Modélisation des écosystèmes planctoniques pélagiques en Méditerranée nord-occidentale : impact des eaux du Rhône à l'échelle du plateau du golfe du Lion et variabilité interannuelle à décennale au large

La dynamique des écosystèmes planctoniques pélagiques au sein du bassin de Méditerranée nord-occidentale a été appréhendée par une approche de modélisation réaliste. Deux cas bien distincts ont été considérés, celui des écosystèmes qui se développent sur les apports du Rhône et celui des écosystèmes du large. Une modélisation haute résolution du panache du Rhône est mise en place pour prendre en compte les spécificités des apports en sels nutritifs dans les eaux du Rhône. Les apports terrestres de matière particulaire et dissoute semblent favoriser la biomasse zooplanctonique qui, par broutage, limite à son tour le dépôt de carbone organique particulaire sur le plateau du golfe du Lion. En stimulant le développement du zooplancton, les lentilles d'eau dessalée qui se détachent du panache du Rhône pourraient favoriser ce processus. Au large, une simulation longue durée de 30 ans montre que la variabilité interannuelle des écosystèmes planctoniques est dirigée par le mélange vertical, en lien avec les forçages atmosphériques. La convection profonde hivernale contrôle la structure de l'assemblage phytoplanctonique, en favorisant le microphytoplancton, et défavorise davantage les communautés zooplanctoniques et bactériennes en moyenne annuelle. Le mélange vertical contrôle également la variabilité interannuelle des exportations de carbone particulaire et dissout Le métabolisme net de la zone MEDOC tend à être faiblement hétérotrophe en période peu convective, atténuant ainsi le caractère puit de la zone MEDOC. Cependant, on observe alors une accumulation accrue de carbone au sein des écosystèmes planctoniques et de la matière détritique.The pelagic planktonic ecosystem dynamics of the north-western Mediterranean Sea has been studied by a realistic modelling approach. The dynamics of coastal ecosystems sustained by the Rhone River nutrient loads, and offshore ecosystems are considered separately. Through an high resolution modelling of the Rhone River plume, a specific calibration dedicated to river plume ecosystems has been proposed and validated on in situ data. Our results suggest a positive influence of river loads on the zooplankton biomass, which in turn limits the organic carbon deposition on the Gulf of Lion shelf through grazing on organic detritus. Low salinity water lenses detached from the Rhone River plume, owing to their positive influence on zooplankton development, could favour this process. Offshore, an extended simulation of 30 years shows that the interannual variability of planktonic ecosystems is controlled by the vertical mixing, in relation with atmospheric forcing. The winter deep convection determines the structure of the phytoplanktonic ecosystem by favouring microphytoplancton, and seems unfavourable to zooplankton and bacterial communities. The vertical mixing also controls the interannual variability of particulate and dissolved organic carbon export. The net metabolism in the MEDOC area tends to be weakly heterotroph during a weak convection period, reducing the sink pattern of the MEDOC area. However, there is an increased accumulation of carbon within planktonic ecosystems and organic detritus

Contrasting responses of the ocean’s oxygen minimum zones to artificial re-oxygenation

Studies assessing potential measures to counteract the marine deoxygenation attributed to anthropogenic activities have been conducted in a few coastal environments and at regional scale, but not yet on a global scale. One way toward global scale artificial oxygenation would be to use oxygen produced as a by-product from hydrogen-production through electrolysis. The low-carbon footprint renewable production of hydrogen from offshore wind energy offers such a possibility. Here, we assessed the potential of this artificial oxygenation method on a global scale using a coupled physical-biogeochemical numerical model. The anthropogenic oxygen source scenario assumes worldwide adoption of hydrogen, considering demographic changes and the feasibility of offshore wind turbine deployment. Following this scenario, artificial oxygenation had a negligible effect on the overall oxygen inventory (an increase of 0.07%) but showed a reduction in the overall volume of Oxygen Minimum Zones (OMZs) between 1.1% and 2.4%. Despite the decrease in the mean OMZ volume globally, OMZs display distinct and contrasting regional patterns notably due to the oxygen impacts on the nitrogen cycle. Artificial oxygenation can inhibit denitrification resulting in a net gain of nitrate that promotes locally and remotely increased biological productivity and consequent respiration. Increased respiration could ultimately lead to an oxygen loss at and beyond injection sites as in the Tropical Pacific and Indian Ocean and particularly expand the Bay of Bengal OMZ. In contrast, the tropical OMZ shrinkage in the Atlantic Ocean is attributed to oxygen enrichment induced by advective transport into the OMZ, while the absence of denitrification in this area precludes any biochemical feedback effect on oxygen levels. These results suggest that the impacts of artificial oxygenation on oxygen concentrations and ecosystems are highly non-linear. It can produce unexpected regional responses that can occur beyond the injection sites which make them difficult to forecast.publishedVersio

