Seasonality of water column methane oxidation and deoxygenation in a dynamic marine environment

Abstract

Most of the methane input to the world’s oceans is intercepted by microorganisms in sediment and the overlying water column and oxidized before it has an opportunity to reach the atmosphere, where it acts as a greenhouse gas. The factors controlling methane consumption in the ocean are not well established and its biogeochemistry in dynamic marine environments is understudied in-part because of challenges in capturing spatial and temporal variability. Our study focused on the factors that structure methane’s biogeochemistry in a dynamic marine environment, the Santa Barbara Basin. The deep-water column of the Santa Barbara Basin experiences seasonal oxygen loss and episodic replenishment which we found to be major factors in structuring the accumulation of methane and the rate at which microorganisms consumed that methane. We found the gradual decline in oxygen that commonly occurs through the summer culminated with a pronounced accumulation of methane in the water column during the fall. Rates of methane oxidation remained low in summer, increased with the buildup of methane in fall, and remained elevated into spring, even after methane concentration had declined. However, results from methane oxidation kinetics experiments revealed a zero-order kinetic dependence on oxygen concentration, indicating that oxygen’s effect on methanotrophy at the ecosystem scale is likely indirect. We also captured an apparent mixing event during fall that drove spatial and temporal variability in oxygen, nitrate and methane concentrations in the Santa Barbara Basin, with stark variations at the investigated timescale of 8 days and along isobaths at a spatial scale of 7 km. Collectively, these results indicate the seasonal development and attenuation of a methanotrophic community associated with restricted circulation, but also of a spatiotemporal variability not previously appreciated for this environment.