Relative Trophic Position Research Articles

Gravel dredging alters diversity and structure of riverine fish assemblages

Summary1. Human activities affect fish assemblages in a variety of ways. Large‐scale and long‐term disturbances such as in‐stream dredging and mining alter habitat and hydrodynamic characteristics within rivers which can, in turn, alter fish distribution. Habitat heterogeneity is decreased as the natural riffle–pool–run sequences are lost to continuous pools and, as a consequence, lotic species are displaced by lentic species, while generalist and invasive species displace native habitat specialists. Sediment and organic detritus accumulate in deep, dredged reaches and behind dams, disrupting nutrient flow and destroying critical habitat for habitat specialist species.2. We used standard ecological metrics such as species richness and diversity, as well as stable isotope analysis of δ13C and δ15N, to quantify the differences in fish assemblages sampled by benthic trawls among dredged and undredged sites in the Allegheny River, Pennsylvania, U.S.A.3. Using mixed‐effects models, we found that total catch, species richness and diversity were negatively correlated with depth (P < 0.05), while species richness, diversity and proportion of species in lithophilic (‘rock‐loving’) reproductive guilds were lower at dredged than at undredged sites (P < 0.05).4. Principal components analysis and manova revealed that taxa such as darters in brood hider and substratum chooser reproductive guilds were predominantly associated with undredged sites along principal component axis 1 (PC1 and manovaP < 0.05), while nest spawners such as catfish and open substratum spawners including suckers were more associated with dredged sites along PC2 (P < 0.05).5. Stable isotope analysis of δ13C and δ15N revealed shifts from reliance on shallow water and benthic‐derived nutrients at undredged sites to reliance on phytoplankton and terrestrial detritus at deep‐water dredged sites. Relative trophic positions were also lower at dredged sites for many species; loss of benthic nutrient pathways associated with depth and dredging history is hypothesised.6. The combination of ecological metrics and stable isotope analysis thus shows how anthropogenic habitat loss caused by gravel dredging can decrease benthic fish abundance and diversity, and that species in substratum‐specific reproductive guilds are at particular risk. The effects of dredging also manifest by altering resource use and nutrient pathways within food webs. Management and conservation decisions should therefore consider the protection of relatively shallow areas with suitable substratum for spawning for the protection of native fishes.

Biomagnification of mercury through lake trout (Salvelinus namaycush) food webs of lakes with different physical, chemical and biological characteristics

Mercury (Hg) biomagnification in aquatic ecosystems remains a concern because this pollutant is known to affect the health of fish-eating wildlife and humans, and the fish themselves. The “rate” of mercury biomagnification is being assessed more frequently using stable nitrogen isotope ratios (δ15N), a measure of relative trophic position of biota within a food web. Within food webs and across diverse systems, log-transformed Hg concentrations are significantly and positively related to δ15N and the slopes of these models vary from one study to another for reasons that are not yet understood. Here we compared the rates of Hg biomagnification in 14 lake trout lakes from three provinces in Canada to understand whether any characteristics of the ecosystems explained this among-system variability. Several fish species, zooplankton and benthic invertebrates were collected from these lakes and analyzed for total Hg (fish only), methyl Hg (invertebrates) and stable isotopes (δ15N; δ13C to assess energy sources). Mercury biomagnification rates varied significantly across systems and were higher for food webs of larger (surface area), higher nutrient lakes. However, the slopes were not predictive of among-lake differences in Hg in the lake trout. Results indicate that among-system differences in the rates of Hg biomagnification seen in the literature may be due, in part, to differences in ecosystem characteristics although the mechanisms for this variability are not yet understood.

Pacific sleeper shark Somniosus pacificus trophic ecology in the eastern North Pacific Ocean inferred from nitrogen and carbon stable‐isotope ratios and diet

Stable-isotope ratios of nitrogen (δ¹⁵N) and lipid-normalized carbon (δ¹³C') were used to examine geographic and ontogenetic variability in the trophic ecology of a high latitude benthopelagic elasmobranch, the Pacific sleeper shark Somniosus pacificus. Mean muscle tissue δ¹³C' values of S. pacificus differed significantly among geographic regions of the eastern North Pacific Ocean. Linear models identified significant ontogenetic and geographic variability in muscle tissue δ¹⁵N values of S. pacificus. The trophic position of S. pacificus in the eastern North Pacific Ocean estimated here from previously published stomach-content data (4·3) was within the range of S. pacificus trophic position predicted from a linear model of S. pacificus muscle tissue δ¹⁵N (3·3-5·7) for fish of the same mean total length (L(T) ; 201·5 cm), but uncertainty in predicted trophic position was very high (95% prediction intervals ranged from 2·9 to 6·4). The relative trophic position of S. pacificus determined here from a literature review of δ¹⁵N by taxa in the eastern North Pacific Ocean was also lower than would be expected based on stomach-content data alone when compared to fishes, squid and filter feeding whales. Stable-isotope analysis revealed wider variability in the feeding ecology of S. pacificus in the eastern North Pacific Ocean than shown by diet data alone, and expanded previous conclusions drawn from analyses of stomach-content data to regional and temporal scales meaningful for fisheries management.

