Controls of High Silicic Acid in a Semi-enclosed Coastal Sea: Inferences from a Coupled Physical-Biological Ocean Model and Data

The Strait of Georgia is a fjord-like semi-enclosed coastal sea on the west coast of Canada. It exhibits an estuarine circulation driven by input from the Fraser River as well as many smaller rivers and streams. It has high levels of dissolved silica (can be >50 μmM even at the surface) and is the site of heavily silicified glass sponge reefs as well as frequent diatom and occasional silicoflagellate blooms. We have developed a coupled physical-biological model representing plankton and nutrient dynamics based on nitrogen uptake and remineralization with a coupled silicon cycle. Silicon in the model may be in the form of dissolved silica, living diatoms, or particulate biogenic silica, and model diatom growth may be limited by nitrogen, light, or dissolved silica availability. We will discuss the challenges involved in accurately representing observed features of the silicon cycle. These include uncertainties in the magnitude of terrestrial inputs and sediment fluxes due to limited observations. We will focus in particular on the relative importance of boundary conditions compared to riverine inputs, as well as impacts of the physical circulation, including deep water renewal events, on the silicon budget in the deepest waters of the Strait. Finally, we will discuss implications of model-data disagreement in terms of the importance of processes not represented, and we will describe how our model can inform understanding of silicon cycling in the Salish Sea.