The influence of body size and season on the feeding ecology of three freshwater fishes with different diets in Lake Erie

Isotopic niche and length-isotope relationships were quantified across the body size of three similarly-sized fish species with different feeding strategies as adults (benthivore: freshwater drum, Aplodinotus grunniens; piscivore: walleye, Sander vitreus; and omnivore: white perch, Morone americana) in Lake Erie's western basin using δ¹³C, δ¹⁵N and δ³⁴S. Stable isotopes demonstrated that resource utilization changed with body size for all three species and length-isotope relationships varied with season. Isotopic niche overlap was lower when modelled with three isotopes (δ¹³C, δ¹⁵N, and δ³⁴S) than with two (δ¹³C and δ¹⁵N), providing greater resolution of feeding ecology among the species. Based on isotopic niches, there was significant overlap in resource use among species and size classes in spring, but overlap decreased in the fall for both. In this study, freshwater fish species with different adult feeding strategies partition resources through different mechanisms that vary through body size and season. Isotopes supported the generally identified feeding ecology of each species but demonstrated that each species underwent significant changes in feeding ecology with increasing body size. Changes in isotopes across season, and body size for each species demonstrate the need for a more thorough understanding of how resource use changes with body size and season in freshwater fish.     

Looking to the past to ensure the future of the world's oldest living vertebrate: Isotopic evidence for multi‐decadal shifts in trophic ecology of the Australian lungfish

Meeting the conservation challenges of long‐lived animal species necessitate long‐term assessments of trophic ecology. The use of dietary proxies, such as ratios of naturally occurring stable isotopes in animal tissues demonstrating progressive growth, has shown considerable promise to reconstruct trophic histories of long‐lived organisms experiencing environmental change. Here, we combine innovative radiocarbon scale‐ageing techniques with stable isotope analysis of carbon and nitrogen from cross sections of scale to reconstruct the trophic ecology of Australian lungfish (Neoceratodus forsteri) across its remaining global distribution. Over a 65‐year period, we found pronounced temporal shifts in the δ¹³C and δ¹⁵N isotopic ratios of lungfish that coincided with a period of hydrological modification by dams and land‐use intensification associated with agriculture and livestock grazing. In the Brisbane and Burnett Rivers, whose hydrology is substantially regulated by large dams, lungfish showed consistent trends of δ¹³C depletion and δ¹⁵N enrichment over time. This may indicate anthropogenic changes in background isotopic levels of basal energy sources and/or that additional seston exported downstream from impoundments represent a carbon source that was previously unavailable, thus shifting lungfish diet from benthic‐dominated primary production typical of unmodified river systems, to pelagic carbon sources. By contrast, δ¹³C ratios of lungfish in the unregulated Mary River were more stable through time, whereas δ¹⁵N ratios increased during a period of dairy industry expansion and increased application of nitrogen fertilization and then subsequently decreased at the same time that rates of pasture development declined and nutrient inputs presumably decreased. In conclusion, we provide evidence for human‐caused alterations in background isotopic levels and potential changes in availability of benthic versus pelagic energy resources supporting Australian lungfish and demonstrate how detectable trophic signals in long‐lived fish scales can reveal long‐term anthropogenic changes in riverine ecosystems.     

Variability in fish and water hydrogen and oxygen stable isotope values in the nearshore region of a large water body