人为营养物质输入对汉丰湖不同营养级生物的影响--稳定C、N同位素分析

PDF HTML阅读 XML下载 导出引用 引用提醒 人为营养物质输入对汉丰湖不同营养级生物的影响--稳定C、N同位素分析 DOI: 10.5846/stxb201101120061 作者: 作者单位: 淡水鱼类资源与生殖发育教育部重点实验室,水产科学重庆市市级重点实验室,西南大学生命科学学院,重庆;,淡水鱼类资源与生殖发育教育部重点实验室,水产科学重庆市市级重点实验室,西南大学生命科学学院,重庆,淡水鱼类资源与生殖发育教育部重点实验室,水产科学重庆市市级重点实验室,西南大学生命科学学院,重庆,淡水鱼类资源与生殖发育教育部重点实验室,水产科学重庆市市级重点实验室,西南大学生命科学学院,重庆,淡水鱼类资源与生殖发育教育部重点实验室,水产科学重庆市市级重点实验室,西南大学生命科学学院,重庆,淡水鱼类资源与生殖发育教育部重点实验室,水产科学重庆市市级重点实验室,西南大学生命科学学院,重庆,中国水产科学研究院长江水产研究所,淡水鱼类资源与生殖发育教育部重点实验室，水产科学重庆市市级重点实验室， 作者简介: 通讯作者: 中图分类号: 基金项目: 公益性行业(农业)科研专项经费资助(200903048-08);中央高校基本科研业务费专项资金项目资助(XDJK2009C196) Effects of anthropogenic nutrient input on organisms from different trophic levels in Hanfeng Lake: evidence from stable carbon and nitrogen isotope analysis Author: Affiliation: Key Laboratory of Freshwater Fish Reproduction and Development (Ministry of Education),Key Laboratory of Aquatic Science of Chongqing，School of Life Science Southwest University,,,,,,,Southwest University Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:以长江一级支流小江上游的汉丰湖为研究对象,设置了4个采样点(影响组:A, B;对照组:C, D),应用碳、氮稳定性同位素探讨人类生活污水和农业面源污染对汉丰湖水生生态系统中不同营养级水平生物类群的影响。结果表明:影响组POM(颗粒有机物)和螺类碳、氮稳定性同位素比值范围分别为-25.93‰--24.63‰、4.12‰-9.86‰,-14.28‰--21.60‰、7.97‰-19.99‰;对照组POM(颗粒有机物)和螺类碳、氮稳定性同位素比值范围分别为-25.62‰--22.51‰、0.01‰-6.56‰,-22.96‰--19.21‰、6.75‰-8.89‰;不同组间POM和初级消费者螺类碳同位素比值无明显空间变化(P>0.05), 而氮稳定性同位素比值空间变化显著(P<0.05)。因此,在汉丰湖食物网中,氮稳定性同位素特征更好地反应了营养物质(人为输入)吸收和富集的信息。与固着藻类、鱼类等相比,POM和软体动物螺类更适合作为环境评价的指示物。影响组A、B样点的部分生物类群已经受到了人为营养物质输入的影响,影响强度B样点区域>A样点区域。结果建议加强汉丰湖水环境保护,控制污水排放量及提高污水处理水平,对于保护小江和三峡库区水质具有十分重要的意义。 Abstract:The carbon and nitrogen stable isotope analysis have been used to study the aquatic ecological system and trophic ecology since the early 1970s. With the increasing of human activities, many kinds of pollution, such as industrial waste water, widespread pollution from the overuse of fertilizers, and domestic sewage and rubbish, were discharged into rivers and lakes, causing aquatic eutrophication, which has resulted in many severe ecological consequences like the disappearance of aquatic plants and the decrease in biodiversity. Therefore, it is vital to monitor the eutrophication as early as possible. The carbon and nitrogen stable isotope analysis are a good method of checking and evaluating the trophic conditions of aquatic ecology. Moreover, many studies were conducted with regard to the organic materials in human sewage as well as the influences of the input of nutrient materials on the aquatic ecology environment. And the organisms of the Hanfeng Lake have been also faced with the threats of domestic sewage water and widespread pollution from the overuse of fertilizers produced by surrounding residents. In the present study, the stable isotope approach was used and we compared the difference of isotopic signatures of the same species between the impacted areas (A and B sites) and the control areas (C and D sites).. So the aims of the present study were trying to investigate the effects of human sewage and agricultural non-point source input in organisms from the different trophic levels (primary producers and primary consumers) in the Hanfeng Lake, an upper reaches of the Yangtze River. The results showed that the carbon and nitrogen stable isotope values of POM and sanil samples were various in different groups. In impact groups, the values of POM samples were -25.93‰ --24.63‰ forδ13C and 4.12 ‰ -9.86‰ for δ15N, the values of snail samples ranged from -14.28‰ - -21.60 ‰ (δ13C) and 7.97 ‰ - 19.99‰ (δ15N); In control groups, the values of bothe POM and snails samples were -25.62- -22.51‰ (δ13C)、0.01-6.56‰ (δ15N),-22.96--19.21‰ (δ13C)、6.75-8.89‰ (δ15N), respectively;.The δ13C values for either POM and snails (primary consumers) were not significantly different between different sampling groups (P>0.05), while the change of δ15N in different sampling groups varied greatly (P<0.05). The relative trophic position of primary producer, such as periphytic algae, POM (mainly composed of phytoplankton) and primary consumers (snails), are higher in impact groups than in those of control groups, whereas there is no significant difference in the relative trophic levels of the other higher trophic level consumers, such as carp (Cyprinus carpio), crucian (Carassius auratus) and Amur catfish (Silurus asotus). Therefore, the nitrogen stable isotope signature was proved to better describe the information on the absorption and enrichment of anthropogenically derived material in food webs of the Hanfeng Lake. Compared with the other organisms(e.g. periphytic algae and fish), POM and snails were more sensitive to pollution than other species, and could be more appropriate to serve as an indicator of assessing the quality of environment. Moreover, the organisms from both impacted groups (site A and site B) in the Hanfeng Lake have been profoundly influenced by anthropogenic inputs of nutrient matter, and site B received a more stronger interference than site A. In conclusion, it has important significance to strengthen the protection of the environment of the Hanfeng Lake, and control the effluent discharge amount for protecting the water quality of the Xiaojiang River and even the Three Gorges Reservoir Area. 参考文献 相似文献 引证文献

