Lower Trophic Levels (lower + trophic_level)

Distribution by Scientific Domains


Selected Abstracts


Ultraviolet-B Radiation Effects on the Structure and Function of Lower Trophic Levels of the Marine Planktonic Food Web

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2006
Gustavo A. Ferreyra
ABSTRACT The impact of UV-B radiation (UVBR; 280,320 nm) on lower levels of a natural plankton assemblage (bacteria, phytoplankton and microzooplankton) from the St. Lawrence Estuary was studied during 9 days using several immersed outdoor mesocosms. Two exposure treatments were used in triplicate mesocosms: natural UVBR (N treatment, considered as the control treatment) and lamp-enhanced UVBR (H treatment, simulating 60% depletion of the ozone layer). A phytoplankton bloom developed after day 3, but no significant differences were found between treatments during the entire experiment for phytoplankton biomass (chlorophyll a and cell carbon) nor for phytoplankton cell abundances from flow cytometry and optical microscopy of three phytoplankton size classes (picoplankton, nanoplankton and microplankton). In contrast, bacterial abundances showed significantly higher values in the H treatment, attributed to a decrease in predation pressure due to a dramatic reduction in ciliate biomass (, 70,80%) in the H treatment relative to the N treatment. The most abundant ciliate species were Strombidinium sp., Prorodon ovum and Tintinnopsis sp.; all showed significantly lower abundances under the H treatment. P. ovum was the less-affected species (50% reduction in the H treatment compared with that of the N control), contrasting with ,90% for the other ones. Total specific phytoplanktonic and bacterial production were not affected by enhanced UVBR. However, both the ratio of primary to bacterial biomass and production decreased markedly under the H treatment. In contrast, the ratio of phytoplankton to bacterial plus ciliate carbon biomass showed an opposite trend than the previous results, with higher values in the H treatment at the end of the experiment. These results are explained by the changes in the ciliate biomass and suggest that UVBR can alter the structure of the lower levels of the planktonic community by selectively affecting key species. On the other hand, linearity between particulate organic carbon (POC) and estimated planktonic carbon was lost during the postbloom period in both treatments. On the basis of previous studies, our results can be attributed to the aggregation of carbon released by cells to the water column in the form of transparent exopolymer particles (TEPs) under nutrient limiting conditions. Unexpectedly, POC during such a period was higher in the H treatment than in controls. We hypothesize a decrease in the ingestion of TEPs by ciliates, in coincidence with increased DOC release by phytoplankton cells under enhanced UVBR. The consequences of such results for the carbon cycle in the ocean are discussed. [source]


Polychlorinated biphenyls and organochlorine pesticides in baltic and atlantic gray seal (Halichoerus grypus) pups

ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 11 2003
Eugen G. Sørmo
Abstract Organochlorine pollutants (OCs), such as polychlorinated biphenyls (PCBs), DDT, chlordanes (CHLs), hexachlorobenzene (HCB) and hexachlorocyclohexanes (HCHs) were determined in blubber biopsies from free-ranging Baltic and Atlantic gray seal (Halichoerus grypus) pups. Well-fed pups from the Baltic Sea had concentrations of DDT, PCBs, and HCHs that were 2 to 10 times higher than in corresponding pups from populations in the Atlantic Ocean. The OC pattern in the Baltic seals differed significantly from that of their Atlantic relatives, reflecting the predominance of regional point source inputs into the Baltic Sea and long-range atmospheric inputs into the Atlantic Ocean. The differences in the pattern of the compounds also indicated an enhanced metabolism of the more metabolizable compounds in the more contaminated Baltic seals. Surprisingly, the proportions of the high chlorinated and low-volatile PCB congeners (>6 Cl atoms) were comparable or lower in the Baltic pups as compared to the Atlantic pups. This difference might be due to Baltic seals occupying a lower trophic level than Atlantic seals and/or to the eutrophication situation in the Baltic Sea, which causes sedimentation of these PCB congeners. Significantly higherOCconcentrations were found in starved and/or abandoned Baltic pups as compared to well-fed pups. The most contaminated Baltic seal pups in the present study had PCB concentrations that are comparable or higher than those reported to impair the immune systems and vitamin A dynamics in phocids. [source]


Ferox Trout (Salmo trutta) as `Russian dolls': complementary gut content and stable isotope analyses of the Loch Ness foodweb

