Factors influencing changes in mercury concentrations in lake water and yellow perch (<Emphasis Type="Italic">Perca flavescens</Emphasis>) in Adirondack lakes

Over the past 20+ years, fish with elevated concentrations of mercury (Hg) have been observed in remote lake districts, including the Adirondack region of New York. Across eastern North America studies have also reported a negative correlation between fish Hg concentration and lake pH. Recent controls in emissions of sulfur dioxide (SO₂) have resulted in some improvement in the acid–base status of acid-impacted surface waters including Adirondack lakes. In addition, there has been an apparent decrease in atmospheric Hg deposition. A synoptic survey of 25 lakes in the Adirondacks was conducted in 1992–1993 to analyze spatial patterns of Hg in the water column and yellow perch (Perca flavescens). The same cluster of 25 lakes was resurveyed in 2005–2006 to evaluate if changes in lake concentrations of Hg species or fish Hg have occurred. We observed a varied response of changes in water chemistry and fish Hg concentrations. In twelve of the resurveyed lakes the yellow perch had lower Hg concentrations, six lakes had yellow perch with higher Hg concentrations, and in seven lakes yellow perch Hg concentrations did not change significantly (α = 0.05). Four variables appear to influence the change in yellow perch Hg concentrations in the Adirondacks: watershed area, elevation, change in pH, and change in fish body condition. We hypothesize that as the acidity in lakes is attenuated, the lakes may become more productive and/or water quality conditions less stressful to fish leading to increasing fish body condition. As fish body condition improves, fish exhibit “growth dilution” of tissue contaminants leading to lower fish Hg concentrations.     

Use of enclosures to detect the contribution of particular zooplankton to growth of young-of-the-year yellow perch (Perca flavescens Mitchell)

Summary Densities of the cladoceran, Holopedium gibberum, were manipulated in 18 enclosures containing juvenile (age 0+) yellow perch (Perca flavescens) and mean-lake densities of other zooplankton. In enclosures, where nearlake densities of all zooplankton species including Holopedium were maintained, young-of-the-year perch grew significantly heavier and longer than in experimental enclosures where Holopedium was excluded. Holopedium comprised between 15–45% of the diet (wet weight) of perch in the first 2 weeks of July in the control treatment (Holopedium at or near ambient lake densities) and only 3–7% of total biomass ingested in the experimental treatment (Holopedium density selectively reduced). Predation on Holopedium decreased dramatically after the 2nd week of July in the control treatment after which Chaoborus, chironomids, and Sida became dominant prey items (by weight) of juvenile perch. These findings suggest that growth and survivorship of age 0+ perch in Precambrian Shield lakes may be coupled to Holopedium abundance. Thus, utilization of Holopedium by young-of-the-year yellow perch may affect recruitment of this species since overwintering survivorship, range of accessible prey sizes or species, and vulnerability of juvenile perch to predation by larger fish depend on body size, which is reduced when Holopedium is excluded from the diet.     

Effect of body mass and water temperature on the standard metabolic rate of juvenile yellow perch, Perca flavescens (Mitchill)

Fish respiration rates that are presumed to represent standard metabolic rates (SMR) may sometimes include an unspecified energy expenditure associated with activity and digestion. This situation may introduce a bias in bioenergetics models because standard metabolism, digestion, and activity may not be affected by the same environmental conditions. The aim of this study was to (1) develop a SMR model for juvenile yellow perch, Perca flavescens (Mitchill), that represent the minimum energy expenditure required to maintain life and (2) compare the results of this study with published perch metabolic rates and bioenergetics models. SMR was estimated for yellow perch over a range of body␣mass (4.4–24.7 g) and water temperature (12–20°C). The intercept of the relationship between fish respiration and swimming velocity obtained during forced swimming experiments was used to determine SMR. SMR estimated by the present study were comparable to values presented by two published studies on Eurasian perch, Perca fluviatilis L. However, estimated SMR were 4.1–20.9 times lower than values of a third respirometry study and predictions of bioenergetics models for perch. The present study suggests that published SMR models may sometimes include a significant fraction of energy expenditures (39.2–75.9%) associated with digestion and activity. This may complicate the implementation and the interpretation of fish bioenergetics models. The present study indicates that the intercept of respiration-velocity relationships and long-term respiration rates during starvation experiments may provide similar and reliable SMR values.     

