Spatial and Temporal Microbial Patterns in a Tropical Macrotidal Estuary Subject to Urbanization

Area of Science:

• Microbial ecology
• Estuarine science
• Environmental microbiology

Background:

• Tropical estuaries like Darwin Harbour are understudied regarding microbial responses to nutrient pollution.
• Urbanization is increasing, leading to localized water quality degradation from urban runoff and sewage effluent.
• Understanding microbial community dynamics is crucial for assessing ecosystem health in these dynamic environments.

Purpose of the Study:

• To examine spatial and temporal patterns of the bacterial community in Darwin Harbour.
• To associate microbial patterns with abiotic factors and human impacts (urban runoff, sewage effluent).
• To determine if a human impact signal is discernible in the microbiota despite strong hydrodynamic forces.

Main Methods:

• Next-generation sequencing of the 16S rRNA gene was used on water and sediment samples.
• Sampling occurred over two years, encompassing neap and spring tides, dry and wet seasons.
• A single impact-double reference design was employed to compare affected and reference creeks.

Main Results:

• Seasonal and tidal patterns were the strongest drivers of the water microbiota, reflecting the macrotidal, wet-dry tropical nature of the estuary.
• Neap tides offered clearer spatial resolution for water microbiota, while sediment microbiota indicated past and present water conditions.
• A microbial signature linked to effluent sources and urban runoff was discernible, with nutrient levels explaining most variation, followed by salinity.

Conclusions:

• The bacterial microbiota is a sensitive indicator of ecosystem health in complex, dynamic estuarine systems like Darwin Harbour.
• While overall water quality is good, localized hypereutrophic areas were identified.
• Hydrodynamics and seasonality strongly influence microbial communities, but human impact signals can still be detected.