A two-dimensional (vertical) steady-state numerical model that simulates water circulation and dissolved-silica distributions is applied to northern San Francisco Bay. The model (1) describes the strong influence of river inflow on estuarine circulation and, in turn, on the biologically modulated silica concentration, and (2) shows how rates of silica uptake relate to silica supply and mixing rates in modifying a conservative behavior. Longitudinal silica distributions influenced by biological uptake (assuming both vertically uniform and vertically decreasing uptake situations) show that uptake rates of 1 to 10 μg-at. l−1 day−1 are sufficient to depress silica concentrations at river inflows of 100–400 m3 s−1, respectively, and that the higher rates appear ineffective at inflows above 400 m3 s−1. The simulations further indicate that higher silica utilization in the null zone is not essential to depress silica concentrations strongly there. Advective water-replacement times at river inflows of 400, 200 and 100 m3 s−1 are computed to be less than 25, 45 and 75 days, respectively, for a 120-km estuary-river system.
