Successive Trophic Levels Research Articles

Multi-taxa marine isoscapes provide insight into large-scale trophic dynamics in the North Pacific

Quantifying large scale trophic dynamics and interactions in marine systems remains a key challenge in ecological research and conservation, particularly in remote habitats which are difficult and costly to access. Stable isotope (SI) analysis has emerged as an effective tool for studying ecosystem interactions, especially when used to construct isotopic landscapes, or “isoscapes”. We used a combination of empirical SI data, global modeled phytoplankton isoscapes, and oceanographic data to investigate how large SI datasets might be harnessed to describe and predict patterns of regional food web dynamics and trophic structuring. We collated data from 99 published and author-contributed datasets of carbon and nitrogen SI data for open-ocean and neritic nektonic fish and invertebrates in the North Pacific Ocean to estimate mean SI values for successive trophic level groups within Longhurst biogeographic provinces. We found that these estimates reflected geographic patterns predicted by modeled phytoplankton isoscapes, suggesting that baseline SI ratios are consistently propagated up through regional food webs. This was further corroborated by significant correlations between our estimated SI values with the same oceanographic and geographic variables known to influence phytoplankton SI ratios. However, we observed variable amounts of isotopic offset between successive trophic level groups within and across provinces, indicating that the underlying drivers of trophic dynamics differ across regions. We found evidence of decreased isotopic differences between higher level trophic groups, suggesting the possibility of trophic compression and highlighting the importance of using taxa- and tissue-specific trophic discrimination factors (TDFs) when comparing or predicting consumer SI ratios. We used stable isotopic niche analyses to demonstrate that coupled use of δ13C and δ15N data increased capacity to differentiate province-specific consumer groups, particularly where adjacent provinces had similar ranges of values for a single SI. We also found that baseline isoscapes may be a viable tool for predicting broad trends of consumer SI ratios by calculating ‘predicted’ δ13C and δ15N values for successive trophic groups across provinces using modeled phytoplankton data and diet-dependent discrimination factors. These predicted values were significantly correlated with our estimates based on empirical data, with similarity of predicted δ13C values increasing for higher trophic level groups. We conclude that regional SI patterns remain consistent across diverse ecosystems. With increased spatiotemporal resolution of SI data across ocean basins and increased availability of species-specific life history traits and physiologies, multi-taxa isoscapes hold great potential for studying large scale marine trophodynamics and movement patterns of highly mobile marine species.

Trophic transfer of microplastics in the aquatic ecosystem of Sundarbans mangrove forest, Bangladesh

Globally microplastics (MPs) contaminations have been widely reported across the large number of organisms in the marine ecosystem. Consequently, trophic transfer of MPs inferred to occur across the organisms of marine food webs. However, scientific evidence on trophic transfer of MPs across the marine organisms is very limited. Therefore, this study aimed to understand the transfer of MPs across the trophic levels in the marine ecosystem. We sampled individuals of different species of primary consumers, secondary consumers, tertiary consumers and quaternary consumers from the aquatic ecosystem of Sundarbans mangrove forest from June 2021 to December 2021. This study found that marine organisms in the aquatic ecosystem of Sundarbans mangrove forest are contaminated with MPs. The abundance of MPs in collected samples varied between 0.56 ± 0.25 items/individual and 6.06 ± 1.20 items/individual. Maximum MPs was recorded as 5.5 ± 1.21 items/individual in predators followed by 5.1 ± 0.85, 4.5 ± 0.39, 1.2 ± 0.26, 1.1 ± 0.28 and 1.01 ± 0.25 in filter feeders, browsers, deposit feeders, selective planktivores and variable feeders, respectively. Maximum MPs abundance was encountered in quaternary consumers (4.17 items/individual) followed by tertiary consumers (3.17 items/individual), secondary consumers (2.74 items/individual) and primary consumers (0.56 items/individual). We found that MPs abundance increases with the increase of trophic levels (R2 = 0.64, p < 0.001) which indicates that transfer of MPs across different trophic levels and also showed the evidence of biomagnification of MPs in successive trophic levels. Our study is the first report of trophic transfer of MPs in sub-tropical mangrove ecosystem and will serve as a guideline to understand the MPs pollution in the coastal ecosystem of Bangladesh.

