Réseau trophique | EPFL Graph Search

vignette|Deux représentations rendant compte du fonctionnement d'un écosystème : pyramide écologique (a) et réseau trophique (b). Un réseau trophique est un ensemble de chaînes alimentaires reliées entre elles au sein d'un écosystème et par lesquelles l'énergie et la biomasse circulent (échanges d'éléments tels que le flux de carbone et d'azote entre les différents niveaux de la chaîne alimentaire, échange de carbone entre les végétaux autotrophes et les hétérotrophes). Le terme trophique se rapporte à tout ce qui est relatif à la nutrition d'un tissu vivant ou d'un organe. Par exemple, une relation trophique est le lien qui unit le prédateur et sa proie dans un écosystème. Dans un écosystème, la structure des réseaux trophiques (les types et réseaux de relations alimentaires entre organismes) influence fortement la quantité, la diversité, la stabilité et la qualité de la biomasse et de la matière organique résiduelle (excrétions, nécromasse) produites par les écosystèmes. La qualit

Silver nanoparticle effects on simple stream food webs and ecosystem processes

Carmen Omayra Gil Allué

With the global nanotechnology market growing rapidly, nanomaterials are being increasingly released into aquatic environments, where they can undergo modifications and sedimentation, which will put benthic organisms at risk. Of particular interest is the study of nanomaterial effects on periphyton, a community of auto- and heterotrophic microorganisms embedded in an extracellular polymer matrix that covers submerged surfaces in aquatic ecosystems. Although periphyton assumes important functions in ecosystems, like primary production, little information is available on its sensitivity to nanomaterials and how these might be transferred to higher trophic levels in food webs. In the present thesis, effects of short- and long-term exposures to citrate-coated silver nanoparticles (AgNP, 35 nm) and silver ions (Ag+, dosed as AgNO3) were assessed on periphyton in microcosm studies for periods between 2 hours and 21 days. After exposure, primary production, secondary production, photosynthetic yield, microbial respiration and potential extracellular enzyme activity were measured. Moreover, addition of an aquatic snail, Physa acuta, feeding on periphyton allowed the assessment of effects of AgNP on an aquatic herbivore and trophic transfer efficiency. Contaminated (AgNP and AgNO3) and uncontaminated periphyton was offered to snails in a 5-day experiment to determine silver uptake and the mortality, feeding rate and reproduction of the snails. Our findings show that all endpoints measured in periphyton were affected by both AgNP and AgNO3 during short-term exposure, but AgNO3 was more toxic on a total silver mass basis than AgNP. Concentrations causing a 50% reduction in the different endpoints (EC50) ranged from 17 to 83 µM AgNP and from 2 to 4 µM AgNO3. Exposure of periphyton in the presence of silver ion ligands to complex Ag+ (ca. 1% of total silver in AgNP suspensions) prevented AgNP toxicity to most endpoints, confirming that the observed toxic effects were caused by the bioavailable Ag+. An exception was the extracellular enzyme leucine aminopeptidase, which was directly affected by AgNP. Concentrations of AgNP and AgNO3 that caused low toxicity during short-term exposure also caused effects on periphyton functions upon long-term exposure, with algae and bacteria being differently affected. Periphyton exposed to 10 µM AgNP for 21 days showed a 63% decrease in primary production and a 246% increase in bacterial secondary production per unit of total periphyton biomass. The community composition exposed to AgNP or AgNO3 differed from that of the control and also from each other. Furthermore, AgNP-exposed communities were more tolerant to subsequent AgNP exposure. Trophic transfer of silver from contaminated periphyton to snails occurred efficiently, independently of the silver form. Feeding rate of snails on AgNO3-exposed communities was 50% lower than on controls, and disturbance of digestion was evident upon feeding on both AgNP- and AgNO3-exposed communities. No changes in the mortality or egg production of the snails were observed. However, the hatching success of eggs from snails that fed on contaminated periphyton was reduced by more than 79%, and 28-day old embryos showed malformations. Overall, this study clearly identified damaging effects of AgNP on periphyton, with consequent indirect impacts on a consumer, trophic relationships and stream ecosystem processes.

EPFL2015

Variation in Detrital Resource Stoichiometry Signals Differential Carbon to Nutrient Limitation for Stream Consumers Across Biomes

Janine Rüegg

Stoichiometric ratios of resources and consumers have been used to predict nutrient limitation across diverse terrestrial and aquatic ecosystems. In forested headwater streams, coarse and fine benthic organic matter (CBOM, FBOM) are primary basal resources for the food web, and the distribution and quality of these organic matter resources may therefore influence patterns of secondary production and nutrient cycling within stream networks or among biomes. We measured carbon (C), nitrogen (N), and phosphorus (P) content of CBOM and FBOM and calculated their stoichiometric ratios (C/N, C/P, N/P) from first- to fourth-order streams from tropical montane, temperate deciduous, and boreal forests, and tallgrass prairie, to compare the magnitude and variability of these resource types among biomes. We then used the ratios to predict nutritional limitations for consumers of each resource type. Across biomes, CBOM had consistently higher %C and %N, and higher and more variable C/N and C/P than FBOM, suggesting that microbial processing results in more tightly constrained elemental composition in FBOM than in CBOM. Biome-specific differences were observed in %P and N/P between the two resource pools; CBOM was lower in %P but higher in N/P than FBOM in the tropical montane and temperate deciduous forest biomes, while CBOM was higher in %P but similar in N/P than FBOM in the grassland and boreal forest biomes. Stable C-13 isotopes suggest that FBOM likely derives from CBOM in tropical and temperate deciduous forest, but that additional non-detrital components may contribute to FBOM in boreal forests and grasslands. Comparisons of stoichiometric ratios of CBOM and FBOM to estimated needs of aquatic detritivores suggest that shredders feeding on CBOM are more likely to experience nutrient (N and/or P) than C limitation, whereas collector-gatherers consuming FBOM are more likely to experience C than N and/or P limitation. Our results suggest that differences in basal resource elemental content and stoichiometric ratios have the potential to affect consumer production and ecosystem rates of C, N, and P cycling in relatively consistent ways across diverse biomes.

SPRINGER2018

UNDERSTANDING THE EXTENT OF FOOD WEB ALTERATIONS ON AQUATIC ECOSYSTEM STRUCTURE AND FUNCTION REQUIRES CAREFUL CONSIDERATION OF SCALE

Janine Rüegg

Species declines due to changing environmental conditions or anthropogenic influences such as fisheries harvest as well as species introductions probably have consequences beyond the species by also altering their food webs. The scope of the effect on the structure and function of aquatic ecosystems likely depends on the role of the affected species (e.g., grazer/consumer, ecosystem engineer, keystone species) and on the spatial and temporal scale considered. Spatial effects can be fairly small, like the introduction of a species in an individual pond or lake, or relatively large, like the decline of anadromous fish migrations affecting thousands of kilometers. Temporal effects may be short-term, such as loss of species due to drought which may be replenished from nearby source populations following rewetting, or long-term, such as the installation of a dam that alters natural flow and temperature regimes of rivers. Ultimately, to gain a better understanding of the implications of food web alterations, the potential pathways of influence need to be explicitly considered across various spatial and temporal scales to identify the challenges aquatic ecosystems face.

2016

Silver nanoparticle effects on simple stream food webs and ecosystem processes

Carmen Omayra Gil Allué

EPFL2015

Variation in Detrital Resource Stoichiometry Signals Differential Carbon to Nutrient Limitation for Stream Consumers Across Biomes

Janine Rüegg

SPRINGER2018