STABLE ISOTOPE ANALYSIS REVEALS ANTHROPOGENIC EFFECTS ON FISH ASSEMBLAGES IN A TEMPERATE RESERVOIR

ABSTRACTEffluent from anthropogenic inputs can affect fish assemblages in aquatic ecosystems by altering species richness, diversity and trophic structure. To investigate the effects of a bleached kraft pulp mill effluent (PME) and municipal sewage treatment plant (STP) discharge on fish assemblages in a temperate reservoir, we combined standard ecological methodologies with stable isotope analysis of δ13C and δ15N. Total catch, species richness, diversity and coefficient of community loss indicated lower species richness and diversity at sites exposed to PME, whereas sewage‐exposed assemblages had intermediate richness and diversity relative to reference sites. Stable isotope analysis of δ13C and δ15N revealed that many species fed at higher relative trophic positions in the presence of both effluents and showed shifts towards increased reliance on littoral‐derived and benthic‐derived carbon at sites downstream of the PME discharge. Fishes at PME‐influenced sites had also generally higher condition factor whereas fishes at STP‐influenced sites were more variable in condition factor. These differences may, in part, be due to decreased species diversity at these sites and to nutrient enrichment manifest from the effluent discharge. While standard fisheries and ecological methodologies and stable isotope analysis have been separately used in other studies of anthropogenic effects on fishes, their integration in this study provides greater resolution than either could alone. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Tracking seasonal changes in North Sea zooplankton trophic dynamics using stable isotopes