Toxic metals distribution, seasonal variations and environmental risk assessment in surficial sediment and mangrove plants (A. marina), Gulf of Kachchh (India)

Toxic metal pollution in the coastal ecosystem is becoming a serious problem, particularly in developing countries as a result of the industrial revolution. In recent years, mangroves are continuously contaminating with toxic metals and receiving global attention due to its toxicity, non-degradability, abundance, subsequent bioaccumulation, and biomagnification through successive trophic levels. This study aims to investigate the toxic metal content and pollution status in mangroves surface sediment and plants. Results showed that toxic metals in sediments were higher than natural background levels indicate anthropogenic sources. Fe, Mn, Sb, Ti found higher in concentration among all toxic metals, and site 9, 15, 18, 19, 21, 31 found the highest total metal load. Contamination indices like enrichment and contamination factor, geo-accumulation index, suggest minimal to extremely high level of contamination, and sediments have found extremely contaminated with Sb and As. Contamination degree and modified contamination degree suggest very high degree of contamination at all sites. Pollution load index indicates significant deterioration of sediment quality. Ecological risk and potential ecological risk index also indicate about 72% of sites come under higher ecological risk. Toxic metal in Avicennia marina was found higher in root than leaf. High bioconcentration factor has observed for Pb, Cu, Mo, Zn. Translocation factor for Cu and Zn at all sites, and As, Ni, Pb, Fe, Sr, Mn at some sites indicate high-efficiency in plants for toxic metal translocation.

Detection of turing patterns in a three species food chain model via amplitude equation

Study of pattern formation in three species spatio-temporal food chain models with the help of amplitude equations within Turing domain is a much less explored area compared to that of two species prey-predator models. These models play a vital role in understanding the resource-consumer interactions at the successive trophic levels of respective ecosystems. In this work, we consider the spatio-temporal extension of a three species food chain model with Holling type-II functional response to describe the grazing pattern of both the intermediate and top predator populations. Conditions for Turing instability are derived. The thresholds for emergence and stability of various Turing patterns near the Turing bifurcation boundary are obtained with the help of amplitude equations using weakly nonlinear analysis. The analytical results are further verified with the help of extensive numerical simulations for the complete nonlinear model and both the results are compared for different sets of parameter values.

Tracing anthropogenic inputs in stream foods webs with stable carbon and nitrogen isotope systematics along an agricultural gradient.

Stable carbon (13C) and nitrogen isotopes (15N) are useful tools in determining the presence of agricultural influences in freshwater ecosystems. Here we examined δ15N and δ13C signatures in nitrate, fish, and mussel tissues, from rivers in Southern Ontario, Canada, that vary in their catchment proportion of agriculture land use, nutrients and organic matter quality. We found comparatively 15N-enriched δ15N values in animal tissues and dissolved nitrates, relative to expected values characterized by natural sources. We also observed a strong, positive correlation between riparian agriculture percentages and δ15N values in animal tissues and nitrates, indicating a significant influence of agricultural land use and the probable dominance of organic fertilizer and manure inputs in particular. The use of a 15N-based equation for the estimation of fish trophic position confirmed dietary analyses is showing all fish species to be tertiary consumers, with a relatively consistent 15N-enrichment in animal tissues between trophic levels. This indicates that variability in 15N-trophic fractionation is minor, and that fish and mussel tissue δ15N values are largely representative of source nitrogen. However, the trophic fractionation value varied from accepted literature values, suggesting strong influences from either local environmental conditions or dietary variation. The δ13C datasets did not correlate with riparian agriculture, and animal δ13C signatures in their tissues are consistent with terrestrial C3 vegetation, suggesting the dominance of allochthonous DOC sources. We found that changes in water chemistry and dissolved organic matter quality brought about by agricultural inputs were clearly expressed in the δ15N signatures of animal tissues from all trophic levels. As such, this study confirmed the source of anthropogenic nitrogen in the studied watersheds, and demonstrated that this agriculturally-derived nitrogen could be traced with δ15N signatures through successive trophic levels in local aquatic food webs. The δ13C data was less diagnostic of local agriculture, due to the more complex interplay of carbon cycling and environmental conditions.

