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The chemical oxygen demand (COD) is an essential indicator of organic pollution that represents the amount of bulk carbon in water. COD is strongly correlated with nutrient cycles and other pollutants in the environment, but it has a limited ability to quantify the amount of organic carbon (OC), of which a large proportion is made up of refractory dissolved organic carbon (RDOC) and is a potential carbon sink. Moreover, the biodegradability of OC in terms of its fate and destination should be explored, as well as how this is reflected by COD. Methods based on particle size, spectroscopy, and isotopic tracing are expected to help with deciphering the bioavailability of COD-responsive OC and explore the processes of biogeochemical cycles. As the pressure on the environment from anthropogenic inputs increases, understanding the bioavailability of OC associated with COD will help with developing more precise scientific indicators for environmental monitoring and identifying how new tools will increase knowledge of the carbon cycle. In this review, we discuss the application, scope, means, and advances of COD measurement. Based on data in the literature, we estimate the global RDOC stock and assess the impact of anthropogenic RDOC on the carbon cycle in offshore bays. This review presents new insights into the behavior of OC in aquatic environments and a potential pathway for ocean negative carbon emissions by expanding the role of RDOC as a carbon sink to offset the effect of anthropogenic carbon emissions.
Lv et al. (Mon,) studied this question.