Zoological Institute of Russian Academy of Sciences, Universitetskaya emb. 1, Saint-Petersburg 199034, Russian Federation. Electronic address: golubkov@zin.ru.
Organic pollution is a serious environmental problem for the coastal zones of seas. The study tested the hypothesis that allochthonous organic carbon derived from St. Petersburg wastewaters is a significant basal resource of carbon for the benthic food webs. We analyzed stable isotope composition of carbon and nitrogen in suspended organic matter in the Neva Estuary and in the tissues of macroinvertebrates and fish. The Stable Isotope Bayesian mixing model showed that waste waters were an important source of carbon for the most of consumers in the Neva Estuary. The autochthonous carbon produced by phytoplankton was a significant source of carbon only for some macroinvertebrates. The main consumers of the carbon derived from waste waters were tubificid worms, chironomid larvae and alien polychaete, which currently dominate in the zoobenthos of the estuary. These species replaced the former dominants, native crustaceans, which to a lesser extent use anthropogenic carbon.
Mosses collected decades ago and stored in herbaria are often used to assess historical nitrogen deposition. This method is effectively based on the assumption that tissue N concentration remains constant during storage. The present study raises serious doubt about the generality of that assumption. We measured tissue N and C concentrations as well as d15N, d13C, Pb and Mg in herbarium and present day samples of seven bryophyte species from six sites across Denmark. While an increase in nitrogen deposition during the last century is well-documented for the study site, we surprisingly found foliar N concentration to be higher in historical samples than in modern samples. Based on d15N values and Pb concentration, we find nitrogen contamination of herbarium specimens during storage to be the most likely cause, possibly in combination with dilution though growth and/or decomposition during storage. We suggest ways to assess contamination and recommend caution to be taken when using herbarium specimens to assess historical pollution if exposure during storage cannot be ruled out.
Baleen as a biomonitor of mercury content and dietary history of North Atlantic minke whales (Balaenopetra acutorostrata): combining elemental and stable isotope approaches.
Baleen is an incrementally-growing tissue of balaenopteran whales which preserves relatively well over time in museums and some archeological sites, and, therefore might be useful for studies examining long-term changes of metal levels in whales. This study examined Hg and stable C and N isotopic composition of baleen plates of the North Atlantic minke whale (Balaenoptera acutorostrata), which continues to be a food source for people in Greenland and elsewhere. We compared the Hg levels and stable isotopes of major tissues (kidney, liver and muscle) with those of baleen plates to see whether baleen could be used as a biomonitor of variations of Hg intake and diet both between individuals and within individuals over time. Mercury was significantly correlated with concentrations in all tissues (kidney, liver and muscle). Stable C and N isotopes in baleen were generally similar to those of muscle, which reflects the recent (approximately one month) feeding of the whale, but in some individuals there were significant differences between baleen and muscle. Sectioning of baleen into 1 cm longitudinal increments showed that these differences were due to marked dietary shifts by some individuals over time that had been recorded in the baleen but were lost from the muscle record. Whole baleen C and N isotopes were better correlated with tissue Hg levels, suggesting that baleen may provide a more reliable indicator of long-term average diet, which in turn may be better related to Hg accumulation in tissues than the shorter-term diet record contained in muscle.
Bayesian stable isotope mixing models (BSIMMs) for d13 C and d15 N can be a useful tool to reconstruct diets, characterize trophic relationships, and assess spatiotemporal variation in food webs. However, use of this approach typically requires a priori knowledge on the level of enrichment occurring between the diet and tissue of the consumer being sampled (i.e. a trophic discrimination factor or TDF). Trophic discrimination factors derived from captive feeding studies are highly variable, and it is challenging to select the appropriate TDF for diet estimation in wild populations. We introduce a novel method for estimating TDFs in a wild population-a proportionally balanced equation that uses high-precision diet estimates from nest cameras installed on a subset of nests in lieu of a controlled feeding study (TDFCAM ). We tested the ability of BSIMMs to characterize diet in a free-living population of gyrfalcon Falco rusticolus nestlings by comparing model output to high-precision nest camera diet estimates. We analysed the performance of models formulated with a TDFCAM against other relevant TDFs and assessed model sensitivity to an informative prior. We applied the most parsimonious model inputs to a larger sample to analyse broad-scale temporal dietary trends. Bayesian stable isotope mixing models fitted with a TDFCAM and uninformative prior had the best agreement with nest camera data, outperforming TDFs derived from captive feeding studies. BSIMMs produced with a TDFCAM produced reliable diet estimates at the nest level and accurately identified significant temporal shifts in gyrfalcon diet within and between years. Our method of TDF estimation produced more accurate estimates of TDFs in a wild population than traditional approaches, consequently improving BSIMM diet estimates. We demonstrate how BSIMMs can complement a high-precision diet study by expanding its spatiotemporal scope of inference and recommend this integrative methodology as a powerful tool for future trophic studies.
Beyond bulk d15N: Combining a suite of stable isotopic measures improves the resolution of the food webs mediating contaminant signals across space, time and communities.
