Water quality changes and shift in mechanisms controlling hypoxia in response to pollutant load reductions: A case study for Shiziyang Bay, Southern China.

Shiziyang Bay, located in the upstream of the Pearl River Estuary, has frequently suffered from hypoxia since 2000, which has persisted in recent years despite effective controls on anthropogenic pollutant loads. To explore the underlying causes, changes in dissolved oxygen (DO), nutrients, chemical oxygen demand (COD), and chlorophyll a (Chl a) along the bay in response to altered pollutant inputs were investigated using observations collected in summers of 2015-2019 and historical data during 2000-2008. In addition, DO sources and sinks were calculated based on data from August 2020 and laboratory incubations for water column respiration (WCR) and sediment oxygen uptake, and were compared with their equivalents in August 2008 to elucidate changes in primary processes controlling hypoxia. The results showed that ammonia has decreased significantly with pollutant control, while other parameters responded in different trends, especially for Chl a (with a substantial increase over time). The intensified eutrophication contributed to high COD levels, leading to a strong WCR (as dominant oxygen depletion) close to that in the 2000s and thereby maintaining low-oxygen conditions despite reduced effluent discharges. The shifted primary oxygen-consuming substances from allochthonous inputs to in-situ phytoplankton production were also evidenced by significant correlation between oxygen consumption rate and Chl a in recent data. Simultaneously, the enhanced algal blooms could also modulate oxygen supply, resulting in higher photosynthetic oxygen production and lower air-sea reaeration compared with the past. Furthermore, the impact of major environmental changes on exacerbated eutrophication was explored and it was speculated that notable declined sediment loads would be important by improving light conditions to promote phytoplankton proliferation in the bay. Collectively, substantial control on eutrophication as well as tracking DO source-to-sink processes is of great importance to mitigate hypoxia in Shiziyang bay.