All Si concentrations are reported as the mass of elemental Si and not the mass of SiO2. All water samples (from river water, porewater, and the hippo pool) were analyzed for dissolved Si concentration on an Inductive Coupled Plasma Atomic Emission Spectrophotometer (ICP-AES, Thermo Fisher Scientific, ICAP 6000 Series).

Grass, feces, and suspended matter sampled on filters were analyzed for biogenic Si concentration in a classical manner. A sample of 25 mg of dry matter (for grass and feces) or the entire filter (for suspended matter) was incubated in 25 ml of 0.5 M NaOH at 80°C for 5 hours. The extracted and dissolved Si was analyzed colorimetrically on a segmented flow analyzer (San++; Skalar, Breda, The Netherlands), after it had passed a dialyzer module separating the analyte from interfering substances (e.g., the yellow color from the nitrocellulose filters). The extraction in 0.1 M NaOH at 80°C has been well established and tested; it is capable of fully dissolving the biogenic Si from plant phytoliths at the solid/solution ratios and extraction time we applied (41).

Sediment, soil, and suspended matter sampled by settlement are usually a mixture of biogenic Si and mineral Si, and a classic analysis will not distinguish between different reactive fractions. Therefore, the samples were extracted using a novel alkaline continuous extraction in NaOH, according to the method from (42). Si and Al concentrations are measured continuously, and the two dissolution curves are fitted together in a first-order equation (Eqs. 1 and 2)Sit=(i=1nAlkExSii × (1eki×t))+b × t(1)Alt=(i=1nAlkExSiiSi/Ali × (1eki×t))+b × tSi/Almin(2)

This method distinguishes between fractions dissolving linearly (poor reactive silica) and those exhibiting a nonlinear behavior (highly reactive silica). Linear fractions are characterized by parameters b (slope of the linear dissolution) and Si/Al (ratio of Si and Al in that fraction); nonlinear fractions are characterized by the amount of AlkExSi (alkaline extractable silica; in mg g−1 of sample), k [reactivity of the fraction in NaOH (in mg g−1 min−1)], and Si/Al (ratio of Si and Al in that fraction). In contrast to the more traditional time course extraction method used for soils (extractions at hours 3, 4, and 5 in a weak base), which also distinguishes between biogenic and mineral silicates based on regression slopes, this new method also allows distinguishing between different reactive silica fractions (with k values > 0.1) according to their origin based on the Si/Al ratio determined by the model: true biogenic Si (Si/Al ratio of ≥4), Si belonging to clay minerals (Si/Al ratio between 1 and 4), and Si absorbed by oxides (Si/Al ratio of ≤1). More details about the continuous alkaline extraction of Si and Al method can be found in (42). We present data of the true biogenic Si (noted as AlkExSi_biogenic Si) and the sum of the other reactive fractions (noted as AlkExSi_non_biogenic Si) in table S2. Nonreactive fractions are not shown.

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