Land use shapes riverine nutrient and sediment concentrations on Moorea, French Polynesia

Human activities drive changes in freshwater ecosystems by altering biogeochemical cycles. On high volcanic tropical islands, human activities can be compartmentalized by steep terrain that delineates watershed boundaries. Patterns of human activities, such as land use, affect adjacent stream ecosystems through runoff of sediment and nutrients, which varies seasonally in the tropics as a result of seasonal rainfall. Here, we sought to reveal human impacts on the nutrient and sediment regimes of tropical rivers by tracking patterns of river chemistry across a series of watersheds on Moorea, French Polynesia, between 2018 and 2019. Repeated sampling of rivers across a gradient of human activities revealed that water chemistry varied seasonally and with respect to rainfall and land use. In particular, dissolved inorganic nitrogen was more concentrated in rivers of watersheds with higher rates of land clearing. Additionally, total suspended solids and phosphate were higher when recent rainfall was high. Our results show that human activities can have a substantial impact on the amounts of nutrients and sediment that tropical rivers transport, which on tropical islands could facilitate movement of materials from land to sea as precipitation increases with intensifying climate change.