FRESHWATER BIOLOGY, Issue 7 2002
J. GREY
1.,Conventional collection methods for pelagic fish species (netting, trawling) are impractical or prohibited in Loch Ness, U.K. To investigate trophic relationships at the top of the Loch Ness food web, an alternative strategy, angling, provided samples of the top predator, the purely piscivorous ferox trout (Salmo trutta). 2.,The gut contents of these fish provided further samples of prey-fish, and subsequent examination of prey-fish guts revealed their dietary intake, analogous to the famous nested `Russian dolls'. Each trophic level separated by gut content analysis provided further complementary samples for stable isotope analysis and thus information on the longer term, assimilated diet. 3.,Ferox trout exhibited considerable cannibalism to supplement a diet of Arctic charr (Salvelinus alpinus). However, conspecifics stemmed from a lower isotopic baseline in relation to charr, so ferox trout exhibited a lower trophic level than predicted (4.3) by using the ,15N values. Charr displayed dietary specialisation with increasing length, and isotopic values supported by the gut data placed the charr at a trophic level of 3.5. The isotope data also indicated that charr carbon was primarily autochthonous in origin. [source]


Ecologically justified charisma: preservation of top predators delivers biodiversity conservation

JOURNAL OF APPLIED ECOLOGY, Issue 6 2006
FABRIZIO SERGIO
Summary 1Because of their popular appeal, top vertebrate predators have frequently been used as flagship or umbrella species to acquire financial support, raise environmental awareness and plan systems of protected areas. However, some have claimed that the utilization of charismatic predators may divert a disproportionate amount of funding to a few glamorous species without delivering broader biodiversity benefits, an accusation aggravated by the fact that the conservation of top predators is often complex, difficult and expensive. Therefore, tests are needed of whether apex predators may be employed to achieve ecosystem-level targets. 2To test such a hypothesis, we compared the biodiversity values recorded at the breeding sites of six raptor species, differing widely in diet and habitat associations, with those observed at three types of control locations, (i) sites randomly chosen in comparable habitat, (ii) breeding sites of a randomly selected bird species of lower trophic level and (iii) breeding sites of a lower trophic level species with specialized ecological requirements. Biodiversity was measured as the richness and evenness of bird, butterfly and tree species. 3Biodiversity levels were consistently higher at sites occupied by top predators than at any of the three types of control sites. Furthermore, sites occupied by top predators also held greater densities of individual birds and butterflies (all species combined) than control sites. 4In a reserve-selection simulation exercise, networks of protected sites constructed on the basis of top predators were more efficient than networks based on lower trophic level species, enabling higher biodiversity coverage to be achieved with a smaller number of reserves. 5Synthesis and applications. Our results provide evidence of a link between the strategic utilization of top predatory species and ecosystem-level conservation. We suggest that, at least in some biological systems, conservation plans based on apex predators could be implemented to deliver broader biodiversity benefits. [source]


Long-term changes in the trophic level of the Celtic Sea fish community and fish market price distribution

JOURNAL OF APPLIED ECOLOGY, Issue 3 2002
J. K. Pinnegar
Summary 1The intensive exploitation of fish communities often leads to substantial reductions in the abundance of target species, with ramifications for the structure and stability of the ecosystem as a whole. 2We explored changes in the mean trophic level of the Celtic Sea (ICES divisions VII f,j) fish community using commercial landings, survey data and estimates of trophic level derived from the analysis of nitrogen stable isotopes. 3Our analyses showed that there has been a significant decline in the mean trophic level of survey catches from 1982 to 2000 and a decline in the trophic level of landings from 1946 to 1998. 4The decline in mean trophic level through time resulted from a reduction in the abundance of large piscivorous fishes and an increase in smaller pelagic species which feed at a lower trophic level. 5Similar patterns of decline in the trophic level of both catches and landings imply that there have been substantial changes in the underlying structure of the Celtic Sea fish community and not simply a change in fishery preferences. 6We suggest that the reported changes in trophic structure result from reductions in the spawning stock biomass of traditional target species associated with intensive fishing, together with long-term climate variability. 7The relative distribution of fish market prices has changed significantly over the past 22 years, with high trophic level species experiencing greater price rises than lower trophic level species. 8Although decreased abundance of high trophic level species will ultimately have negative economic consequences, the reduction in mean trophic level of the fish community as a whole may allow the system to sustain higher fishery yields. 9Management objectives in this fishery will depend on the relative values that society attaches to economic profit and protein production. [source]


Growth and food availability of silver and bighead carps: evidence from stable isotope and gut content analysis