Incorporating organismal hydrogen (δ²H or δD) and oxygen (δ¹⁸O) stable isotope ratios into aquatic food web research can help elucidate relative reliance on shoreline inputs and/or feeding at different trophic levels; however, their interpretation is complicated by the fact that aquatic organisms derive hydrogen and oxygen from both their diet and the ambient water in which they reside. We examined spatial and seasonal patterns in δ²H and δ¹⁸O of field-collected water and small-bodied fishes. Samples were gathered from nearshore Lake Michigan, a dynamic region of a large, increasingly oligotrophic freshwater lake. We examined seasonal and spatial δ²H and δ¹⁸O values of surface and bottom water, and compared spatial δ²H and δ¹⁸O values of young-of-year yellow perch (Perca flavescens) and seasonal δ²H and δ¹⁸O values of round goby (Neogobius melanostomus). Nearshore δ²H and δ¹⁸O water values were more variable than previously-described offshore values. Variation in nearshore water δ²H and δ¹⁸O values was likely related to differential precipitation and/or discharges from tributaries, and there may be a surface dilution effect which led to lower δ¹⁸O values in nearshore surface waters than bottom waters. In our study, fish tissue δ²H and δ¹⁸O also varies more spatially than seasonally, and some findings suggest that fish tissue δ²H may reflect feeding at higher trophic levels. Though characteristics of the study system affect their interpretation, we suggest that δ²H and δ¹⁸O can be another tool to assess food web structure, with δ²H in particular having potential to resolve questions when δ¹³C or δ¹⁵N are inconclusive.     

Large variation in periphyton δ13C and δ15N values in the upper Great Lakes: Correlates and implications

Stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) are frequently used to examine food web structure. Despite periphyton's importance to lake food webs, little is known about spatial variation of periphyton δ¹³C and δ¹⁵N values in the Great Lakes. We present periphyton δ¹³C and δ¹⁵N values from 28 sites the upper Great Lakes, including Lake Superior, the north shore of Lake Michigan, and Green Bay. We also examined variation in periphyton isotope values relative to several water quality parameters (TP, TN, TKN, NO₃⁻, K_d) as well as periphyton C:N. There was a large range in both periphyton δ¹³C (range = 13.5‰) and δ¹⁵N (range = 10.2‰) among sites. Periphyton in more eutrophic sites had more depleted δ¹³C and more enriched δ¹⁵N compared to more oligotrophic sites. Our finding of high variability in periphyton isotope values in the Upper Great Lakes has implications for stable isotope-based reconstructions of food web structure.     

Diet,trophic position of upper St. Lawrence River round goby giants reveals greater dependence on dreissenids with increasing body size

Round gobies (Neogobius melanostomus) from the upper St. Lawrence River (USLR) have an abundance of some of the largest individuals recorded from the Great Lakes (>230-mm total-length). We found a distinct separation in diet and isotopic signatures (δ¹⁵N and δ¹³C) between round goby classified as small (≤130-mm; n = 63) and large (>130-mm total-length; n = 75) from USLR coastal bays. At small sizes, round gobies had variable diets indicative of generalist and opportunistic feeding on native and non-indigenous benthic prey. Between 100 and 130-mm total-length, signatures of assimilated carbon (δ¹³C) indicated a directed shift towards a dreissenid-centric diet and once larger than 130-mm total-length, round gobies appeared to feed proportionally more on dreissenid mussels. We also found that large round gobies fed proportionally more on Hydrobiidae than small round gobies. A weak negative relationship between δ¹⁵N (indicative of trophic position) and round gobies total length resulted where smaller round gobies had slightly higher δ¹⁵N values than larger conspecifics. Round gobies larger than 180-mm total-length were common in nearshore habitats (≤2-m) during the spring, and dreissenid mussels and Hydrobiidae were the most frequently encountered prey. Our results demonstrate elevated reliance on dreissenids as round goby increased body size, but the diversity of prey suggest more complex trophodynamic pathways associated with coastal bay habitats.     