The Shrimp Caridina nilotica in Lake Victoria (East Africa), Before and After the Nile Perch Increase

The shrimp Caridina nilotica is a major prey of the introduced Nile perch in Lake Victoria. In spite of heavy predation, the density of shrimps increased after the Nile perch boom and the concomitant disappearance of the haplochromine cichlids. In the same period, the mean size of gravid shrimps and the size at first maturity declined. This seems to indicate an increased predation pressure on adult shrimps. Before the Nile perch upsurge, specialised shrimp eaters and piscivores, among the haplochromine cichlids, only took adult shrimps, whereas we assume that most haplochromines used to include juvenile shrimps into their diet. Another important predator on adult shrimps was Bagrus docmak. The combined density of predators on adult shrimps in the pre-Nile perch era was estimated at 10 kg ha⁻¹ and the potential predators on juveniles were estimated at 170 kg ha⁻¹. After the Nile perch upsurge, only Nile perch up to 10 cm TL and Rastrineobola argentea fed on juvenile shrimps (ca. 36 kg ha⁻¹) and Nile perch from 10 to 50 cm TL (ca. 13 kg ha⁻¹) fed on adults. These rough estimates of the biomass of predators on shrimps before and after the Nile perch upsurge indicate a reduced predation pressure on juvenile shrimps. The disappearance of the haplochromines may have released competition with small Nile perch for juvenile shrimps, thus enhancing the recruitment of Nile perch.     

Temporal variation of δ<Superscript>13</Superscript>C of larch leaves from a montane boreal forest in Mongolia

This paper reports the temporal variation (2002–2004) in foliar δ¹³C values, which are indicative of long-term integrated photosynthetic and water use characteristics, of Siberian larch (Larix sibirica Ledeb.) trees in a montane forest at Mongonmorit, NE Mongolia. At the stand, the δ¹³C value for understory shaded leaves was more negative by 2‰ on average than that for sunlit leaves sampled concurrently from open and sun-exposed environments in a forest gap. The δ¹³C value of both sunlit and shaded leaves showed pronounced intra- but relatively small inter-seasonal variations. The δ¹³C value was more positive for juvenile than mature leaves. We conjecture that juvenile leaves may derive carbon reserves in woody tissues (e.g., stems). Regardless of leaf habitats, the δ¹³C value was also affected by insect herbivores occurred in mid summer of 2003, being more negative in newly emerging leaves from the twigs after defoliation than in non-defoliated mature leaves. This pattern seems to contrast with that for the juvenile leaves in the early growing season. We surmise that the newly emerging leaves used stored organic carbon that was depleted due to fractionation during remobilization and translocation for leaf regrowth. There was also intra- and inter-seasonal variation in the foliar N concentrations and C:N ratios. A good positive (negative) correlation between the foliar δ¹³C values and N concentrations (C:N ratios) was also observed for both sunlit and shaded leaves, suggesting that the relationship between water and nitrogen use is a crucial factor affecting the plant carbon–water relationship in this mid latitude forest with a cold semiarid climate. Our isotopic data demonstrate that the larches in NE Mongolia exhibits relatively higher water use efficiency with a distinct within-season variability.     

Changes in trophic linkages to shortfin eels (<Emphasis Type="Italic">Anguilla australis</Emphasis>) since the collapse of submerged macrophytes in Lake Ellesmere,New Zealand