AQUACULTURE RESEARCH, Issue 14 2009
Qiong Zhou
Abstract A 2-year investigation of growth and food availability of silver carp and bighead was carried out using stable isotope and gut content analysis in a large pen in Meiliang Bay of Lake Taihu, China. Both silver carp and bighead exhibited significantly higher ,13C in 2005 than in 2004, which can probably be attributed to two factors: (i) the difference between isotopic compositions at the base of the pelagic food web and (ii) the difference between the compositions of prey items and stable isotopes. The significantly positive correlations between body length, body weight and stable isotope ratios indicated that isotopic changes in silver carp and bighead resulted from the accumulation of biomass concomitant with rapid growth. Because of the drastic decrease in zooplankton in the diet in 2005, silver carp and bighead grew faster in 2004 than in 2005. Bighead carp showed a lower trophic level than silver carp in 2005 as indicated by stable nitrogen isotope ratios, which was possibly explained by the interspecific difference between the prey species and the food quality of silver carp and bighead. [source]


Patterns of organic contaminants in eggs of an insectivorous, an omnivorous, and a piscivorous bird nesting on the Hudson River, New York, USA

ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 10 2010
Christine M. Custer
Abstract Belted kingfisher (Ceryle alcyon), spotted sandpiper (Actitus macularia), and tree swallow (Tachycineta bicolor) eggs were collected in 2004 from the upper Hudson River, New York, USA. This area is one of the most polychlorinated biphenyl (PCB)-contaminated locations in North America. Multivariate analyses indicated among species differences in the concentration and composition of PCB congeners, polychlorinated dibenzo- p -dioxin (PCDD), and dibenzofuran (PCDF, PCDD-F when combined with PCDDs) congeners, and chlorinated pesticides. Total PCB concentrations followed the typical food chain biomagnification paradigm of higher concentrations in piscivorous bird eggs and lower concentrations in eggs of species that feed at lower trophic levels. Concentrations in the insectivorous swallows (geometric mean,=,6.8,µg/g wet wt) were approximately half the concentrations present in the piscivorous kingfisher (11.7,µg/g) or omnivorous sandpiper (12.6,µg/g). In contrast, PCB toxic equivalents (TEQs) were higher in swallows (1,790,pg/g wet wt) than in either kingfishers (776,pg/g) or sandpipers (881,pg/g). This difference can be mainly attributed to higher PCB77 concentrations in swallows relative to the other two species. Also contrary to the accepted food-chain paradigm, the sum of PCDD-F concentrations and the sum of their TEQs were higher in swallows than in either sandpipers or kingfishers. Metabolic pathway differences in the respective food chains of the three species probably accounted for the differences observed in PCB TEQ, total PCDD-F, and PCDD-F TEQ concentrations among species. Environ. Toxicol. Chem. 2010;29:2286,2296. © 2010 SETAC [source]


Ecological effects of regime shifts in the Bering Sea and eastern North Pacific Ocean

FISH AND FISHERIES, Issue 2 2002
Ashleen J Benson
Abstract Large-scale shifts occurred in climatic and oceanic conditions in 1925, 1947, 1977, 1989 and possibly 1998. These shifts affected the mix and abundance of suites of coexisting species during each period of relative environmental stability,from primary producers to apex predators. However, the 1989 regime shift was not a simple reversal of the 1977 shift. The regime shifts occurred abruptly and were neither random variations nor simple reversals to the previous conditions. Timing of these anomalous environmental events in the North Pacific Ocean appears to be linked to physical and biological responses in other oceanic regions of the world. Changes in the atmospheric pressure can alter wind patterns that affect oceanic circulation and physical properties such as salinity and depth of the thermocline. This, in turn, affects primary and secondary production. Data from the North Pacific indicate that regime shifts can have opposite effects on species living in different domains, or can affect similar species living within a single domain in opposite ways. Climatic forcing appears to indirectly affect fish and marine mammal populations through changes in the distribution and abundance of their predators and prey. Effects of regime shifts on marine ecosystems are also manifested faster at lower trophic levels. Natural variability in the productivity of fish stocks in association with regime shifts indicates that new approaches to managing fisheries should incorporate climatic as well as fisheries effects. [source]


How well are velocity effects on ,13C signatures transmitted up the food web from algae to fish?

FRESHWATER BIOLOGY, Issue 6 2010
JOSEPH B. RASMUSSEN
Summary 1. Benthic algae fractionate carbon isotopes less at low water velocities because of reduced boundary layer exchange, and this effect on ,13C is passed on to consumers via trophic transfer. This study examines the relationships between ,13C signatures of consumers (invertebrates and salmonid fishes) and water velocity in the Sainte Marguerite River, QC, Canada, and compares them to patterns for periphyton, both along the river main-stem and in a small tributary. 2. Relationships of ,13C signatures of herbivore/grazers and collector/gatherers with water velocity were strong and similar to those of periphyton, but relationships for filter-feeders were weak, probably reflecting the effect of spatial averaging of their food supply as a result of downstream transport. 3. Velocity effects on salmonid signatures were much weaker than those of lower trophic levels, being barely significant except in the small tributary where the fish were resident and isolated from the main river. In the river main-stem, even when reach standardised (reach mean subtracted from each data point), fish signatures were only weakly related to water velocity. 4. The fidelity with which velocity effects are transmitted to consumers from benthic algae is highly variable, and depends on a combination of consumer and resource movements, in addition to the trophic position of the consumer. [source]


