We report a large data-set of 295 interfacial CO 2 flux measurements obtained in the Scheldt estuary (November 2002 and April 2003), using the floating chamber method. From concomitant measurements of the air-water CO 2 gradient, we computed the gas transfer velocity of CO 2 . The gas transfer velocity is well correlated to wind speed and a simple linear regression function gives the most consistent fit to the data. Based on water current measurements, we estimated the contribution of water current induced turbulence to the gas transfer velocity, using the conceptual relationship of O' Connor and Dobbins (1958). This allowed us to construct an empirical relationship to compute the gas transfer velocity of CO 2 , that accounts for the contribution of wind and water current. Based on this relationship, the spatial and temporal variability of the gas transfer velocity in the Scheldt estuary was investigated. Water currents contribute significantly to the gas transfer velocity in the Scheldt, but the spatial and temporal variability (from daily to seasonal scales) is mainly related to wind speed variability. REFERENCESO'Connor D.

[1] A two-dimensional, nested grid, hydrodynamic, and reactive-transport model of the macrotidal Scheldt estuary (B/NL) and its tributaries has been developed to identify the driving forces controlling the temporal and spatial dynamics of primary production during a summer diatom bloom. The hydrodynamic model indicates that energy dissipation reaches its maximum 90 km upstream from the mouth, closely followed by a minimum farther upstream. Suspended particulate matter (SPM) dynamics is simulated to provide the transient light conditions in the water column. Results show that the spatial distribution of SPM mirrors closely the profile of energy dissipation. The temporal SPM dynamics is highly sensitive to fluctuations in river discharge, whose influence decreases downstream. Peaks in SPM are triggered by high discharges and can be recorded as far as 50 km seaward of the upstream model boundary. Results from the phytoplankton model demonstrate the fast response of diatom growth to changes in the physical environment, especially those due to daily variations in river discharge which continuously modify the SPM concentrations and residence times. Episodes of persistent low flow conditions lead to a progressive depletion of dissolved silica. Simulated diatom growth becomes increasingly controlled by silica availability, until primary production collapses. The spatiotemporal evolution of primary production is explored over the entire domain of forcing conditions. The distribution of the daily maximum of net primary production and its location reveal that four different system states can be identified in the forcing planes. The transition from one state to the other characterizes the diatom growth response in the estuary.Citation: Arndt, S., J.-P. Vanderborght, and P. Regnier (2007), Diatom growth response to physical forcing in a macrotidal estuary: Coupling hydrodynamics, sediment transport, and biogeochemistry,

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A fully coupled, two-dimensional hydrodynamic and reactive-transport model of C, N, O 2 and Si along a river-estuarinecoastal zone system is presented. It is applied to the Scheldt continuum, a macrotidal environment strongly affected by anthropogenic perturbations. The model extends from the upper tidal river and its tributaries to the southern Bight of the North Sea. Five dynamically linked nested grids are used, with a spatial resolution progressively increasing from 33 m to 2.7 km. The biogeochemical reaction network consists of aerobic degradation, nitrification, denitrification, phytoplankton growth and mortality, as well as reaeration. Diagnostic simulations of a typical summer situation in the early 1990s are compared to field data taken from the OMES database (>300 samples per variable). Results demonstrate that the process rates in the tidal river are very high and far larger than in the saline estuary, with maximum nitrification rates in the water column up to 70 mM N day − 1 , and maximum aerobic respiration and denitrification up to 70 and 40 mM C day − 1 , respectively. Phytoplankton production is about one order of magnitude lower, a result which confirms the dominance of heterotrophic processes in this system. The influence of secondary and tertiary wastewater treatment in the catchment is then assessed. Results show a significant decrease of organic matter and ammonium concentrations above Antwerp, which in turn leads to a partial restoration of oxygen levels. The model also predicts a reduction of denitrification rates, which locally results in a 4-fold increase of the nitrate concentration. Mass budgets for carbon, nitrogen and oxygen are established for the saline estuary (km 0 to 100) and for the tidal river network (km 100 to 160). Three scenarios, corresponding to the situation in the early 1990s, the years 2000 and the situation expected in 2010 are considered. They show that the tidal river and the estuary contribute almost equally to the overall biogeochemical cycling of these elements, despite the very different water volumes involved. For the simulated periods, the large decrease in nitrogen input (> 55%) expected between 1990 and 2010 will not lead to a significant decrease of N export to the coastal zone during the summer period.

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