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

Abstract

Abstract. A transient, vertically resolved, analytical model for the early diagenesis of silica has been developed to quantify 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 intensity 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 ε [6×10−3–3.6×10−1 d−1]), diffusion coefficients of dissolved silica (DSi ε [35×10−6–35×10−5 m2 d−1]) and duration of dissolved silica depletion in the water column (wPDSI ε [1–3 month]). Results show that the diffusive silica flux responds with a time delay of 20 to 120 days to the biogenic silica deposition pulse. For high max kSi, simulated time lags are shortest and completely determined by the dissolution kinetics. However, decreasing max kSi leads to a slower benthic flux response. In addition, the variability increases due to the increasing importance of transport processes. The sensitivity study also allows us to constrain the uncertainties of a system-scale simulation, where a large number of benthic compartments (>50 000) are coupled 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. Benthic processes are mainly modulated by the combined influence of local hydrodynamic conditions and pelagic primary production dynamics, and show therefore a high degree 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 (=8.0 mmol m2 d−1) and 7.8×104 mol d−1 (=4.8 mmol m2 d−1) in mid-August. The spatially integrated diffusive flux reaches a maximum of 1.5×104 mol d−1 at the end of a pelagic silica depletion period in September. However, the total amount of dissolved silica released from the estuarine 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.