Effects of Stratification on the Depth, Intensity and Community Composition of Deep Chlorophyll Maxima in the Western Tropical Pacific

Abstract

The Northern Mariana Islands are located within the West Pacific Warm Pool, a region characterized by year-round warm sea surface temperatures, low salinity, and strong vertical stratification. This stratification limits upward nutrient flux, resulting in oligotrophic surface waters and low satellite-observed chlorophyll concentrations. Satellite observations are limited to the surface layers and often fail to detect the subsurface Deep Chlorophyll Maximum (DCM). The Mariana Volcanic Arc hosts over 60 seamounts, creating variable bathymetry that locally weakens stratification through enhanced mixing, which increases nutrient supply and affects the DCM structure. Using data collected aboard R/V Thomas G. Thompson Cruise TN-450, we investigated whether decreasing stratification intensity at seamounts was associated with (1) shallower DCM depth, (2) increased chlorophyll concentration, and (3) a shift in plankton community composition towards a higher trophic level. CTD fluorescence was calibrated against processed chlorophyll water samples, resulting in a moderate calibration R2 (0.552) that raises uncertainty for calculating DCM metrics. Correlations between stratification and DCM depth, thickness, and intensity were weak and statistically insignificant (p > 0.05), suggesting submesoscale variability and episodic mixing had more influence than stratification. Dominance of Radiolaria was observed across all stations at DCM. The minimal difference offshore and at seamounts indicates that community structure is more strongly influenced by the Island Mass Effect than local stratification. These results indicate that stratification alone does not explain DCM metrics in this region. Future studies should incorporate nutrient, PAR, and temperature data to evaluate the primary driver of DCM characteristics.