Shifts In Food Webs Research Articles

Seabird inputs drive changes in Cladocera assemblages in freshwater ponds

Seabirds are biovectors that transport large concentrations of nutrients from their marine feeding areas to terrestrial breeding grounds. Here, we used subfossil cladoceran assemblages to assess if, and how, changes in the world’s largest colony of Leach’s Storm Petrels affected the structure of Cladocera assemblages over the past ~1700 years. Using sediment cores from four ponds impacted by Leach’s Storm-Petrel colonies on Baccalieu Island (NL, Canada), we observed a consistent transition in cladoceran assemblages from benthic/littoral to pelagic taxa in association with high seabird presence. This shift aligns with previously published limnological changes that tracked the growth of the colony. Compared to trends in sedimentary chlorophyll-a, pelagic cladoceran taxa lagged behind algal shifts driven by seabird activity. The main drivers of cladoceran assemblage shifts were likely alterations to the physical habitat structure and food availability driven by seabird inputs. Furthermore, deposition of calcium from seabirds may have also contributed to changing the composition of cladoceran communities. Our study provides information on food web shifts associated with seabird-driven eutrophication, which can be compared to future paleoecological studies.

Intra-lake origin and rapid expansion of invasive pelagic three-spined stickleback in Lake Constance

The rapid expansion of three-spined stickleback (Gasterosteus aculeatus Linnaeus, 1758 (Perciformes, Gasterosteida)) in the pelagic zone of Lake Constance, Central Europe, since 2012 contributed to stark ecosystem-wide effects, such as food-web shifts and declines in native biodiversity, including commercially important fish species. Yet, the origin of this invasive pelagic population remains unclear. Using RAD-sequencing of Lake Constance sticklebacks, we show that the pelagic Lake Constance population likely arose recently within the lake, potentially from the littoral population. We did not detect any substantial genome-wide genetic differentiation between individuals from different habitats, supporting a recent origin of the pelagic population and/or ongoing gene flow. This is further supported by minimal differences in meristic and morphometric traits. However, we also identified multiple outlier loci between littoral and pelagic individuals across the genome, potentially suggesting early signs of adaptation despite high connectivity. In this study, we provide an important example of rapid within-lake ecological diversification of an invasive species from standing genetic variation. Ultimately, our findings will have major implications for the management of invasive pelagic stickleback, as they indicate that the stickleback population has to be managed as a whole and that management efforts cannot only focus on the hyper-abundant pelagic population.

Insect decline in forests depends on species’ traits and may be mitigated by management

Insects are declining, but the underlying drivers and differences in responses between species are still largely unclear. Despite the importance of forests, insect trends therein have received little attention. Using 10 years of standardized data (120,996 individuals; 1,805 species) from 140 sites in Germany, we show that declines occurred in most sites and species across trophic groups. In particular, declines (quantified as the correlation between year and the respective community response) were more consistent in sites with many non-native trees or a large amount of timber harvested before the onset of sampling. Correlations at the species level depended on species’ life-history. Larger species, more abundant species, and species of higher trophic level declined most, while herbivores increased. This suggests potential shifts in food webs possibly affecting ecosystem functioning. A targeted management, including promoting more natural tree species composition and partially reduced harvesting, can contribute to mitigating declines.

Ecological effects of mosquito control with Bti: evidence for shifts in the trophic structure of soil- and ground-based food webs

The microbial control agent Bacillus thuringiensis var. israelensis (Bti) has been successfully used worldwide to reduce abundances of biting Nematocera (Diptera), often with little direct impact on non-target organisms observed. However, the potential for additional indirect effects on other ecosystem properties, including on trophic linkages within food webs, is poorly known. We investigated the effects of multiple-year mosquito control treatments using the Bti product VectoBac®-G on the stable isotope composition of epigeal and soil-based consumers inhabiting replicate floodplains along the River Dalälven, Sweden. We observed significant changes in the isotopic composition of detritivores feeding at the base of floodplain food webs. Enchytraeid worms were characterised by 3.5% higher δ13C values in treated floodplains, suggesting increased consumption of δ13C-enriched food. The overall range of community-wide δ15N values was 56% greater in the treated floodplains, whilst δ15N values of oribatid mites were elevated by 97%. These results suggest extra fractionation in the transfer of nitrogen through floodplain food chains. We conjecture that the ecological mechanisms driving these food web shifts are (1) the mass mortality of high δ13C A. sticticus larvae, which leaves high concentrations of dead mosquito biomass deposited on soils at local scales, after the floodwaters have receded and (2) incorporation of the very high δ13C-enriched corn particles comprising the bulk of the VectoBac®-G product into floodplain food webs. Our results suggest that repeated applications of Bti might have wider, still largely unknown implications for nutrient and energy cycles within floodplain ecosystems.