Plankton based energy transfer rates in four Cuban coastal lagoons

Biomass transfer rates between successive trophic levels in plankton based food chains frequently produce inverted pyramids but when productivity data are used instead, such a methodologically caused thermodynamic impossibility is eliminated. A study of plankton based energy transfer rates in 4 Cuban coastal lagoons, including each ecosystem major component biomass, age, productivity, and energy contents values, is presented to illustrate how inverted pyramids are produced, and eliminated.Duplication times and mean ages, variable between lagoons, were 25–77 h for suspended detritus; 48–74 h for phytoplankton, and 60–90 days for zooplankters, 78% of which were copepods. For intertidal sessile filter feeders attached to the red mangrove aerial roots average ages were 0.8–2.5 months for barnacles; 1.1–2.3 months for scorched mussel; 2.9–3.5 months for flat tree-oyster; 4.5–4.8 months for mangrove oyster, and 6.1 months average for soft bodied epibionts such as tunicates and sponges.The average biomass transfer rate for the 4 lagoons from phytoplankton to zooplankton was 141%, and from detritus plus phytoplankton to sessile filter feeders was 423% both reduced to 5% and 8%, respectively, when productivity values were used, demonstrating that biomass transfer rates are intrinsically incorrect, and misleading, and that only productivity values, preferably in energy units, should be considered in trophic ecology studies.

Transfer of ornithogenic influence through different trophic levels of the Arctic terrestrial ecosystem of Bjørnøya (Bear Island), Svalbard

Despite widespread recognition of the crucial role seabirds play in the fertilization of nutrient-poor polar terrestrial ecosystems, no studies have attempted a concurrent analysis of the entire or large proportion of an ornithogenically-supported food web. The aim of the current study was to assess the significance of allochthonous nutrient enrichment of key elements of the Bjørnøya (Svalbard) terrestrial ecosystem by investigating how different seabird species influenced the characteristics of soil, vegetation, and soil invertebrates (direct ornithogenic effects), and also how those characteristics were interrelated (indirect ornithogenic effects). We sampled in the vicinity of a little auk (Alle alle) colony, and in areas occupied by great skua (Stercorarius skua) and glaucous gull (Larus hyperboreus) nests. Our data demonstrate clear, multi-trophic-level, ornithogenic impacts across the terrestrial ecosystem, with most of the measured parameters of soil, vegetation and invertebrates being altered by proximity to bird nesting areas, though to varying degrees. The ornithogenic effects tended to weaken with increasing complexity of interactions between the ecosystem components, with progression through successive trophic levels. The clearest responses were observed for soil (higher nitrogen stable isotope ratio δ15N, nitrogen and water content) and vegetation characteristics (higher δ15N, N content and total cover, lower diversity and species number, and modified community composition). The responses seen in the invertebrate communities were less clear (community composition change), and were only apparent when major invertebrate groups were considered together and for the assumed decomposers: springtails and oribatid mites, while not in the case of predators (mesostigmatid mites and one spider species). There were also suggestions in the data that different seabird species may have different impacts on the surrounding environment, probably due to their different diet and nesting area topography. However, generally, the species of bird was a weaker factor than the presence of a seabird colony or nest itself.

