Vertical Structure of Chlorophyll-a and Deep Chlorophyll-a Maximum Dynamics in the Southern Lombok Strait under Spring-Neap Tidal Forcing

The Deep Chlorophyll Maximum (DCM) is a defining feature of phytoplankton distribution in stratified tropical oceans, yet its response to episodic physical forcing remains poorly constrained. We investigate the vertical chlorophyll-a structure and associated physical–biogeochemical drivers in the southern Lombok Strait, Indonesia, a region influenced by strong internal wave activity and pronounced spring–neap tidal variability. Using 112 CTD profiles collected at three adjacent stations from 12 to 27 March 2021 (0–500 m), complemented by satellite-derived photosynthetically active radiation, biogeochemical model nutrients, and tidal observations, we characterize DCM depth, magnitude, and variability. The DCM consistently occurred at 50–100 m near the upper thermocline, but its intensity and vertical expression were modulated by tidal-driven mixing, with stronger spring-tide turbulence enhancing nutrient supply to the euphotic zone. Our results demonstrate that short-period variability in mixing exerts a first-order control on subsurface phytoplankton structure in this dynamically forced strait system, providing new insight into the coupling between physical processes and biogeochemical responses in tropical marginal seas.