Variabilité de structure et de fonctionnement d'un écosystème de bord est: Application à l'upwelling de Californie

The California Current System (CCS) is one of the major eastern boundary upwelling systems, which are characterized by a seasonal wind regime that upwells deep nutrient-rich water to the surface, favorable to high biological activity at coast. In the long term, the CCS ecosystem reveals still unexplained shifts in marine communities. In this context, this thesis aims at understanding how pluriannual wind variability influences the structure and functioning of the lower trophic levels of the CCS ecosystem. With this end in view, we carry out process studies based on a numerical approach. In a first study, we show that the low frequency variability of coastal upwelling and cross-shore transport is strongly correlated with that of alongshore wind stress and with the North Pacific Gyre Oscillation (NPGO). This latter mode was recently discovered and is known to explain part of the low frequency variability of the upwelling winds and chlorophyll in the CCS. At a finer scale, we show a strong relationship between the seasonal variability of the upwelling winds and the NPGO, which is translated into temporal modulation of the upwelling onset in the CCS. We investigate the influence of such a delay in upwelling onset on a planktonic ecosystem. Nearshore, in an early upwelling scenario, the ecosystem is immediately more productive. Offshore, the ecosystem is also influenced via cross-shore transport processes. The effect on zooplankton species is more pronounced than on phytoplankton species and may impact higher trophic levels.Le système du Courant de Californie (CCS) est l'un des grands systèmes d'upwelling de bord est de la planète, caractérisés par un régime saisonnier de vents qui provoque des remontées d'eaux profondes (upwelling côtier), riches en nutriments, favorisant une forte activité biologique. À long terme, l'écosystème du CCS révèle des alternances de dominance de communautés marines, encore inexpliquées. L'objet de cette thèse est de comprendre l'effet de la variabilité pluriannuelle des vents sur la structure et le fonctionnement des premiers maillons trophiques de l'écosystème du CCS à partir d'études de processus reposant sur une approche numérique. Une première étude a permis de montrer que l'upwelling côtier et le transport côte-large ont une variabilité à basse fréquence fortement corrélée à celle de la tension de vent parallèle à la côte et au mode North Pacific Gyre Oscillation (NPGO), mis en évidence récemment et connu pour capturer une part de la variabilité à basse fréquence des vents d'upwelling et de la chlorophylle dans le CCS. Une étude fine de ces vents a permis de mettre en évidence une relation forte entre leur variabilité saisonnière et le mode NPGO, avec une modulation temporelle du déclenchement de la saison d'upwelling du CCS. L'impact d'un tel déphasage de l'upwelling sur un écosystème planctonique a pu ensuite être testé. À la côte, l'écosystème répond directement à un scénario d'upwelling précoce par une productivité plus forte. Au large, les incidences sur l'écosystème s'opèrent via les processus de transport côte-large. L'effet sur le zooplancton est plus prononcé que sur le phytoplancton et est susceptible d'affecter les niveaux trophiques supérieurs

Variabilité de structure et de fonctionnement d'un écosystème de bord est (application à l'upwelling de Californie)