Experimental Detritus Manipulations Unite Surface and Cave Stream Ecosystems Along a Common Energy Gradient

Subsidies of detritus from donor habitats are important energy sources for many ecosystems, but understanding their role in structuring recipient food webs requires comparative experimental studies along the full spectrum of detrital fluxes. Here we report results from an experimental addition of maize (Zea mays L.) litter to a detritus-poor cave stream ecosystem, which we then compare with analogous, past experiments using detritus-rich surface stream ecosystems that similarly have detritus-based food webs and extremely low in situ primary production. Bulk-tissue and compound-specific stable isotope analyses showed that maize litter carbon (C) was rapidly assimilated by microbes and transferred via successive trophic levels to the top of the cave stream food web (omnivorous crayfishes and predatory salamanders). All trophic levels increased in abundance and biomass, but only facultative cave taxa, that is those also found in surface streams, contributed to this numerical response. The lack of response by obligate cave species presumably occurred because evolutionary trade-offs associated with adaptations to low-C environments constrained their population-level responses during the one-year period of the litter addition. Comparison of the responses of the cave community with the analogous litter manipulation experiments in surface streams showed strong convergence in the functional relationship between invertebrate and detritus biomass (R 2 = 0.72, P < 0.0001). Our results suggest that these seemingly disparate stream food webs lie along a single, common gradient of detritus supply, occupied at its extreme minimum by communities of obligate cave taxa adapted to low-energy environments.

Seasonal succession in zooplankton feeding traits reveals trophic trait coupling

AbstractThe seasonal forcing of pelagic communities invokes a succession of the dominant phytoplankton and zooplankton species. Here, we characterize the seasonal succession of the plankton traits and their interactions using observations and model simulations of the plankton community in the western English Channel. We focus on activity traits that characterize the defensive and feeding abilities of zooplankton and distinguish between low risk, low return ambush feeders and high risk, high return feeding‐current feeders. While the phytoplankton succession depends on traits related to nutrient acquisition and photosynthesis, it also depends on grazing which couples feeding and motility traits across trophic guilds. Despite interannual variations in the species dominating the protist plankton community, the seasonal trait distribution reveals robust and repeatable seasonal patterns, changing between non‐motile cells flourishing in spring and motile community dominating during summer. The zooplankton community is dominated by active feeding‐current feeders with peak biomass in the late spring declining during summer. The model reveals how zooplankton grazing reinforces protist plankton seasonal succession and shows how the physical environment controls the vertical structure of plankton communities, where ambush feeders exhibit a preference for greater depths during summer. We characterize the seasonal succession as trophic trait coupling and conjecture that this coupling leads to a trophic trait cascade where successive trophic levels alternate in their expression of activity traits further up in the food chain.

Measuring the in situ carbon isotopic composition of distinct marine plankton populations sorted by flow cytometry

AbstractThe carbon isotope ratio (δ13C value) of marine particulates is a potentially useful tracer for elucidating pathways of carbon flow in the marine environment. Different species of phytoplankton vary in fractionation vs. CO2 by up to 24‰ in laboratory cultures under varying nutrient and growth conditions, a signal that should propagate through the microbial food web. However, such contrasts have been difficult to confirm in field measurements due to analytical limitations. Here, we combine fluorescence‐activated cell sorting (FACS) with a specialized micro‐combustion interface and isotope‐ratio mass spectrometry (SWiM‐IRMS) to provide some of the first direct measurements of whole‐cell δ13C values for specific phytoplankton populations in the wild. For three samples collected off Scripps Pier in 2010–2011, Synechococcus averages δ13C values of −25.7 ± 2.0‰, Prochlorococcus averages −23.0 ± 1.3, and diatoms average −20.8 ± 1.7‰. Diatoms were ∼3‰ enriched in 13C when measured during a bloom (March 2011) as compared with mid‐summer (July 2010). Sorted particles thought to represent living heterotrophic bacteria averaged −25.4 ± 2.5‰, whereas total filterable particles averaged −19.6 ± 1.0‰, indicating a strong similarity to diatom biomass. These variations demonstrate that in situ differences in δ13C among different populations of particles can be exploited to follow carbon flow through successive trophic levels, and throughout organic matter remineralization, sinking, and preservation.

