Nitrogen Stable Isotope Signatures Research Articles

Sources of primary production supporting food webs in an arid coastal embayment

Penaeid shrimp can be useful ecological indicators of linkages between shallow tropical coastal habitats, acting as integrators of carbon and nitrogen sources due to their generalist feeding habits and their mobility between habitats and with tidal cycles. In the current study, the contribution of mangrove, seagrass and microbial mat to the nutrition of two penaeid shrimp species, Penaeus semisulcatus and Metapeneus ensis, in a shallow arid embayment in the Arabian Gulf was assessed through a combination of analysis of stomach contents and dual carbon and nitrogen stable isotope signatures. Shrimp tissue stable isotope signatures identified seagrass as a major source of carbon and nitrogen for both species, contributing 21–38 % (1–99 ‰). Microbial mat was also detected as a significant nutritional source for early-stage Metapenaeus affinis postlarvae (1–27 %). However, mangroves were not identified as a significant source, with the range of results including the possibility of a zero contribution. Moreover, the greatest possible contribution of mangroves as source of carbon was less than for the other primary producers. This may be due the high salinity and wide temperature range limiting mangrove productivity as well as the low export of dissolved and particulate organic material out of the mangroves due to low rainfall.

Stream community and ecosystem responses to the boom and bust of an invading snail

Invasive species can fundamentally change ecosystems, but there remains surprisingly little understanding of how they alter ecosystems through time. New Zealand mudsnails (NZMS, Potamopyrgus antipodarum) are invading North American aquatic ecosystems with potentially enormous impacts on stream communities and ecosystems. Here we present a unique 10 year time series of stream benthic invertebrates in two sites in a California stream—one invaded site where NZMS populations boomed and then busted, and one upstream reference site that was not invaded to the same degree. As NZMS populations rose and fell, the abundance of native grazing herbivores crashed and then recovered, evidence of direct negative competitive impacts of NZMS. However, NZMS had a positive indirect effect on piercing herbivores, probably mediated by a shift in the dominant algal type. Retrospective stable isotope analyses and Bayesian isotopic mixing models revealed that NZMS invasion decreased the reliance of native taxa, from a variety of functional feeding groups, on periphyton-derived carbon. Furthermore, NZMS invasion was associated with depleted nitrogen stable isotope signatures, evidence that NZMS alter stream ecosystem nitrogen cycling. The reference site did not exhibit these community- or ecosystem-level dynamics. Thus, invasive herbivores can have strong top-down and bottom-up influences on invaded ecosystems, but these impacts can be extremely temporally dynamic.

Nutrient Enrichment with Salmon Carcass Analogs in the Columbia River Basin, USA: A Stream Food Web Analysis

AbstractAnadromous fishes represent an important ecosystem linkage between marine and inland aquatic and terrestrial habitats. These fishes carry organic matter and marine‐derived nutrient (MDN) subsidies across a vast landscape, often with profound influences on recipient ecosystem food web structure and function. In the Columbia River basin, century‐long declines in the abundance of anadromous fish populations have focused attention on potential mitigation efforts to address MDN deficits. In this study, we evaluate components of the stream food web response (periphyton, macroinvertebrate, and fish) to pasteurized salmon carcass analog (SCA) treatments in 15 streams across the Columbia River basin. Periphyton standing crop, macroinvertebrate density, and salmonid fish growth rates and stomach fullness measures increased following the addition of SCA. We found no significant change in dissolved nutrient concentrations after treatment, suggesting that biological demand exceeded supply. Nitrogen stable isotope signatures confirmed trophic transfer from SCA to lower trophic levels but were noticeably weak in fish tissue samples despite our marked growth and stomach fullness measures. These data indicate that SCA has the potential to increase the productivity of nutrient‐limited freshwater ecosystems and may provide a nutrient mitigation tool in ecosystems where MDNs are severely limited or unavailable.

