Asian Journal of Biology

Background: Estuarine and mangrove ecosystems are dynamic zones where microbial processes regulate nutrient cycling and water quality but are increasingly impacted by pollution and seasonal changes. An integrated assessment of physicochemical and microbial indicators is essential to understand water quality dynamics, especially in tropical estuaries like Can Gio.

Aims: To assess organic pollution, microbial indicators, and surface water quality in the Can Gio estuary, Vietnam, and to compare their seasonal and spatial variation using previously published monitoring data.

Study Design: Secondary analysis of an existing surface water quality dataset.

Place and Duration of Study: Can Gio estuary, Ho Chi Minh City, Vietnam, using monitoring data collected at 14 sampling sites during the dry season (March 2023) and rainy season (September 2023).

Methodology: Published numerical data from previous studies were reorganized and interpreted using descriptive statistics, graphical comparison, and a simple regression to examine the five-day biochemical oxygen demand (BOD₅) - dissolved oxygen (DO) pattern. The assessed parameters included DO, BOD₅, chemical oxygen demand (COD), ammonium (NH₄⁺), nitrite (NO₂^-), phosphate (PO₄^3-), total coliform, and Escherichia coli.

Results: Water quality showed clear seasonal and spatial variation. BOD₅ and COD were generally higher in the dry season, whereas DO tended to be slightly higher in the rainy season. Organic pollution was unevenly distributed among sampling sites, with several hotspots likely influenced by aquaculture, domestic wastewater, and other local human activities. Among nutrient variables, NO₂^- showed the clearest seasonal fluctuation, while NH₄⁺ and PO₄^3- varied less consistently. Microbial indicators remained high in both seasons, and E. coli fluctuated more strongly in the rainy season. The BOD₅ - DO relationship at site scale was weak, indicating that DO was influenced not only by organic loading but also by hydrodynamic and environmental conditions.

Conclusion: The Can Gio estuary is under pressure from mixed organic and microbial pollution, with the dry season being more sensitive to organic load accumulation. The study provides an integrated basis for water quality monitoring and management in tropical mangrove estuaries.