Mercury bioaccumulation in cartilaginous fishes from Southern New England coastal waters: Contamination from a trophic ecology and human health perspective

This study examined total mercury (Hg) concentrations in cartilaginous fishes from Southern New England coastal waters, including smooth dogfish (Mustelus canis), spiny dogfish (Squalus acanthias), little skate (Leucoraja erinacea), and winter skate (Leucoraja ocellata). Total Hg in dogfish and skates were positively related to their respective body size and age, indicating Hg bioaccumulation in muscle tissue. There were also significant inter-species differences in Hg levels (mean ± 1 SD, mg Hg/kg dry weight, ppm): smooth dogfish (3.3 ± 2.1 ppm; n = 54) > spiny dogfish (1.1 ± 0.7 ppm; n = 124) > little skate (0.4 ± 0.3 ppm; n = 173) ∼ winter skate (0.3 ± 0.2 ppm; n = 148). The increased Hg content of smooth dogfish was attributed to its upper trophic level status, determined by stable nitrogen (δ15N) isotope analysis (mean δ15N = 13.2 ± 0.7‰), and the consumption of high Hg prey, most notably cancer crabs (0.10 ppm). Spiny dogfish had depleted δ15N signatures (11.6 ± 0.8‰), yet demonstrated a moderate level of contamination by foraging on pelagic prey with a range of Hg concentrations, e.g., in order of dietary importance, butterfish (Hg = 0.06 ppm), longfin squid (0.17 ppm), and scup (0.11 ppm). Skates were low trophic level consumers (δ15N = 11.9–12.0‰) and fed mainly on amphipods, small decapods, and polychaetes with low Hg concentrations (0.05–0.09 ppm). Intra-specific Hg concentrations were directly related to δ15N and carbon (δ13C) isotope signatures, suggesting that Hg biomagnifies across successive trophic levels and foraging in the benthic trophic pathway increases Hg exposure. From a human health perspective, 87% of smooth dogfish, 32% of spiny dogfish, and <2% of skates had Hg concentrations exceeding the US Environmental Protection Agency threshold level (0.3 ppm wet weight). These results indicate that frequent consumption of smooth dogfish and spiny dogfish may adversely affect human health, whereas skates present minimal risk.

Methylmercury in Biota Downstream of Arivaca Lake, Arizona, USA

Concentrations of total mercury (Hg) and methylmercury (MeHg) were determined in water, sediment, periphyton, spiders, and amphibians from the streams and desert marsh downstream from Arivaca Lake, Arizona, to better understand their distribution and bioaccumulation. Mean concentrations of MeHg in water ranged from 0.09 to 0.93ng/L, and mean concentrations of total Hg in sediment ranged from 10.4 to 126μg/kg. Hg and MeHg in water and sediments downstream from Arivaca Lake were low enough that they did not exceed human health or ecological thresholds. Hg and MeHg between sites ranged from 0.11 to 1.90μg/g Hg and 0.01 to 0.3μg/g MeHg in periphyton, from 0.09 to 0.25μg/g Hg and 0.04 to 0.10μg/g MeHg in spiders, and from 0.15 to 0.38μg/g Hg and 0.14 to 0.35μg/g MeHg in adult bullfrogs. No Hg toxicity data exist for periphyton or spiders, but MeHg concentrations in tadpoles (0.04±0.005μg/g) were lower than those known to cause sublethal effects and subchronic mortality. The mean total Hg concentration in adult bullfrogs in the present study was 0.24μg/g, which is slightly lower than the mean (0.37μg/g) from an Hg-contaminated wetland in California. MeHg bioaccumulated at each successive trophic level, and MeHg bioconcentration factors from the Arivaca watershed were similar to those for periphyton but greater than amphibians in other studies. Local resource managers can use these data to determine if water should be released from Arivaca Lake to recharge the aquifer downstream or to decrease Hg methylation in the reservoir.

