Particulate organic matter as causative factor to eutrophication of subtropical deep freshwater: Role of typhoon (tropical cyclone) in the nutrient cycling

•Typhoons caused the steep and prolonged increases of the nutrients in bottom waters.•Typhoons drove strong and prolonged increases of pico-POM (i.e., 0.2–3 μm).•Pico-POM (i.e., bacteria) is key indicator of nutrient cycling in deep waters.•Typhoons boost nitrogen and phosphorus cycling and accumulation in deep waters.•Overall, typhoons can increase the risk of eutrophication in deep freshwater. Intense storms pose a serious threat to ecosystem functioning and services. However, the effects of typhoons (tropical cyclones) on the biogeochemical processes mediating risk of eutrophication in deep freshwater ecosystems remain unclear. Here, we conducted a three-year study to elucidate linkages between environmental change, stable isotopes and the stoichiometry of particulate organic matter (POM), and nutrient cycling (i.e., carbon, nitrogen and phosphorus) in a subtropical deep reservoir subjected to typhoon events. The typhoons significantly changed the nutrient levels in the deep waters as well as the thermocline position. Increased typhoon-driven organic matter input, algae sinking and heterotrophic decomposition interacted with each other to cause steep and prolonged increases of total nitrogen, ammonium nitrogen and total phosphorus in the bottom waters of the reservoir. Small-sized or pico-sized POM (i.e., 0.2–3 μm) showed a substantial increase in bottom waters, and it exhibited stronger response than large-sized POM (i.e., 3–20, 20–64, 64–200 μm) to the typhoons. Our results also indicated that typhoons boost the nutrient cycling in deep waters mainly through pico-sized POM. [Display omitted]