Hydrodynamic control on the interannual variability of algal bloom in the
Pearl River Estuary from 2021 to 2024: evidences from buoy data and hydrodynamic modeling

1、Introduction：

Ocean phytoplankton plays a vital role in global primary production and carbon cycling (Lu et al., 2025), and supports marine ecosystems and nutrient cycles, especially in coastal and estuarine waters (Breitburg et al., 2018). Understanding the drivers of phytoplankton dynamics is therefore key to coastal environmental management and sustainability. Phytoplankton dynamics are governed by both local factors and physical transport processes (Qin and Shen, 2017). Local controls include nutrient supply, temperature, light availability, and grazing pressure,

which directly regulate growth (Glibert, 2016). Physical processes, such as horizontal advection and vertical mixing, influence phytoplankton distribution and accumulation (Qin and Shen, 2019). Studies show that increased nutrient inputs and altered N/P ratios can intensify algal blooms in estuaries and coastal waters (Jia et al., 2025; Kumar et al., 2018; Oduor et al., 2023; Turner and Rabalais, 1994; Wåhlstrm et al., 2024; Zheng and Zhai, 2023; Zhou et al., 2008) and shift communities from diatoms to harmful species (Anderson et al., 2002; Glibert and Burford, 2017; Xiao et al., 2018). Improved light penetration due to reduced sediment loads (Gao and Wang, 2008) and rising sea

temperatures under climate change (Dai et al., 2023) have also been linked to more frequent blooms. However, compared with local drivers, the role of physical transport processes in shaping phytoplankton dy- namics remains less studied.
Phytoplankton dynamics are shaped by physical transport processes, including both horizontal and vertical mixing through advection and diffusion. Horizontal circulation driven by tides, winds, and river discharge regulates estuarine water exchange and redistributes algal biomass and nutrients (Paerl et al., 2014). Under extreme weather conditions such as typhoons or winter storms, this exchange intensifies significantly (Zhang et al., 2021). Weaken circulation decreases flushing between estuaries and coastal waters, often increases phytoplankton accumulation and bloom frequency (Andrade et al., 2022; Wan et al.,

2013). It is also important to note the role of oceanic fronts, narrow transition zones where distinct water masses meet, marked by sharp gradients in temperature and salinity (Cao et al., 2026). Fronts can aggregate nutrients and phytoplankton along boundaries through cross- front convergence, or conversely restrict their distribution (Cao et al., 2026; Lv et al., 2022; Woodson et al., 2012). Moreover, upwelling and downwelling circulations strongly influence water exchange in bays and estuaries. Upwelling transports cold, nutrient-rich water into the euphotic zone, boosting productivity and enhancing offshore exchange (Pitcher et al., 2010; Shute et al., 2022), whereas downwelling sup- presses exchange through onshore transport (Likumahua et al., 2026). Estuarine and coastal currents may also carry algae into regions conducive to harmful algal blooms (Kuang and Lee, 2005). Vertical

stratification confines phytoplankton to sunlit surface layers, favoring growth under high light (Masson and Pe 2009). In contrast, strong vertical mixing induced by tides or wind events generally inhibits algal bloom formation (Wong et al., 2007).
The Pearl River Estuary (PRE) is situated in the Guangdong-Hong Kong-Macau Greater Bay Area of southern China (Fig. 1). In the PRE, major local factors influencing phytoplankton growth and species composition include increased nutrient inputs, elevated N/P ratios, and reduced light attenuation from declining sediment loads (winter: 4.33; summer: 242.50 in 2020) (Ma et al., 2022; Ke et al., 2023). Despite substantial nutrient inputs, algal blooms in the PRE are less frequent and

smaller in scale than in many other eutrophic coastal systems. This is attributed to dynamic physical processes, such as river discharge, estu- arine circulation, wind-driven currents, and wind/tidal mixing, as well as potential phosphorus limitation, which collectively regulate bloom occurrence (Harrison et al., 2008). Under these coupled physical- biological controls, blooms typically occur in the middle estuary dur- ing the dry season (October to April) when residence time (RT) is long, and shift to the lower estuary or offshore areas in the wet season (May to September) when RT is shorter (Lu and Gan, 2015). Moreover, vertical mixing must remain below a certain threshold for blooms to develop in HK waters of the PRE (Guo et al., 2020; Wong et al., 2009). Although

monthly/bimonthly and cruise-based observations have revealed sea- sonal and annual bloom patterns and highlighted the role of local fac- tors, the hydrodynamic control on the episodic and interannual bloom dynamics remains poorly understood. This gap stems from a lack of long-term, high-frequency (e.g., daily-scale) hydrodynamic and envi- ronmental monitoring in the region.

To address this issue, long-term daily observations of six marine environmental buoys and a 3D hydrodynamic model was used to 1) investigate the interannual variability of algal blooms in the PRE during 2021–2024, (2) to identify the key physical processes in controlling this variability and (3) to determine the relative importance of RT versus vertical stability.