A model for the benthic-pelagic coupling of silica in estuarine ecosystems: sensitivity analysis and system scale simulation

A transient, vertically resolved, analytical model for the early diagenesis of silica has been developed to quan- tify the importance of benthic-pelagic coupling in estuarine biogeochemical silica cycling. A sensitivity analysis based on Monte-Carlo simulations is carried out to assess the in- tensity and timing of benthic diffusive fluxes in response to a pelagic diatom bloom. The diffusive flux dynamics are analyzed over a realistic range of dissolution rate constants (max kSi [6×10−3−3.6×10−1 d−1]), diffusion coefficients of dissolved silica (DSi [35×10−6−35×10−5m2 d−1]) and duration of dissolved silica depletion in the water column (wPDSI  [1–3 month]). Results show that the diffusive sil- ica flux responds with a time delay of 20 to 120 days to the biogenic silica deposition pulse. For high maxkSi, simu- lated time lags are shortest and completely determined by the dissolution kinetics. However, decreasing maxkSi leads to a slower benthic flux response. In addition, the vari- ability increases due to the increasing importance of trans- port processes. The sensitivity study also allows us to con- strain the uncertainties of a system-scale simulation, where a large number of benthic compartments (>50 000) are cou- pled to a high-resolution (100×100 m) pelagic model of a macrotidal river and estuary (Western Scheldt, B/NL). The model is applied to a diatom bloom event recorded in 2003, characterized by pelagic silica depletion in August. Ben-thic processes are mainly modulated by the combined in- fluence of local hydrodynamic conditions and pelagic pri- mary production dynamics, and show therefore a high de- gree of spatial heterogeneity over short distances. Spatially integrated deposition fluxes and dissolution rates of biogenic silica are high throughout the growth period, with maxima of 1.3×105 mol d−1 (=8.0mmolm2 d−1) and 7.8×104 mol d−1 (=4.8mmolm2 d−1) in mid-August. The spatially integrated diffusive flux reaches a maximum of 1.5×104 mool d−1 at the end of a pelagic silica depletion period in September. How- ever, the total amount of dissolved silica released from the es- tuarine sediments between June and December 2003 is small (2×106 mol) compared to the much higher riverine influx of dissolved silica (5.9×107 mol) and plays a minor role in the pelagic primary production dynamics.