This study aimed to assess whether ecological inferences from isotopic functional indices (IFIs) are impacted by changes in isotopic baselines in aquatic food webs. We used sudden CO
-outgassing and ...associated shifts in DIC-
C brought by waterfalls as an excellent natural experimental set-up to quantify impacts of changes in algal isotopic baselines on ecological inferences from IFIs.
Carbon (
C) and nitrogen (
N) stable isotopic ratios of invertebrate communities sharing similar structure were measured at above- and below-waterfall sampling sites from five rivers and streams in Southern Quebec (Canada). For each sampled invertebrate community, the six Laymans IFIs were then calculated in the
-space (
C vs.
N).
As expected, isotopic functional richness indices, measuring the overall extent of community trophic space, were strongly sensitive to changes in isotopic baselines unlike other IFIs. Indeed, other IFIs were calculated based on the distribution of species within
-space and were not strongly impacted by changes in the vertical or horizontal distribution of specimens in the
-space. Our results highlighted that IFIs exhibited different sensitivities to changes in isotopic baselines, leading to potential misinterpretations of IFIs in river studies where isotopic baselines generally show high temporal and spatial variabilities. The identification of isotopic baselines and their associated variability, and the use of independent trophic tracers to identify the actual energy pathways through food webs must be a prerequisite to IFIs-based studies to strengthen the reliability of ecological inferences of food web structural properties.
Understanding the processes that regulate food chain length in nature is a classic theme in ecology. Two factors expected to explain variation in food chain length are resource availability ...(productivity) and environmental stress. We examined the impact of anthropogenic activities, both a source of stress and productivity, on lotic food webs using two simple food web descriptors based on stable nitrogen isotopes (δ¹⁵N). We used 1) trophic position of small fish and 2) slopes of δ¹⁵N-size class relationships in the invertebrate community as variables related to food chain length. Trophic positions and δ¹⁵N-size slopes changed significantly across the 23 study sites selected to vary in productivity and impact from industrial activities (trophic position=0.4 to 1.9; slopes=-1 to +2.8). Fish trophic position was not affected by productivity (chlorophyll a) or environmental stress (number of factories on the watershed) and was not modulated by the proportion of predatory invertebrates in the food web leading to fish. This lack of relationship suggests that variation in trophic position is likely influenced by fish feeding behaviour and is therefore not a good integrator of changes in food chain length induced by anthropogenic activities. δ¹⁵N-size slopes significantly increased with productivity and decreased with stress, suggesting that trophic organization by size within invertebrate communities is affected by these factors. These slopes also tended to be greater when the proportion of predators in the community increased in larger size classes. These results suggest that when productivity is high and stress is low, proportions of higher δ¹⁵N-value predators in lotic food webs tend to increase, contributing to the lengthening of food chains. They also suggest that δ¹⁵N-size relationships of invertebrate communities may be used as indicators of food web structure alteration by human activities.
Stable isotopes (particularly C and N) are widely used to make inferences regarding food web structure and the phenology of consumer diet shifts, applications that require accurate isotopic ...characterization of trophic resources to avoid biased inferences of feeding relationships. For example, most isotope mixing models require that endmembers be adequately represented by a single probability distribution; yet, there is mounting evidence that the isotopic composition of aquatic organisms often used as mixing model endmembers can change over periods of weeks to months. A review of the literature indicated that the δ
13
C values of five aquatic primary consumer taxa, commonly used as proxies of carbon production sources (i.e., trophic baselines), express seasonally dynamic cycles characterized by an oscillation between summer maxima and winter minima. Based on these results, we built a dynamic baseline mixing model that allows a growing consumer to track temporal gradients in the isotopic baselines of a food web. Simulations showed that the ability of a consumer to maintain or approach isotopic equilibrium with its diet over a realistic growth season was strongly affected by both the rate of change of the isotopic baseline and equilibration rate of the consumer. In an empirical application, mixing models of varying complexity were used to estimate the relative contribution of benthic vs. pelagic carbon sources to nine species of juvenile fish in a fluvial lake of the St. Lawrence River system (Québec, Canada). Estimates of
p
(proportion of carbon derived from benthic sources) derived from a static mixing model indicated broad interspecific variation in trophic niche, ranging from complete benthivory to >95% reliance on pelagic food webs. Output from the more realistic dynamic baseline mixing model increased estimated benthivory by an average of 36% among species. Taken together, our results demonstrate that failing to identify dynamic baselines when present, and (or) matching consumers with baseline taxa that possess substantially different equilibration rates can seriously bias interpretation of stable isotope data. Additionally, by providing a formalized framework that allows both resources and consumers to shift their isotopic value through time, our model demonstrates a feasible approach for incorporating temporally dynamic isotope conditions in trophic studies of higher consumers.