Trophodynamics of meso-zooplankton in the North Sea (NS) were assessed at a site in the southern NS, and at a shallow and a deep site in the central NS. Offshore and neritic species from different ecological niches, including Calanus spp., Temora spp. and Sagitta spp., were collected during seven cruises over 14 months from 2007 to 2008. Bulk stable isotope (SI) analysis, phospholipid-derived fatty acid (PLFA) compositions, and δ 13CPLFA data of meso-zooplankton and particulate organic matter (POM) were used to describe changes in zooplankton relative trophic positions (RTPs) and trophodynamics. The aim of the study was to test the hypothesis that the RTPs of zooplankton in the North Sea vary spatially and seasonally, in response to hydrographic variability, with the microbial food web playing an important role at times. Zooplankton RTPs tended to be higher during winter and lower during the phytoplankton bloom in spring. RTPs were highest for predators such as Sagitta sp. and Calanus helgolandicus and lowest for small copepods such as Pseudocalanus elongatus and zoea larvae (Brachyura). δ 15NPOM-based RTPs were only moderate surrogates for animals’ ecological niches, because of the plasticity in source materials from the herbivorous and the microbial loop food web. Common (16:0) and essential (eicosapentaenoic acid, EPA and docosahexaenoic acid, DHA) structural lipids showed relatively constant abundances. This could be explained by incorporation of PLFAs with δ 13C signatures which followed seasonal changes in bulk δ 13CPOM and PLFA δ 13CPOM signatures. This study highlighted the complementarity of three biogeochemical approaches for trophodynamic studies and substantiated conceptual views of size-based food web analysis, in which small individuals of large species may be functionally equivalent to large individuals of small species. Seasonal and spatial variability was also important in altering the relative importance of the herbivorous and microbial food webs.

Nursery habitat use and foraging ecology of the brown stingray Dasyatis lata determined from stomach contents, bulk and amino acid stable isotopes

Identification of nursery habitats and knowledge of the trophic ecology and habitat use of juvenile fishes within these habitats are fundamental in developing sound management and con- servation strategies. The brown stingray Dasyatis lata is a large benthic predator that inhabits the coastal waters of Hawai'i. Although abundant in these ecosystems, little is known about its basic eco - logy. Stomach content, bulk and amino acid stable isotope analyses were used to assess diet and habitat use of juvenile brown stingrays and to examine the possibility of competitive interactions with juvenile scalloped hammerhead sharks Sphyrna lewini that are sympatric with brown stingrays in K¯ ane'ohe Bay, Oahu. Based on stomach contents, brown stingrays fed almost exclusively on crus- taceans. An ontogenetic shift in stingray diet and an increase in relative trophic position (TP) were apparent from stomach content and stable isotope analysis. Stingray bulk δ 13 C and δ 15 N values indi- cated long-term foraging fidelity to subregions of the bay. Use of K¯ ane'ohe Bay as a nursery habitat was supported by nitrogen isotopic analysis of individual amino acids from stingray muscle samples. Our results clearly demonstrated that stingrays foraged within the bay for the majority of their juve- nile lives then shifted to offshore habitats with the onset of sexual maturity. Trophic enrichment fac- tors used to estimate TPs from amino acid analysis in previous studies may underestimate TPs in elas- mobranchs owing to urea retention for osmoregulation. Potential prey resources were partitioned between stingrays and juvenile scalloped hammerhead sharks, and TP estimates from each analyti- cal method indicated that juvenile scalloped hammerhead sharks forage on higher TP prey than do juvenile brown stingrays. These results show that the study of foraging ecology and habitat use of marine animals can greatly benefit from integrating traditional stomach content and bulk stable isotopic analyses with nitrogen isotopic analyses of individual amino acids.

Methylmercury levels and bioaccumulation in the aquatic food web of a highly mercury-contaminated reservoir

The low Ebro River basin (NE Spain) represents a particular case of chronic and long-term mercury pollution due to the presence of an industrial waste (up to 436 μg/g of Hg) coming from a chlor-alkali plant Albeit high total mercury (THg) levels have been previously described in several aquatic species from the surveyed area, methylmercury (MeHg) values in fish individuals have never been reported. Accordingly, in order to investigate bioaccumulation patterns at different levels of the aquatic food web of such polluted area, crayfish and various fish species, were analysed for THg and MeHg content. At the hot spot, THg mean values of crayfish muscle tissue and hepatopancreas were 10 and 15 times, respectively, greater than the local background level. Higher mean THg concentrations were detected in piscivorous (THg = 0.848 ± 0.476 μg/g wet weight (ww); MeHg = 0.672 ± 0.364 μg/g ww) than in non-piscivorous fish (THg = 0.305 ± 0.163 μg/g ww; MeHg = 0.278 ± 0.239 μg/g ww). Although these results indicated that THg in fish increased significantly with increasing trophic position, the percentage of the methylated form of Hg was not strongly influenced by differences in relative trophic position. This is an important finding, since the fraction of THg as MeHg in the top fish predator was unexpectedly lower than for other species of the aquatic food chain. Moreover, mean THg concentrations in piscivorous fish exceed the maximum level recommended for human consumption. From our findings, it is clear that for this specific polluted system, speciation becomes almost mandatory when risk assessment is based on MeHg, since single measurements of THg are inadequate and could lead to an over- or under-estimation of contamination levels.