Quantitative Assessment of Polycyclic Aromatic Hydrocarbons and Heavy Metals in Fish Roasted with Firewood, Waste Tyres and Polyethylene Materials

The bioaccumulation and biomagnification of PAHs along successive trophic levels is an important means of exposure to human and animal life. This study evaluated Polycyclic Aromatic Hydrocarbons (PAHs) and heavy metals in fresh fish and fish samples processed/ roasted with smoke/heat/flame generated from firewood, waste tyre and polyethylene materials. PAHs and heavy metals were determined with Gas chromatography with flame ionization detector (GC-FID) and Atomic Absorption Spectrophotometer (AAS) respectively. Significant differences (P<0.05) were observed in the concentrations of PAHs and heavy metals in the heat-processed fish when compared to fresh fish. Appreciable amount of carcinogenic PAHs; benzo(a)pyrene, benz(a)anthracene and dibenz(a,h)anthracene benz(a)anthracene and dibenz(a,h)anthracene and heavy metals such as cadmium (Cd), zinc (Zn) and lead (Pb) were detected in processed/roasted fish samples.

Mercury concentration in the eggs of four Canadian Arctic-breeding shorebirds not predicted based on their population statuses

Methylmercury is a toxic form of mercury which persists in food webs for long periods of time and biomagnifies up successive trophic levels. Shorebirds breeding in the Arctic are exposed to methylmercury, derived from both natural and anthropogenic sources, when they ingest their invertebrate prey. Populations of many shorebird species are believed to be declining and one hypothesis for these declines is that they are due to detrimental effects of contaminants, including methylmercury. To test this hypothesis, we assessed mercury contamination in eggs of four Canadian Arctic-breeding shorebird species: black-bellied plover (Pluvialis squatarola), ruddy turnstone (Arenaria interpres), semipalmated plover (Charadrius semipalmatus) and white-rumped sandpiper (Calidris fuscicollis). Black-bellied plovers and ruddy turnstones are declining in the western hemisphere, whereas white-rumped sandpipers and semipalmated plovers have stable or slightly increasing populations. We found no relationship between egg mercury concentration and population trend for these four shorebird species. Intraspecific variation in mercury concentration was high. Notably, the mercury concentrations were much higher than levels found in a previous study of eggs of the same shorebird species from this same site, suggesting that mercury contamination may be subject to substantial inter-annual variation in the Canadian Arctic food web.

Wild versus Farmed Fish: Metal Concentration and Biomagnification Trends in the Food Web of a Chinese Reservoir

Natural and anthropogenic elemental contamination is widespread in the environment. This is especially true in China, a country with rapid industrial and economic growth. Some metals can be toxic at very low concentrations, rapidly accumulating within fish tissues and other organisms. As fish are an important part of the Chinese diet, food web transfer and biomagnification trends of As, Cd, Co, Cr, Cu, K, Ni, Pb, Rb, Se and Zn are assessed for fish from a reservoir in eastern China. This study site, with 1,078 islands and prized freshwater fish species, contains 13 families and 86 fish species. Commonly-consumed farmed fish from a range of trophic levels were sampled, including Silurus asotus, Aristichthys nobilis, Ictalurus punctatus, Carassius auratus, Ctenopharyngodon idella, Micropterus salmoides and Hypothalmichthys molitrix. Wild piscivorous fish sampled include Siniperca chuatsi, Erythroculter mongolicus, Erythroculter illishaeformis and Channa argus argus. Our questions for this study are: (1) Are metal accumulation trends and concentrations in wild versus farmed fish species different? (2) Do trends in metal concentration between fish markets in a large city and a fishing town differ? Concentrations of Cd, Co and Pb were below reliable ICP-OES detection limits. As, Cu and Zn concentrations were highest in farmed species, whereas Cr, K, Ni, Rb and Se were highest in wild species. δ15N values were used to estimate trophic position. Results obtained suggest significant biomagnification of Se at successive trophic levels and marginally significant biomagnification trends for Rb. All other relationships between elemental concentration and trophic position indicated a lack of biomagnification. Anthropogenic sources from industrial activities surrounding the reservoir may contribute to elevated Cr and Se in fish, while ambient pH and temperature differentials could play a role in the increased bioavailability of Rb in the region.

