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Ecosystem model (MODECOGeL) of the Ligurian Sea revisited. Seasonal and interannual variability due to atmospheric forcing.
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A one-dimensional coupled hydrodynamical–biological model, MODèle d'ECOsystème du G.H.E.R. et du L.O.V. (MODECOGeL), of the water column is developed and applied to the Ligurian Sea (North Western Mediterranean). It is an extended and improved version of the model presented by Lacroix and Nival [J. Mar. Syst. 16 (1998) 23]. The hydrodynamic model is a 1D version of the 3D turbulent closure G.H.E.R. model, which takes into account momentum and heat surface fluxes computed from a real meteorological data set. The ecosystem model is defined by a nitrogen cycle described by 12 biological state variables including several plankton size classes and an explicit description of the bacterial loop. One data set coming from the FRONTAL missions is used to initialise and validate the model. To assess the impact of the interannual variability of the meteorological conditions on the ecosystem dynamics, the coupled model is run with 4-year real meteorological conditions (October 1984–September 1988). The model estimated percentages of the interannual variability of the annual mean biomass of phytoplankton, zooplankton and bacteria respectively of 31.0%, 16.2% and 16.3%. The contribution of the zooplankton related to the total plankton biomass (phytoplankton, zooplankton and bacteria) has been found to be the most sensitive to the meteorological conditions variations (21%), followed by the phytoplankton (12%) and finally, by the bacteria (5%). The model estimated percentages of interannual variability of the annual gross primary production, the annual mean f-ratio and the annual bacterial production respectively of 27.9%, 18.5% and 13.4% although the interannual variability of the real winds conditions is only of 11.3%. Due to the more windy and less sunny conditions prevailing during the years “1985–1986” and “1986–1987”, the annual primary production was found higher than during the years “1984–1985” and “1987–1988”. The bacterial production is always greater than the primary production, showing the importance of the bacteria in such an oligotrophic environment. On a seasonal scale, the highest interannual variability of the primary production and the f-ratio is found in spring like for the wind intensity.
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Atmospheric CO2 flux from mangrove surrounding waters.
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The partial pressure of CO 2 (pCO2) was measured at daily and weekly time scales in the waters surrounding mangrove forests in Papua New Guinea, the Bahamas and India. The pCO2 values range from 380 to 4800 æatm. These data, together with previously published data, suggest that overall oversaturation of CO2 with respect to atmospheric equilibrium in surface waters is a general feature of mangrove forests, though the entire ecosystems (sediment, water and vegetation) are probably sinks for atmospheric CO2. The computed CO2 fluxes converge to about +50 mmolC m -2 day-1. If this conservative value is extrapolated for worldwide mangrove ecosystems, the global emission of CO2 to the atmosphere is about 50 106 tC year-1. Based on this tentative estimate, mangrove waters appear to be regionally a significant source of CO2 to the atmosphere and should be more thoroughly investigated, especially at seasonal time scale.
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Exchange processes and nitrogen cycling on the shelf and continental slope of the Black Sea basin.
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A 3D coupled biogeochemical-hydrodynamical model has been applied to the Black Sea to simulate nitrogen cycling and to estimate the exchange of biogeochemical components at the shelf break and between the continental slope and the deep sea. It was found that biological processes on the northwestern shelf are in approximate balance. Primary production is fueled by river discharge, nitrate input from the open sea at the shelf break, and in situ remineralization. The input of nitrate from the open sea is roughly equivalent to the river nitrate discharge but is half the nitrate export from the shelf toward the open sea. Also, the Black Sea shelf acts throughout the year as a nitrate source for the open sea. The amount of shelf production not remineralized in the euphotic layer is 22.2% and is exported to lower layers (20%) or offshore (2.2%). We estimate that the export of carbon from the shelf to the interior of the basin represents 2.5% of the new production of the open sea. The upper slope adjoining the northwestern shelf is the site of downwelling events responsible for the downward transport to the intermediate layer of the continental slope of biogeochemical components exported from the shelf in the upper layer. The shelf has been found to be an efficient trap for the refractory material discharged by the Danube.
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The age as a diagnostic of the dynamics of marine ecosystem models.
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The constituent-oriented age theory (CAT) worked out by Delhez et al. (1999) is a flexible tool that can be applied to diagnose complex models. It is shown here how this can be used to quantify the pace at which an ecosystem model works. At the cost of the introduction of one additional evolution equation for each compartment of the ecosystem model, the mean age of the biological material forming these compartments can be computed. The information obtained in this way complements the information provided by the concentration data; while the latter measures the standing stocks, the former provides an integrated assessment of the interaction rates and matter fluxes. The benefits of the method are demonstrated with a simple Lotka–Volterra system and a one-dimensional vertical model of the nitrogen cycle in the Ligurian Sea. The theory can be used to study the biological compartments individually or the ecosystem as a whole. In particular, the age is a valuable tool to quantify the overall cycling rate of nitrogen in the food web.
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Remotely sensed seasonal dynamics of phytoplankton in the Ligurian Sea in 1997-1999.
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Remotely sensed data and a one-dimensional hydrophysical model were used to study the seasonal dynamics of surface plant pigments concentration in the Ligurian-Provençal basin. The variations of phytoplankton biomass were estimated from the observations of the Coastal Zone Color Scanner (1978–1986) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) (September 1997 to October 1999) radiometers. The factors of physical environment analyzed included remotely sensed sea surface temperature (from advanced very high resolution radiometers), wind, air temperature, and atmospheric precipitation. The Geohydrodynamics and Environment Research (GHER) model was used to explain the observed correlations between the physical forcing and the response of phytoplankton biomass. The general pattern of phytoplankton seasonal dynamics was typical to subtropical areas: maximum biomass during cold season from October to April and low biomass during summer months. The intensity of winter/spring bloom significantly varied during different years. The correlation was revealed between the summer/autumn air temperature contrast (expressed as the difference between the air temperatures in August and in November) and the maximum monthly averaged surface chlorophyll concentration during the subsequent winter/spring bloom. The features of seasonal dynamics of phytoplankton are regulated by the physical impacts influencing water stratification. The difference between two seasonal cycles (from September 1997 to October 1999) illustrates the response of phytoplankton growth to local meteorological conditions. In March–April 1999 the vernal bloom was much more pronounced; it resulted from deeper winter cooling and more intensive winter convection. Heating of surface water layer, wind mixing, and freshwater load with rains and river discharge either stimulate or depress the development of phytoplankton, depending on what limiting environmental factor (light or nutrient limitation) prevailed.
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Changements environnementaux postglaciaires et action de l'homme dans le bassin du Buech et en Champsaur (Hautes Alpes, France). Premier bilan d'une étude pluridisciplinaire
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Archéologie et paléoenvironnement dans les Alpes méridionales françaises - hauts massifs de l’Argentiérois, du Champsaur et de l’Ubaye (Hautes-Alpes et Alpes-de-Hautes-Provence) : Néolithique Final - début de l’Antiquité
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Structures pastorales d’altitude et paléoenvironnement. Alpes méridionales françaises du Néolithique final à l’âge du Bronze
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Bilan des études environnementales à Ribemont
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L’environnement végétal et climatique
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