Lake Whitefish Feeding habits and condition in Lake Michigan

Lake whitefi sh (Coregonus clupeaformis) have experienced declines in condition in some areas of the Great Lakes. The hypothesis tested was that condition—in terms of relative weight, percent lipid and docosahexaenoic acid (DHA)—was greater in regions where larger proportions of high quality prey (e.g., Diporeia) were included in the diet. Samples of spawning lake whitefi sh from four regions around Lake Michigan (northwest, Naubinway, Elk Rapids and southeast) had distinct mean carbon and nitrogen stable isotope signatures. Lake whitefi sh may be using a variety of prey items, especially the Naubinway population where fi sh occupy the largest stable isotopic niche space. However, trophic niche width inferred from stable isotopes did not vary among regions. Relative weight was highest in the southeast and lowest for all northern regions. The mean measured lipid from lake whitefi sh dorsal, skinless, muscle biopsies were highest for northwest fi sh. DHA was signifi cantly different among regions, with high mean values in Elk Rapids and the northwest. No correlations were found between stable isotope measures and condition metrics. The results suggest that lake whitefi sh are coping with declining Diporeia abundances by feeding on alternate prey. Overall results do not substantiate the hypothesis of a relationship between condition and prey use, although lake whitefi sh from Elk Rapids and the northwest had high quality prey and good condition.

Temporal and Ontogenetic Variability in Trophic Role of Four Groundfish Species—Walleye Pollock, Pacific Cod, Arrowtooth Flounder, and Pacific Halibut—around Kodiak Island in the Gulf of Alaska

AbstractChanges in trophic position estimates of commercial fishery catches are used as an ecosystem‐based indicator for sustainability, but often these estimates do not incorporate species‐specific seasonal feeding dynamics and ontogenetic diet changes. Using stable isotope analysis, we obtained a fine‐scale resolution of ontogenetic and temporal (interannual and seasonal) variations in the trophic roles of four commercially and ecologically important groundfish species in the Gulf of Alaska: walleye pollock Theragra chalcogramma, Pacific cod Gadus macrocephalus, arrowtooth flounder Atheresthes stomias, and Pacific halibut Hippoglossus stenolepis. For each groundfish taxon, the nitrogen stable isotope signature (δ15N) increased with total length. In contrast, the lipid‐normalized carbon stable isotope signature (δ13C′) significantly varied with size‐class only for walleye pollock. There were species‐specific differences in trophic position; adult Pacific cod fed at the highest trophic position, and walleye pollock fed at the lowest trophic position. Walleye pollock also had the lowest δ13C′, indicating a mainly pelagic diet, while Pacific halibut and Pacific cod had the highest δ13C′, indicating a mostly benthic diet. Interannual differences in trophic position were detected for each species. Pacific cod, arrowtooth flounder, and Pacific halibut fed at a significantly lower trophic position in 2003 than in 2001, 2002, or 2004. All species had a significantly lower average δ13C′ (i.e., a more benthic diet) in 2001 and a higher average δ13C′ (a more pelagic diet) in 2003. Pacific cod, Pacific halibut, and walleye pollock had a significantly more pelagic diet in the summer. Walleye pollock, arrowtooth flounder, and Pacific halibut showed a significantly more benthic diet in the fall, which probably corresponded with their seasonal migrations. Temporal variations in stable isotope signatures were observed but were relatively small (<1.25‰). Length‐based ontogenetic diet shifts, however, were much larger (up to 6‰) and must be considered when trophic position is used as an ecosystem indicator of sustainability.

Differences in carbon and nitrogen stable isotope signatures amongst wild and released pheasant populations

Game birds such as pheasants Phasianus colchicus (Horsfield) are bred and released to supplement wild living populations prior to the hunting season. The total bag records of these birds are, therefore, not suitable for monitoring the development of wild populations because they are heavily influenced by game management. Wild and released pheasants can be expected to be genetically similar or identical to wild birds because wild birds are captured and used for breeding. It is, therefore, not possible to use genetic methods for identifying released individuals. Heavy stable isotopes are incorporated in animal tissues in a predictable manner and depend on the concentration of heavy isotopes in the diet. The heavy isotopes 13C and 15N accumulate in the food chain because the more common lighter isotopes are excreted at a higher rate than the heavy isotopes. Bred and released pheasant feed on a uniform plant-based diet, and it can, therefore, be expected that these animals have lower and less variable levels of heavy isotopes than individuals from a wild populations which feed on a diverse diet of plants and invertebrates. We investigated the isotope signatures in feathers from wild and released pheasant populations and compared the levels and variation in isotope signature amongst the populations. Bred and released pheasants are clearly identified in comparison to released pheasants as wild types have higher δ 15N.