Diet and resource use among Greenland sharks (Somniosus microcephalus) and teleosts sampled in Icelandic waters, using δ13C, δ15N, and mercury

Stable carbon (δ13C) and nitrogen (δ15N) isotopes and total mercury (Hg) were used to investigate diet and resource use among Greenland sharks ( Somniosus microcephalus ) and 14 teleosts inhabiting Icelandic waters. Greenland shark stomachs contained 11 of the teleosts sampled, along with other fishes and marine mammal tissues. Teleost resource use ranged from pelagic (e.g., Argentina silus ) to benthic (e.g., Anarhichas lupus ) based on δ13C, and relative trophic positions (TP, based on δ15N) ranged from 3.0 ( Mallotus villosus ) to 3.8 (e.g., Brosme brosme ). Greenland shark δ13C indicated feeding on benthic and pelagic resources, with a high input of pelagic carbon, and δ15N indicated a relative TP of 4.3. Log[Hg] increased with δ15N (i.e., TP) from teleosts to Greenland sharks and was higher in offshore vs. inshore teleosts. Linear regressions revealed that log[Hg] was better described by both δ15N and δ13C-assigned resource use than by δ15N alone. Hg was useful for supporting the TPs suggested by δ15N, and the higher Hg in offshore fishes could help explain the high Hg of Greenland sharks. Results from this study demonstrated the potential use of Hg as a dietary tracer in marine fishes.

Nitrogen isotopes in otoliths reconstruct ancient trophic position

The ratio of 15 N/ 14 N( δ 15 N) from consumer and prey tissue is commonly used in ecological studies to determine trophic level, food web structure, and mean trophic level in aquatic ecosystems. There is a predictable positive relationship between the δ 15 N values in tissue and trophic level, caused by the bioaccumulation of 15 N in tissues of consumers with each step up the food chain. Reconstructing trophic structure or food chain length over time may provide resource managers with insights about ecosystem biodiversity and resilience. Yet, in many marine systems the absence of baseline information before anthropogenic disturbances makes comparative stud- ies addressing ecosystem responses extremely diffi- cult. Here we attempt to retrospectively reconstruct trophic position in four species of fish from the upper Gulf of California, Mexico before perturbations such as overfishing or the damming of the Colorado River. We first validated if otolith δ 15 N approximates the δ 15 N observed in fish tissue. We then used the δ 15 N encapsulated in ancient fish otoliths that are between 1,000 and 5,500 years old to define the food web structure. Our results suggested that δ 15 N in otoliths has slightly more positive δ 15 N than soft tissue. The δ 15 N values from ancient otoliths appropriately defined the fishes' relative trophic position. We found significant differences in δ 15 N between functional

Trophic positions of omnivores are not always flexible: Evidence from four species of freshwater crayfish

Omnivores are generally believed to be flexible in their diet and trophic position: seasonal, ontogenetic and site-based differences in trophic position have been observed. We compared consumed and assimilated diet among four species within a group of omnivorous freshwater crayfish, to determine whether species that occur together at a site occupy different trophic positions. Diets of Geocharax falcata, Gramastacus insolitus, Cherax destructor and Euastacus bispinosus (Decapoda: Parastacidae) were compared using stable isotopes (δ13C and δ15N) and gut content analysis across nine sites that varied in their species composition. Gramastacus insolitus consumed mainly plant material across all sites. Geocharax falcata consumed either plants or animals or both at different sites. Its trophic level was consistently similar to G. insolitus, despite differences in gut contents and source for dietary carbon. Cherax destructor consumed animals and had a relatively stable trophic position among sites. Relative trophic position of these three species was consistent across sites and regardless of food consumed, they were positioned as omnivores at a lower trophic level than predators but higher than primary producers and herbivores. Euastacus bispinosus occupied a higher trophic level than other invertebrate species but δ13C levels did not differ among sites. Cherax destructor and G. falcata may show flexibility in food sources and in the assimilation of food that determines their trophic position relative to other crayfish species. In contrast, G. insolitus and E. bispinosus are likely to show both a more fixed diet and less flexible trophic position. Therefore, not all omnivores show the flexible diet and trophic position generally reported in the literature. Some species of omnivorous crayfish may maintain a relatively constant trophic position across sites, seasons or changes in food availability regardless of whether their consumed diet alters or not.

