A spatiotemporally explicit framework for deriving HAB-associated pre-bloom nutrient benchmarks in the Bohai Sea.

Characterizing nutrient conditions that commonly precede harmful algal blooms (HABs) remains challenging in eutrophic coastal seas because nutrient observations are rarely aligned with the bloom-related states they are intended to represent. Here, we developed a spatiotemporally matched framework to derive HAB-associated pre-bloom nutrient benchmarks in the Bohai Sea. Long-term HAB records were used to delineate recurrent hotspot-associated subregions, within which pre-bloom observations were extracted to estimate region- and season-specific benchmarks for dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and the molar N/P ratio. Two historical reference datasets and a random-window analysis were used to assess the contextual position and temporal specificity of the derived values. The benchmarks showed pronounced regional and seasonal heterogeneity. Across five subregions, May-July estimates ranged from 0.12 to 0.38 mg/L for DIN, 0.005 to 0.011 mg/L for DIP, and 45.2 to 189.0 for N/P; the corresponding August-October ranges were 0.18-0.41 mg/L, 0.005-0.015 mg/L, and 60.8-143.1. The N/P benchmarks reflected consistently high DIN relative to DIP. Comparisons with historical reference datasets showed several positive contrasts, including a 24% higher grouped May-July DIN estimate relative to the non-hotspot reference, although none remained significant after false-discovery-rate correction. Temporal specificity was most evident for DIP and N/P, particularly during August-October. Environmental analyses further suggested that benchmark variability was associated with hydrographic and ecological heterogeneity. This framework provides a practical basis for deriving region- and season-specific pre-bloom nutrient benchmarks and can support regional HAB screening and monitoring prioritization in heterogeneous coastal systems.