Natural and human influences on dissolved silica export from watershed to coast in Southeast China

Abstract

Natural and human factors controlling dissolved silica (DSi) export from the watershed were investigated for the Jiulong River watershed in Southeast China, which is a subtropical region where tectonic activity and high runoff combine to create a global hot spot for DSi flux. Results showed that river DSi was fairly high (246 ± 76 µmol L−1) due to granite lithology and increased human perturbation. The percentage of granite, paddy soil, arable land, and forest land each explain 47–69% of the spatial variation in DSi level at the subwatershed scale. Land use change (deforestation for agriculture) and soil erosion increased DSi transfer from terrestrial systems towards the river. Discharge explains over 95% of the variation in DSi fluxes, and a discrepancy of up to 16% of watershed DSi export was observed between low and high flow conditions. DSi retention is more significant in the dry season, likely due to biological uptake by diatoms in reservoirs under low flow conditions and higher residence time. However, this may be offset by high runoff flush in the wet season, which might cause unfavorable conditions for diatom growth and DSi depletion. There has been little overall change in DSi in estuarine water in recent decades. However, seasonal variation and occasionally fluctuation of DSi during extreme hydrological events (rainstorms) does exist, with important implications for the coastal ecosystem. This research confirmed that human activities and land use change have substantially altered Si delivery, cycling and riverine export to the coast in this region.