Michèle Loijens

Application of a transport-reaction model to the estimation of biogas fluxes in the Scheldt Estuary

AbstractIn the frame of the BIOGEST project, the fulltransient, one-dimensional, reactive-transportmodel CONTRASTE has been extended for thecomputation of biogases in the Scheldt estuary. The CONTRASTE model (Coupled, Networked, Transport-Reaction Algorithm for Strong T> idal Estuaries) provides a satisfactorydescription of the estuarine residualcirculation (including daily freshwaterdischarge and a complete description of thetide) and a flexible implementation of thevarious physico-chemical and biologicaltransformations, including bothkinetically-controlled and equilibriumreactions. The model allows resolution of thecomplex, nonlinear collective behaviour of thistype of system and investigation of thenon-steady-state phenomena which governestuarine dynamics. Variables currentlyimplemented in the model include salinity,suspended matter, oxygen, inorganic carbonspecies, degradable organic carbon andnitrogen, inorganic nitrogen species,freshwater and marine phytoplankton. Biologicalprocesses described are heterotrophicrespiration, primary production, nitrificationand denitrification. Equilibrium formulationsallow for DIC and NH4+/NH3speciation. Physical processes include gastransfer at the water/air interface, dependingon both wind speed and current velocity. pHprofiles are explicitly computed and constitutea very sensitive check of the overall modelconsistency. Results of the CONTRASTE model arein very good agreement with the measuredlongitudinal distribution of the variablesconsidered, in particular O2, pH,pCO2 and N2O concentrations. However,discrepancies are observed between thecalculated fluxes of CO2 and thoseestimated using an in situ floatingchamber. It is shown that the evaluation of gastransfer can be affected by serious errors ifthe variations due to changes in currentvelocity and water depth during one tidal cycleare not taken into consideration. The modelalso shows that the fluxes of biogases inestuaries are greatly influenced by thequasi-exponential increase of the exchangesurface area with decreasing distance to thesea. Our estimation of the total daily flux ofO2, CO2 and N2O is equal to+28500, −19000 and −17 kmoles.day−1respectively for the Scheldt estuary in July 1996.

The response of phytoplankton production to periodic upwelling and relaxation events at the Iberian shelf break: estimates by the 14C method and by satellite remote sensing

Abstract The primary productivity of the NW Iberian margin has been determined by a combination of 14 C incorporation experiments on natural phytoplankton assemblages and satellite remote sensing with the ocean colour sensor, SeaWiFS. The approach to the field determination of phytoplankton production was Eulerian, involving measurements on a fixed grid of stations covering the region from 41°30′N to 43°00′N and 09°00′W to 10°30′W. Three different laboratories made primary production measurements over a 3-year period. Each group used variations of the 14 C method—in situ incubations and two modelling approaches based on P – E parameter derivation. An intercalibration experiment showed acceptable agreement between the three estimates. Measurements by the three groups on eight cruises have been merged to provide estimates of regional productivity. However, the temporal coverage of the data was too limited to provide robust seasonal estimates since few experiments were done in the autumn and winter months. A second approach to the estimation of primary production utilised surface pigment concentrations derived from satellite remote sensing. Two models were used. Firstly, an empirical relationship was found between depth-integrated primary production and surface chlorophyll concentration, which explained 78% of the variance in the production estimates. The second approach used a semi-analytical model which incorporates irradiance and temperature as well as chlorophyll-derived parameters. Both models have been applied to SeaWiFS-derived, surface chlorophyll concentrations to estimate primary production during 1998, 1999 and 2000 for three regions—the shelf, the slope and the open ocean of the region of study. Using the semi-analytical model, the mean primary production for the 3 years is estimated to be 319 g C m −2 year −1 for the shelf, 280 g C m −2 year −1 for the slope and 217 g C m −2 year −1 for the adjacent open ocean. Interannual variation in production on the shelf ranged from 334 g C m −2 year −1 in 1998 to 301 g C m −2 year −1 in 2000. It is estimated that upwelling increased primary production, relative to that in the open ocean, by ∼50%. Satellite-derived estimates of the ratio of new production to primary production were 0.37, 0.28 and 0.40 for the shelf region during the upwelling seasons of 1998, 1999 and 2000.

Study of the Scavenging of Trace Metals in Marine Systems Using Radionuclides

The rate of scavenging of Mn, Co, Zn, and Cd by particles and the possible role of living organisms in the uptake of these metals has been investigated using water samples collected in various marine environments. The method consists of measuring the transfer of the corresponding radioactive trace metals from the dissolved to the particulate phase simultaneously with measurements of the uptake of labeled 14C bicarbonate, as classically performed for estimations of primary production.