Top predators are used as indicators of contaminant trends across space and time. However, signals are integrated over complex food webs, and variation in diet may confound such signals. Trophic position, assessed by bulk d15N, is widely used to infer the variation in diet relevant to contamination, yet a single variable cannot completely describe complex food webs. Thus, we examined relationships across three aquatic systems varying from a single species to a small food web using bulk values from four isotopes and 21 amino acid-specific values. Because variation in baseline ('source') d15N can confound estimates of trophic position , we calculated trophic position from the difference between d15Ntrophic (d15N for amino acids that change with trophic position) and d15Nsource (d15N for amino acids that do not change with trophic position). Across all three systems, variation in d15Nsource explained over half of the variation in bulk d15N, and stable isotope values that reflected the base of the food web (d13C, d18O, d34S) predicted contaminants as well or better than d15N-which was supported by a meta-analysis of other studies. In ospreys feeding in lakes, variation in d15Nsource across space created a spurious relationship between SDDT and apparent trophic position, and masked a relationship between SPCB and trophic position. In a seabird guild, changes in diet over time obscured temporal variation in contaminants over five decades. In Arctic fish and invertebrates, more accurate trophic magnification factors were calculated using d15Ntrophic-source. Thus, (1) using d15Ntrophic-source, instead of bulk d15N, avoided incorrect conclusions and improved accuracy of trophic magnification factors necessary to assess risk to top predators; and (2) diet assessed with multiple spatial isotopes, rather than d15N alone, was essential to understand patterns in contaminants across space, time and biological communities. Trophic position was most important for lipophilic 'legacy' contaminants (SDDT, SPCB) and habitat was most important for other contaminants (SPBDE, SPFAS, mercury). We argue that the use of amino acid-specific analysis of d15N alongside 'non-trophic' isotopes should be a core feature of any study that examines the influence of trophic position on chemical pollution, as required for a chemical to be added to international conventions such as the Stockholm Convention.
Department of Environmental Sciences, University of Helsinki, P.O. Box 65, Viikinkaari 2a, 00014 Helsinki, Finland. Electronic address: sirkku.manninen@helsinki.fi.
The deposition of reactive nitrogen (N) compounds currently predominates over sulphur (S) deposition in most of the cities in Europe and North America. Acidophytic lichens growing on tree trunks are known to be sensitive to both N and S deposition. Given that tree species and climatic factors affect the composition of epiphytic lichen communities and modify lichen responses to air pollution, this study focused on the impact of urban air pollution on acidophytes growing on boreal conifer trunks. The study was performed in the Helsinki metropolitan area, southern Finland, where annual mean nitrogen dioxide (NO2) concentrations range from 4-5µgm-3 to >50µgm-3. In addition, background forest sites in southern and northern Finland were included. The results demonstrated elevated N contents (=0.7%) in Hypogymnia physodes and Platismatia glauca at all the sites where the species occurred. In the Helsinki metropolitan area, a higher frequency of green algae+Scoliociosporum chlorococcum and reduced numerical frequencies of other indicator lichen species (e.g. Pseudevernia furfuracea, Bryoria spp., Usnea spp.) were associated with elevated atmospheric concentrations of NO2 and particulate matter containing N, as well as elevated concentrations of inorganic N in bark. The N isotope values (d15N) of lichens supported the uptake of oxidized N mainly originating from road traffic. Sulphur dioxide (SO2) also negatively affected the most sensitive species, despite the current low levels (1-4µgm-3yr-1). Critical levels of 5µgNO2m-3yr-1 and 0.5µgNH3m-3yr-1, and a critical load of 2-3kgNha-1yr-1 are proposed for protecting the diversity of boreal acidophytes. This study calls for measurements of the throughfall of various N fractions in urban forest ecosystems along precipitation and temperature gradients to verify the proposed critical levels and loads.
Differential ecophysiological response of deciduous shrubs and a graminoid to long-term experimental snow reductions and additions in moist acidic tundra, Northern Alaska.
Changes in winter precipitation that include both decreases and increases in winter snow are underway across the Arctic. In this study, we used a 14-year experiment that has increased and decreased winter snow in the moist acidic tussock tundra of northern Alaska to understand impacts of variation in winter snow depth on summer leaf-level ecophysiology of two deciduous shrubs and a graminoid species, including: instantaneous rates of leaf gas exchange, and d(13)C, d(15)N, and nitrogen (N) concentrations of Betula nana, Salix pulchra, and Eriophorum vaginatum. Leaf-level measurements were complemented by measurements of canopy leaf area index (LAI) and depth of thaw. Reductions in snow lowered summer leaf photosynthesis, conductance, and transpiration rates by up to 40% compared to ambient and deep snow conditions for Eriophorum vaginatum, and reduced Salix pulchra conductance and transpiration by up to 49%. In contrast, Betula nana exhibited no changes in leaf gas exchange in response to lower or deeper snow. Canopy LAI increased with added snow, while reduced winter snow resulted in lower growing season soil temperatures and reduced thaw depths. Our findings indicate that the spatial and temporal variability of future snow depth will have individualistic consequences for leaf-level C fixation and water flux by tundra species, and that these responses will be manifested over the longer term by changes in canopy traits, depth of thaw, soil C and N processes, and trace gas (CO2 and H2O) exchanges between the tundra and the atmosphere.