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

Primary consumers as bioindicator of nitrogen pollution in lake planktonic and benthic food webs

The increased nitrogen loading from anthropogenic sources has affected aquatic ecosystems and has cascaded through food webs worldwide. Therefore, the evaluation of ecological impacts of anthropogenic nitrogen has become increasingly important. In this paper, we investigated the effect of nutrient enrichment in the planktonic and benthic food webs of a hypereutrophic urban lake using stable nitrogen isotope analysis. The stable isotope mixing model revealed that zooplanktons rely mainly on planktonic nitrogen, and that Chironomus and Oligochaete rely mainly on benthic nitrogen. The stable nitrogen isotope signatures of seston and the sediment organic matter can be a sensitive indicator, because they are correlated with the nitrogen content of water and sediment. Furthermore, the relationships between the stable nitrogen isotope values of primary production and primary consumers indicate that polluted nitrogen has transferred through the planktonic and benthic food webs, respectively. For primary consumers, the stable nitrogen isotope values of zooplanktons were correlated to nitrogen concentrations of water, and the stable nitrogen values of Chironomus and Oligochaete were correlated to the nitrogen content of sediment, respectively. This finding indicates that these primary consumers can be used as promising bioindicators of the anthropogenic nitrogen input in planktonic and benthic food webs, respectively. Our results suggest that stable nitrogen isotope of primary consumers can act as bioindicators to detect the dispersal patterns of anthropogenic contamination and understand the incorporation into and movement of waste nitrogen in pelagic and benthic food chains.

Geographic and trophic patterns of OCs in pelagic seabirds from the NE Atlantic and the Mediterranean: A multi-species/multi-locality approach

Trophic ecology and geographic location are crucial factors explaining OC levels in marine vertebrates, but these factors are often difficult to disentangle. To examine their relative influence, we analyzed PCBs, DDTs and stable-nitrogen isotope signatures (δ15N) in the blood of 10 pelagic seabird species across 7 breeding localities from the northeast Atlantic and western Mediterranean. Large scale geographic patterns emerged due to the confined character and greater historical OC inputs in the Mediterranean compared to the Atlantic basin. Spatial patterns also emerged at the regional scale within the Atlantic basin, probably associated with long-range pollutant transport. Trophic ecology, however, was also a major factor explaining OC levels. We found clear and consistent OC differences among species regardless of the sampled locality. However, species δ15N and blood OC levels were not correlated within most breeding localities. Petrel species showed significantly greater OC burdens than most shearwater species but similar trophic positions, as indicated by their similar δ15N signatures. This pattern probably results from Petrel species feeding on mesopelagic fish and squid that migrate close to the sea surface at night, whereas shearwater species mainly feed on epipelagic diurnal prey. In sum, this study illustrates the lasting and unequal influence of past human activities such as PCB and DDT usage across different marine regions. In addition, our results suggest that multi-species designs are powerful tools to monitor geographic patterns of OCs and potentially useful to assess their vertical dynamics in the marine environment.

Hunter–gatherer foraging ranges, migrations, and travel in the middle Holocene Baikal region of Siberia: Insights from carbon and nitrogen stable isotope signatures

Foraging ranges, migrations, and travel among Middle Holocene hunter–gatherers in the Baikal region of Siberia are examined based on carbon and nitrogen stable isotope signatures obtained from 350 human and 203 faunal bone samples. The human materials represent Early Neolithic (8000–6800 cal BP), Late Neolithic (6000–5000 cal BP), and Early Bronze Age periods (∼5000–4000 cal BP) and come from the following four smaller areas of the broader region: the Angara and upper Lena valleys, Little Sea of Baikal’s northwest coast, and southwest Baikal. Forager diets from each area occupy their own distinct position within the stable isotope spectrum. This suggests that foraging ranges were not as large as expected given the distances involved and the lack of geographic obstacles between the micro-regions. All examined individuals followed a similar subsistence strategy: harvesting game and local fishes, and on Lake Baikal also the seal, and to a more limited extent, plant foods. Although well established in their home areas, exchange networks with the other micro-regions appear asymmetrical both in time and direction: more travel and contacts between some micro-regions and less between others. The Angara valley seems to be the only area with the possibility of a temporal change in the foraging strategy from more fishing during the Early Neolithic to more ungulate hunting during the Late Neolithic–Early Bronze Age. However, the shift in stable isotope values suggesting this change can be viewed also as evidence of climate change affecting primary productivity of the Baikal–Angara freshwater system.