The trophic ecology of castes in harvester ant colonies

Summary1.The castes of social insects can vary dramatically in physiology, morphology and behaviour, and are a model of phenotypic plasticity. Dietary variation has been implicated in the evolution of castes and is involved in developmental differentiation. This study uses the elemental and isotopic composition of castes to infer the diets they consumed during larval development, when caste was determined.2.We analysed the elemental (C : N) and isotopic (δ13C and δ15N) composition of individuals of all castes in colonies ofPogonomyrmex badius, the Florida seed harvester ant, at each of two sites. Sampled herbivores and insectivores were used to calibrate measures of colony trophic position. The proportions of insects and seeds foraged by colonies correlated with inferred trophic position, strengthening the inference of diet from isotopic composition.3.Pupae of each of the four castes (male, reproductive female or gyne, major worker and minor worker) are distinguishable based on their composition and inferred diet during the larval phase.4.Within colonies, larval diet inferred from δ15N suggests that the largest individuals assimilated more insect/protein relative to seeds. This result was consistent across colonies despite high variation in a colony’s relative trophic position.5.Carbon to nitrogen ratio (C : N) of each caste was also different, and differences were consistent across colonies and sites. While females increase in N content with increasing size, the pattern is reversed in males. This result may reflect basic life history differences (e.g. life span) between males and females. Since female reproductive potential correlates with female size in this species, the decrease in N content with increasing female caste size supports that reproductive development is N‐limited, and that workers are trophically castrated due to N deprivation during larval development.

Stable isotope analysis and satellite tracking reveal interspecific resource partitioning of nonbreeding albatrosses off Alaska

Albatrosses (Diomedeidae) are the most threatened family of birds globally. The three North Pacific species ( Phoebastria Reichenbach, 1853) are listed as either endangered or vulnerable, with the population of Short-tailed Albatross ( Phoebastria albatrus (Pallas, 1769)) less than 1% of its historical size. All North Pacific albatross species do not currently breed sympatrically, yet they do co-occur at-sea during the nonbreeding season. We incorporated stable isotope analysis with the first simultaneous satellite-tracking study of all three North Pacific albatross species while sympatric on summer (nonbreeding season) foraging grounds off Alaska. Carbon isotope ratios and tracking data identify differences in primary foraging domains of continental shelf and slope waters for Short-tailed Albatrosses and Black-footed Albatrosses ( Phoebastria nigripes (Audubon, 1839)) versus oceanic waters for Laysan Albatrosses ( Phoebastria immutabilis (Rothschild, 1893)). Short-tailed and Black-footed albatrosses also fed at higher trophic levels than Laysan Albatrosses. The relative trophic position of Black-footed and Laysan albatrosses, however, appears to differ between nonbreeding and breeding seasons. Spatial segregation also occurred at a broader geographic scale, with Short-tailed Albatrosses ranging more north into the Bering Sea than Black-footed Albatrosses, which ranged more to the southeast, and Laysan Albatrosses more to the southwest. Differences in carbon isotope ratios among North Pacific albatross species during the nonbreeding season likely reflect the relative proportion of neritic (more carbon enriched) versus oceanic (carbon depleted) derived nutrients, and possible differential use of fishery discards, rather than latitudinal differences in distribution.

Combining Organic Matter Source and Relative Trophic Position Determinations to Explore Trophic Structure

Stable isotope ratios of carbon and sulfur were used to assess organic matter utilization of numerically abundant consumers present in Apalachicola Bay, FL, USA. These results were used to infer nitrogen isotopic enrichment of organic matter sources in an effort to establish baseline δ15N enrichment for trophic evaluations. We compared results from concentration-independent and concentration-dependent mixing models and found that the two methods resulted in widely different conclusions about the importance of organic matter sources that varied ninefold in sulfur concentrations. Nitrogen isotopic enrichment was used to determine relative trophic positions of consumer organisms. Source elemental concentrations of nitrogen were also considered in the calculation of relative trophic levels in a concentration-dependent approach. Concentration-independent and concentration-dependent methods of calculating trophic results were compared. While relative trophic levels of individual species varied continuously from approximately 1.7 to 3.5, comparisons of trophic level among consumers indicated four possible trophic groupings. Filter feeders (mussels and oysters) made up the lowest trophic tier while teleost fishes made up the highest trophic tier. Invertebrates sampled were assigned intermediate nondiscrete trophic levels. Because δ15N values of important organic matter sources in the system were similar, the concentration-independent and concentration-dependent methods did not result in significantly different conclusions about trophic level for any of the consumers examined. However, a comparison of the two methods applied to a hypothetical case found that differences in base δ15N values ranging approximately 4‰ resulted in significantly different trophic-level assignments when comparing the concentration-dependent and concentration-independent methods of trophic-level calculations. Our results confirm that consideration of the elemental concentrations of the base organisms is an important factor in determining source contributions and may affect trophic-level calculations in systems with a sufficient range of base nitrogen enrichment. However, this result depended on the relative isotopic signatures of the chosen sources and their elemental concentrations and should be considered individually for each system.

