Diurnal Variation of Deep Chlorophyll Maximum in the Bay of Bengal

Using high-resolution, diurnally resolved observations, this study examines the physical and biogeochemical controls on the deep chlorophyll maximum (DCM) in the southwestern Bay of Bengal during the post-northeast monsoon transition (February 2024). Co-located hydrographic, optical, dissolved oxygen, and nutrient measurements were collected along an offshore latitudinal transect spanning Chennai, Puducherry, and Karaikal. A persistent DCM was observed between 55 and 80 m, aligned with the base of the barrier layer and the upper thermocline, where light levels were reduced to ~0.5-1% of surface photosynthetically available radiation. Chlorophyll-a concentrations within the DCM ranged from 0.6 to 1.2 mg m⁻³ and deepened northward in response to reduced haline stratification, deeper thermoclines, and improved optical clarity. Despite pronounced diurnal heating-cooling cycles at the surface, the DCM depth exhibited only minor fluctuations, indicating strong density-controlled confinement. Nutrient concentrations within the DCM ranged from 1.5-3 µmol L⁻¹ nitrate, 0.10-0.30 µmol L⁻¹ ammonium, and 2.5 - 5 µmol L⁻¹ silicate, while dissolved oxygen remained relatively uniform (3.4 - 4.0 mg L⁻¹). These results indicate that the Bay of Bengal DCM is closely associated with a stable density structure and exhibits variability consistent with photoacclimation and potential in situ nutrient recycling, rather than vertical displacement.