Cycle biogéochimique | EPFL Graph Search

Un cycle biogéochimique est le processus de transport et de transformation cyclique (recyclage) d'un élément ou composé chimique entre les grands réservoirs que sont la géosphère, l'atmosphère, l'hydrosphère, dans lesquels se retrouve la biosphère. Un tel cycle induit souvent des passages de l'état organique à l'état minéral au sein de la biosphère. Les divers cycles en interaction confèrent à la biosphère une capacité de régulation, appelée homéostasie. Celle-ci est à la base de la pérennité des écosystèmes, grâce à la grande stabilité qu'elle assure, tout du moins en dehors des interventions humaines et phénomènes géoclimatiques exceptionnels. Lorsque l'élément impliqué est un nutriment essentiel, ce processus correspond à un cycle des nutriments. Cycles Les cycles les plus importants sont :

le cycle de l'azote ;
le cycle du carbone ;
le cycle de l'eau ;
le cycle de l'hydrogène ;
le cycle de l'oxygène ;
le cycle du phosphore ;
le cycle du sélénium ;
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Towards an improved understanding of biogeochemical processes across surface-groundwater interactions in intermittent rivers and ephemeral streams

Luis Gómez Gener, Oleksandra Shumilova

Surface-groundwater interactions in intermittent rivers and ephemeral streams (IRES), waterways which do not flow year-round, are spatially and temporally dynamic because of alternations between flowing, non-flowing and dry hydrological states. Interactions between surface and groundwater often create mixing zones with distinct redox gradients, potentially driving high rates of carbon and nutrient cycling. Yet a complete understanding of how underlying biogeochemical processes across surface-groundwater flowpaths in IRES differ among various hydrological states remains elusive. Here, we present a conceptual framework relating spatial and temporal hydrological variability in surface water-groundwater interactions to biogeochemical processing hotspots in IRES. We combine a review of theIRES biogeochemistry literature with concepts of IRES hydrogeomorphology to: (i) outline common distinctions among hydrological states in IRES; (ii) use these distinctions, together with considerations of carbon, nitrogen, and phosphorus cycles within IRES, to predict the relative potential for biogeochemical processing across different reach-scale processing zones (flowing water, fragmented pools, hyporheic zones, groundwater, and emerged sediments); and (iii) explore the potential spatial and temporal variability of carbon and nutrient biogeochemical processing across entire IRES networks. Our approach estimates the greatest reach-scale potential for biogeochemical processing when IRES reaches are fragmented into isolated surface water pools, and highlights the potential of relatively understudied processing zones, such as emerged sediments. Furthermore, biogeochemical processing in fluvial networks dominated by IRES is likely more temporally than spatially variable. We conclude that biogeochemical research in IRES would benefit from focusing on interactions between different nutrient cycles, surface-groundwater interactions in non-flowing states, and consideration of fluvial network architecture. Our conceptual framework outlines opportunities to advance studies and expand understanding of biogeochemistry in IRES.

2021

Tall and old Amazonian forests are more resilient to precipitation variability

Francesco Giardina

Climate change is altering the natural equilibrium of the Amazon ecosystem, which is in turn deeply connected to the Earth’s biogeochemical cycles. Understanding this relationship aids optimal rainforests management in the future. Previous research has investigated the role of environmental variables and climatic drivers, neglecting the influence of forest height and age. This study uses remote sensing observations of precipitation, vapour-pressure deficit and canopy height along with model estimates of tree age and aboveground biomass to show that tall and old Amazonian trees are three times less sensitive to precipitation variability than younger and smaller forests. Canopy size is found to be equally important to mean precipitation in controlling the response to interannual climate variability. Our results align with those of in-situ experiments. These findings could help better predict the response of the Amazon forest to future droughts and water stress.

2017

Long-term changes in phytoplankton functional groups and primary production in Lake Geneva: A Modelling Approach

Emile Barbe

In this study, a hydrodynamical model is coupled to a biogeochemical model, and then used to simulate the main physical, chemical and biological characteristics of Lake Geneva. The study period, going from 1981 to 2016 was marked by two major changes with concomitant effects: re-oligotrophication by reducing the phosphorus loading and climate change. Thanks to the calibrated model, changes of the ecosystem related to climate change and re-oligotrophication were disentangled. It resulted in a positive effect of the former on phytoplankton biomass and primary production, and a negative effect of the latter. The combined effects of these two majors changes are negative. However, these results have a large uncertainty related to the capacity of the model to reproduce the biological characters of Lake Geneva. The calibration procedure must be improved to ensure a higher relevance of the results.

2020

Towards an improved understanding of biogeochemical processes across surface-groundwater interactions in intermittent rivers and ephemeral streams

Luis Gómez Gener, Oleksandra Shumilova

2021