Vertical distribution of nitrate concentration in interstitial water of marine sediments with nitrification and denitrification

Vertical concentration profiles of nitrate in interstitial water of sediment in the Sluice Dock at Ostend (Belgium) commonly show a maximum in nitrate concentration at a few centimeters depth where sediments are sandy and poor in organic matter, while in muddy and organic-rich sediments, nitrate is lower in interstitial water than in the overlying water and decreases rapidly with depth. Direct measurements of the activity of autotrophic nitrifying bacteria in the sediments show nitrification in the upper few centimeters of sandy sediments but not in muddy sediments. A mathematical model is proposed to analyze quantitatively these experimental results, taking into account nitrification, denitrification, diffusion, and sedimentation. Seasonal variations of nitrate concentration in overlying water are slow enough to justify the use of a stationary (steady state) model. When appropriate values are used for the parameters (rate of nitrification, depth of the sedimentary layer in which nitrification occurs, rate of denitrification, diffusion coefficient), some being experimentally determined, the model predicts concentration profiles in good agreement with experimental data

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

In 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

Planktonic primary production in estuaries: comparison of ¹⁴C, O₂ and ¹⁸O methods

Rates of primary production were measured in 2 estuaries (Randers Fjord, Denmark, and the Scheldt estuary, Belgium/The Netherlands) using 3 different incubation methods: (1) the oxygen light-dark method (O2-LD), (2) 14C incorporation and (3) 18O labeling. Estimates based on the 14C incorporation technique were not significantly different from those obtained using the O2-LD technique. The 18O approach provided rates significantly lower than the 2 other techniques. Ratios of O2-LD to 18O-based rates (range: 0.99 to 3.54) were often statistically significantly higher than 1 and increased with decreasing salinities and/or lower oxygen concentrations. The underestimation of gross primary production by the 18O method may be due to an intracellular recycling of labeled oxygen which increased in magnitude with decreasing external oxygen conditions. These results suggest that the 18O method must be used with extreme care in nutrient-rich, low oxygen systems

Modelling photosynthesis-irradiance curves: effects of temperature, dissolved silica depletion and changing community assemblage on community photosynthesis

Sets of photosynthesis-irradiance (P-I) curves yield more information about community photosynthesis when analyzed with proper models in mind. Based on ecosystem-specific considerations regarding the factors that explain spatial and temporal patterns of photosynthesis, theWebb model of photosynthesis can be extended and fitted to P-I data. We propose a method based on a series of nested models of increasing complexity to test whether supposed effects of environmental factors are reflected in the P-I data, whether more complex models fit the data significantly better than more simple models, and whether parameters describing the presumed dependencies can be estimated from the data set. We compare a direct approach, fitting the extended model to all P-I data at once, with a two-step approach in which photosynthetic efficiencies and maximum photosynthetic rates of individual P-I curves are determined first, and then related to environmental variables. A nested model approach prevents overfitting of multiparameter models. Monte Carlo analysis sheds light on the error structure of the model, by separating parameter and model uncertainty, and provides an assessment of the performance of the formulations used in ecosystem models. We demonstrate that the two-step approach underperforms when used to compute photosynthetic rates. We apply the proposed method to an extensive P-I data set from the Schelde estuary, where spatiotemporal patterns of photosynthesis arise from a combination of seasonality, silica depletion, phytoplankton community composition, and salinity effects

Modeling complex multi-component reactive-transport systems: Towards a simulation environment based on the concept of a Knowledge Base

A modelling framework within which transport processes in the hydrosphere can be described and interfaced with relevant biogeochemical reactions is presented. Three key elements of this simulation environment are discussed: (1) a numerical engine for solving sets of coupled non-linear process equations; (2) an automated procedure for model code generation ('Automatic Code Generator'); (3) a Web-distributed Knowledge Base (KB) of processes. The Automatic Code Generator translates the information selected in the KB into computer algorithms using the principles defined in the numerical engine. The code CONTRASTE is a first attempt at developing such a modelling framework. It allows one to easily select, adapt and combine a specific set of biogeochemical processes relevant to a user-defined application. The workings of CONTRASTE are described by means of examples which demonstrate how the various components of the simulation environment are coupled and automated. Prospects for future developments towards a fully automated model generation procedure are discussed. © 2002 Elsevier Science Inc. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe