Key drivers of hypoxia revealed by time-series data in the coastal waters of Muping, China.

Coastal hypoxia (low dissolved oxygen in seawater) is a cumulative result of many physical and biochemical processes. However, it is often difficult to determine the key drivers of hypoxia due to the lack of frequent observational oceanographic and meteorological data. In this study, high-frequency time-series observational data of dissolved oxygen (DO) and related parameters in the coastal waters of Muping, China, were used to analyze the temporal pattern of hypoxia and its key drivers. Two complete cycles with the formation and destruction of hypoxia were captured over the observational period. Persistent thermal stratification, high winds and phytoplankton blooms are identified as key drivers of hypoxia in this region. Hypoxia largely occurs due to persistent thermal stratification in summer, and hypoxia can be noticeably relieved when strong wind mixing weakens thermal stratification. Furthermore, we found that northerly high winds are more efficient at eroding stratification than southerly winds and thus have a greater ability to relieve hypoxia. This study revealed an episodic hypoxic event driven by a phytoplankton bloom that was probably triggered by terrestrial nutrient loading, confirming the causal relationship between phytoplankton blooms and hypoxia. In addition, we found that the lag time between nutrient loading, phytoplankton blooms and hypoxia can be as short as one week. This study could help better understand the development of hypoxia and forecast phytoplankton and hypoxia, which are beneficial for aquaculture in this region.

Investigation of the causes and mechanisms of hypoxia in the central Bohai Sea in the summer of 2022.

The deep-water area in the central Bohai Sea (BS) serves as a spawning ground and nursery for fish, shrimp, and crabs. Frequent hypoxia will affect the ecological environment in the central BS. Data from an on-site investigation of the central BS in the spring and summer of 2022 were used to analyze the relevant factors generating the occurrence of hypoxia in the central BS through the eutrophication index E, apparent oxygen consumption (AOU), and Spearman correlation. The hypoxia area was largely distributed in the study area's deep water section, and stratification was the main cause of hypoxia at the bottom. Organic matter mineralization, degradation, and biological respiration further exacerbated the hypoxia. In the summer of 2022, temperature stratification was the dominant factor influencing hypoxia.