Air-sea CO_ fluxes in the coastal zone are controlled by complex physical and biological interactions. The important physical processes include the presence of sea ice, upwelling, and tidal sloshing. Biological processes of interest include photosynthesis and respiration. This study examined these controls, interactions, and the resultant patterns of flux, in three coastal seas off of Barrow in Alaska, Bodega Bay in northern California, and San Diego in southern California with distinctive overarching controls on net CO_ flux. CO_ flux was measured by eddy covariance and bulk method. The eddy covariance system on a mobile platform was also tested for measuring CO_ flux at kelp forests near the coast of San Diego. CO_ flux from the nearshore water off Barrow and San Diego showed high sinks of CO_ to the ocean, and correlation analyses with environmental parameters revealed that these systems were highly regulated by biological control. The alongshore water of Barrow was a strong sink of CO_ (up to 2 g C m__ day__) in summer despite terrestrial inputs, and the temporal patterns of CO_ flux was highly influenced by physical and biological influence of ice sheets and phytoplankton bloom. On the other hand, inter-annual pattern of CO_ sink in the alongshore water in San Diego was likely related to primary productivity enhanced by upwelling and adjacent kelp stands. The long-term record of CO_ flux from San Diego revealed the study area was a sink of CO_ with the average annual value of 200 g C m__ yr__. On the contrary, the coastal sea off of Bodega Bay was consistently a source of CO_ to the atmosphere of 420 g C m__ yr__, and the magnitude of CO_ flux was highly controlled by upwelling intensity that was reflected in sea surface temperature, salinity and chlorophyll density. The magnitude of CO_ fluxes from all the three sites were about two orders higher than variations of CO_ flux typically observed in the open ocean suggesting an importance of the role of the coastal seas in understanding the global carbon budget.