Seasonal dynamics of multiple coastal pollutants in the Bhatiari coastal waters, Chattogram, Bangladesh

Abstract

Coastal waters adjacent to the Bhatiari area were assessed through an integrated multipollutant approach to capture monsoon-driven changes in physicochemical conditions, nutrients, heavy metals (HMs), oil and grease (OG), microplastics (MPs), and microbial indicators. Nearshore sites were generally more contaminated than offshore sites. Pre-monsoon (PM) waters showed higher salinity, electrical conductivity (EC), total dissolved solids, turbidity, and suspended solids, while monsoon conditions increased dissolved oxygen, chlorophyll a (Chl a), and dissolved nutrients, particularly nitrate, nitrite, ammonium, and silicate. One-way ANOVA confirmed a strong seasonal contrast for conservative mixing properties, with salinity differing significantly between seasons (p < 0.001). Correlation analysis showed that salinity, EC, and total dissolved solids were almost perfectly coupled (r = 1.000, p < 0.01), while Chl a was strongly inversely related to the salinity-linked gradient (r = -0.892 to −0.904, p < 0.01), collectively indicating a shift from more saline PM conditions to a fresher, more productive monsoon regime. HMs profiles were seasonally restructured, with monsoon increases in Fe and Zn, hotspot detectability of Cd and Pb, and generally lower dissolved Cr and Cu. OG persisted at low to moderate levels (0.6–1.5 mg/L) with a recurrent hotspot at Station 3. MPs were widespread and dominated by microfibers (> 65%), with nearshore declines during monsoon but higher abundance and diversity offshore. Most microbial indicators were elevated nearshore and declined during the monsoon, although Pseudomonas spp. and total viable count increased. Bhatiari represents a low to moderate, multiple-stressor coastal system requiring season-specific integrated monitoring and management. This study highlights its novelty by presenting one of the first integrated, seasonally resolved multi-stressor assessments within a single analytical framework for a ship-breaking influenced coastal ecosystem in Bangladesh. Elevated levels of HMs, MPs, and microbial contamination, along with their spatial and seasonal variability influenced by nearby ship-breaking activities and monsoon-driven hydrodynamics, indicate significant ecological and public health risks, underscoring the need for continuous monitoring and risk-informed coastal management.