Physical control on methanotrophic potential in waters of the Santa Monica Basin, Southern California

Methane concentrations and turnover rates were measured throughout the water column at nine stations in the Santa Monica Basin (SMB), at one station in the San Pedro Basin (SPB), and at one station in the Santa Catalina Basin (SCtB) in July 2007 and September 2009. Methane concentrations were elevated throughout the water column, with subsurface (4-15 nmol L-1), midwater (7.5-100 nmol L-1), and bottom-water (5-242 nmol L-1) maxima. The SMB water column was divided into four depth-dependent regimes, based on temperature-salinity relationships and patterns in methane concentration and fractional turnover rate. We propose that distinct physical controls on methane dynamics distinguish these regimes. In the upper water column, methanotrophic activity appears to be controlled by borderland-scale, seasonal fluctuations in circulation. The midwaters represent a transition regime between northern-sourced shallow waters and southern-sourced deep waters, wherein methanotrophic activity decreases with depth to its minimum at 500 m, coinciding with the midwater methane maxima. Below 500 m, methanotrophic activity increases to the sill depth of 737 m, coincident with increasingly restricted circulation of water. Bottom waters are restricted from through-basin circulation. These waters were oxygen depleted, but methane input sustained an active psychrotolerant methanotrophic community and had some of the fastest aerobic methane oxidation rates yet reported in the marine environment.