Use of stable nitrogen isotope signatures of anthropogenic organic matter in the coastal environment: The case study of the Kosirina Bay (Murter Island, Croatia)

In this study stable nitrogen isotope ratios of particulate matter POM, zooplankton and selected biota such as Mytilus galloprovincialis were used to assess the impact of anthropogenically derived organic matter from the untreated domestic sewage, municipal and industrial effluents on the coastal ecosystem of the Kosirina Bay (Murter Island). The differences in δ15N values observed in POM and organisms collected in Kosirina Bay as compared to POM and biota sampled at unaffected sites from the southern part of the Kornati Island and highly impacted Pirovac Bay revealed only a very minor effects of anthropogenic inputs of nutrients and organic matter which is most probably derived from a sewage outfall south of the Tužbina Island.

Quantitative Links Between Pacific Salmon and Stream Periphyton

Species’ impacts on primary production can have strong ecological consequences. In freshwater ecosystems, Pacific salmon (Oncorhynchus spp.) may influence stream periphyton through substrate disturbance during spawning and nutrient subsidies from senescent adults. The shape of relationships between the abundance of spawning salmon and stream periphyton, as well as interactions with environmental variables, are incompletely understood and may differ across the geographic range of salmon. We examined these relationships across 24 sockeye salmon (Oncorhynchus nerka) spawning streams in north-central British Columbia, Canada. The influence of salmon abundance and environmental variables (temperature, light, dissolved nutrients, water velocity, watershed size, and invertebrate grazer abundance) on post-spawning periphyton abundance and nitrogen stable isotope signatures, which can indicate the uptake of salmon nitrogen, was evaluated using linear regression models and Akaike Information Criterion. Periphyton nitrogen stable isotope signatures were best described by a positive log-linear relationship with an upstream salmon abundance metric that includes salmon from earlier years. This suggests the presence of a nutrient legacy. In contrast, periphyton abundance was negatively related to the spawning-year salmon density, which likely results from substrate disturbance during spawning, and positively related to dissolved soluble reactive phosphorus prior to spawning, which may indicate phosphorus limitation in the streams. These results suggest that enrichment from salmon nutrients does not always translate into elevated periphyton abundance. This underscores the need to directly assess the outcome of salmon impacts on streams rather than extrapolating from stable isotope evidence for the incorporation of salmon nutrients into food webs.

Impact of diazotrophy on N stable isotope signatures of nitrate and particulate organic nitrogen: case studies in the north-eastern tropical Atlantic Ocean

During two independent cruises in the north-eastern tropical Atlantic Ocean, we applied two different approaches to investigate the impact of diazotrophy on nitrogen stable isotope signatures in nitrate and particulate organic nitrogen (PON) of the food-web constituents. The first approach, used during the Poseidon cruise 348 in the Mauritanian upwelling, investigated the long-term influence of diazotrophy on the natural abundance of δ15N-NO− 3 and PON. The second approach, adopted during the Cape Verde field cruise, applied stable isotope tracer addition experiments. These served to determine the instantaneous transfer of diazotrophic N to the higher trophic level. Both approaches showed that N2 fixation was compatible with the pattern and the magnitude of the isotopic depletion of dissolved NO− 3 during the Mauritanian upwelling cruise, as well as PON in zooplankton and phytoplankton during the Cape Verde cruises. An N-budget using 15N incorporation rates and diazotrophic N2 fixation rates showed that 6 % of the daily N2 fixation was potentially taken up by the mesozooplankton community. Direct grazing accounted for 56 % of gross mesozooplanktonic N incorporation, while 46 % occurred due to channelling through the microbial loop.

Chemical characterization of microbial-dominated soil organic matter in the Garwood Valley, Antarctica

Despite its harsh environmental conditions, terrestrial Antarctica contains a relatively large microbial biomass. Natural abundance carbon and nitrogen stable isotope signatures of organic materials in the dry valleys indicate mixed provenance of the soil organic matter (SOM) with varying proportions of contributions from lichens, mosses, lake-derived algae and cyanobacteria. Here we employed two complementary analytical techniques, biomarker measurements by gas chromatography/mass spectrometry and solution-state 1H nuclear magnetic resonance spectroscopy, to provide further information at a molecular-level about the composition and possible source of SOM in the Garwood Valley, Antarctica. The predominance of branched alkanes and short-chain lipids in the solvent extracts indicates that the primary contribution to the SOM was microbial-derived. Chemical structures in the NaOH extracts from soils were also dominated by amide, peptides, and a CH 3-dominating aliphatic region that were characteristic of microbial signatures. Furthermore, the SOM in the Garwood Valley contained compounds that were different from those in the cyanobacteria-dominated mat from a nearby lake (including monoethyl alkanes and enriched side-chain protons). This observation suggests that easily degradable carbon sources from the nearby lake did not dominate the SOM, which is consistent with a fast turnover of the mat-derived organic matter found in the valley. This study highlights the important role of native soil microbes in the carbon transformation and biogeochemistry in terrestrial Antarctica.

