The overall objective of the proposed research is to increase our understanding of the biological Si processing in tropical river systems. We will investigate bio-Si cycling in large tropical wetlands. We will conduct studies in two tropical systems, which can be considered hot-spots for biological Si cycling: the Okavango Delta (Botswana) and the Fly River (Papua New Guinea). Research fits in the growing recognition that biota control the global silica cycle, which has tight connections to ocean and terrestrial carbon sinks.