Seasonal differences in the phytoplankton community between the inshore and offshore areas in Northern Beibu Gulf, South China Sea

Seasonal water mass dynamics and terrigenous inputs (aquaculture, industrialization, sewage discharge, river inputs, etc.) create environmental gradients between inshore and offshore the Beibu Gulf. This study analyzed summer-autumn phytoplankton community structure, revealing higher inshore nutrients (NO₃⁻, NO₂⁻, NH₄⁺, PO₄³⁻ and SiO₃²⁻) and phosphorus limitation in northern inshore waters. Inshore phytoplankton assemblages showed higher abundance but lower diversity than those in the offshore areas. Inshore riverine silicate inputs skewed Si/N/P ratios above Redfield values, causing nutrient imbalance that promoted the growth of red tide species (e.g., <i>Skeletonema costatum</i>). Seasonal water masses transported coastal nutrients offshore, reducing the offshore phytoplankton diversity and fostering late-autumn harmful algal blooms (HABs) by <i>Phaeocystis globosa</i> and <i>Thalassiosira subtilis</i>. These HAB species exhibit small cell size, high specific surface area and enhanced material exchange capacity, enabling ecological dominance under nutrient-enriched conditions.