Lake Ellesmere (Te Waihora) is a nationally important coastal brackish lake in New Zealand, however degradation in water quality and loss of submerged macrophytes over past decades have raised concerns in regards to the declining status of the lake’s commercial and customary fisheries, predominantly targeted at shortfin eels (Anguilla australis). We investigated foodweb dynamics and trophic linkages to shortfin eels in Lake Ellesmere using a combination of abundance assessments, dietary studies, and stable isotope analyses. Data from our study are compared with historical data sets on benthic invertebrate community composition and shortfin eel diets to trace changes in the trophic linkages to top predators that have occurred since the late 1960s. Stable isotope analyses indicate that the foodweb is predominantly driven by epipelic and phytoplankton derived carbon sources, although it was difficult to discriminate between these two carbon pools because of wind-driven resuspension of lake sediments. Comparison of our survey results with historical data sets indicates a clear shift in benthic biota from being dominated by phytofaunal species such as Potamopyrgus antipodarum (comprising 90% of total invertebrate biomass) during the 1960s, to now being almost entirely comprised of subterranean species such as Chironomus zealandicus and oligochaetes (together comprising 82% of total invertebrate biomass). This shift in benthic communities has resulted in significant changes in the size-specific diet of juvenile shortfin eels (<400 mm) from those reported for Lake Ellesmere during the mid 1970s, with Chironomus larvae now comprising 65% of the diets of juvenile eels, whereas historically P. antipodarum was the dominant food item (>30% of total biomass). This shift towards foraging on smaller sediment-dwelling species could have implications for juvenile eel bioenergetics, and may help explain why juvenile shortfin growth rates have significantly decreased in past decades. Juvenile shortfins now appear to switch to foraging on preyfish (mainly common bullies, Gobiomorphus cotidianus) at a smaller size (≈400 mm) than historically recorded (>500 mm). Dietary and stable isotope signatures indicated that small shortfins (100–299 mm) have considerable overlap in trophic position (δ¹³C = −20.4‰, δ¹⁵N = 13.6‰) with common bullies (δ¹³C = −20.5‰, δ¹⁵N = 13.7‰), the dominant fish in Lake Ellesmere (92% of total abundance CPUE), potentially indicating that these two species may directly compete for food resources. These findings again highlighted the importance of C. zealandicus in sustaining the fish populations of the lake. Handling editor: S. Declerck     

Piscivorous birds as top predators and fishery competitors in the lagoon ecosystem

Trophic patterns of omnivorous freshwater shrimps, Exopalaemon modestus and Macrobrachium nipponensis, were investigated in two shallow eutrophic lakes by using stable isotope analysis. δ¹⁵N and δ¹³C of M. nipponensis and E. modestus increased with increasing body weight, which might be attributed to larger individuals ingesting organisms that feed higher up the food chain and/or increased assimilation of benthic food items with enriched isotopic signatures. Of the freshwater shrimps occurring in the studied lakes, those from Lake Taihu had significantly elevated δ¹⁵N and δ¹³C values (4.3‰ and 1.8‰, respectively) compared with those from the less eutrophic Lake Chaohu, indicating that the isotopic signature might partially reflect the trophic states of their habitats. Mixing model results suggested that the benthic food web provides the primary carbon source for both shrimp species, and that E. modestus assimilated relatively more pelagic food sources than M. nipponensis in these lakes. Handling editor: S. Wellekens     

Feeding behavior and modeled energetic intake of common loon (Gavia immer) adults and chicks on small lakes with and without fish

We examined the behavior of common loons, Gavia immer (Brünnich), breeding on small, shallow lakes in central Alberta, Canada that were naturally fishless or contained only small-bodied fishes (minnow lake). For both lake types, adults spent >90% of their time on the nesting lake and >50% of their time foraging. Adult loons on fishless lakes dove more frequently, but dives were of shorter duration than loons on lakes with fish. On two intensively studied fishless lakes, adults fed chicks macroinvertebrates, particularly leeches, whereas on a focal minnow lake, fish made up >70% of prey items delivered by adults. Chicks >36 days of age on a minnow lake spent >50% of their time foraging, whereas older chicks on fishless lakes were highly dependent on food provisioning by adults. Models based on observed foraging patterns indicated that prey size was a better predictor of success in meeting energetic requirements than was feeding behavior (e.g., dive rate, dive success). For most models, estimated energetic intake was higher for loons on minnow lakes than on fishless lakes. Our behavioral observations and model results are consistent with surveys in central Alberta that indicate that breeding Common Loons frequently establish territories on small lakes, but that chicks hatched on lakes completely lacking fish rarely fledge and only if sufficient large invertebrates such as leeches are available.     