Elevated methylmercury concentration and trophic position of the non-native bloody red shrimp (Hemimysis anomala) increase biomagnification risk in nearshore food webs

The establishment of non-native species can result in complex shifts in food-web structure and ecosystem function that alter the bioaccumulation, transfer, and biomagnification of contaminants. Hemimysis anomala (the bloody red shrimp), a mysid native to the Ponto-Caspian region that is now established throughout the Laurentian Great Lakes region (North America) and Europe, is characterized by high densities and a trophic ecology that may be consequential to mercury fluxes and bioaccumulation. We combined methylmercury (MeHg) and stable isotope (δ¹⁵N and δ¹³C) analyses of invertebrates from Seneca Lake (NY, USA) to test the hypotheses that mercury concentrations differ among (1) H. anomala and native or naturalized analogs due to food-web position and (2) stages of H. anomala due to ontogenetic diet shifts. The MeHg concentration and δ¹⁵N of H. anomala exceeded other littoral invertebrates (such as amphipods, dreissenid mussels, and zooplankton >153 μm) and were as high as the pelagic, native Mysis diluviana. These taxa-specific patterns indicate intensified biomagnification of MeHg is possible in nearshore and reef-spawning fish following the establishment of H. anomala, an energy-dense prey item for economically and ecologically important fish. Larger, adult H. anomala had higher MeHg concentrations and more enriched δ¹⁵N than juveniles, and H. anomala collected at a littoral site had higher MeHg than a canal site. These intraspecies contrasts are consistent with the shift toward zooplanktivory in adult H. anomala. As both putative prey for fish and competitors for shared zooplankton resources, H. anomala may impact fish in compounding ways.     

Riverine Landscape Patches Influence Trophic Dynamics of Riparian Ants

Food webs in riparian corridors are increasingly viewed as embedded in complex riverine landscapes characterized by an amalgam of aquatic, semi‐aquatic, and terrestrial habitats. However, the influence of riverine landscape pattern on trophic dynamics of riparian consumers remains largely unknown. We used naturally abundant stable isotope ratios (δ¹⁵N) to compare trophic structure of ants (Formica subsericea) among riparian patch types (crop, grass/herbaceous, gravel bar, lawn, mudflat, shrub, swamp, and woody vegetation) at 12 riverine landscapes distributed along an urban‐rural landscape gradient of the Scioto River, Ohio, USA. We expected that the diet of F. subsericea, a common generalist consumer, would reflect local prey availability and thus differences in trophic dynamics among patch types. Mean ant δ¹⁵N was higher in crop patches than in any other patch type, and was lowest in grass/herbaceous, lawn, shrub, and woody vegetation patches, suggesting that patch type was associated with trophic position of F. subsericea. We also found that the range of δ¹⁵N, and thus trophic breadth, was significantly different by patch type, with woody vegetation exhibiting the greatest spread. Variability in canopy, tree and shrub cover, and the degree of urban development was positively correlated with δ¹⁵N range (R² = 0.78), pointing to the role of habitat structure in mediating trophic breadth of riparian ants. These findings provide evidence that riverine landscape pattern can strongly influence trophic dynamics of riparian arthropods. Copyright © 2016 John Wiley & Sons, Ltd.     

Temporal trends of phytoplankton and zooplankton stable isotope composition in tropical Lake Malawi

Stable isotope ratios of three seston size classes (20–100 μm, 2–20 μm, and 0.2–2 μm) and zooplankton species were analyzed to determine the plankton food web structure of Lake Malawi. Over an annual cycle, seston δ¹³C varied between ?20.41‰ and ?27.43‰ with a mean value of ?24.27‰ ± 1.2 while δ¹³C values for zooplankton fluctuated between ?22‰ and ?25‰ with a mean of ?23.84‰ ± 0.77. Seston δ¹³C fluctuations appeared to be related to changes in physical and meteorological conditions in the lake that ultimately control nutrient availability. The highest seston δ¹³C values observed during the rainy and mixed seasons likely result from high phytolankton growth rates. δ¹⁵N of plankton was temporally variable, suggesting short term changes in N cycling dynamics that control the supply of N to phytoplankton. Very low seston δ¹⁵N values recorded during the mixing season suggest excess NO₃^? availability resulting from upwelling and vertical mixing. In contrast to expectations the calanoid Tropodiaptomus cunningtoni appeared to feed at a trophic level higher than that of all other zooplankton species, including the cyclopoid, Mesocyclops aequatorialis aequatorialis. δ¹⁵N values indicate that zooplankton were nearly 2 trophic levels above seston in the early stratified season. This implies that adult zooplankton could be utilizing forms of food other than phytoplankton during this period, such as nauplii or protozoans. This extra step in the food web, and the trophic positions of large zooplankton species, may alter estimates of food web efficiency and potential fish production for Lake Malawi.     