Shifts in food webs and niche stability shaped survivorship and extinction at the end-Cretaceous

It has long been debated why groups such as non-avian dinosaurs became extinct whereas mammals and other lineages survived the Cretaceous/Paleogene mass extinction 66 million years ago. We used Markov networks, ecological niche partitioning, and Earth System models to reconstruct North American food webs and simulate ecospace occupancy before and after the extinction event. We find a shift in latest Cretaceous dinosaur faunas, as medium-sized species counterbalanced a loss of megaherbivores, but dinosaur niches were otherwise stable and static, potentially contributing to their demise. Smaller vertebrates, including mammals, followed a consistent trajectory of increasing trophic impact and relaxation of niche limits beginning in the latest Cretaceous and continuing after the mass extinction. Mammals did not simply proliferate after the extinction event; rather, their earlier ecological diversification might have helped them survive.

Elevated Mercury Concentrations and Isotope Signatures (N, C, Hg) in Yellowfin Tuna (Thunnus albacares) from the Galápagos Marine Reserve and Waters off Ecuador

We examined how dietary factors recorded by C and N influence Hg uptake in 347 individuals of yellowfin tuna (Thunnus albacares), an important subsistence resource from the Galápagos Marine Reserve (Ecuador) and the Ecuadorian mainland coast in 2015-2016. We found no differences in total Hg (THg) measured in red muscle between the two regions and no seasonal differences, likely due to the age of the fish and slow elimination rates of Hg. Our THg concentrations are comparable to those of other studies in the Pacific (0.20-9.60 mg/kg wet wt), but a subset of individuals exhibited the highest Hg concentrations yet reported in yellowfin tuna. Mercury isotope values differed between Δ199 Hg and δ202Hg in both regions (Δ199 Hg = 2.86 ± 0.04‰ vs. Δ199 Hg = 2.33 ± 0.07‰), likely related to shifting food webs and differing photochemical processing of Hg prior to entry into the food web. There were significantly lower values of both δ15 N and δ13 C in tuna from Galápagos Marine Reserve (δ15 N: 8.5-14.2‰, δ13 C: -18.5 to -16.1‰) compared with those from the Ecuadorian mainland coast (δ15 N: 8.3-14.4‰, δ13 C: -19.4 to -11.9‰), of which δ13 C values suggest spatially constrained movements of tuna. Results from the pooled analysis, without considering region, indicated that variations in δ13 C and δ15 N values tracked changes of Hg stable isotopes. Our data indicate that the individual tuna we used were resident fish of each region and were heavily influenced by upwellings related to the eastern Pacific oxygen minimum zone and the Humboldt Current System. The isotopes C, N, and Hg reflect foraging behavior mainly on epipelagic prey in shallow waters and that food web shifts drive Hg variations between these populations of tuna. Environ Toxicol Chem 2022;41:2732-2744. © 2022 SETAC.

Seasonal variability of invertebrate prey diet and selectivity of the dominant forage fishes in Lake Huron

