Drivers of nutrient dynamics and chlorophyll-a variability: The role of freshwater input, upwelling, and tropical cyclones near Saint Martin's Island, Bangladesh

Authors

• Mohammad Sadman Alam, The University of Texas Rio Grande Valley
• Subrata Sarker, Shahjalal University of Science and Technology
• Nabanita Das, Shahjalal University of Science and Technology
• Shashowti Chowdhury Riya, Shahjalal University of Science and Technology
• Myung Hwangbo, The University of Texas Rio Grande ValleyFollow
• Jongsun Kim, The University of Texas Rio Grande ValleyFollow

Document Type

Article

Publication Date

2-1-2026

Abstract

The Bay of Bengal (BoB) receives significant amounts of discharge from river runoff as the largest delta in the world, which affects the dynamic of coastal and estuarine systems. Thus, Saint Martin Island (SMI), located in the northeastern part of the BoB, faces a significant influx of sediments and numerous contaminants from upstream sources, which are affecting the biogeochemistry of SMI. It is hypothesized that both river influx and frequent tropical cyclones that occur in the BoB significantly influence the coastal ecosystem of the Island. Therefore, to understand the coastal ecosystem of SMI, the spatiotemporal variability of nutrients and chl-a (July 2022–June 2023) was explored. This study identified the northeast (NE) monsoon as an important season for primary productivity with the concentration of chl-a, inorganic nitrogen, phosphate, and silicate as 1.03 μg L-1, 64.09 μmol L-1, 3.86 μmol L-1, 86.17 μmol L-1, respectively. Seasonal and regional nutrient ratios were near-Redfield (N:P:Si ≈ 17–18:1:17–18), with modest monsoonal shifts and comparatively stronger N-Si supply toward the estuary. The correlation of rain, wind stress, river discharge, Ekman transport, salinity, nutrients, temperature, chl-a, and dissolved oxygen was evaluated by using a coast-normal wind and Ekman framework together with structural equation modelling (SEM). SEM indicated positive effects of discharge, rainfall, and Ekman transport on nutrients. Especially, chl-a was positively associated with salinity and DIP and negatively with DIN and temperature, while the direct Ekman-to-chl-a path was not significant. After the cyclone Sitrang, the mixed-layer was deepened (∼1.33 m), and a narrowed temperature-salinity pattern toward higher salinity, DIP, and DIN increases in the upper 50 m was observed. During the southwest (SW) monsoon, river discharge dominated nutrient supply, whereas the reduced rainfall was correlated with wind- and eddy-assisted vertical inputs that sustained productivity near the estuary during the NE monsoon. This study highlighted monthly horizontal and vertical variations of hydrological parameters and their interrelationship with algal growth and nutrient dynamics.

Comments

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Recommended Citation

Alam, Mohammad Sadman, Subrata Sarker, Nabanita Das, Shashowti Chowdhury Riya, Myung Hwangbo, and Jongsun Kim. "Drivers of nutrient dynamics and chlorophyll-a variability: The role of freshwater input, upwelling, and tropical cyclones near Saint Martin's Island, Bangladesh." Continental Shelf Research (2025): 105624. https://doi.org/10.1016/j.csr.2025.105624

Publication Title

Continental Shelf Research

DOI

10.1016/j.csr.2025.105624