Three new interstitial dorvilleids (Annelida: Polychaeta) from the Cymodocea nodosa meadows of the Canary Islands

Resource–consumer relationships in Lake Victoria were investigated by use of stable isotope data. ¹³C and ¹⁵N signatures were determined for organisms at a deep (22 m) and a littoral (5 m) site in the Napoleon Gulf near Jinja, Uganda. Results suggest that two food chains operate at the deep site, one leading from a shrimp (Caridina nilotica) to juvenile Nile perch (Lates niloticus), the second leading from zooplankton (copepods and cladocerans) to a cyprinid (Rastrineobola argentea) and lake flies (Chaoborus). Isotopic evidence suggests that shrimp eat suspended particulates and benthos, not crustacean zooplankton or water hyacinth (Eichhornia crassipes). Resource–consumer relationships revealed in this study have implications for understanding future yields of the economically important Nile perch fishery.     

Selenium and Mercury in Native and Introduced Fish Species of Patagonian Lakes,Argentina

A survey of mercury (Hg) and selenium (Se) contents was performed in fish collected from lakes located in two National Parks of the northern patagonian Andean range. Two native species, catfish (Diplomystes viedmensis) and creole perch (Percichthys trucha), and three introduced species, brown trout (Salmo trutta), rainbow trout (Oncorhynchus mykiss), and brook trout (Salvelinus fontinalis), were caught from lakes Nahuel Huapi, Moreno, Traful, Espejo Chico, and Guillelmo belonging to Nahuel Huapi National Park and from lakes Futalaufquen and Rivadavia, Los Alerces National Park. In lake Moreno, fish diet items were analyzed and rainbow trout grown in a farm. Hg and Se were measured in muscle and liver tissues by instrumental neutron activation analysis. The average concentrations in muscle of Hg for all species, ages, and lakes are between 0.4 to 1.0 μg g⁻¹ dry weight (DW) with a few fish, mainly native, exceeding the United States Environmental Protection Agency health advisory for freshwater fish limited consumption, and from 0.8 to 1.5 μg g⁻¹ DW for Se. Average concentrations in liver of Hg in all species range from 0.4 to 0.9 μg g⁻¹ DW. Brown trout, the top predator in these lakes, showed the lowest average Hg burden in both tissues. Se concentrations in the liver of brown and rainbow trout, up to 279 μg g⁻¹ DW, are higher than those expected for nearly pristine lakes, exceeding 20 μg g⁻¹ DW, the threshold concentration associated with Se toxicity. These species show lower Hg contents in muscle, suggesting a possible detoxification of Hg by a Se-rich diet. Creole perch and velvet catfish livers have lower Se concentrations, with a narrower span of values (2.3 to 8.5 μg g⁻¹ and 3.3 to 5.5 μg g⁻¹ DW respectively).     

Mercury and Selenium in Blood and Epidermis of Bottlenose Dolphins (Tursiops truncatus) from Sarasota Bay, FL: Interaction and Relevance to Life History and Hematologic Parameters

Blood and epidermal biopsies from free-ranging Tursiops truncatus captured and released during either summer or winter health assessments in Sarasota Bay, FL, were evaluated for concentrations of mercury, selenium, stable isotopes (δ¹³C and δ¹⁵N), and blood glutathione peroxidase activity in conjunction with routine hematology and serum chemistry panels. Major objectives were to: 1) quantify and describe relationships among mercury, selenium, glutathione peroxidase, and stable isotopes of C and N in blood and epidermis; 2) elucidate major parameters that influence blood mercury and glutathione peroxidase activity; 3) relate measures of tissue mercury, selenium, and glutathione peroxidase to specific ecological, hematological, morphological, or life history parameters, including season, sex, age, and trophic level. Mercury in both tissues examined is almost exclusively methylmercury. Epidermal concentrations of mercury and selenium reflect their respective amounts in blood, albeit at several times blood concentrations of mercury. The strong association between blood mercury and serum selenium, in conjunction with a lack of significant correlation between blood mercury and glutathione peroxidase, implies that a substantial proportion of blood mercury is affiliated with another selenium-containing moiety or is related to recent dietary intakes (e.g., trophic level, intensive fish consumption). Circulating blood mercury may be described in terms of serum selenium concentration, along with interaction terms among serum selenium, blood δ¹⁵N, and age. Current selenium concentrations in Sarasota Bay dolphins appear adequate for maintenance of blood glutathione peroxidase activity. However, dolphins evidently are subject to seasonal exacerbation of oxidative stress, which might render them more vulnerable to toxic effects of mercury. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Potential risk to common loons (Gavia immer) from methylmercury exposure in acidified lakes