Predators can shift their diets and even selectivity in response to changing environmental conditions. Since the early 2000s, Lake Huron experienced major food-web shifts that have caused changes in the prey availability and quality for consumers at multiple trophic levels. Previous studies have reported declining energetic condition for key planktivorous fishes, such as bloater (Coregonus hoyi) and rainbow smelt (Osmerus mordax), which play a key role in supporting commercially and recreationally important piscivorous fishes. To improve understanding of how changes in the invertebrate prey community have influenced foraging by rainbow smelt and bloater, we processed diets and calculated selectivity along two transects in Lake Huron during 2012. Diet proportions for both species varied seasonally and consisted of mostly calanoid copepods during spring and summer, specifically Leptodiaptomus sicilis and diaptomid copepodites, and Daphnia galeata mendotae in autumn. Bloater selectivity varied primarily by season and transect with Limnocalanus macrurus or Mysis diluviana as the most preferred prey during spring, Chironomidae pupae or L. sicilis during summer, and Chironomidae pupae or Mysis during autumn. Rainbow smelt selectivity was consistent across seasons and transects with Mysis being the species most commonly selected and Bythotrephes longimanus the second most. Both fish species selected for relatively large invertebrate prey, but declining densities of Mysis and the benthic amphipod Diporeia have caused fish to consume smaller prey with much lower energy density. Our results illustrate how food-web changes underlie the reduced energetic condition of bloater and rainbow smelt, which ultimately reduces the growth potential for recreationally and commercially important piscivorous fish.

Phytodetrital quality (C:N ratio) and temperature changes affect C and N cycling of the intertidal mixotrophic foraminifer Haynesina germanica

The combination of lower diet quality and increased metabolic rates is assumed to cause cascading effects on organismic C cycling. Future changes in CO2 levels or terrestrial nutrient discharges in marine ecosystems can lead to increased phytoplankton C:N ratios relative to consumer C:N ratios, lowering the quality of the food source. In this study, we compared the single and interactive effects of diet quality and temperature on the feeding behavior and C and N intake and release of a common and abundant intertidal mixotrophic protist, the foraminifer Haynesina germanica. Two batches of artificially produced and dual isotope-labeled (13C/15N) chlorophyte detritus with different C:N ratios (5.6 and 7.1) were fed to the foraminifer at 3 different temperatures (15, 20, 25°C). We observed a strong interactive effect of temperature and diet. A very strong increase in feeding rates was observed at 20°C for the low-quality food source. Respiration rates of carbon derived from the low-quality diet (C:N ratio of 7.1) were lower than those of the high-quality diets and increased at 25°C. This indicates that a high C content of the diet might be of advantage in calcifying mixotrophs, since respired excess C could be advantageous for test calcification. Additionally, respired excess C could be a useful resource of CO2 for kleptoplast photosynthesis and functionality in the mixotrophic lifestyle of H. germanica. Further, the observed effects of diet and temperature could impact nutrient fluxes in the habitat of H. germanica, possibly leading to food-web shifts in the future.

Mercury source changes and food web shifts alter contamination signatures of predatory fish from Lake Michigan

To understand the impact reduced mercury (Hg) loading and invasive species have had on methylmercury bioaccumulation in predator fish of Lake Michigan, we reconstructed bioaccumulation trends from a fish archive (1978 to 2012). By measuring fish Hg stable isotope ratios, we related temporal changes in Hg concentrations to varying Hg sources. Additionally, dietary tracers were necessary to identify food web influences. Through combined Hg, C, and N stable isotopic analyses, we were able to differentiate between a shift in Hg sources to fish and periods when energetic transitions (from dreissenid mussels) led to the assimilation of contrasting Hg pools (2000 to present). In the late 1980s, lake trout δ202Hg increased (0.4‰) from regulatory reductions in regional Hg emissions. After 2000, C and N isotopes ratios revealed altered food web pathways, resulting in a benthic energetic shift and changes to Hg bioaccumulation. Continued increases in δ202Hg indicate fish are responding to several United States mercury emission mitigation strategies that were initiated circa 1990 and continued through the 2011 promulgation of the Mercury and Air Toxics Standards rule. Unlike archives of sediments, this fish archive tracks Hg sources susceptible to bioaccumulation in Great Lakes fisheries. Analysis reveals that trends in fish Hg concentrations can be substantially affected by shifts in trophic structure and dietary preferences initiated by invasive species in the Great Lakes. This does not diminish the benefits of declining emissions over this period, as fish Hg concentrations would have been higher without these actions.

