Global patterns of dissolved silica export to the coastal zone: Results from a spatially explicit global model

Importance of silicon

Silicon (Si) clearly shows the link between rock and life. Silicon dioxide (SiO₂) or silica is the most abundant component of the earth’s crust. It occurs as silicate minerals in igneous, metamorphic and sedimentary rocks. These minerals undergo physical and chemical weathering, which is the major natural source of dissolved silica (dissolved SiO₂, hereafter referred to as DSi) in aquatic ecosystems. On its way through soils, aquifers, and riparian zones Si exerts control over the cycling and fate of carbon (C), nitrogen (N), phosphorus (P) and other nutrients.

While riverine N loads have increased during the past decades and similar changes have occurred for P, Si loads have remained constant or even decreased in many rivers primarily as a result of Si retention in reservoirs and lakes through eutrophication and increased diatom productivity. When diatom growth is compromised by Si limitation, non-diatoms may be competitively favored, with dominance of flagellated algae including noxious bloom-forming communities. Thus the biogeochemical cycling of C, N and P and food web dynamics leading to fisheries harvests are affected by shifts in the availability of Si.

Model for river export of dissolved SiO₂

We present a multiple linear regression model developed for describing global river export of dissolved SiO₂ (DSi) to coastal zones. This model was developed as part of an international interdisciplinary effort to model river export of multiple bioactive elements (C, N, P, and Si) and elemental forms (dissolved/particulate, inorganic/organic) called Global Nutrient Export from Watersheds (Global NEWS). Global NEWS and is a project under LOICZ. We hereafter refer to our model as “NEWS-DSi”.

The NEWS-DSi model, with river-basin spatial scale and an annual temporal scale, is based on four variables with a significant influence on DSi yields (soil bulk density, precipitation, slope and area with volcanic lithology), for the pre-dam situation. Cross validation showed that the model is robust with respect to the selected model variables and coefficients. The calculated global river export of DSi is 380 Tg yr^-1 (340-427 Tg yr^-1). Most of the DSi is exported by global rivers to the coastal zone of the Atlantic Ocean (41%), Pacific Ocean (36%) and Indian Ocean (14%). South America and Asia are the largest contributors (25% and 23%, respectively). DSi retention in reservoirs in global river basins may amount to 18 – 19 %.

Uncertainties

Inevitably there is considerable uncertainty associated with our predictions. Nonetheless, as the first attempt to develop a robust, internally consistent, and spatially-explicit global model of DSi river export, NEWS-DSi constitutes a significant advance in its own right. With the NEWS-DSi model now available, it is possible to analyze the exported ratios of N, P and Si in all different forms, also including information on the effect of dam construction.

Data

The data used to develop the model can be downloaded (ZIP, 5,6KB)