Methane Concentrations Increase in Bottom Waters During Summertime Anoxia in the Highly Eutrophic Estuary, Chesapeake Bay, USA

The Chesapeake Bay is a highly eutrophic estuary that undergoes seasonal bottom water hypoxia and its sediments contain methane (CH4) gas bubbles within the upper few cm. Yet, little is known if this greenhouse gas escapes to the water column or the atmosphere. Here, we hypothesized that when bottom waters become anoxic, CH4 is released from sediments, builds-up under the pycnocline, and is released at the end of the anoxia. To test this, bottom water from the Chesapeake Bay was continuously collected using OsmoSamplers from April to October 2013 to give a record of CH4 concentrations every 4 days. In April, CH4 concentrations were low (~1 μM) when bottom waters were still fully oxygenated and steadily increased as hypoxic conditions set-in. By mid-July, CH4 concentrations peaked and reached as high as 40 μM and returned to background levels when normoxic conditions returned in late September. The correlation with oxygen supports our hypothesis, although we can not rule out organic matter input and temperature as contributing, or even dominant drivers of this pattern. Surface water concentrations were 6 times higher in September compared to June, suggesting that CH4 is also released at the end of seasonal hypoxia. We postulate that this process of anoxia enhanced CH4 flux is not specific to our field site and thus that other highly eutrophic estuaries may also experience a similar phenomenon. Collectively, such estuaries may be a larger source of atmospheric CH4 than originally thought.