Evaluation of Bioaccumulation Using In Vivo Laboratory and Field Studies

A primary consideration in the evaluation of chemicals is the potential for substances to be absorbed and retained in an organism's tissues (i.e., bioaccumulated) at concentrations sufficient to pose health concerns. Substances that exhibit properties that enable biomagnification in the food chain (i.e., amplification of tissue concentrations at successive trophic levels) are of particular concern due to the elevated long-term exposures these substances pose to higher trophic organisms, including humans. Historically, biomarkers of in vivo chemical exposure (e.g., eggshell thinning, bill deformities) retrospectively led to the identification of such compounds, which were later categorized as persistent organic pollutants. Today, multiple bioaccumulation metrics are available to quantitatively assess the bioaccumulation potential of new and existing chemicals and identify substances that, upon or before environmental release, may be characterized as persistent organic pollutants. This paper reviews the various in vivo measurement approaches that can be used to assess the bioaccumulation of chemicals in aquatic or terrestrial species using laboratory-exposed, field-deployed, or collected organisms. Important issues associated with laboratory measurements of bioaccumulation include appropriate test species selection, test chemical dosing methods, exposure duration, and chemical and statistical analyses. Measuring bioaccumulation at a particular field site requires consideration of which test species to use and whether to examine natural populations or to use field-deployed populations. Both laboratory and field methods also require reliable determination of chemical concentrations in exposure media of interest (i.e., water, sediment, food or prey, etc.), accumulated body residues, or both. The advantages and disadvantages of various laboratory and field bioaccumulation metrics for assessing biomagnification potential in aquatic or terrestrial food chains are discussed. Guidance is provided on how to consider the uncertainty in these metrics and develop a weight-of-evidence evaluation that supports technically sound and consistent persistent organic pollutant and persistent, bioaccumulative, and toxic chemical identification. Based on the bioaccumulation information shared in 8 draft risk profiles submitted for review under the United Nations Stockholm Convention, recommendations are given for the information that is most critical to aid transparency and consistency in decision making.

Using stable isotopes and a multiple-source mixing model to evaluate fish dietary niches in a mesotrophic lake

Christensen, D.R. and B.C. Moore. 2009. Using stable isotopes and a multiple-source mixing model to evaluate fish dietary niches in a mesotrophic lake. Lake Reserv. Manage. 25:167–175. We used stable isotope analysis (SIA) of δ13C and δ15N and a multiple-source mixing model to evaluate dietary niches within the Twin Lakes, Washington, fish community to identify potential for exploitative competition, resource partitioning, and predation. The SIA revealed distinct spatial feeding niches; pelagic species were δ13C depleted, while littoral organisms were δ13C enriched. Trophic feeding niches were identified from δ15N enrichment with each successive trophic level. We found δ15N progression with increasing largemouth bass size, suggesting ontogenetic diet variability. Model results suggested that the smaller bass (i.e., ⩽299 mm length) fed principally on golden shiner, crayfish, and macroinvertebrates while larger bass (⩾300 mm) primarily consumed brook trout and golden shiner. Isotopic signatures for golden shiner were intermediary, indicating both pelagic and littoral feeding sources, including phantom midges (nocturnal and pelagic) and damselflies (littoral) as principal diet sources, suggesting possible horizontal diel migration in golden shiner. Pelagic zooplankton was indicated as the most important food source for rainbow trout. Food resources appeared to be partitioned among the Twin Lakes fish community, possibly limiting competitive interactions. Piscivory in largemouth bass was focused on golden shiner and brook trout, suggesting possible top-down regulation of lower trophic levels. Stable isotope analysis and multiple-source mixing models are useful tools that can improve lake and fishery management decisions by providing data on food web dynamics such as competition, resource partitioning, and predation in lakes.

