Abstract
Seagrasses are globally recognized as bioindicators of marine eutrophication and contamination. Seagrasses also harbor a distinct root microbial community that largely reflects the conditions of the surrounding environment as well as the condition of the seagrass. Hence monitoring changes in the root microbial community could act as an additional biological indicator that reflects both the seagrass health condition, as well as potential deterioration in coastal waters. We used 16S rRNA gene sequencing combined with analysis of seagrass nutrients (C, N, δ15N, δ13C) and tissue metal concentrations to investigate potential links between seagrass (Halophila ovalis) root bacteria and seagrass nutrient and metal concentrations within an anthropogenically influenced estuary. We found seagrass tissue nitrogen (%) and δ15N values were 2–5 times higher than global averages for this species. Seagrass root associated bacteria formed distinct communities that clustered by site and were correlated to both seagrass nutrient and metal concentration, with some putative sulfide oxidizing bacteria (Sulfurimonas and Sulfurovum) correlated with greater nutrient concentrations, and putative iron cycling bacteria (Lewinella and Woeseia) correlated with greater Fe and As concentrations. Our findings shed further light on the relationship between seagrass and their microbes, as well as provide additional assessment of the use of both seagrass and their microbes as indicators of estuarine and seagrass condition.
Highlights
Estuaries, which are transitional zones between the land and the sea, trap sediments and pollutants from rivers and streams and reduce the extent to which these pollutants enter the ocean
Dissolved oxygen (DO) concentration of seagrass beds ranged from 7.9 mg L−1 (99% air saturation) to 15.5 mg L−1 (202% air saturation), whilst DO in stormwater drains flowing into seagrass meadows ranged from 0.1 mg L−1 (1% air saturation) to 2.8 mg L−1 (32% air saturation)
The stormwater drains tended to have high concentrations of soluble reactive phosphorus (SRP) and N-NH4 compared to the seagrass meadows in the estuary, with some of these nutrient values above the Australian and New Zealand guidelines for fresh and marine water quality (ANZECC and ARMCANZ; Table 1)
Summary
Estuaries, which are transitional zones between the land and the sea, trap sediments and pollutants from rivers and streams and reduce the extent to which these pollutants enter the ocean. Poor catchment management can lead to increased sediment, nutrient and pollutant loads to estuaries which can cause a decline in the health of the estuary and increase pollution export to the Anthropogenic Stress and Seagrass Bacteria ocean (Dafforn et al, 2012; Statham, 2012). Excess nutrients and heavy metals are both common and serious pollutants of estuaries (Jiang et al, 2001; Sutherland et al, 2017). Excess nutrients in estuaries can lead to cultural eutrophication and associated deoxygenation events, which is a leading cause of impairment of freshwater and coastal ecosystems globally (Statham, 2012). Under certain physicochemical conditions (e.g., water hypoxia and anoxia), these contaminated sediments can become a source of continued pollution for estuarine ecosystems (Bennett et al, 2012)
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