Piscivorous birds and mammals in areas remote from point sources of Hg contamination may be exposed to dietary methylmercury concentrations that are sufficiently high to cause reproductive impairment. Common loons (Gavia immer) were observed to show aberrant nesting behavior and low overall reproductive success when Hg concentrations in prey (small fish and crayfish) averaged > 0.3 µg g^–1 wet weight (Barr, 1986), levels known to occur in fish from many lakes in central Ontario. We used data on Hg in Ontario fish to estimate the proportion of lakes where fish small enough for loons to eat (< 250 g) had Hg concentrations that exceeded estimated thresholds for reproductive impairment. Up to 30 % of lakes exceeded thresholds for reproductive impairment, depending on the species of fish and the threshold Hg concentrations chosen. There was a significant negative correlation between fish-Hg concentration and lake pH in most fish species examined. For these species, reductions in sulfate deposition rates are predicted to result in a corresponding reduction of lakes in Ontario having fish with potentially toxic concentrations of Hg.     

Costs of reproduction in circumpolar <Emphasis Type="Italic">Parnassia palustris</Emphasis> L. in light of global warming

In frequently burnt savannas, saplings face the formidable challenge of both recovering from, and eventually growing tall enough to escape from, frequent fire damage. The aim of this study was to explore how saplings allocate carbon to achieve these ends through carbon partitioning, storage and remobilization. Lignotuber total non-structural carbohydrate (TNC) concentrations and δ¹³C values of Acacia karroo (Fabaceae; Mimosoideae) were determined in plants from two different juvenile stages. These were one year after a fire when the plant consisted of numerous leafy shoots or coppices (“coppicing” stage), and three years after a fire when the plant consisted of one pole-like stem (“Gulliver” stage). Gulliver lignotubers were found to have significantly larger TNC pools (150 g vs. 97 g) and larger TNC concentrations (33% vs. 24%, w/w) than coppice lignotubers showing that post-coppice Gullivers recharged TNC in the lignotuber. δ¹³C values from the stems of plants in the Gulliver stage were significantly enriched (>1‰) in ¹³C compared to both coppicing (P < 0.01) and adult (P < 0.05) plants. Changes in both the amount of stored carbon and in the δ¹³C values indicated dependence on stored carbon reserves, and partially heterotrophic growth for initial resprouting. The plants appeared to use both current photosynthate and stored carbon reserves for growth of the Gulliver stem. The use of stored carbon is hypothesized to promote fast stem growth rates to a height where saplings escape fire injury.     

Responses of breeding common loons to human activity in upper Michigan

Breeding populations of the common loon (Gavia immer) in Michigan have declined in the past several decades, resulting in classification of this species as threatened under state law. Factors responsible for the decline are unknown, but may include toxic contaminants, mortality in commercial fish nets, and human disturbance of breeding sites. To assess the latter possibility, 960 hours of observation were devoted to observing human-loon interactions on two sets of lakes, one with restricted human use (minimal or no shoreline development and open to canoe use only), and another with unrestricted use (varying amounts of shoreline development and motorboat traffic). Six mated pairs of loons on six restricted use (r-u) lakes were compared to eight loon pairs on seven open-use (o-u) lakes. The number of nests that hatched young per nest started was not significantly different between the two sets of lakes (7 of 13 (0.62) on r-u lakes vs 8 of 17 (0.47) on o-u lakes), despite significantly more human activity on o-u lakes. Chicks hatched per pair of loons were likewise not significantly different (1.1 vs 1.2 on r-u and o-u lakes, respectively). Fledging success was significantly lower on r-u lakes (7 chicks fledged of 11 hatched) than on o-u lakes (13 fledged of 13 hatched). Human activity on o-u lakes was 2–3 times that on r-u lakes during chick rearing, but time spent by adult loons tending and feeding chicks was not significantly different between the two types of lakes. The larger size of most o-u lakes may have allowed loons a greater opportunity to avoid human disturbance. Higher levels of human activity did not affect production of chicks by loons under the conditions observed, but these results should not be extrapolated to lakes experiencing much higher human use.     