Top-down and bottom-up stabilizing mechanisms in eelgrass meadows differentially affected by coastal upwelling

Trophic cascades usually regulate structure and dynamics of valuable benthic vegeta- tion communities (e.g. kelp forests, salt marshes), resulting in alternating high and low abundances from predators at the highest level, down through herbivores, to basal producers at the lowest trophic level. While herbivores may have direct strong negative effects on kelp and salt marsh vegetation, grazing invertebrates (mesograzers) within seagrass meadows consume and thus control the biomass of opportunistic epiphyte algae that can outcompete seagrasses for light. Small fish predators, how- ever, may control mesograzer abundances, thus releasing epiphytes from grazing pressure, with resultant widespread losses of seagrasses under eutrophic conditions. In order to assess whether trophic cascades operate in seagrass systems, we compared patterns of abundance and composition of the benthic vegetation community, the associated epifaunal invertebrates, and their predators in eelgrass meadows distributed across a natural gradient of algal productivity. Within this gradient we selected and sampled in summer and winter 3 eelgrass meadows differentially affected by strong coastal upwelling. High mesograzer abundance repeatedly coincided with low epiphyte biomass, emphasizing top-down control of epiphytes, despite high nutrient availability. Consumption of live eelgrass tissue by mesograzers was potentially important at the lower end of the productivity gradi- ent. Alternating patterns of abundances between successive trophic levels under intermediate and high productivity conditions suggest that small predator abundances may ultimately govern the suc- cess of eelgrass in eutrophic meadows. When pipefish were abundant and eelgrass complexity was low, patterns were consistent with a destabilizing trophic cascade. Evidence supporting a trophic cascade dynamic from small fish through to epiphytes may indicate susceptibility of eelgrass systems to higher order interactions.

Anticipatory Reproduction and Population Growth in Seed Predators

Mast seeding, the intermittent, synchronous production of large seed crops by a population of plants, is a well-known example of resource pulses that create lagged responses in successive trophic levels of ecological communities. These lags arise because seed predators are thought capable of increasing reproduction and population size only after the resource pulse is available for consumption. The resulting satiation of predators is a widely cited explanation for the evolution of masting. Our study shows that both American and Eurasian tree squirrels anticipate resource pulses and increase reproductive output before a masting event, thereby increasing population size in synchrony with the resource pulse and eliminating the population lag thought to be universal in resource pulse systems.

Benthic trophic network in the Bay of Banyuls-sur-Mer (northwest Mediterranean, France): An assessment based on stable carbon and nitrogen isotopes analysis

The benthic trophic network in the Bay of Banyuls-sur-Mer was studied through the carbon and nitrogen isotopic characterization of a large set of soft-bottom macrobenthic invertebrates, fishes and potential food sources. Continental inputs as well as seagrass meadows did not contribute significantly to this benthic trophic network as indicated by: (1) the difference between their δ 13C signatures (respectively −28.4‰ and −9.5‰) and those of sampled animals (between −21.0‰ and −14.6‰); and (2) their low inputs to the bay. Benthic primary consumers fed mostly on surface sediment organic matter (SSOM), which tightly interacts with suspended particulate organic matter (SPOM) and sedimenting organic matter (STOM) due to sediment resuspension. Our results also suggest the occurrence of a transfer between marine SPOM and some invertebrates (e.g. Veretillum cynomorium) and fishes (e.g. Boops boops and Spicara melanurus) through zooplankton. Moreover, the different types of primary consumers (i.e., suspension-feeders, interface-feeders, surface deposit-feeders and subsurface deposit-feeders) preferentially used distinct fractions of the heterogeneous SPOM-STOM-SSOM pool. These differences were mostly related with feeding depth and resulted in distinct isotopic signatures. Differences in the stable isotopic ratios of suspension and interface-feeders could also partly reflect the use of microphytobenthos by the later. Assuming a 15N-enrichment factor of 3.4‰ between the lower and upper ranges of two successive trophic levels, we estimated that the benthic food web of the Bay of Banyuls-sur-Mer was composed of 4 trophic levels. The comparison with our δ 13C values suggests that the whole trophic food chain is affected by continental inputs at the immediate vicinity of the Rhône River mouth even though these effects are maximal for deposit-feeding and carnivorous polychaetes.