Seasonal Dynamics of Diatom–Dinoflagellate Relative Dominance and Its Environmental Determinants in a Shallow Coastal Nature Reserve: Insights from the Qingdao Amphioxus Municipal Nature Reserve, Yellow Sea

The Qingdao Amphioxus Municipal Nature Reserve (QAMR), located on the coast of the Yellow Sea, is an important habitat for Qingdao amphioxus (Branchiostoma belcheri tsingtauense), which is listed as endangered on the China Species Red List. Phytoplankton community shifts reflect environmental conditions and may influence food resources for benthic filter-feeders. This study examined the relative dominance patterns of diatoms versus dinoflagellates and their associations with environmental factors using data from four seasonal cruises in 2018. Diatoms dominated throughout the year, accounting for 86.8–98.7% of total abundance and 82.4–88.6% of taxon richness. Both metrics were lowest in summer and highest in autumn. The relative dominance shifts were associated with the combined effects of water-column physical structure and nutrient stoichiometry rather than nutrient concentration alone. Summer stratification may have favored dinoflagellates, whereas autumn vertical mixing and nutrient resupply reinforced diatom dominance. The QAMR remained diatom-dominated, with no clear shift toward dinoflagellate dominance. These findings provide baseline data for understanding seasonal variations in nearshore phytoplankton communities and may inform ecological status assessments and B. belcheri tsingtauense habitat management.