We examined the influence of Montreal wastewater treatment plant (WWTP) effluents on two top predators, Walleye (
Sander vitreus
) and Sauger (
Sander canadensis
), with a focus on δ
15
N isotopic ...signatures and per- and polyfluoroalkyl substances (PFAS). These two fish species were collected in the summer 2013 in the St. Lawrence River upstream and downstream from a major WWTP, as well as in background sites (semi-remote lakes). Most of the δ
15
N variations for Sauger and Walleye are attributable to
1
) δ
15
N values of the primary producers and sewage-derived particulate organic matter (SDPOM) at the base of the trophic food chain,
2
) agricultural activities combined with biogeochemical processes, and
3
) food web length. δ
15
N was significantly lower in fish collected in the effluent-mixed water masses than other sites of the St. Lawrence River, attributed to the SDPOM of the WWTP effluent. Relative to the background sites, certain PFAS were present at much higher levels in the St. Lawrence River, with profiles dominated by perfluoroalkyl sulfonates (PFSA). However, PFSA profiles generally remained consistent along the St. Lawrence River. PFOS levels in fish from the St. Lawrence exceeded the current Federal Environmental Quality Guidelines for protecting piscivorous mammals or birds. However, the human chronic daily intake of PFOS remained below current thresholds suggested by national agencies.
We used variation in algal δ13C between river habitats to study the spatial scale of energy flow through river food webs. We found a strong negative relationship between herbivore δ13C (which ...reflects algal δ13C) and water velocity in three productive Northern California rivers but not in unproductive streams. The contrast among habitats suggests that water velocity affects algal δ13C most strongly when CO2availability is low relative to photosynthetic rates. Our results help explain the wide variation in published river biota δ13C and show that past studies using carbon isotope analyses may have significantly underestimated the importance of algal-derived carbon to river food webs. While flow-related variation in δ13C complicates this common application of carbon isotope analysis, we show that it provides a natural tracer of the flux of algal production derived from different habitats within rivers to higher trophic levels. Measurements of consumer δ13C showed that most invertebrate and vertebrate consumers relied on local production, except for filter-feeding insects and steelhead trout, which relied on production derived from multiple sources. Stable carbon isotopes may thus be used to spatially delineate the habitats that support river food webs, providing previously unavailable information for understanding and managing river ecosystems.
Environmental homogenization in coastal ecosystems impacted by human activities may be an important factor explaining the observed decline in fish species richness. We used fish community data (>200 ...species) from extensive surveys conducted in two biogeographic provinces (extent >1,000 km) in North America to quantify the relationship between fish species richness and local (grain <10 km(2)) environmental heterogeneity. Our analyses are based on samples collected at nearly 800 stations over a period of five years. We demonstrate that fish species richness in coastal ecosystems is associated locally with the spatial heterogeneity of environmental variables but not with their magnitude. The observed effect of heterogeneity on species richness was substantially greater than that generated by simulations from a random placement model of community assembly, indicating that the observed relationship is unlikely to arise from veil or sampling effects. Our results suggest that restoring or actively protecting areas of high habitat heterogeneity may be of great importance for slowing current trends of decreasing biodiversity in coastal ecosystems.
Ecological subsidies have been hypothesized to be key factors affecting the dynamics of the recipient ecosystems. We assessed the impact of the flood regime on the assimilation of a trophic subsidy ...produced in the floodplain of a large fluvial lake, Lake Saint-Pierre (Quebec, Canada) on five invertivore littoral fish species (yellow perch (Perca flavescens), pumpkinseed (Lepomis gibbosus), bridle shiner (Notropis bifrenatus), golden shiner (Notemigonus crysoleucas), and banded killifish (Fundulus diaphanus)). We hypothesized that the incorporation of floodplain-derived carbon into fish biomass and individual fitness would increase in years with higher magnitude and longer duration floods. Studying four consecutive years with contrasting spring-flood events, we used a stable isotope mixing model to estimate floodplain contribution, and linear mixed models to analyze the impact of hydrology on floodplain contribution and fish body condition. Floodplain and main river’s invertebrates exhibited statistically distinct isotopic ratios (t test: p value < 0.001). Floodplain contribution and body condition were positively correlated with flooding intensity and duration for all species. Body condition was positively correlated with the assimilation of floodplain-subsidy for golden shiner and banded killifish. Our study identified flooding intensity as a main driver influencing the assimilation of floodplain-derived carbon by littoral fish and their fitness in a large river ecosystem.
