and showed greater inter-individual isotope differences. Anadromous Arctic charr appear to affect ecology and feeding of sympatric freshwater species, but effects are more subtle than those seen for semelparous anadromous species.     

Stable isotopes of carbon and nitrogen as indicators of diet and trophic structure of the fish community in a shallow hypereutrophic lake

Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes were employed to elucidate energy flows and trophic interactions in Lake Apopka, a hypereutrophic lake in central Florida, U.S.A. Isotope compositions of lake biota ranged from −27·1 to −3·0‰ for δ¹³C, and from 3·7 to 13·9‰ for δ¹⁵N. The food web was based primarily on plankton production with diatoms, Microcystis and zooplankton dominating the diet of fish. Carbon isotope evidence showed that pico- and nano-phytoplankton were not a direct carbon source for fish, but were important to zooplankton. δ¹⁵N mass balance estimates indicated that planktivorous fish obtained 48–85% of their diets from zooplankton. The ∼3‰ range of δ¹⁵N in gizzard shad reflected increasing dependence on zooplankton as fish grew whereas the positive relationship between total length and δ¹⁵N of largemouth bass reflected increasing predation on larger planktivorous fish with growth. The broad ranges of δ¹³C (−25·9 to −9·5‰) and δ¹⁵N (5·8 to 14·4‰) of blue tilapia were indicators of diet diversity. Two presumed omnivores (brown bullhead and white catfish) and piscivores (black crappie, largemouth bass and Florida gar) were found to depend on planktivorous fish. However, stable isotope data revealed no trophic links between blue tilapia, an abundant fish in the near-shore area, and piscivores.     

Evidence for genetic differences in the offspring of two sympatric morphs of Arctic charr

The sub-arctic Lake Fjellfrøsvatn, northern Norway, has two morphs of Arctic charr that are reproductively isolated because they spawn 5 months apart. The smaller morph (≤14 cm L_F ) is confined to the profundal zone of the lake and the larger morph is mainly littoral. Three hypotheses were tested: (i) the offspring of the profundal Arctic charr grow slower than the offspring of the littoral Arctic charr under identical conditions, thus indicating a genetic basis for the slow growth of the profundal Arctic charr in the wild; (ii) the wild phenotypes of the two morphs are morphometrically different and the differences are persistent in the offspring; (iii) the offspring of the two morphs have different behaviour traits under similar treatments. The first hypothesis was rejected; offspring of the profundal morph grew slightly better than offspring of the littoral morph at 10° C in the laboratory. The second and third hypotheses were supported by the data. Wild-caught fish of the two morphs differed in several morphometric characters and most of the differences persisted in the offspring. In the laboratory, offspring of the littoral morph were more active, more aggressive and more pelagic than offspring of the profundal morph and naive offspring of the profundal morph were more effective in eating live chironomid larvae than were offspring of the littoral morph. The data for morphometry and behaviour, but not growth, provide evidence for genetic differences between the two Arctic charr morphs of Fjellfrøsvatn.     

Morphometric analysis of two ecologically distinct forms of Arctic charr, Salvelinus alpinus (L.), in Loch Rannoch, Scotland

Gill–netted samples of Arctic charr from Loch Rannoch, Scotland were bimodal when tested by univariate and multivariate morphometric analyses. The separation into two morphs corresponded very closely (95–98%) with fish classified subjectively in the field as benthic or pelagic, based largely on colour differences and ecological observations. The benthic charr had relatively longer heads, larger eyes and more powerful jaws than the pelagic charr. Unlike sympatric morphotypes described from Scandinavia and Greenland, neither Rannoch morph is dwarfed.     

Assimilate partitioning in Pachyrhizus erosus tubers under short days

The distribution of ¹⁴C products in yam bean ( Pachyrhizus erosus ) plants was measured as a function of time after a ¹⁴CO₂ pulse-chase experiment under short-day conditions. The ¹⁴C declined by more than 70% in leaves during the first two hours, indicating that they actively exported carbon. Tubers were strong storage sinks for carbon and accumulated more than 80% of the total incorporated ¹⁴C after a 72-h chase. The data show that, although sucrose represented about 21% of the tuber non-structural carbohydate, i.e. 15% of the tuber dry weight, ¹⁴C did not accumulate in sucrose but in glucose, fructose and starch. The data indicate the existence of a separate sucrose pool which is affected only very slowly by recent assimilates. As a result, recent photosynthates, temporarily stored, may contribute to the reservoir of carbon available for nitrogen fixation. The data also suggest the existence of two distinct pools of amino acids in the tuber, one utilized mostly for protein synthesis and the other probably stored in the vacuole.     

Patterns of stable isotope signatures in willow warbler Phylloscopus trochilus feathers collected in Africa

We conducted stable isotope analyses of nitrogen and carbon on feathers obtained from willow warblers in Africa to find an explanation for a previously observed pattern of different δ¹⁵N and δ¹³C values across a migratory divide in central Scandinavia. A new data set confirms that north Scandinavian birds of the subspecies P. t. acredula have higher δ¹⁵N values than south Scandinavian birds of the subspecies P. t. trochilus . In Africa, we found significant differences for both δ¹⁵N and δ¹³C values among feathers collected from major geographical regions as well as between countries within regions. Isotope signatures of δ¹⁵N and δ¹³C in Scandinavian acredula matched well with those of willow warblers sampled in southern parts of Africa, but differed from samples obtained in East and West Africa. Isotope signatures in Scandinavian trochilus did not agree with the pattern in any of the three African regions (West, East or South). However, a more detailed analysis of the isotopic data in feathers from countries within West Africa, which is the wintering region of Swedish trochilus based on ringing recoveries, revealed a correspondence with samples from Liberia, the Ivory Coast and Nigeria.     

Lipid‐rich zooplankton subsidise the winter diet of benthivorous Arctic charr (Salvelinus alpinus) in a subarctic lake

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