Limited evidence of zebra mussel (Dreissena polymorpha) consumption by freshwater drum (Aplodinotus grunniens) in Lake Winnipeg

Freshwater drum (Aplodinotus grunniens) may be a predator of the invasive zebra mussel (Dreissena polymorpha), which established in Lake Winnipeg in 2013. In this study, the diets, trophic position, and growth of 51 freshwater drum collected in 2019 (six years post-zebra mussel invasion) were compared to 64 freshwater drum sampled in 2000. Benthic insect larvae were the dominant food items in both years. Although mollusks occur in high densities in Lake Winnipeg, they were only consumed by a few freshwater drum in either sample year. Zebra mussels were not a frequent prey item in 2019 as they were only consumed by four of the sampled freshwater drum. Stable isotope analysis of white muscle tissue yielded similar δ¹³C and δ¹⁵N values in both years and were consistent with a benthic, insectivorous diet. Length-at-age data derived from otoliths revealed that the 2019 population had at least an equal growth rate to the 2000 population. Weight-at-length data suggested that fish condition was greater in 2019 than in 2000, which coincided with increased benthic macroinvertebrate density in Lake Winnipeg. Based on these findings, Lake Winnipeg freshwater drum continue to feed predominantly on insect larvae and not zebra mussels.     

The effects of impoundment and non‐native species on a river food web in Mexico's central plateau

Habitat modifications, non‐native species and other anthropogenic impacts have restructured fish communities in lotic ecosystems of central Mexico. Conservation of native fishes requires understanding of food web changes resulting from the introduction of non‐native species, flow alteration and other human impacts. Using δ¹³C and δ¹⁵N analysis of fishes and invertebrates we investigated the effects of non‐native species, and reservoirs on food webs of the Laja river ecosystem (Guanajuato, central Mexico). We estimated trophic position (TP), relative trophic niche and food web dispersion at 11 reservoir, river and tailwater sites. Reservoirs and non‐native fishes modified food webs in the Laja. Food web dispersion was greater in reservoir than in tailwater and river sites. Reservoir food webs had the greatest range of δ¹³C values, indicating a more diverse resource base compared to rivers. δ¹³C values increased with distance downstream from reservoirs, suggesting declining subsidies of river food webs by reservoir productivity. Stable isotopes revealed potential effects of non‐native fishes on native fishes via predation or competition. Non‐native Micropterus salmoides were top predators in the system. Non‐natives Cyprinus carpio, Oreochromis mossambicus and Carassius auratus exhibited lowest TP in the Laja but overlapped significantly with most native species, indicating potential for resource competition. Native Chirostoma jordani was the only species with a significantly different trophic niche from all other fish. Many rivers in central Mexico share similar anthropogenic impacts and similar biotas, such that food web patterns described here are likely indicative of other river systems in central Mexico. Copyright © 2008 John Wiley & Sons, Ltd.     