Water temperature determines strength of top-down control in a stream food web

FRESHWATER BIOLOGY, Issue 8 2005
DAISUKE KISHI
Summary 1. We examined effects of water temperature on the community structure of a three trophic level food chain (predatory fish, herbivorous caddisfly larvae and periphyton) in boreal streams. We used laboratory experiments to examine (i) the effects of water temperature on feeding activities of fish and caddisfly larvae and on periphyton productivity, to evaluate the thermal effects on each trophic level (species-level experiment), and (ii) the effects of water temperature on predation pressure of fish on abundance of the lower trophic levels, to evaluate how temperature affects top-down control by fish (community-level experiment). 2. In the species-level experiment, feeding activity of fish was high at 12 °C, which coincides with the mean summer temperature in forested streams of Hokkaido, Japan, but was depressed at 3 °C, which coincides with the mean winter temperature, and also above 18 °C, which coincides with the near maximum summer temperatures. Periphyton productivity increased over the range of water temperatures. 3. In the community-level experiments, a top-down effect of fish on the abundance of caddisfly larvae and periphyton was clear at 12 °C. This effect was not observed at 3 and 21 °C because of low predation pressure of fish at these temperatures. 4. These experiments revealed that trophic cascading effects may vary with temperature even in the presence of abundant predators. Physiological depression of predators because of thermal stress can alter top-down control and lead to changes in community structure. 5. We suggest that thermal habitat alteration can change food web structure via combinations of direct and indirect trophic interactions. [source]


Effects of predatory ants on lower trophic levels across a gradient of coffee management complexity

JOURNAL OF ANIMAL ECOLOGY, Issue 3 2008
S. M. Philpott
Summary 1Ants are important predators in agricultural systems, and have complex and often strong effects on lower trophic levels. Agricultural intensification reduces habitat complexity, food web diversity and structure, and affects predator communities. Theory predicts that strong top-down cascades are less likely to occur as habitat and food web complexity decrease. 2To examine relationships between habitat complexity and predator effects, we excluded ants from coffee plants in coffee agroecosystems varying in vegetation complexity. Specifically, we studied the effects of eliminating ants on arthropod assemblages, herbivory, damage by the coffee berry borer and coffee yields in four sites differing in management intensification. We also sampled ant assemblages in each management type to see whether changes in ant assemblages relate to any observed changes in top-down effects. 3Removing ants did not change total arthropod densities, herbivory, coffee berry borer damage or coffee yields. Ants did affect densities of some arthropod orders, but did not affect densities of different feeding groups. The effects of ants on lower trophic levels did not change with coffee management intensity. 4Diversity and activity of ants on experimental plants did not change with coffee intensification, but the ant species composition differed. 5Although variation in habitat complexity may affect trophic cascades, manipulating predatory ants across a range of coffee agroecosystems varying in management intensity did not result in differing effects on arthropod assemblages, herbivory, coffee berry borer attack or coffee yields. Thus, there is no clear pattern that top-down effects of ants in coffee agroecosystems intensify or dampen with decreased habitat complexity. [source]


Contrasting effects of the extent of sea-ice on the breeding performance of an Antarctic top predator, the Snow Petrel Pagodroma nivea

JOURNAL OF AVIAN BIOLOGY, Issue 4 2001
Christophe Barbraud
Recent studies have shown that the Antarctic Circumpolar Wave and the related sea-ice cover around the Antarctic continent may have a profound effect on the lower trophic levels of the marine environment. In particular, extensive sea-ice cover enhances the survival of krill. However, the effects of sea-ice cover on top predators remain poorly understood. Using time series from 1973 to 1999, we examine the influence of regional sea-ice extent on a number of indices of breeding performance of an avian predator, the Snow Petrel, in Antarctica. The percentage of breeding pairs was highly variable and there were fewer birds breeding when sea-ice cover was extensive during July. By contrast, overall breeding success and fledgling body condition were improved during years with extensive sea-ice cover during the preceding November and July,September. These results indicate that the same sea-ice conditions may have different effects on the breeding performance of a species. The overall increase in winter sea-ice extent during the last decade appears to have resulted in an overall improvement of the quality of fledglings produced, and thus probably of future recruitment. [source]