Science Highlights

Task 3: Marine Ecology

DO (dissolve oxygen) was the key variable shaping the microbiota communities in water and sediment. A hypoxic zone (DO level <2.8mg/L) was observed at the mouth of Pearl River Estuary (PRE), which is close to the coasts of Macau and Hong Kong. The sediment from close to the coastline of Lantau Island in Hong Kong had the least diversity and richness among the sampled Stations, including less DO (1.1 mg/L) and pH, but comparatively higher NH₄⁺ (170.8 µg/kg) and organic content (7.8%) than the other most Stations of PRE. Organic matter in sediment is inversely associated with the bulk density in sediment and DO in seawater, due to the increased colonization of heterotrophic bacteria and oxygen consumption during remineralization of organic matter which further reduced the microbiota. Positive correlations were found between DO and sulfate-reducing bacteria in Desulfococcus and GOUTA19 genera in the sediment of the SCS. Nutrients, salinity and phytoplankton (Chl a) are significantly associated with the microbial community dynamics in seawater and sediment. Correlation analysis found that Fluviicola and Nitrosopumilus genera were significantly positively associated with the nutrients (NO₂⁻, NO₃⁻ and SiO₃²⁻) in onshore of PRE, but negatively associated with salinity. Nitrosopumilus bacteria consume oxygen and helps to make more hypoxic condition (e.g. less DO) by oxidizing ammonia to nitrite. Low concentrations of perfluoroalkyl substances (PFASs) were not found to exert significant effects on the hypoxia, eutrophication, acidic conditions and microbial structure of PRE compared to other environmental factors. However, the levels of benzophenone-4 (BP-4) decreased with increasing water temperature and acidic conditions and ethylhexyl methoxycinnamate (EHMC) was degraded more easily from estuary waters to the open sea waters. Moreover, water temperature positively related to hypoxic conditions. The results concluded that the hypoxic and acidic conditions enhanced the degradation of contaminants such as BP-4. Offshore of the SCS had more oxygen-producing bacteria (Synechococcus, Prochlorococcus, and photosynthetic cyanobacteria) than the onshore sites. Positive correlations were observed among per- or polyfluoroalkyl acids (PFAAs), nutrients, and Chl a, suggesting that they share the same source, potentially being carried by Pearl River to the estuary and increased amounts of nutrients facilitated the eutrophication by reproduction of phytoplankton in PRE. This further supported by their inverse relationships with salinity because concentrations of PFAAs, EHMC, nutrients and Chl a will gradually decrease from onshore to offshore when salinity is increasing. Finally, the hypoxic condition, eutrophication, acidification and different contaminants including, organic UV-filters and PFASs concurrently decreased from estuary to open seawater of the SCS.