El Niño as a predictor of round sardinella distribution along the northwest African coast

The El Niño Southern Oscillation (ENSO) produces global marine environment conditions that can cause changes in abundance and distribution of distant fish populations worldwide. Understanding mechanisms acting locally on fish population dynamics is crucial to develop forecast skill useful for fisheries management. The present work addresses the role played by ENSO on the round sardinella population biomass and distribution in the central-southern portion of the Canary Current Upwelling System (CCUS). A combined physical-biogeochemical framework is used to understand the climate influence on the hydrodynamical conditions in the study area. Then, an evolutionary individual-based model is used to simulate the round sardinella spatio-temporal biomass variability. According to model experiments, anomalous oceanographic conditions forced by El Niño along the African coast cause anomalies in the latitudinal migration pattern of the species. A robust anomalous increase and decrease of the simulated round sardinella biomass is identified in winter off the Cape Blanc and the Saharan coast region, respectively, in response to El Niño variations. The resultant anomalous pattern is an alteration of the normal migration between the Saharan and the Mauritanian waters. It is primarily explained by the modulating role that El Niño exerts on the currents off Cape Blanc, modifying therefore the normal migration of round sardinella in the search of acceptable temperature conditions. This climate signature can be potentially predicted up to six months in advance based on El Niño conditions in the Pacific.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Book of Abstracts submitted to the IV Congress of Marine Sciences

Sardinella aurita, or round sardinella, has been the main small pelagic fish species harvested off Senegal and Mauritania coasts over the last 4 decades and plays a central role for sub-regional food security and economic incomes. The landings of this species have strongly declined recently. Intense exploitation and climate change are acting together but population&#8217;s dynamics are too poorly understood to disantangle the different drivers and to clearly evaluate the current state of S. aurita&#8217;s population. In the present study, we developed a bio-physical, individual based model for S. aurita population off North-West Africa. The hydrodynamic environment was simulated by a regional model ("ROMS") configuration covering the area 5°-40°N and 5°-30°W, with a 8km resolution and 32 sigma-levels in our area of interest. The biogeochemical compartments were simulated using the PISCES model coupled with ROMS. Fish schools of S. aurita were represented by active lagrangian markers affected with ad hoc larval, juvenile and adult fish swimming behavior. Individual&#8217;s physiology was described following the local temperature and food availability by the Dynamic Energy Budget model "DEB". The extended kinesis algorithm ruled the horizontal fish movement and depends on food research, individual temperature preference and spawning migration, whereas fish vertical position in the water column was set for each stage according to scientific knowledge. We investigate the predicted seasonal migrations pattern of S. aurita off West Africa over the period 1980-2006. Then we also evaluate the fluctuations of fish biomass available for coastal fisheries (h<200m) in 4 distinct areas from the western Saharan bank (24°N) down to the Bijagos islands (11°N) and compare with the available fish landings data on this period. Finally, we calculate seasonal connectivity indices between the populations in the 4 areas selected as the percentage of fish present in an area that was born in another area

Insight from PREFACE & AWA on Tropical Atlantic Tuna ecology and effects on western African fisheries economies

International audienc

Modélisation des écosystèmes planctoniques pélagiques en Méditerranée nord-occidentale (impact des eaux du Rhône à l'échelle du plateau du golfe du Lion et variabilité interannuelle à décennale au large)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Spatial characterization of biogeochemical fluxes in the North West African upwelling [résumé]

International Conference AWA (ICAWA), Dakar, SEN, 09-/12/2014 - 11/12/201

Effect of climate variability on decadal changes in small pelagic fisheries in the West African upwelling Ecosystem : the case of Sardinella aurita in Senegal [résumé]