The invasive ctenophore Mnemiopsis leidyi: a threat to fish recruitment in the North Sea?

The invasive ctenophore Mnemiopsis leidyi has a strong reputation as a threat to fish stocks. Apart from competitive relationships between M. leidyi and fish larvae, direct predation by the ctenophore on both eggs and larvae is considered an important factor linking ctenophore and fish populations. We therefore estimated abundances of both the ctenophore and its potential prey in the spring of 2008. No significant correlations were detected between ctenophore numbers and the abundance of fish eggs. We further carried out stable isotope analyses to investigate the trophic position of M. leidyi. Carbon and nitrogen stable isotope signatures of three potential prey groups (fish eggs, small plankton and larger plankton) showed that M. leidyi primarily feeds on plankton, while fish eggs are of minor importance. Mnemiopsis leidyi was located roughly one trophic level above the indigenous cteno- phore Bolinopsis infundibulum, whereas its trophic position was more similar to another native ctenophore, Pleurobrachia pileus. A feeding selection experiment, with fish eggs and copepods offered in the same proportion, corroborated these find- ings. Mnemiopsis leidyi ingested significantly more copepods; feeding on fish eggs was not significantly different from zero. Based on these experiments, we conclude that in the North Sea, M. leidyi has no serious potential as a direct predator of fish eggs, but individuals of this species might compete for food with larval fish as well as with the native ctenophore P. pileus.

Sea cucumber habitat differentiation and site retention as determined by intraspecific stable isotope variation

Stable carbon and nitrogen isotope ratios (δ13C/δ15N) were assessed as a means to ascertain the recent in situ feeding history of the common New Zealand sea cucumber Australostichopus mollis in relation to nutrient enrichment from a longline green-lipped mussel (Perna canaliculus) farm in Northern New Zealand. δ13C and δ15N isotopic signatures and the ratios of sea cucumbers sampled from within the impact footprint of the mussel farm were compared with those of sea cucumbers residing on adjacent natural reefs. Sea cucumbers from beneath mussel farming longlines had significantly different δ13C stable isotope signatures in comparison with sea cucumbers collected from neighbouring natural reef habitats. This difference supports the hypothesis that sea cucumbers in the same bay maintain distinctly different feeding histories, with those residing beneath mussel farming longlines deriving tissue carbon from sediment impacted by farming activities. This hypothesis is further supported by the finding that the isotope signature of sediment collected from beneath the mussel farm is consistent with the expectation that sea cucumbers were feeding on and consuming sediment enriched with bivalve waste (faeces and pseudo-faeces). In contrast, the nitrogen stable isotope signature (δ15N) was found to be similar between sites for both sea cucumbers and assumed food sources. Both findings lend support to the viability of future sea cucumber/green-lipped mussel farm polyculture systems. Sea cucumbers in different locations (mussel farm, natural reef) possessed distinctly different isotope signatures, suggesting that mixing of sea ranched sea cucumbers with natural reef populations would be negligible or non-existent. Similarities between the isotope signatures in low metabolic tissue of sea cucumbers residing at the mussel farm site to that of mussel farm-impacted sediment suggest that cucumbers beneath mussel farms appear to have high rates of retention at the farm site.

Food partitioning of leaf-eating mangrove crabs ( Sesarminae): Experimental and stable isotope ( 13C and 15N) evidence

The feasibility of mangrove leaves as a full diet for sesarmid crabs has been questioned for decades. Since these leaves are nitrogen-poor, sesarmids probably obtain nitrogen from other sources to sustain growth. The aim of this study was to assess the food partitioning of the sesarmid species Neoepisesarma versicolor with emphasis on nitrogen allocation. The preference for animal tissue when crabs were pre-fed diets of different nitrogen content was determined in the laboratory. Furthermore, the possible in situ diet composition of N. versicolor was established from carbon and nitrogen stable isotope signature ( δ 13C and δ 15N) of freshly caught individuals and their potential food sources, using a concentration-dependent mixing model. N. versicolor showed significantly higher feeding preferences for fish meat when pre-fed leaf material without than with access to meat, indicating that this crab species can meet its nitrogen demand by ingesting animal tissue. The stable isotope mixing model based on in situ materials suggests that the diet of N. versicolor consists of ∼60% leaves in terms of biomass, leaving ∼40% for other sources such as animal tissue and benthic microorganisms. The biomass contribution from animal tissues, in form of e.g. other crustaceans and fish carcasses, was found to account for ∼15%. Despite the relative low biomass fraction, animal food sources may contribute with up to half of the nitrogen in the diet of N. versicolor. The quantity of ingested sediment most likely exceeds that of animal tissues. However, due to the low concentration of assimilable microalgae and other microorganism, we propose that sediment associated sources are less important as a nitrogen source for N. versicolor than hitherto presumed.