Use of sediment elemental and isotopic compositions to record the eutrophication of a polymictic reservoir in central Texas, USA

Paleolimnological studies are rarely performed on reservoirs because of concern that sediments might not accurately chronicle reservoir history. Eutrophication indicators might behave differently in polymictic reservoirs and stratified natural lakes because of system and/or mixing regime differences. Particulate organic carbon (POC), particulate organic nitrogen (PON), and total phosphorus (TP) concentrations, carbon:nitrogen (C:N) and nitrogen:phosphorus (N:P) ratios, and carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes from a sediment core were measured to demonstrate that sufficient information can be derived from sediments to permit a historical reconstruction. The scattered POC data were likely biased by seasonal/annual variability in allochthonous organic matter (OM) loading. The upwardly increasing PON in the sediment core supported historic primary productivity (PP) data, suggesting PON could be a better PP indicator than POC. The upwardly increasing TP documented historic P enrichment. The upwardly decreasing C:N ratio identified an OM source shift from allochthonous to increasingly autochthonous sources with reservoir age. The upwardly increasing N:P ratio implied that N‐fixation rates have increased with reservoir age, to compensate for increasing N limitation as the P loading increased. The δ¹³C decreased as the PP increased with reservoir age producing an atypical relationship compared to stratified natural lakes. The OM source shifts likely biased the δ¹³C–PP relationship, and might weaken δ¹³C‐inferred PP reconstructions in similar reservoirs. The δ¹⁵N increased with reservoir age, likely resulting from dissolved inorganic N (DIN) source changes, rather than nitrate utilization. Watershed urban growth and dairy operation intensification potentially contributed greater loads of isotopically heavy DIN. This study demonstrated that paleolimnology has great potential to assist eutrophication assessment and management efforts in reservoirs.     

Spatial and Ontogenetic Variability of Sea Lamprey Diets in Lake Superior

Invasive sea lamprey (Petromyzon marinus) remain an important source of fish mortality in the Laurentian Great Lakes, yet assessing their impact is hindered by lack of quantitative diet information. We examined nitrogen and carbon stable isotope ratios (δ¹⁵N and δ¹³C) of sea lamprey and host species in six ecoregions of Lake Superior, mainly in 2002–2004. Data implied that most sea lamprey fed primarily on upper trophic level species, including forms of lake trout (Salvelinus namaycush). However, in Ontario waters, particularly semi-enclosed Black Bay, sea lamprey relied heavily on lower trophic levels, such as coregonines (Coregonus spp.) and suckers (Catostomus spp.). Sea lamprey δ¹⁵N and δ¹³C generally increased with sea lamprey size, implying dependence on higher trophic levels later in life. Most parasitic sea lamprey that we captured were attached to either lean lake trout (35% of observed attachments), lake whitefish (Coregonus clupeaformis; 25%), or cisco (C. artedii; 25%); the latter sea lamprey were typically < 15 g. Survey- and fishery-dependent wounding rate data compiled from 1986–2005 suggest that lean and siscowet lake trout were selectively parasitized by sea lamprey, which is consistent with our stable isotope data. Our results largely support the notion that lake trout are the principal host species in Lake Superior. However, stable isotope evidence that sea lamprey feed at lower trophic levels in some regions argues for comprehensive monitoring of sea lamprey impacts throughout the fish community in systems that sea lamprey have invaded.     

Metal accumulation in Lake Michigan prey fish: Influence of ontogeny,trophic position,and habitat

Developing an understanding of factors that influence the accumulation and magnification of heavy metals in fish of the Laurentian Great Lakes is central to managing ecosystem and human health. We measured muscle tissue concentrations of heavy metals in Lake Michigan prey fish that vary in habitat use, diet, and trophic position, including alewife, bloater, deepwater sculpin, round goby, rainbow smelt, and slimy sculpin. For each individual, we measured tissue concentrations of four metals (chromium [Cr], copper [Cu], manganese [Mn], and total mercury [THg]), stable isotope ratios for trophic position (δ¹⁵N and δ¹³C), and individual fish attributes (length, mass). Total mercury concentration was positively related to total length and δ¹⁵N. Of all species, round goby had among the greatest increases in mercury per unit growth and was most isotopically distinct from other species. Profundal species (bloater, deepwater sculpin, slimy sculpin) had similar high THg tissue concentrations, possibly due to slower growth due to cold temperatures, whereas other species (alewife, round goby, rainbow smelt) showed more variation in THg. In contrast, other metals (Cr, Cu, Mn) had either a negative or no relationship to total length and δ¹⁵N, suggesting no bioaccumulation or biomagnification. Potential incorporation of mercury by sportfish may thus be related to species, age, diet, trophic position, and habitat of prey fish. Our findings serve as a foundation for understanding how heavy metals accumulate in Lake Michigan food webs and highlight the continued need for management of metal input and cycling in Lake Michigan.     