Changes in Sport Fish Mercury Concentrations from Food Web Shifts Suggest Partial Decoupling from Atmospheric Deposition in Two Colorado Reservoirs

Partial decoupling of mercury (Hg) loading and observed Hg concentrations ([Hg]) in biotic and abiotic samples has been documented in aquatic systems. We studied two Colorado reservoirs to test whether shifts in prey for sport fish would lead to changes in [Hg] independent of external atmospheric Hg deposition. We compared sport fish total mercury concentrations ([T-Hg]) and macroinvertebrate (chironomids and crayfish) methylmercury concentrations ([MeHg]) before and after food web shifts occurred in both reservoirs. We also monitored wet atmospheric Hg deposition and sediment [T-Hg] and [MeHg] at each reservoir. We found rapid shifts in Hg bioaccumulation in each reservoir's sport fish, and these changes could not be attributed to atmospheric Hg deposition. Our study shows that trends in atmospheric deposition, environmental samples (e.g., sediments), and samples of species at the low trophic levels (e.g., chironomids and crayfish) may not accurately reflect conditions that result in fish consumption advisories for high trophic level sport fish. We suggest that in the short-term, monitoring fish [Hg] is necessary to adequately protect human health because natural and anthropogenic perturbations to aquatic food-webs that affect [Hg] in sport fish will continue regardless of trends in atmospheric deposition.

Cascading ecological effects of landscape moderated arthropod diversity

Understanding the mechanisms regulating the diversity and distribution of arthropods is essential to understanding food web interactions and ecosystem functioning. Local arthropod diversity is known to be linked to features of surrounding landscapes, including the area of human‐developed land. Yet, how such landscape moderation of diversity affects processes within local sites remains understudied. We report on a study that 1) measured the impacts of human development surrounding old‐field habitats of arthropods on arthropod food web structure within those habitats and 2) determined if these shifts were associated with cascading impacts on the plant community. We sampled the arthropod community in 16 old‐fields that span an urban‐rural gradient throughout southern New England, USA. In each field, we also established paired mesocosms enclosing vegetation, one of which allowed arthropod herbivory while the other excluded such interactions, to isolate impacts of arthropod herbivory on three functional groups of plants: grasses, goldenrod and non‐goldenrod forbs. Biomass of both herbivorous and predatory arthropods were positively related to the proportion of natural area surrounding a field early in the growing season (June). This relationship persisted later into the season for predatory arthropods (through July), but not for herbivorous arthropods. We found no evidence that the biomass of predators was related to the abundance of herbivorous arthropods in a field; or that biomass of herbivores was correlated to change in plant biomass between the two types of mesocosms. We did, however, find that in fields with low predator abundance there was greater herbivory on grasses (nutritious host), but that in high predator fields goldenrod was increasingly impacted (safe host), as is predicted by past work in old‐field ecosystems. The findings support the generalizability of landscape moderated biodiversity to non‐agricultural systems and suggests that observed shifts in food webs have implications for community and ecosystem dynamics.

Rescaling stable isotope data for standardized evaluations of food webs and species niches

Many human and natural events can impact aquatic populations and communities, leav- ing strong imprints as altered food web dynamics. Stable C and N isotopes in fish can record these altered trophic dynamics in an integrated way, and a new simple methodology is presented to extract measures of food web change from isotope measure- ments of fish species. Measured C and N isotope data are re-scaled as modified Z-scores to equalize the effects of average food inputs, then graphed in (x,y) space to obtain food web shapes. Pairs of food webs are compared for shifts in Euclidean distance of isotope values for individual species and pairs of spe- cies, and these shifts are used to assess overall con- stancy of species niches and fish food webs. Appli - cation of the methodology shows that (1) on average, estuarine fish species are relatively fixed in their trophic niches, with only 14 to 24% variability in the niches; (2) relatively undisturbed natural fish com- munities often show substantially different food web structure even when the same species of fish are present; and (3) factors such as species invasions and dredging result in marked differences between food webs. Isotope evaluations of possible food web shifts should be valuable in many conservation and resto- ration contexts.

