Hypoxia formation triggered by the organic matter from subsurface chlorophyll maximum in a large estuary-shelf system

This study reports, for the first time, the role of shoreward transport of organic matter (OM) from subsurface chlorophyll maximum (SCM) in triggering hypoxia off the Pearl River Estuary (PRE, an outstanding example of typical estuary-shelf systems) based on field measurements. Compared to frequently observed hypoxia driven by surface eutrophication and terrestrial OM during large river discharge, we demonstrate that the upslopetransported SCM played a critical role in forming offshore hypoxia during low river discharge. Together with the plume-sourced OM trapped below the surface plume front, upslope-transported OM originating from the SCM accumulated underneath the pycnocline and consumed dissolved oxygen (DO), enhancing the bottom hypoxia. The DO consumption induced by the SCM-associated OM was estimated to contribute similar to 26% (+/- 23%) of the DO depletion under the pycnocline. Based on coherent and consistent physical and biogeochemical evidence and reasoning, this study reveals the contribution of SCM to bottom hypoxia off the PRE, which is unreported and likely occurs in other coastal hypoxic systems.

Automated summary

This study reports the role of shoreward transport of organic matter from subsurface chlorophyll maximum in triggering hypoxia off the Pearl River Estuary based on field measurements. The upslope-transported SCM and plume-sourced OM accumulated underneath the pycnocline and consumed dissolved oxygen, contributing to bottom hypoxia. This study reveals the contribution of SCM to bottom hypoxia off the PRE and its potential occurrence in other coastal hypoxic systems.