Bioaccumulation of perfluoroalkyl substances in a Lake Ontario food web

Per- and polyfluoroalkyl substances (PFAS) are a group of synthetic chemicals detected throughout the environment. To better understand the distribution of PFAS in an aquatic system (the Laurentian Great Lakes), stable isotope enrichment (δ¹³C and δ¹⁵N), fatty acid (FA) profiles, and PFAS were measured in various species from the Lake Ontario (LO) aquatic food web. Sampled organisms included top predator fish, prey fish, and benthic and pelagic macroinvertebrates. The trophic level of each species in the LO food web was determined using δ¹⁵N, and FA concentrations (range: <1–139 mg/g wet weight (ww)). The individual PFAS concentrations in the LO food web were ~1.5 to 5 times lower than previously reported. The highest PFAS concentrations were observed in deepwater sculpin (Myoxocephalus thompsonii, 150 ± 35.7 ng/g ww) suggesting a potential source of PFAS from the offshore benthic zone or sediment. The concentration of PFOS and long chain (C9-C14) perfluoroalkyl carboxylic acids (PFCAs) were significantly higher than short chain PFAS indicating the significant impact of hydrophobicity on the bioaccumulation of PFAS in organisms from the food web. However, high molecular weight PFCAs (>C8) did not exhibit increasing biomagnification factors (BMFs) and trophic magnification factors (TMFs) with log K_ow, suggesting hydrophobicity does not govern the movement of PFAS from low to high trophic levels in the LO food web.     

The Impact of Low Flow on Riverine Food Webs in South‐Central Newfoundland

Low‐flow events can reduce food availability and decrease the feeding niche of consumers within rivers. Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope and stomach content analyses were employed to evaluate resource use and overlap between fish species in a natural and regulated river in normal and low‐flow years, with the use of multiple methodological approaches providing the best means of understanding short‐term and long‐term observations on fish feeding and resource overlap under changing flow conditions. Diet analyses generally indicated significant inter‐specific differences in the diets of key fish species within rivers and similarities in resource use between rivers. In comparison with fish from the natural river, fish from the regulated river had lower and less inter‐annually variable δ¹³C values. In the natural river, there was a significant reduction and increase, respectively, in δ¹³C and δ¹⁵N variation in the low‐flow year. Intra‐annual or inter‐annual differences in trophic niche area were not apparent in the regulated river, whereas within the natural river, intra‐annual and inter‐annual differences in trophic niche were found. Resource overlap between key fish species was also higher in the low‐flow year and lower in the spring and higher in the summer as a result of differences in flow. Resource overlap was also higher between rivers in the low‐flow year. High resource overlap between rivers during decreased summer flow indicates a strong effect of flow on river organisms, where both fish and their invertebrate prey resources are concerned. Copyright © 2014 John Wiley & Sons, Ltd.     

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

Effluent 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 & Sons, Ltd.     