Resurrecting the Ecological Underpinnings of Ocean Plankton Blooms

Nutrient and light conditions control phytoplankton division rates in the surface ocean and, it is commonly believed, dictate when and where high concentrations, or blooms, of plankton occur. Yet after a century of investigation, rates of phytoplankton biomass accumulation show no correlation with cell division rates. Consequently, factors controlling plankton blooms remain highly controversial. In this review, we endorse the view that blooms are not governed by abiotic factors controlling cell division, but rather reflect subtle ecosystem imbalances instigated by climate forcings or food-web shifts. The annual global procession of ocean plankton blooms thus represents a report on the recent history of predator-prey interactions modulated by physical processes that, almost coincidentally, also control surface nutrient inputs.

Selective zooplanktivory of an invasive Ponto-Caspian mysid and possible consequences for the zooplankton community structure of invaded habitats

The impact of invasive alien species on native species is of increasing global concern. Invasive species can cause food-web shifts that have severe consequences for native species and ecosystems. However, the mechanisms by which the invaders influence the native communities are poorly understood. Here we investigated the interactions of the invasive Ponto-Caspian mysid Limnomysis benedeni with native freshwater zooplankton in laboratory and mesocosm experiments. This mysid migrates between benthic and pelagic zones and thereby forms a potential trophic link between these habitats. In laboratory predation experiments, L. benedeni fed both on Daphnia galeata and D. magna, and predation rates depended on the sizes of predator and prey but not on the availability of light. However, no predation was observed at prey sizes greater than 2 mm, which appears to be the upper size limit for a successful prey capture by L. benedeni. In outdoor mesocosm experiments, L. benedeni strongly decreased the densities of cladocerans, rotifers and copepod nauplii within a few days, while the densities of copepods were unaffected. Prey selection indices provide further evidence for strongly selective predation of L. benedeni on different zooplankton taxa. The presence of phytoplankton as an additional resource for the omnivorous mysid led to a lower predation pressure of L. benedeni on Cladocera and rotifers, indicating that the presence of alternative prey modulates the strength of the top-down effect of invasive mysids on the zooplankton community. These results suggest that the invasion of L. benedeni can have profound and complex impacts on the community structure of the native zooplankton fauna and that mysid invasions potentially have whole-ecosystem consequences.

Heterogeneity of bacterial communities within the zebra mussel ( Dreissena polymorpha) in the Laurentian Great Lakes Basin

We analyzed and compared the structure of bacterial communities associated with zebra mussel mantle cavity fluid, gills, and gut samples collected from Lake Loon, an inland lake in Michigan's Lower Peninsula (U.S.A.) using partial 16S rRNA gene sequencing. A total of 713 cloned 16S ribosomal gene sequences were checked for similarity to existing 16S sequences in two public databases: the Ribosomal Database Project and BLAST. Based on a 98% sequence similarity threshold, a total of 355 phylotypes belonging to 12 bacterial phyla and the phylum Bacillariophyta (diatoms) were identified in zebra mussel samples. A dominance of sequences belonging to the class γ-proteobacteria was observed in the mantle cavity clone libraries ( P < 0.0001). Significant sample-specific sequence associations ( P < 0.001) included members of the orders Pseudomonadales and Vibrionales in mantle cavity fluid and gut clone libraries, members of both the phylum Actinobacteria and the class δ-proteobacteria in gill clone libraries, and the Cyanobacteria/Bacillariophyta group in gut clone libraries. Furthermore, our results suggest that the zebra mussel may serve as a reservoir for facultative and opportunistic pathogenic bacteria, e.g., Clostridium spp., Flavobacterium spp. and Mycobacterium spp., for many aquatic and terrestrial animals. This work constitutes the first account of the heterogeneity of bacterial communities associated with multiple compartments within the zebra mussel. The information gained in this study significantly contributes to what is known regarding the microbial ecology of the zebra mussel and its role in disease ecology and food-web shifts in the Great Lakes ecosystem.

