Fluxes of N 2, O 2, and CO 2 in nearshore waters off Martha's Vineyard

Abstract

Accurate, high-resolution time series measurements of aqueous CO 2, O 2, and N 2 were used to investigate the fluxes and transformations of these gases in a complex and dynamic nearshore environment. The measurements were made at 5-m depth over 10 days in June 2002 at the Martha's Vineyard Observatory, MA (41°19.722′ N and 70°33.096′ W). The average depth of the water column at this location was 13 m. Supporting measurements include water temperature, salinity, fluorescence, and local meteorological conditions. For the analysis, the data set was partitioned into discrete events characterized by similar environmental conditions. Approximately 30% of the total data set was chosen for detailed analysis: two ‘wind events’ where 5< U 10<11 m s −1, and one ‘calm period’ where U 10<5 m s −1. Heat and salt budgets were used to select data appropriate for analysis using a 1-D interpretation. During the wind events, budgets of biologically inactive N 2 provided estimates of air–sea gas transfer rates, which were then scaled using appropriate air–sea gas exchange models, and used to calculate air–sea CO 2 and O 2 fluxes. Variability in O 2 and CO 2 during the stratified calm period were used to estimate biologically controlled carbon fluxes. The air–sea carbon fluxes during the wind events were 9% and 34% of the biological fluxes during the calm period. A second estimate of air–sea O 2 flux was derived from the non-biologically controlled O 2 variability, based on Redfield ratios and knowledge that dissolved O 2 will equilibrate with the atmosphere via air–sea gas exchange faster than CO 2. For one wind event, when fluxes were large, both estimates agreed to within 37%. These observations provide quantitative estimates of air–sea gas exchange rates in the complex nearshore zone, elucidate the role of biophysical interactions in controlling air–sea CO 2 and O 2 exchange, and demonstrate the feasibility of new methods to quantify air–sea gas fluxes. While this study was conducted in nearshore waters, the methods can also be applied to waters of the continental shelf and open ocean.