Le système du Courant de Californie (CCS) est l un des grands systèmes d upwelling de bord est de la planète, caractérisés par un régime saisonnier de vents qui provoque des remontées d eaux profondes (upwelling côtier), riches en nutriments, favorisant une forte activité biologique. À long terme, l écosystème du CCS révèle des alternances de dominance de communautés marines, encore inexpliquées. L objet de cette thèse est de comprendre l effet de la variabilité pluriannuelle des vents sur la structure et le fonctionnement des premiers maillons trophiques de l écosystème du CCS à partir d études de processus reposant sur une approche numérique. Une première étude a permis de montrer que l upwelling côtier et le transport côte-large ont une variabilité à basse fréquence fortement corrélée à celle de la tension de vent parallèle à la côte et au mode North Pacific Gyre Oscillation (NPGO), mis en évidence récemment et connu pour capturer une part de la variabilité à basse fréquence des vents d upwelling et de la chlorophylle dans le CCS. Une étude fine de ces vents a permis de mettre en évidence une relation forte entre leur variabilité saisonnière et le mode NPGO, avec une modulation temporelle du déclenchement de la saison d upwelling du CCS. L impact d un tel déphasage de l upwelling sur un écosystème planétonique a pu ensuite être testé. À la côte, l écosystème répond directement à un scénario d upwelling précoce par une productivité plus forte. Au large, les incidences sur l écosystème s opèrent via les processus de transport côte-large. L effet sur le zooplancton est plus prononcé que sur le phytoplancton et est susceptible d affecter les niveaux trophiques supérieurs.The California Current System (CCS) is one of the major eastern boundary upwelling systems, which are characterized by a seasonal wind regime that upwells deep nutrient-rich water to the surface, favorable to high biological activity at coast. In the long term, the CCS ecosystem reveals still unexplained shifts in marine communities. In this context, this thesis aims at understanding how pluriannual wind variability influences the structure and functioning of the lower trophic levels of the CCS ecosystem. With this end in view, we carry out process studies based on a numerical approach. In a first study, we show that the low frequency variability of coastal upwelling and crossshore transport is strongly correlated with that of alongshore wind stress and with the North Pacific Gyre Oscillation (NPGO). This latter mode was recently discovered and is known to explain part of the low frequency variability of the upwelling winds and chlorophyll in the CCS. At a finer scale, we show a strong relationship between the seasonal variability of the upwelling winds and the NPGO, which is translated into temporal modulation of the upwelling onset in the CCS. We investigate the influence of such a delay in upwelling onset on a planktonic ecosystem. Nearshore, in an early upwelling scenario, the ecosystem is immediately more productive. Offshore, the ecosystem is also influenced via cross-shore transport processes. The effect on zooplankton species is more pronounced than on phytoplankton species and may impact higher trophic levels.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

On the sensitivity of plankton ecosystem models to the formulation of zooplankton grazing

International audienceModel representations of plankton structure and dynamics have consequences for a broad spectrum of ocean processes. Here we focus on the representation of zooplankton and their grazing dynamics in such models. It remains unclear whether phytoplankton community composition, growth rates, and spatial patterns in plankton ecosystem models are especially sensitive to the specific means of representing zooplankton grazing. We conduct a series of numerical experiments that explicitly address this question. We focus our study on the form of the functional response to changes in prey density, including the formulation of a grazing refuge. We use a contemporary biogeochemical model based on continuum size-structured organization, including phytoplankton diversity, coupled to a physical model of the California Current System. This region is of particular interest because it exhibits strong spatial gradients. We find that small changes in grazing refuge formulation across a range of plausible functional forms drive fundamental differences in spatial patterns of plankton concentrations, species richness, pathways of grazing fluxes, and underlying seasonal cycles. An explicit grazing refuge, with refuge prey concentration dependent on grazers’ body size, using allometric scaling, is likely to provide more coherent plankton ecosystem dynamics compared to classic formulations or size-independent threshold refugia. We recommend that future plankton ecosystem models pay particular attention to the grazing formulation and implement a threshold refuge incorporating size-dependence, and we call for a new suite of experimental grazing studies

On the sensitivity of plankton ecosystem models to the formulation of zooplankton grazing.

Model representations of plankton structure and dynamics have consequences for a broad spectrum of ocean processes. Here we focus on the representation of zooplankton and their grazing dynamics in such models. It remains unclear whether phytoplankton community composition, growth rates, and spatial patterns in plankton ecosystem models are especially sensitive to the specific means of representing zooplankton grazing. We conduct a series of numerical experiments that explicitly address this question. We focus our study on the form of the functional response to changes in prey density, including the formulation of a grazing refuge. We use a contemporary biogeochemical model based on continuum size-structured organization, including phytoplankton diversity, coupled to a physical model of the California Current System. This region is of particular interest because it exhibits strong spatial gradients. We find that small changes in grazing refuge formulation across a range of plausible functional forms drive fundamental differences in spatial patterns of plankton concentrations, species richness, pathways of grazing fluxes, and underlying seasonal cycles. An explicit grazing refuge, with refuge prey concentration dependent on grazers' body size, using allometric scaling, is likely to provide more coherent plankton ecosystem dynamics compared to classic formulations or size-independent threshold refugia. We recommend that future plankton ecosystem models pay particular attention to the grazing formulation and implement a threshold refuge incorporating size-dependence, and we call for a new suite of experimental grazing studies

Fate of floating plastic debris released along the coasts in a global ocean model

International audienc

California coastal upwelling onset variability: cross-shore and bottom-up propagation in the planktonic ecosystem.