Temporal and spatial patterns in the diet of northern fulmars Fulmarus glacialis in the Canadian High Arctic

AQUATIC BIOLOGY Aquat Biol Vol. 10: 181–191, 2010 doi: 10.3354/ab00277 Published online August 31 Temporal and spatial patterns in the diet of northern fulmars Fulmarus glacialis in the Canadian High Arctic M. L. Mallory 1, *, N. J. Karnovsky 2 , A. J. Gaston 3 , K. A. Hobson 4 , J. F. Provencher 5 , M. R. Forbes 6 , G. L. Hunt Jr. 7 , T. Byers 8 , T. A. Dick 9 Canadian Wildlife Service, Box 1714, Iqaluit, Nunavut X0A 0H0, Canada Department of Biology, Pomona College, 175 W. 6th St., Claremont, California 91711, USA National Wildlife Research Centre, Environment Canada, Carleton University, Ottawa, Ontario K1A 0H3, Canada Environment Canada, 11 Innovation Blvd., Saskatoon, Saskatchewan SYN 3H5, Canada Department of Biology, University of Victoria, Victoria, British Columbia V8W 3N5, Canada Department of Biology, Carleton University, Ottawa, Ontario K1S 5B6, Canada Department of Ecology and Evolutionary Biology, University of California – Irvine, Irvine, California 92697, USA Byers Environmental Studies, Box 1049, Teulon, Manitoba R0C 3B0, Canada Department of Zoology, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada ABSTRACT: The northern fulmar Fulmarus glacialis is a medium-sized seabird with a broad, circum- polar range in the northern hemisphere, and is the only petrel that inhabits the High Arctic. We used stomach analysis and stable isotopes (δ 15 N, δ 13 C) of muscles to examine the diet of 179 fulmars dur- ing the breeding season at 4 locations in Arctic Canada, to compare diet to those from studies con- ducted in these regions > 2 decades earlier. Across sampling locations, cephalopods, polychaetes and crustaceans dominated dietary remains in fulmars, although there was some regional variation. Both stable isotopes and stomach dissections showed that a seasonal shift in diet occurred in May, after which fulmars fed at a higher trophic level, suggesting a difference in winter/migration diet and breeding season diet. After migration, fulmar digestive organs decreased markedly in size, and by the time chicks were hatching, these organs were still 17 to 39% smaller than their size when birds arrived at the colony. Despite ongoing changes in the marine environment in much of the Arctic due to global warming, recent fulmar diet samples were similar to samples collected in the 1970s and 1980s, except that a higher proportion of recent collections contained fish. KEY WORDS: Arctic · Cephalopod · Crustacean · Procellariiformes · Reproduction Resale or republication not permitted without written consent of the publisher INTRODUCTION Invertebrate and fish fauna vary across oceano- graphic zones in the North Atlantic in response to differing oceanographic conditions, often on small geographic scales (e.g. Karnovsky et al. 2003). Many marine birds are generalist predators that exploit invertebrates and fish (e.g. Woo et al. 2008). Conse- quently, monitoring the diet of marine birds provides a measure of the diversity and availability of different types of organisms in marine food webs and allows detection of shifts in food webs due to changes in oceanographic conditions (e.g. Gaston et al. 2003, Frederiksen et al. 2004, 2006). In the North Atlantic, the northern fulmar Fulmarus glacialis is the only Procellariiform seabird (petrel) found in Boreal, Low Arctic and High Arctic oceano- graphic zones (Salomonsen 1965, Hatch & Nettleship 1998). It feeds on zooplankton, cephalopods, fish and carrion across its range in Boreal and Low Arctic waters (Furness & Todd 1984, Hamer et al. 1997, Phillips et al. 1999, Garthe et al. 2004). Relatively less is known about *Email: mark.mallory@ec.gc.ca © Inter-Research 2010 · www.int-res.com

Binary dietary responses of northern gannets Sula bassana indicate changing food web and oceanographic conditions

