Dynamics of harmful algal bloom species are independent of the extent of the Changjiang Diluted Water Intrusion Off Western Jeju Island in the Northern East China Sea

Abstract

The western Jeju Island region, located on the continental shelf of the northern East China Sea, is influenced by Changjiang Diluted Water (CDW), Yellow Sea Bottom Cold Water, and branches of Kuroshio Current. Due to its proximity to the South Sea of Korea, an area historically subject to annual harmful algal blooms (HABs) but experiencing a recent decline in bloom frequency, we investigated variations in the HAB species occurrence during the CDW intrusion in June 2022 and 2023, when the extent of CDW intrusion drastically differed. Additionally, current flows from the study region toward the South Sea were investigated using a data-assimilative ocean circulation model that integrated multiple satellite and in situ observations. Microscopy and 18S rRNA sequencing analyses and photosynthetic efficiency measurements using Phyto-PAM II revealed that more CDW intrusion increased nutrient stress for diatoms and dinoflagellates, mainly due to extreme phosphorus limitation. However, HAB species such as Pseudo-nitzschia spp. rapidly responded to increased nitrogen availability. In contrast, reduced CDW intrusion mitigated nutrient stress caused by imbalanced stoichiometry, and small-sized HAB species, including Aureococcus anophagefferens, Karlodinium veneficum, Prorocentrum obtusidens dominated under conditions of the DOP availability and light-limitation. Furthermore, ocean current simulations showed strong flows toward the southern coastal waters of Korea during summer. Collectively, our findings suggest that, regardless of the extent of CDW inflow, different types of HAB species can be developed in western Jeju, and ocean currents potentially transport them to the South Sea of Korea.