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

Human activities drive changes in freshwater ecosystems by altering biogeochemical cycles. Freshwater networks provide important ecosystem services to human societies by purifying water and serving as an intermediary between terrestrial and marine systems. On high volcanic tropical islands, human activities are compartmentalized by steep terrain that delineates watershed boundaries. Patterns of land use affect adjacent stream ecosystems through runoff of sediment and nutrients, which fluctuates in the tropics as a result of seasonal rainfall. Here, we sought to reveal human impacts on 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 amounts of land clearing during the rainy season, and total suspended solids and phosphate were higher when recent rainfall was high. Importantly, the water quality of the rivers on Moorea repeatedly exceeded safe water quality standards established for similar high tropical islands in the Pacific. Our results show that differential land use across the landscape 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.