Abstract
Darwin Harbour in northern Australia is an estuary in the wet-dry tropics subject to increasing urbanization with localized water quality degradation due to increased nutrient loads from urban runoff and treated sewage effluent. Tropical estuaries are poorly studied compared to temperate systems and little is known about the microbial community-level response to nutrients. We aimed to examine the spatial and temporal patterns of the bacterial community and its association with abiotic factors. Since Darwin Harbour is macrotidal with strong seasonal patterns and mixing, we sought to determine if a human impact signal was discernible in the microbiota despite the strong hydrodynamic forces. Adopting a single impact–double reference design, we investigated the bacterial community using next-generation sequencing of the 16S rRNA gene from water and sediment from reference creeks and creeks affected by effluent and urban runoff. Samples were collected over two years during neap and spring tides, in the dry and wet seasons. Temporal drivers, namely seasons and tides had the strongest relationship to the water microbiota, reflecting the macrotidal nature of the estuary and its location in the wet-dry tropics. The neap-tide water microbiota provided the clearest spatial resolution while the sediment microbiota reflected current and past water conditions. Differences in patterns of the microbiota between different parts of the harbor reflected the harbor's complex hydrodynamics and bathymetry. Despite these variations, a microbial signature was discernible relating to specific effluent sources and urban runoff, and the composite of nutrient levels accounted for the major part of the explained variation in the microbiota followed by salinity. Our results confirm an overall good water quality but they also reflect the extent of some hypereutrophic areas. Our results show that the microbiota is a sensitive indicator to assess ecosystem health even in this dynamic and complex ecosystem.
Highlights
Estuarine ecosystems are a focal point of impacts from the land and seaward side and experience increasing pressures from growing populations and human activities worldwide (Jennerjahn and Mitchell, 2013)
During eight sampling rounds over 2 consecutive years (2013– 2014), two dry and wet seasons and four neap and spring tides, 286 water and 208 sediment samples were collected from 32 sites from two areas in Darwin Harbour, namely Shoal Bay and East Arm creeks (Figure 1)
Samples were rarefied to 17,000 sequences, which resulted in a mean Goods coverage of 90.2% for water samples (n = 277) and 79.2% for sediment samples (n = 192)
Summary
Estuarine ecosystems are a focal point of impacts from the land and seaward side and experience increasing pressures from growing populations and human activities worldwide (Jennerjahn and Mitchell, 2013). Within these ecosystems, tidal creeks are especially dynamic environments renowned for their complexity and productivity (Holland et al, 2004). The few reports available for tropical systems have shown that sediment microbial communities were spatially and temporally dependent and associated with changes in temperature, nutrient load, dissolved oxygen, salinity and pH (Sun et al, 2011; Zhang et al, 2014). One study in a macrotidal estuary in the Australian wet-dry tropics found that salinity was the determining factor for temporal variations of nitrogen-cycle related bacteria in the sediment (Abell et al, 2009)
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