Preliminary assessment of Greenland halibut diet in Cumberland Sound using stable isotopes

We provide preliminary carbon (δ13C) and nitrogen (δ15N) stable isotope assessment of the Greenland halibut (Reinhardtius hippoglossoides) diet in Cumberland Sound, with focus on two possible prey sources: pelagic represented by capelin (Mallotus villosus) and epibenthic represented by shrimp (Lebbeus polaris). The δ13C for the Greenland halibut stock indicated a pelagic carbon source in Cumberland Sound while stable isotope mixing models, IsoSource and MixSIR, indicated a 99% dietary composition of capelin relative to the shrimp. The δ15N did not vary across Greenland halibut size ranges and placed them at a fourth trophic position relative to a primary herbivore. This study provides the starting point for more elaborate Cumberland Sound research on the local Greenland halibut feeding ecology by confirming pelagic feeding and establishing relative trophic position as well as identifying stable isotopes as a useful tool for the study of diet in cold water fish species.

Mercury Cycling in Stream Ecosystems. 3. Trophic Dynamics and Methylmercury Bioaccumulation

Trophic dynamics (community composition and feeding relationships) have been identified as important drivers of methylmercury (MeHg) bioaccumulation in lakes, reservoirs, and marine ecosystems. The relative importance of trophic dynamics and geochemical controls on MeHg bioaccumulation in streams, however, remains poorly characterized. MeHg bioaccumulation was evaluated in eight stream ecosystems across the United States (Oregon, Wisconsin, and Florida) spanning large ranges in climate, landscape characteristics, atmospheric Hg deposition, and stream chemistry. Across all geographic regions and all streams, concentrations of total Hg (THg) in top predator fish and forage fish, and MeHg in invertebrates, were strongly positively correlated to concentrations of filtered THg (FTHg), filtered MeHg (FMeHg), and dissolved organic carbon (DOC); to DOC complexity (as measured by specific ultraviolet absorbance); and to percent wetland in the stream basins. Correlations were strongest for nonurban streams. Although regressions of log[Hg] versus delta15N indicate that Hg in biota increased significantly with increasing trophic position within seven of eight individual streams, Hg concentrations in top predator fish (including cutthroat, rainbow, and brown trout; green sunfish; and largemouth bass) were not strongly influenced by differences in relative trophic position. Slopes of log[Hg] versus delta15N, an indicator of the efficiency of trophic enrichment, ranged from 0.14 to 0.27 for all streams. These data suggest that, across the large ranges in FTHg (0.14-14.2 ng L(-1)), FMeHg (0.023-1.03 ng L(-1)), and DOC (0.50-61.0 mg L(-1)) found in this study, Hg contamination in top predatorfish in streams likely is dominated by the amount of MeHg available for uptake at the base of the food web rather than by differences in the trophic position of top predator fish.

Characterizing dietary variability and trophic positions of coastal calanoid copepods: insight from stable isotopes and fatty acids

The spring zooplankton community in the Strait of Georgia (British Columbia, Canada) is characterized by the presence of several calanoid copepod species which collectively make up ~90% of the mezozooplankton biomass. Here, we investigate interspecific, interannual, and geographic variability in the diets and trophic positions of these copepods using a combination of fatty acids and stable isotopes. To characterize geographic variability in diet, we compare our findings from the Strait of Georgia with similar data from Ocean Station P in the subarctic northeast Pacific. Both fatty acid and stable isotope signatures indicate the existence of three trophic levels, even within the limited size range of these copepods: Neocalanus plumchrus and Calanus marshallae are primarily omnivorous, while Euchaeta elongata is carnivorous and Eucalanus bungii is herbivorous. Fatty acid markers of trophic position (e.g., DHA/EPA, 18:1n-9/18:1n-7) correlate significantly with δ15N, while markers indicating the proportion of diatoms to flagellates in the diet (e.g., 16PUFA/18PUFA and DHA/EPA) correlate significantly with δ13C, after the effect of lipid concentration on δ13C is accounted for. Despite the general correlation between stable isotopes and fatty acids, the former are not sensitive enough to capture the range of interannual variability observed in the latter, and can only capture substantial shifts in the diet over geographic scales. However, regardless of variability in food quality, the relative trophic positions of these copepods do not change significantly either spatially or temporally.

