Divergent trajectories of N:P stoichiometry and a chemical eutrophication index under low-DIP conditions in the Bohai Sea, summer 2017–2024

Nutrient assessments in semi-enclosed coastal seas can become difficult to interpret when dissolved inorganic phosphorus (DIP) approaches analytical detection limits. Using an eight-year summer dataset (2017-2024) from 15 stations in the Bohai Sea, we examined dissolved inorganic nitrogen (DIN), DIP, N:P stoichiometry, and a composite chemical eutrophication index (EI). Mean DIN increased from 0.080 mg/L in 2017 to 0.140 mg/L in 2020 and subsequently declined to 0.054 mg/L in 2024, whereas mean DIP decreased markedly after 2019, reaching 0.0007 mg/L in 2021-2022. Neither variable showed a statistically significant monotonic trend over the full study period. DIP non-detection reached 73.3% of stations in 2021-2022, introducing substantial uncertainty into calculated N:P ratios. Under LOD/2 substitution, annual mean N:P exceeded the Redfield reference value of 16:1 in all years and reached a nominal 486.26 in 2022; sensitivity analysis showed that absolute values during low-DIP years were strongly substitution-dependent. In contrast, EI remained within the mesotrophic range throughout the study period and peaked at 0.225 in 2018. This divergence reflects the opposing mathematical dependence of the two indicators on DIP: declining DIP increases N:P but suppresses EI. Spatially, elevated DIN repeatedly occurred near the Bohai Bay-Laizhou Bay junction, whereas DIP showed no persistent hotspot. These findings demonstrate that N:P and EI provide complementary chemical diagnostics and should be interpreted jointly under low-DIP conditions.