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The formation of periodic hypoxia within tributary estuaries, and its relationship to the spring-neap tidal cycle, has been well documented in several systems along the US east coast. However, the importance and scale of other physical and biological processes, which ultimately control the frequency and spatial extent of hypoxia, are less well understood. This study synthesized in situ measurements, metabolic incubations, and high-resolution water quality monitoring into a spatially explicit, temporally integrated mass balance to examine the significance of multiple organic matter sources and oxygen sinks in relation to hypoxia in the York River estuary (YRE), Virginia, USA. Results highlight episodic peaks in gross primary production (GPP) mostly unrelated to the spring-neap cycle, with phytoplankton accounting for the bulk of total GPP. Despite extensive shoals, microphytobenthos contributed under 20% and typically under 10% of total GPP. Respiration rates were sufficient to drive bottom waters to hypoxia during transitions from spring to neap tides. While GPP generally appeared to be relatively balanced with respiration, results indicated an area at the mesohaline–polyhaline boundary that was net heterotrophic. Phytoplankton production dominated the estimated inputs of organic carbon from the tributaries and surrounding watersheds, and was 1.5 times greater than inputs from the Chesapeake Bay, which roughly balanced exports. Management efforts to alleviate hypoxia should focus on reducing internal phytoplankton production, although inputs of labile organic matter from the Chesapeake Bay represent an important source that can only be controlled by more regional efforts.

Internal versus external drivers of periodic hypoxia in a coastal plain tributary estuary: the York River, Virginia