Seasonal variability in primary productivity and associated shifts in bacterial community structure at chlorophyll maximum depths in the eastern Arabian Sea

Abstract

The eastern Arabian Sea experiences a seasonal increase in primary productivity during the southwest monsoon. However, how this seasonal increase restructures bacterial communities and ecological dynamics at the chlorophyll maximum depths remains insufficiently resolved. This study examines how monsoon-driven hydrographic changes regulate bacterial community structure and ecological functions across coastal and offshore stations off Goa, Mangalore, and Trivandrum, integrating in situ primary productivity measurements, physicochemical profiling, and 16S rRNA gene amplicon sequencing. Monsoon intensification resulted in a shallower mixed layer (25–50 m), with enhanced nutrients, and higher mean primary productivity (65 mg C m⁻³ d⁻¹), and microbial processes within a narrower upper-water column. A significant positive correlation between primary productivity and bacterial carbon biomass demonstrated strong coupling between autotrophic production and bacterial responses. This productivity enhancement coincided with pronounced seasonal restructuring of community composition. Non-monsoon bacterial communities were dominated by Proteobacteria and Firmicutes, adapted to relatively nutrient-limited conditions. In contrast, the monsoon favored Cyanobacteria along with diverse heterotrophic lineages, including Bacteroidetes, Actinobacteria, and several Candidatus phyla, reflecting enhanced processing of freshly produced organic substrates. Multivariate analyses confirmed that primary productivity and physicochemical variables significantly structured bacterial communities across seasons. Predictive functional profiling indicated a shift from heterotroph-dominated carbon recycling during the non-monsoon to increased representation of phototrophic and carbon assimilation functions during the monsoon, suggesting a seasonal transition in microbial trophic strategies. These findings suggest that monsoon-driven hydrographic variability influences microbial trophic balance, linking physical forcing to bacterial community restructuring and inferred carbon-processing pathways in tropical marine systems.