The effect of ethanol fixation on stable isotope signatures in benthic organisms

Stable isotope analyses are often used for the estimation of food web structures based on empirical fractionation (i.e., enrichment of carbon isotope signatures at 0–1‰ with one trophic level, and that of nitrogen at 3–4‰). We examined the effect of 80% ethanol fixation on both carbon and nitrogen stable isotope signatures of six species of macrobenthos samples. The effect of fixation was inconsistent (i.e. both enrichment and depletion occurred). The shift of nitrogen isotope signatures due to fixation ranged from −0.3 to +0.8‰, and the carbon signatures of fixed samples showed enrichment by −0.3 to +1.3‰ depending on the species. These changes would clearly affect estimation of trophic levels and energy flow in food webs. Ethanol fixation may not be a smart choice because of the inconsistency of the effect on isotope signatures. However, if the effect was corrected for each species, ethanol-fixed samples can be practically useful for food web analyses using stable isotopes.

Variability of stable nitrogen isotopic baselines and its consequence for trophic modeling

Ecologists frequently use stable isotopes to measure the pattern and process of interested ecosystems.Comparisons of food web structure among and within ecosystems rely on a baseline isotopic signature for each system.The goal of a trophic baseline is to reflect the isotopic signature of the primary source of materials and nutrients for the food web.In general isotope baseline selection depends on the specific research question and must be tailored to a particular study system.We reviewed the use of the stable nitrogen isotopic signatures of primary producers and primary consumers to develop the baseline in aquatic ecology.Each of these studies used a slightly different method of baseline correction that was tailored to address a question of specific ecological interest. However,choosing an appropriate baseline depends on the inter-and intraspecific spatial,temporal variability of isotopic baselines and on the spatial and temporal context of the ecological question under consideration.We also evaluated to what extent isotopic heterogeneity actually influence consumer's trophic positions at the temporal and intraspecifical scales.The result of trophic position output estimated using an isotopic mixing model was generally variable,provided that baseline variability for trophic position estimation were sufficiently distinct in dietary mixing models.

Stable isotope signatures and methane use by New Zealand cold seep benthos

The carbon isotopic composition of seep faunal tissue represents a time-integrated view of the interaction between biology and the biogeochemical gradients within the environment. Here we provide an initial description of carbon and nitrogen stable isotope signatures of dominant symbiont-bearing megafauna and heterotrophic mega- and macrofauna from 10 methane-seep sites on the continental margin of the North Island of New Zealand (662–1201 m water depth). Isotopic signatures suggest that sulfide oxidation supports symbiont-bearing taxa including solemyid and vesicomyid bivalves, and methanotrophic symbionts are present in the seep mussel Bathymodiolus sp. Multiple species of Frenulata (Siboglinidae) are present and have a range of isotopic values that are indicative of both thiotroph- and methanotroph-based nutrition. Isotopic composition of the tubeworm Lamellibrachia sp. varied by 23.3‰ among individuals although there was no consistent difference among sites. Variation in methane use by heterotrophic fauna appears to reflect the availability of hard vs. soft substrate; macrofauna on hard substrates had high δ 13C signatures, reflecting consumption of photosynthetic-derived organic matter. Two unique, biotic assemblages were discovered to be fueled largely by methane: a hard-substrate, multi-phyla sponge-associated community (inhabiting the sponge Pseudosuberites sp.) and a soft-sediment assemblage dominated by ampharetid polychaetes. Isotope signatures yield estimates of 38–100% and 6–100% methane-derived carbon in sponge associates and ampharetid-bed macrofauna, respectively. These estimates are comparable to those made for deeper methane seeps at the Florida Escarpment (3290 m) and Kodiak, Alaska seeps (4445 m). The overall high use of methane as a carbon source by both symbiont-bearing and heterotrophic fauna suggests that New Zealand methane seeps are an ideal model system to study the interaction among metazoans, bacteria, archaea, and their resulting effect on methane cycles.