Trophic ecology of salmonine predators in northern Lake Huron with emphasis on Atlantic salmon (Salmo salar)

Pacific salmon were introduced to the Great Lakes in the 1960s and now support major recreational fishery. Population declines resulting from invasive species have prompted agencies to consider diversifying sport fisheries through stocking. Atlantic salmon are native to Lake Ontario, but a small fishery has developed in northern Lake Huron since the 1990s that appears suited to the Lake Huron food web leading to requests for increased stocking by anglers and consideration by agencies. However, no study has evaluated the trophic ecology of Atlantic salmon in relation to other salmonine predators in northern Lake Huron. In this study, we used stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N), along with mercury (Hg) concentrations to assess resource use, niche overlap, and contaminant accumulation in Atlantic salmon compared to select Lake Huron predators. Atlantic salmon exhibited considerable niche overlap with Chinook and coho salmon but were strongly differentiated from lake trout. In addition, we observed that Atlantic salmon had similar Hg concentrations as coho but were lower than both Chinook salmon and lake trout. Based upon the relationship between fish size, δ¹⁵N, and Hg, Atlantic salmon bioaccumulate Hg similarly to Pacific salmon but likely have lower consumptive demands than Chinook salmon. Continued attention should be placed on understanding how Atlantic salmon fit into the current Lake Huron food web in order to evaluate the long-term efficacy of the Atlantic salmon stocking program.     

Use of Stable Isotopes to Trace Municipal Wastewater Effluents into Food Webs within a Highly Developed River System

Many anthropogenic inputs, such as municipal wastewater effluents (MWWEs), affect stable isotope signatures (δ¹³C and δ¹⁵N) at the base of exposed food webs creating spatial patterns reflecting their incorporation into aquatic food webs. The Grand River in southern Ontario, Canada, is a heavily modified, rapidly urbanizing river that assimilates wastewater from 30 municipal wastewater treatment plants. Stable isotope analysis was applied to resident aquatic invertebrates and fish influenced by three different wastewater outfalls in early, middle, and late summer to determine how values shifted seasonally and with differing effluent quality. There was a slight increase in δ¹³C in both invertebrates and fish in late summer downstream from the three outfalls, but it is difficult to separate effects of the effluents from downstream gradients. Downstream of two of the three outfalls, the δ¹⁵N tended to increase relative to upstream, while the remaining effluent, of the poorest quality, decreased δ¹⁵N values of both invertebrates and fish. Spatial trends in stable isotopes became more pronounced as the summer progressed with the greatest between‐site differences occurring in late summer. This study reflects the complex nutrient dynamics associated with MWWE inputs to rivers and contributes to our understanding and application of stable isotope analysis in impacted lotic ecosystems. Copyright © 2014 John Wiley & Sons, Ltd.     

Differential habitat use patterns of yellow perch Perca flavescens in eastern Lake Michigan and connected drowned river mouth lakes

Understanding stock structure and habitat use is important for sustainable fisheries management and conservation of genetic and phenotypic diversity. In eastern Lake Michigan, yellow perch Perca flavescens is found in both the nearshore region of Lake Michigan proper and drowned river mouth (DRM) lakes, small estuary-like systems directly connected to Lake Michigan. Recent genetic analyses suggest complex stock structure between these habitats and the potential migration of Lake Michigan yellow perch into the profundal zone of DRM lakes. We quantified carbon (δ¹³C) and oxygen (δ¹⁸O) stable isotope ratios of yellow perch otolith cores to index natal origins, and measured muscle δ¹³C, nitrogen (δ¹⁵N), δ¹⁸O, and hydrogen (δ²H) isotope ratios to reflect recent diet and habitat use. Stable isotope ratios of otolith cores and muscle samples support the existence of resident populations in nearshore Lake Michigan and DRM lakes, as well as Lake Michigan migrants using DRM lakes. Most fish caught in DRM lakes that had natal and recent stable isotope values similar to Lake Michigan fish were collected during fall in DRM lake profundal zones. Comparison of otolith core and muscle stable isotope ratios of individual yellow perch suggest that individuals that recently migrated to DRM lakes also spent early life in Lake Michigan. Differential habitat use patterns of yellow perch in eastern Lake Michigan may have important implications for harvest estimates and fishing regulations. Migration by Lake Michigan fish into DRM lakes does not appear to be related to reproduction, and the underlying benefits of these migrations remain unclear.