Trophic ecology of invasive Argentine ants in their native and introduced ranges

Although the ecological effects of invasions often become obvious soon after introduced species become established, more gradual effects may take years to manifest and can thus require long-term data for quantification. We analyzed an 8-year record of stable isotope data on Argentine ants (Linepithema humile) from southern California to infer how the trophic position of this widespread invasive species changes over time as native ant species are displaced. We couple this longitudinal analysis with a biregional comparison of stable isotope data (delta(15)N) on ants from Argentina (native range) and California (introduced range) to quantify (i) how the trophic position of L. humile differs between native and introduced populations, and (ii) how relative trophic position as estimated by delta(15)N values of Argentine ants compare with those of other ants at the same site. Both long-term and biregional comparisons indicate that the Argentine ant's relative trophic position is reduced at sites with a longer history of occupation. Over the course of 8 years, the relative trophic position of L. humile remained high at the leading edge of an invasion front but declined, on average, behind the front as native ants disappeared. Relative to native populations, where L. humile is among the most carnivorous of ants, Argentine ants from California occupied lower trophic positions. These results support the hypothesis that Argentine ants shift their diet after establishment as a result of resource depletion and increasing reliance on plant-based resources, especially honeydew-producing Hemiptera. Our results demonstrate the value of long-term and biregional data in uncovering ecological effects of invasions.

Stable isotope natural abundances ( δ13C and δ15N) of the earthworm Eisenia fetida and other soil fauna living in two different vermicomposting environments

Manure heaps and vermicomposting systems are hotspots of heterotrophic activity supporting a high-detritivore biomass where epigeic earthworms interact intensively with bacteria, fungi and other soil fauna. We carried out a prospective study of the vermicomposting food webs using the natural abundance of stable C and N isotopes in 66 samples of soil fauna and the substrates in which the animals live in two systems: (i) a high-feeding-rate vermireactor, fed with pig slurry, and (ii) a farm manure vermicomposting heap fed with cattle manure. The aims of the study were specifically (i) to test the extent to which the isotopic signals in the earthworms resemble those of the substrates in which they live, (ii) to further our knowledge of the ontogenic changes in resource utilization of the earthworm Eisenia fetida, and (iii) to obtain information about the relative trophic position of the soil fauna in the food web of vermicomposting systems. Tissues of earthworms were significantly 15N-enriched (by 4–8‰) relative to fresh and mature manures in both vermicomposting systems. The δ 13C values of adult earthworms were not different from those of the fresh animal wastes in both vermicomposting systems, suggesting that adult worms preferred fresh manure than worked materials as carbon source. The little but significant enrichment in 15N observed in hatchlings living in the pig slurry vermicomposting bins relative to adult tissues likely reflect different feeding strategies, not observed in the cattle manure heap. Besides, hatchlings in the cattle manure heap appeared markedly depleted in 13C (by ∼5‰) relative to the adult earthworms, suggesting the use of a different source of carbon in the early stage. Diptera larvae presented very low values of δ 13C, likely suggesting a relevant role of methanotrophic bacteria in their diet. Based on the shifts in δ 15N, a taxon in the pig slurry vermicomposting bins may be assigned at least to three relative trophic positions separated by a 15N shift of 2‰, with Enchytraeida clearly in the lower position, adults and hatchlings of E. fetida and nematodes in an intermediate level, and Collembola at the higher position showing an enrichment of 9‰ relative to the substrate. In the cattle manure heap three trophic levels may be also identified, with larvae of Diptera and Coleoptera as the less 15N-enriched level, a general detritivore group in intermediate position, and finally a predatory taxa with a +9‰ shift comprised by Staphylinidae.

Influence of feeding ecology on blood mercury concentrations in four species of turtles

Mercury is a relatively well-studied pollutant because of its global distribution, toxicity, and ability to bioaccumulate and biomagnify in food webs: however, little is known about bioaccumulation and toxicity of Hg in turtles. Total Hg (THg) concentrations in blood were determined for 552 turtles representing four different species (Chelydra serpentina, Sternotherus odoratus, Chrysemys picta, and Pseudemys rubriventris) from a Hg-contaminated site on the South River (VA, USA) and upstream reference sites. Methylmercury and Se concentrations also were determined in a subset of samples. Because the feeding ecology of these species differs drastically, stable isotopes of carbon (delta13C) and nitrogen (delta15N) were employed to infer the relationship between relative trophic position and Hg concentrations. Significant differences were found among sites and species, suggesting that blood can be used as a bioindicator of Hg exposure in turtles. We found differences in THg concentrations in turtles from the contaminated site that were consistent with their known feeding ecology: C. serpentina > or = S. odoratus > C. picta > P. rubriventris. This trend was generally supported by the isotope data, which suggested that individual turtles were feeding at more than one trophic level. Methylmercury followed similar spatial patterns as THg and was the predominant Hg species in blood for all turtles. Blood Se concentrations were low in the system, but a marginally positive relationship was found between THg and Se when species were pooled. The blood THg concentrations for the turtles in the present study are some of the highest reported in reptiles, necessitating further studies to investigate potential adverse effects of these high concentrations.