Phytoplankton blooms near the Pearl River Estuary induced by Typhoon Nuri

Abstract

The authors investigate two phytoplankton blooms near or off the Pearl River Estuary (PRE) triggered by Category 2 Typhoon Nuri with moderate wind intensity in the South China Sea using both remotely sensed and in situ data sets. An offshore triangular phytoplankton bloom (chlorophyll a (Chl a) > 0.5 mg m−3) was observed near Dongsha Archipelago along Nuri's track, prolongating southeastward to 18°N 1 week after the typhoon's passage; a stronger nearshore phytoplankton increase (Chl a > 1.5 mg m−3) with high total suspended sediment appeared southwest of the PRE from the coast toward the South China Sea. After the typhoon's passage, sea surface cooling (∼3°C), strong wind (>20 m s−1), and heavy rainfall (>100 mm d−1) were also observed in the region, especially offshore. In addition, the freshwater discharge from the PRE during the typhoon passage increased fivefold in comparison with the August climatology in the nearshore bloom region. The nearshore bloom was probably due to the increased discharge from the PRE and favorable current, as well as mixing, upwelling, and near‐inertial resonance driven by strong typhoon wind. The offshore bloom may be triggered by upwelling and entrainment induced by strong typhoon wind and the topography of islands and islets. The present study suggests that one typhoon may nourish phytoplankton biomass by inducing transport of nutrient‐rich water from both the Pearl River Estuary to offshore and the sublayer to surface.