Planktonic methane production and oxidation within the algal maximum of the pycnocline: seasonal fine-scale observations in an anoxic estuarine basin

Abstract

High resolution profiles of methane concentration and potential methane-oxidation rates were used to test the importance of in situ production and oxidation in controlling methane structure and dynamics in the southern anoxic basin of the Pettaquamscutt Estuary, Rhode Island, USA. Profiles conducted in both April and August showed low but increasing methane concentrations in the surface oxic layer, a minor maximum about 0.75 m above the Eh = 0 boundary, a methane minimum just above Eh = 0, and a rapid increase to a major maximum about 0.75 m below Eh = 0. Methane concentrations in both maxima increased between April and August while concentrations in the methane minimum decreased. This methane minimum in the suboxic zone was strongly associated with a minor oxygen maximum and a major peak in chlorophyll due to the oxygenic alga Euglena proxima. April incubations in the presence of added oxygen at 2C supported oxidation rates as high as 9% d-' in and just below the methane minimum at the Eh = 0 boundary, due to psychrophilic methane-oxidation. Similar experiments conducted in August at 22C showed a similar narrow peak in potential methaneoxidation rates in and just below the methane minimum at the Eh = 0 boundary, suggesting that the boundary was now dynamically controlled by rapid mesophilic methane-oxidation. Analysis of the August incubations at 2C showed a decline in methane at the depths of the major methane maximum for 4 d, followed by a 4 d increase in methane concentration at the same depths, due presumably to the exhaustion of dissolved oxygen and the cessation of the masking oxidation. These results indicate that the seasonal increase in the major methane maximum is generated Ln situ by psychrophilic methaneproduction, a production that IS largely masked by a concomitant psychrophilic methane-oxidation. The shift from an initial dominance of methane-oxidation to a final dominance of methane-production in the same water sample bottles indicates just how sensitive the structure of the major methane maximum is to the bacterial processes of methanogenesis and methanotrophy, changes in water temperature, and oxygen inputs by in situ primary production and physical intrusions. Such processes have made it bfficult to document a major production of methane in the pycnocllne.