The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Over the last 50 years, there has been a significant decrease in the amount of dissolved oxygen in the oceans, amounting to a 2% loss of O_, in addition to a 500% increase in the number of coastal environments considered hypoxic (with a DO of < 2mg/l O₂). As we look to model future changes to ocean oxygen levels, one area of key importance is the stoichiometry of the organic matter involved in ocean biogeochemical cycles. While the Redfield ratio is useful as a rule of thumb for organic matter stoichiometry, it is not entirely accurate when considering all environments. This is the case several different oceanographic settings, as organisms change their stoichiometry in response to variable conditions.  The purpose of this study is to understand how the stoichiometry of organic matter changes in the coastal environment, as these environments are subject to greater levels of anthropogenic and terrestrial inputs than those found in the deeper ocean. This has been determined via the respiration quotient, which uses measurements of particulate organic carbon (POC) and particulate chemical oxygen demand (PCOD) to determine the proportions of carbon and oxygen specifically in the particulate matter. The waters of Hong Kong, in particular Tolo Harbour (year-round) and the Pearl River Estuary (selected months), have been selected for testing as they represent two different coastal environments (semi-enclosed and estuarial), both of which are close to sources of large anthropogenic activities. From this, we will understand how the oxidation state of organic matter changes both on a temporal and spatial scale in coastal environments and see how this may affect future deoxygenation.
 

coastal seasonal hypoxia, particulate organic carbon, stoichiometric ratio, Greater Bay Area