The variability of the California Current System (CCS) is primarily driven by variability in regional wind forcing. In particular, the timing of the spring transition, i.e., the onset of upwelling-favorable winds, varies considerably in the CCS with changes in the North Pacific Gyre Oscillation. Using a coupled physical-biogeochemical model, this study examines the sensitivity of the ecosystem functioning in the CCS to a lead or lag in the spring transition. An early spring transition results in an increased vertical nutrient flux at the coast, with the largest ecosystem consequences, both in relative amplitude and persistence, hundreds of kilometers offshore and at the highest trophic level of the modeled food web. A budget analysis reveals that the propagation of the perturbation offshore and up the food web is driven by remineralization and grazing/predation involving both large and small plankton species

North Pacific Gyre Oscillation modulates seasonal timing and ecosystem functioning in the California Current upwelling system

International audienceOn interannual and longer time scales, dynamical and biogeochemical fluctuations in the North Pacific are dominated by two modes of variability, namely the Pacific Decadal Oscillation and the North Pacific Gyre Oscillation (NPGO). In this study the regional expression of the NPGO in the California Current System (CCS) is detailed. The statistical relationship between the NPGO index and nearshore wind variability (mostly upwelling favorable) along the U.S. West coast is strongest in the wintertime (December to March) off Central California. Most importantly, NPGO fluctuations are associated with a seasonal shift of 1-2 months in the onset of the upwelling season. Regional numerical simulations show that an early (late) onset of upwelling during the positive (negative) phase of the NPGO leads to a more (less) productive planktonic ecosystem throughout spring and summer, i.e., several months after the direct NPGO effects on the system have ceased. These results bring new light on the California ecosystem variability as observed in atypical years such as 2005 and 2007

California Coastal Upwelling Onset Variability: Cross- Shore and Bottom-Up Propagation in the Planktonic Ecosystem

The variability of the California Current System (CCS) is primarily driven by variability in regional wind forcing. In particular, the timing of the spring transition, i.e., the onset of upwelling-favorable winds, varies considerably in the CCS with changes in the North Pacific Gyre Oscillation. Using a coupled physical-biogeochemical model, this study examines the sensitivity of the ecosystem functioning in the CCS to a lead or lag in the spring transition. An early spring transition results in an increased vertical nutrient flux at the coast, with the largest ecosystem consequences, both in relative amplitude and persistence, hundreds of kilometers offshore and at the highest trophic level of the modeled food web. A budget analysis reveals that the propagation of the perturbation offshore and up the food web is driven by remineralization and grazing/ predation involving both large and small plankton species

Cross-shore transport variability in the California Current: Ekman upwelling vs. eddy dynamics

International audienceThe low-frequency dynamics of coastal upwelling and cross-shelf transport in the Central and Southern California Current System (CCS) are investigated using the Regional Ocean Modeling System (ROMS) over the period 1965-2008. An ensemble of passive tracers released in the numerical model is used to characterize the effects of linear (Ekman upwelling) and non-linear (mesoscale eddies) circulation dynamics on the statistics of advection of coastal waters. The statistics of passive tracers released in the subsurface show that the low-frequency variability of coastal upwelling and cross-shelf transport of the upwelled water mass are strongly correlated with the alongshore wind stress, and are coherent between the central and southern CCS. However, the offshore transport of tracers released at the surface is not coherent between the two regions, and is modulated by intrinsic mesoscale eddy activity, in particular cyclonic eddies. The transport of cyclonic eddies extends with depth and entrains water masses of southern origin, advected by the poleward California Undercurrent (CUC). The CUC water masses are not only entrained by eddies but also constitute a source for the central California upwelling system. The interplay between intrinsic (eddy activity) and deterministic (Ekman upwelling) dynamics in controlling the cross-shelf exchanges in the CCS may provide an improved framework to understand and interpret nutrients and ecosystem variabilit