ICAWA : International Conference AWA, Dakar, SEN, 13-/12/2016 - 15/12/2016In Northwest African upwelling system, the populations of Sardinella aurita show evidence of important log-term natural fluctuations in their abundance, which have implications for medium and long-term forecasting of catches. These fluctuations seem to be related, among other factor, to large-scale climatic variability, raising important scientific and economics concerns. Understanding the processes affecting recruitment/abundance is a fundamental objective of fisheries biology. Thus, this study assesses the effect of climatic variability on the abundance of S. aurita in Northwest African upwelling system. Monthly data indicating the abundance of sardinella were first estimated from commercial statistics, using Generalized Linear Model techniques over the period 1966- 2011. Abundance indices were then compared with environmental indices, at the local scale, a Coastal Upwelling Index (CUI) and a coastal Sea Surface Temperature (SST) index, and on a large scale, the North Atlantic Oscillation (NAO), the Atlantic Multidecadal Oscillation (AMO) and the Multivariate El Niño Southern Oscillation Index (MEI), using times series analyses, linear models and generalized additive models. The results showed that the abundance of sardinella is determined by a strong seasonal pattern and inter-annual fluctuations. The abundance of S. aurita peaked in spring and in autumn. The trend of the sardinella abundance was significantly correlated with the CUI, especially in autumn and spring. Interannual fluctuations of S. aurita abundance are respectively driven by the precocity and the duration of the upwelling season that is attributed to distinct migration patterns. Sardinella species also respond with a delay of around 4 years to the winter NAO index and the autumn CUI, and the AMO index respectively, either related to migration patterns. The wide variations in sardinella biomass are caused by variations in environmental conditions, which should be considered in the implementation of an ecosystem-based approach in sardinella stocks management

Similarities and Contrasts in Time-Mean Striated Surface Tracers in Pacific Eastern Boundary Upwelling Systems: The Role of Ocean Currents in Their Generation

Eastern boundary upwelling systems feature strong zonal gradients of physical and biological properties between cool, productive coastal oceans and warm, oligotrophic subtropical gyres. Zonal currents and jets (striations) are therefore likely to contribute to the transport of water properties between coastal and open oceanic regions. For the first time, multi-sensor satellite data are used to characterize the time-mean signatures of striations in sea surface temperature (SST), salinity (SSS), and chlorophyll-a (Chl-a) in subtropical eastern North/South Pacific (ENP/ESP) upwelling systems. In the ENP, tracers exhibit striated patterns extending up to ~2500 km offshore. Striated signals in SST and SSS are highly correlated with quasi-zonal jets, suggesting that these jets contribute to SST/SSS mesoscale patterns via zonal advection. Striated Chl-a anomalies are collocated with sea surface height (SSH) bands, a possible result of mesoscale eddy trains trapping nutrients and forming striated signals. In the ESP, the signature of striations is only found in SST and coincides with the SSH bands, consistently with quasi-zonal jets located outside major zonal tracer gradients. An interplay between large-scale SST/SSS advection by the quasi-zonal jets, mesoscale SST/SSS advection by the large-scale meridional flow, and eddy advection may explain the persistent ENP hydrographic signature of striations. These results underline the importance of quasi-zonal jets for surface tracer structuring at the mesoscale

Effect of climate variability on decadal changes in small pelagic fisheries in the West African upwelling Ecosystem : the case of Sardinella aurita in Senegal [résumé]

ICAWA : International Conference AWA, Dakar, SEN, 13-/12/2016 - 15/12/2016In Northwest African upwelling system, the populations of Sardinella aurita show evidence of important log-term natural fluctuations in their abundance, which have implications for medium and long-term forecasting of catches. These fluctuations seem to be related, among other factor, to large-scale climatic variability, raising important scientific and economics concerns. Understanding the processes affecting recruitment/abundance is a fundamental objective of fisheries biology. Thus, this study assesses the effect of climatic variability on the abundance of S. aurita in Northwest African upwelling system. Monthly data indicating the abundance of sardinella were first estimated from commercial statistics, using Generalized Linear Model techniques over the period 1966- 2011. Abundance indices were then compared with environmental indices, at the local scale, a Coastal Upwelling Index (CUI) and a coastal Sea Surface Temperature (SST) index, and on a large scale, the North Atlantic Oscillation (NAO), the Atlantic Multidecadal Oscillation (AMO) and the Multivariate El Niño Southern Oscillation Index (MEI), using times series analyses, linear models and generalized additive models. The results showed that the abundance of sardinella is determined by a strong seasonal pattern and inter-annual fluctuations. The abundance of S. aurita peaked in spring and in autumn. The trend of the sardinella abundance was significantly correlated with the CUI, especially in autumn and spring. Interannual fluctuations of S. aurita abundance are respectively driven by the precocity and the duration of the upwelling season that is attributed to distinct migration patterns. Sardinella species also respond with a delay of around 4 years to the winter NAO index and the autumn CUI, and the AMO index respectively, either related to migration patterns. The wide variations in sardinella biomass are caused by variations in environmental conditions, which should be considered in the implementation of an ecosystem-based approach in sardinella stocks management