This study examines the temporal and spatial variability of the floodplain trophic contribution to the fish community of Lake Saint-Pierre, a large (≈ 300 km²), shallow (≈ 3 m) widening of the St. ...Lawrence River (Québec, Canada). We hypothesized that floodplain contribution to fish diet would be maximal during the early growing period following the seasonal flood and at sites located closest to the floodplain. Carbon stable isotope ratios (δ¹³C) of invertebrates and 35 fish species were analyzed at the beginning and at the end of two growing seasons, at increasing distances from the floodplain. Floodplain contribution, in the form of exported aquatic invertebrates, was estimated at the fish community (littoral versus main river), species and individual fish level, using a stable isotopic mixing model and a linear mixed model selection. As hypothesized, fish captured in the early part of the growing season near the floodplain benefitted from the largest floodplain contribution, which decreased during the late growing season. In addition, floodplain contribution differed among fish species, being higher in small or immature littoral species (northern pike, yellow perch, blackchin and golden shiners) than for larger taxa inhabiting the main river channel (shorthead redhorse, white sucker and channel catfish). Our study revealed that floodplain production during the early growing season represents a key food resource to a large portion of the riverine food web, thus highlighting the essential role of the floodplain in human-altered temperate rivers.
Emerging PFAS were recently reported at sites impacted by aqueous film-forming foams (AFFFs) and near major manufacturing centers; however, few studies have evaluated whether these can occur far from ...release sites. Here, newly identified PFAS were investigated in wild sporting fish from boreal freshwater ecosystems (background sites, 2013–2014 summer seasons), compared to fish impacted by a major AFFF release (summer 2013 and autumn 2014). Different freshwater wild sporting fish species (Esox lucius, Esox masquinongy, Micropterus dolomieu, Sander vitreus, Perca flavescens, and Semotilus corporalis, n = 74) were collected from 13 ecosystems (lakes, reservoirs, and rivers) across Eastern Canada. Of 29 quantitative PFAS, 15 compounds were detected in fish from background sites, including perfluorocarboxylates (C6,8–14), perfluoroalkane sulfonates (C6,8,10), perfluorooctane sulfonamide (FOSA), 6:2 fluorotelomer sulfonate (6:2 FTSA), 7:3 fluorotelomer carboxylic acid (7:3 FTCA), and a zwitterionic PFAS—perfluorooctane sulfonamidoalkyl betaine (PFOSB). To our knowledge, this is the first report of PFOSB in biota. It is also one of the first reports of anionic fluorotelomers (6:2 FTSA, 7:3 FTCA, 9:3 FTCA) in wildlife from background sites. Long-chain fluorotelomer sulfonamidoalkyl betaines (e.g., 8:2 and 10:2 FTAB), fluorotelomer betaines (e.g., 9:3 and 9:1:2 FTB), and fluorotelomer sulfone propanoic acids (e.g., 8:2 FT(SO2)-PA, 10:2 FT(SO2)-PA)) were solely prevalent (up to 97% of summed suspect PFAS) in Smallmouth Bass (M. dolomieu) from the AFFF-impacted site. Perfluorobutane sulfonamide (FBSA), perfluorohexane sulfonamide (FHxSA), 6:2 FTSA and 7:3 FTCA were detected in at least one Smallmouth Bass sample both at the AFFF-impacted and background sites. According to the estimated chronic daily intake and current tolerable daily intake suggested by national agencies, the observed PFOS levels would not pose a health risk to anglers who might consume these wild-caught fish.
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•Newly identified PFAS were investigated in wild fish from boreal ecosystems.•15 compounds were detected in fish from background sites.•Certain ECF-based sulfonamide precursors were detected at background sites.•Specific fluorotelomers were detected in fish from a recently impacted AFFF site.•The fish PFOS levels do not seem to pose a health risk to consumers.
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