Spatial and Diurnal Variability of Chlorophyll-a and Its Associations with Dissolved Inorganic Nitrogen and Phosphorus in Nha Trang Bay, Vietnam, during the Northeast Monsoon

Abstract

Chlorophyll-a (Chl-a) variability in coastal waters reflects interactions among nutrient availability, hydrodynamic processes, and short-term phytoplankton dynamics. This study examined the spatial and diurnal variability of Chl-a and its associations with dissolved inorganic nutrients and other water-quality variables in Nha Trang Bay, Vietnam, during a January 2026 survey conducted during the Northeast Monsoon season. Water samples were collected at 34 stations in surface and bottom waters, complemented by observations at 6-h intervals over three days at one station. Chl-a ranged from 0.60 to 4.13 µg L⁻¹ with higher mean concentrations in bottom waters (1.52 ± 0.83 µg L⁻¹) than in surface waters (1.29 ± 0.54 µg L⁻¹). Elevated concentrations occurred near the Tac River estuary and aquaculture areas, whereas lower values generally occurred offshore. Bottom waters also had higher mean TSS concentrations. Spearman correlation analysis showed the strongest association between Chl-a and total suspended solids (TSS; rₛ = 0.800, p < 0.001), followed by ammonium. Short-term observations at one station revealed temporal variation in Chl-a and dissolved nutrient concentrations. However, these observations do not establish the mechanisms underlying the vertical differences or temporal patterns. The findings highlight the value of integrating spatial surveys, paired-depth sampling, and short-term observations in coastal water-quality monitoring.