Wildfire Particulates Enhance Phytoplankton Growth and Alter Communities in the South China Sea Under Wind-Driven Upwelling

Abstract

Extreme wildfire events and cyclones are on the rise across tropical regions in response to climate change. Despite assumptions about their impact on phytoplankton through nutrient supplies, field evidence is lacking, and their combined effects remain unclear. In an on-site microcosm experiment conducted in the Xisha Islands, South China Sea (SCS) after Typhoon Noru, we observed enhanced phytoplankton growth in response to exposure to total suspended particulates (TSP) from wildfires (2 mg/L and 6 mg/L) under wind-driven upwelling conditions. Upwelled nutrients had a limited effect on Chl-a concentration due to phosphate depletion, by contrast, wildfire TSP contributed nutrients enriched in nitrogen and phosphate, resulting in a 3.30–5.61-fold increase in Chl-a. However, upwelled nutrients increased the diatom-to-dinoflagellate ratio from the initial 11.0 to 12.7, TSP at low and high levels reduced the ratio to 0.3–0.8 and significantly altered the communities, with 61.8% of species, including two dominant diatoms, negatively correlated with N and/or P supplies. Species diversity declined significantly at high TSP levels. These findings suggest that enhanced primary productivity by wildfires may come at the cost of an altered phytoplankton community. This field study improves understanding of the effects of simultaneous occurrences of multiple extreme climate events on marine ecosystems.