Nitrogen availability patterns in white-sand vegetations of Central Brazilian Amazon

Addressing spatial variability in nitrogen (N) availability in the Central Brazilian Amazon, we hypothesized that N availability varies among white-sand vegetation types (campina and campinarana) and lowland tropical forests (dense terra-firme forests) in the Central Brazilian Amazon, under the same climate conditions. Accordingly, we measured soil and foliar N concentration and N isotope ratios (δ¹⁵N) throughout the campina-campinarana transect and compared to published dense terra-firme forest results. There were no differences between white-sand vegetation types in regard to soil N concentration, C:N ratio and δ¹⁵N across the transect. Both white-sand vegetation types showed very low foliar N concentrations and elevated foliar C:N ratios, and no significant difference between site types was observed. Foliar δ¹⁵N was depleted, varying from −9.6 to 1.6‰ in the white-sand vegetations. The legume Aldina heterophylla had the highest average δ¹⁵N values (−1.5‰) as well as the highest foliar N concentration (2.1%) while the non-legume species had more depleted δ¹⁵N values and the average foliar N concentrations varied from 0.9 to 1.5% among them. Despite the high variation in foliar δ¹⁵N among plants, a significant and gradual ¹⁵N-enrichment in foliar isotopic signatures throughout the campina–campinarana transect was observed. Individual plants growing in the campinarana were significantly enriched in ¹⁵N compared to those in campina. In the white-sand N-limited ecosystems, the differentiation of N use seems to be a major cause of variations observed in foliar δ¹⁵N values throughout the campina–campinarana transect.     

Involvement of polyamines in cytoplasmic male sterility of stem mustard (Brassica juncea var. tsatsai)

Changes in carbon isotope composition(¹³C) and leaf morphology associated withvegetative phase change were monitored in Metrosiderosexcelsa Sol. ex Gaertn. (family Myrtaceae). Plants of threeontogenetic states were used: juvenile seedlings, micropropagated plants in arejuvenated state, and reproductively mature plants bearing leaves with adultcharacteristics. The effects of temperature regime (32/24 °C,24/16 °C, and 16/8 °C day/night) and plantarchitecture (branched and single-stemmed plants) were studied in two separateexperiments. Although both juvenile and rejuvenated plants exhibited juvenileleaf morphology at the start of the experiments, there was no differencebetweenleaf ¹³C in these plants and that in adultplantsat this time (mean ca. –27%). Vegetative phase change occurred injuvenileand rejuvenated plants grown at 24/16 °C, and there was acorresponding increase in leaf ¹³C (from ca.–27% to –23%) in these two groups of plants. Leaf¹³C in adult plants remained relatively constant(ca. –26%) at 24/16 °C. There was little change in leaf¹³C in all plant states maintained at 32/24°C or 16/8 °C, and vegetative phase change didnot occur in juvenile and rejuvenated plants grown under these two temperatureregimes. Rejuvenated plants grown in a greenhouse also exhibited a progressivedevelopment of adult leaf morphology, accompanied by an increase in leaf¹³C, an effect that was more pronounced insingle-stemmed (from –26.4% to ca. –24%) than in branched plants. Itis suggested that increasing ¹³C in juvenile andrejuvenated plants undergoing phase change is a result of reduced sink strengthin single-stemmed plants, and to a lesser extent within each branch of branchedplants, causing reduced stomatal conductance and photosynthesis.     

Investigation of mercury concentrations in fur of phocid seals using stable isotopes as tracers of trophic levels and geographical regions