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 352:213-220 (2007) - DOI: https://doi.org/10.3354/meps07075 Binary dietary responses of northern gannets Sula bassana indicate changing food web and oceanographic conditions W. A. Montevecchi* Cognitive and Behavioural Ecology Program, Memorial University, St. John’s, Newfoundland A1B 3X9, Canada *Email: mont@mun.ca ABSTRACT: Interactions between seabirds and their prey are shaped largely by the dynamics of the marine ecosystems in which they are embedded. Physical oceanographic processes can drive the distributions of ectothermic and planktonic prey and hence their availability to marine birds. Owing to the complex nature of these processes and interactions, the signal-to-noise ratios of avian indicators of prey conditions are variable, often low and further degraded (buffered) by seabird behaviour and life-history features. Cairns (1987, Biol Oceanogr 5:261–271) detailed seabird responses operating over a range of temporal scales to variation in food supplies, and suggested that interval relationships might be developed between avian responses and environmental variation. While this may be possible in some instances, it appears unrealistic in most instances to expect interval relationships between seabird responses and prey conditions that are often nonlinearly related. The present paper focuses on binary data (e.g. breeding success versus failure) derived from seabirds that can provide robust information about major shifts in prey and oceanographic conditions and that are particularly informative when accumulated over decadal and large ocean scales. Inter-annual and decadal variations in specific and nominally categorized (warm- versus cold-water) prey landings of northern gannets Sula bassana at a large oceanic colony in the NW Atlantic reflect shifts in pelagic food webs induced by changes in regional sea surface temperature. Binary patterns emphasize decadal shifts in food webs and yield predictive indication of systemic change. KEY WORDS: Binary patterns . Bio-indicators . Decadal scales . Seabirds . Ecosystem shift Full text in pdf format PreviousNextCite this article as: Montevecchi WA (2007) Binary dietary responses of northern gannets Sula bassana indicate changing food web and oceanographic conditions. Mar Ecol Prog Ser 352:213-220. https://doi.org/10.3354/meps07075 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 352. Online publication date: December 20, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Aquatic invasive species harming U.S. ecosystems, fisheries

Biological invasions are fundamentally changing the structure and function of many ecosystems around the world, Gregory Ruiz, senior scientist with the Smithsonian Environmental Research Center in Edgewater, Maryland, told a 20 June U.S. Congressional hearing on aquatic invasive species.“Not only have many [nuisance aquatic species] become established, but many communities are now dominated by [them] in terms of number or organisms, biomass, and ecological processes,” Ruiz said. “In some cases, it is clear that invasions have caused dramatic shifts in food webs, chemical cycling, disease outbreaks, and commercial fisheries.”

Nitrogen in aquatic ecosystems.

Aquatic ecosystems respond variably to nutrient enrichment and altered nutrient ratios, along a continuum from fresh water through estuarine, coastal, and marine systems. Although phosphorus is considered the limiting nutrient for phytoplankton production in freshwater systems, the effects of atmospheric nitrogen and its contribution to acidification of fresh waters can be detrimental. Within the estuarine to coastal continuum, multiple nutrient limitations occur among nitrogen, phosphorus, and silicon along the salinity gradient and by season, but nitrogen is generally considered the primary limiting nutrient for phytoplankton biomass accumulation. There are well-established, but nonlinear, positive relationships among nitrogen and phosphorus flux, phytoplankton primary production, and fisheries yield. There are thresholds, however, where the load of nutrients to estuarine, coastal and marine systems exceeds the capacity for assimilation of nutrient-enhanced production, and water-quality degradation occurs. Impacts can include noxious and toxic algal blooms, increased turbidity with a subsequent loss of submerged aquatic vegetation, oxygen deficiency, disruption of ecosystem functioning, loss of habitat, loss of biodiversity, shifts in food webs, and loss of harvestable fisheries.

Stable isotope evidence for the food web consequences of species invasions in lakes

Species invasions pose a serious threat to biodiversity and native ecosystems1,2; however, predicting and quantifying the impacts of invasive species has proven problematic3,4,5,6. Here we use stable isotope ratios to document the food-web consequences of the invasion of two non-native predators, smallmouth bass and rock bass, into Canadian lakes. Invaded lakes had lower littoral prey-fish diversity and abundance than uninvaded reference lakes. Consistent with this difference, lake trout from invaded lakes had more negative δ13C values (-29.2‰ versus -27.4‰) and reduced trophic positions (3.3 versus 3.9) than those from reference lakes, indicating differences in food-web structure. Furthermore, a comparison of the pre- and post-invasion food webs of two recently invaded lakes showed that invasion was followed by substantial declines in littoral prey-fish abundance and the trophic position of lake trout, reflecting a shift in the diet of lake trout towards zooplankton and reduced dependence on littoral fish. This study demonstrates the use of stable isotope techniques to detect changes in food-web structure following perturbations; in this instance, bass-induced food-web shifts may have severe consequences for native species and ecosystems.