Lancaster University, Soil and Ecosystem Ecology Laboratory, Institute of Environmental and Natural Sciences, Lancaster LA1 4YQ, UK. edayres@nrel.colostate.edu
Mosses are one of the most diverse and widespread groups of plants and often form the dominant vegetation in montane, boreal and arctic ecosystems. However, unlike higher plants, mosses lack developed root and vascular systems, which is thought to limit their access to soil nutrients. Here, we test the ability of two physiologically and taxonomically distinct moss species to take up soil- and wet deposition-derived nitrogen (N) in natural intact turfs using stable isotopic techniques (15N). Both species exhibited increased concentrations of shoot 15N when exposed to either soil- or wet deposition-derived 15N, demonstrating conclusively and for the first time, that mosses derive N from the soil. Given the broad physiological and taxonomic differences between these moss species, we suggest soil N uptake may be common among mosses, although further studies are required to test this prediction. Soil N uptake by moss species may allow them to compete for soil N in a wide range of ecosystems. Moreover, since many terrestrial ecosystems are N limited, soil N uptake by mosses may have implications for plant community structure and nutrient cycling. Finally, soil N uptake may place some moss species at greater risk from N pollution than previously appreciated.
Stable isotope analysis is widely used to reconstruct diet, delineate trophic interactions, and determine energy pathways. Such ecological inferences are based on the idea that animals are, isotopically, what they eat but with a predictable difference between the isotopic ratio of a consumer and that of its diet, coined as the discrimination factor. Providing correct estimates of diet-consumer isotopic discrimination in controlled conditions is key for a robust application of the stable isotopes technique in the wild.
Using a Finnigan Mat Delta Plus isotope-ratio mass spectrometer, we investigated isotopic discrimination of carbon and nitrogen isotope ratios (d13 C and d15 N values) in guard hairs of four Arctic predators; the wolf (n?=?7), the wolverine (n?=?2), the grizzly bear (n?=?2), and the polar bear (n?=?3). During a 3-month trial, carnivores were fed a mixed diet. The d13 C and d15 N values, and the mass (g) of diet items, were monitored weekly for each individual to determine their Total Diet Average ratios.
Diet-hair isotopic discrimination (?x) varied according to species, ranging [1.88?±?0.69‰: 3.2?±?0.69‰] for d13 C values, and [1.58?±?0.17‰: 3.81?±?0.22‰] for d15 N values. Adult wolves ?13 C average (2.03?±?0.7‰) was lower than that of young wolves (2.60?±?0.8‰) and any other species (combined average of 2.59?±?0.28‰), except for the wolverine (2.12?±?0.23‰). Wolves ?15 N averages (juveniles: 3.51?±?0.34‰, adults: 3.68?±?0.28‰) were higher than those of any other species (combined average: 2.50?±?0.58‰).
The discrimination factors for d13 C and d15 N values calculated in this study could be used in ecological studies dealing with free-ranging animals, with implications for non-invasive research approaches. As in other controlled discrimination studies, we recommend caution in applying our discrimination factors when the population structure is heterogeneous.
Dual fuels: intra-annual variation in the relative importance of benthic and pelagic resources to maintenance, growth and reproduction in a generalist salmonid fish.
Ecological systems are often characterized as stable entities. However, basal productivity in most ecosystems varies between seasons, particularly in subarctic and polar areas. How this variability affects higher trophic levels or entire food webs remains largely unknown, especially in these high-latitude regions. We undertook a year-long study of benthic (macroinvertebrate) and pelagic (zooplankton) resource availability, along with short (day/days: stomach content)-, medium (month: liver d(13)C and d(15)N isotopes)- and long-term (season: muscle d(13)C and d(15)N isotopes) assessments of resource use by a generalist fish, the European whitefish, in a deep, oligotrophic, subarctic lake in northern Europe. Due to the long ice-covered winter period, we expected to find general benthic reliance throughout the year, but also a seasonal importance of zooplankton to the diet, somatic growth and gonadal development of whitefish. Benthic and pelagic resource availability varied between seasons: peak littoral benthic macroinvertebrate density occurred in mid-winter, whereas maximum zooplankton density was observed in summer. Whitefish stomach content revealed a reliance on benthic prey items during winter and pelagic prey in summer. A seasonal shift from benthic to pelagic prey was evident in liver isotope ratios, but muscle isotope ratios indicated a year-round reliance on benthic macroinvertebrates. Whitefish activity levels as well as somatic and gonadal growth all peaked during the summer, coinciding with the zooplankton peak and the warmest water temperature. Stable isotopes of muscle consistently depicted the most important resource, benthic macroinvertebrates, whereas short-term indicators, that is, diet and stable isotopes of liver, revealed the seasonal significance of pelagic zooplankton for somatic growth and gonad development. Seasonal variability in resource availability strongly influences consumer growth and reproduction and may also be important in other ecosystems facing pronounced annual weather fluctuations.