Recent studies have shown that the complementary analysis of mercury (Hg) concentrations and stable isotopic ratios of nitrogen (δ¹⁵N) and carbon (δ¹³C) can be useful for investigating the trophic influence on the Hg exposure and accumulation in marine top predators. In this study, we propose to evaluate the interspecies variability of Hg concentrations in phocids from polar areas and to compare Hg bioaccumulation between both hemispheres. Mercury concentrations, δ¹⁵N and δ¹³C were measured in fur from 85 individuals representing 7 phocidae species, a Ross seal (Ommatophoca rossii), Weddell seals (Leptonychotes weddellii), crabeater seals (Lobodon carcinophagus), harbour seals (Phoca vitulina), grey seals (Halichoerus grypus), ringed seals (Pusa hispida) and a bearded seal (Erignathus barbatus), from Greenland, Denmark and Antarctica. Our results showed a positive correlation between Hg concentrations and δ¹⁵N values among all individuals. Seals from the Northern ecosystems displayed greater Hg concentrations, δ¹⁵N and δ¹³C values than those from the Southern waters. Those geographical differences in Hg and stable isotopes values were likely due to higher environmental Hg concentrations and somewhat greater number of steps in Arctic food webs. Moreover, dissimilarities in feeding habits among species were shown through δ¹⁵N and δ¹³C analysis, resulting in an important interspecific variation in fur Hg concentrations. A trophic segregation was observed between crabeater seals and the other species, resulting from the very specific diet of krill of this species and leading to the lowest observed Hg concentrations.     

Spatial and temporal variabilities of δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N within lower trophic levels of a large lake: implications for estimating trophic relationships of consumers

Stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) often have unique values among lake habitats (e.g. benthic, littoral, pelagic), providing a widely used tool for measuring the structure and energy flow in aquatic food webs. However, there has been little recognition of the spatial and temporal variabilities of these isotopes within habitats of aquatic ecosystems. To address this, δ¹³C and δ¹⁵N were measured in seston, zebra mussels (Dreissena polymorpha) and young-of-year (YOY) yellow (Perca flavescens), and white perch (Morone americana) collected from four sites across the offshore habitat of the western basin of Lake Erie during June–September 2009. Values of δ¹³C and δ¹⁵N showed significant spatial and temporal variations, with month accounting for >50% of the variation, for both stable isotopes and all the species except seston. Such variation in isotope values has the potential to significantly influence or confound interpretation of stable isotopes in measures, such as trophic position (TP) which use lower trophic level organisms as their baseline. For example, TP was found to vary up to 0.7 for yellow and white perch (TP = δ¹⁵N_fish − δ¹⁵N_{zebra mussel}/diet-tissue fractionation factor) depending on the zebra mussel data used (e.g., from a different location or a different collection month). As the use of stable isotopes continues to move from qualitative to more quantitative measures of trophic structure, food web research must recognize the importance of stable isotopes' variability in lower trophic level organisms, especially in large lake systems.     

Mercury and cadmium concentrations in the tissues of three species of southern albatrosses

Cadmium and mercury concentrations were measured in the tissues of 64 individual albatrosses [23 wandering albatrosses (Diomedea exulans), 9 royal albatrosses (Diomedea epomophora) and 32 shy albatrosses (Thalassarche cauta)] which were killed as by-catch in longline fishing activities between 1991 and 1994. Mercury concentrations were also determined for 33 shy albatross eggs (excluding shells). The birds were all sexed and assigned to one of two age classes (immature and adult). The three species exhibited differences both in overall concentrations of cadmium and mercury, and also in the pattern of accumulation of metals with age and sex. Wandering albatrosses exhibited the highest mercury concentrations with a mean concentration in adult liver samples of 920.0 ± 794.1 μg g⁻¹ dry weight. Shy albatrosses had the lowest mercury concentrations with mean concentrations in adult livers of 36.3 ± 21.4 mg g⁻¹ dry weight. The highest mercury concentration was 1800 μg g⁻¹ for an adult female wandering albatross. Cadmium concentrations were less variable, with adult royal albatrosses having the highest average concentrations (180.0 ± 165.0 in adult kidneys) and adult shy albatrosses the lowest (40.1 ± 20.0 in adult kidney). The highest individual cadmium concentration was 287 μg g⁻¹ for a juvenile wandering albatross. There was no evidence of increased accumulation of cadmium with age in any of the species, but wandering albatrosses showed higher mercury concentrations in adults than juveniles. Female wandering albatrosses also had significantly higher mercury concentrations than males. The mercury contents of the shy albatross eggs were very low, with a maximum concentration of 5.4 μg g⁻¹. The results of this study are consistent with the findings of previous work on albatrosses and support the notion that the life-history strategy of these species (i.e. long-lived with low reproductive output) may be an important determinant in the concentrations of some metals found in their tissues. Accepted: 15 February 1999