Air-sea CO₂fluxes in the near-shore and intertidal zones influenced by the California Current

Abstract

[1] The study of air-sea CO 2 fluxes (FCO 2 ) in the coastal region is needed to better understand the processes that influence the direction and magnitude of FCO 2 and to constrain the global carbon budget. We implemented a 1 year (January through December 2009) paired study to measure FCO 2 in the intertidal zone (the coastline to 1.6 km offshore) and the near-shore ($3 km offshore) off the north-western coast of Baja California (Mexico); a region influenced by year-round upwelling. FCO 2 was determined in the intertidal zone via eddy covariance; while in the near-shore using mooring buoy sensors then calculated with the bulk method. The near-shore region was a weak annual net source of CO 2 to the atmosphere (0.043 mol CO 2 m À2 y À1 ); where 91% of the outgassed FCO 2 was contributed during the upwelling season. Sea surface temperature (SST) and DpCO 2 (from upwelling) showed the strongest relationship with FCO 2 in the near-shore, suggesting the importance of meso-scale processes (upwelling). FCO 2 in the intertidal zone were up to four orders of magnitude higher than FCO 2 in the near-shore. Wind speed showed the strongest relationship with FCO 2 in the intertidal zone, suggesting the relevance of micro-scale processes. Results show that there are substantial spatial and temporal differences in FCO 2 between the near-shore and intertidal zone; likely a result of heterogeneity. We suggest that detailed spatial and temporal measurements are needed across the coastal oceans and continental margins to better understand the mechanisms which control FCO 2 , as well as reduce uncertainties and constrain regional and global ocean carbon balances.

Temporal variations in air‐sea CO2 exchange near large kelp beds near San Diego, California

“…Nonetheless, we used linear regressions, as there is no well‐defined function to describe relations between CO 2 flux and coastal processes. Air‐sea gas transfer can occur independently from the chemical process within seawater, and there can be a lag between CO 2 flux and relevant coastal processes [ Reimer et al ., ]. However, no systematic lag between CO 2 flux and environmental parameters was detected by a cross‐correlation analysis in our data set with maximum time lag of 24 hours.…”

“…However, AWV was not determined to be an important parameter at any temporal scales, and high wind speeds simply increased air‐sea gas transfer and enhanced CO 2 uptake from the ocean on an hourly scale. Therefore, the influence of the outgassing due to upwelling on CO 2 flux was not detected at this study site unlike other coastal upwelling zones [ Torres et al ., ; Ikawa et al ., ; Reimer et al ., ].…”

“…To our knowledge, there are few studies that use EC for coastal seas possibly inhabited by macrophytes [ Ikawa et al ., ; Reimer et al ., ]. However, EC systems used by Ikawa et al .…”

“…The statistical level of significance was set at p < 0.05. As discussed in Reimer et al ., [], the influences of various coastal processes on CO 2 flux may not necessarily be linear. Nonetheless, we used linear regressions, as there is no well‐defined function to describe relations between CO 2 flux and coastal processes.…”

“…Since the development of the eddy covariance (EC) technique in the early 1970s [ Kaimal et al ., ; Garratt , ], CO 2 flux has been measured widely over terrestrial ecosystems, for the understanding of temporal and spatial patterns of CO 2 exchange between terrestrial biospheres and the atmosphere [ Baldocchi et al ., ]. Along with the development of reliable instruments and secure research environments for buoys, ships, and piers, EC has been applied for oceanic flux measurements [ Iwata et al ., ; McGillis et al ., ; Kondo and Tsukamoto , ; Miller et al ., ; Reimer et al ., ]. However, partly because of difficulties in accessibility and maintenance, EC measurements over the ocean have generally been quite limited in space and time.…”