Coastal CO2 climatology of Oahu, Hawaii: Six years of high resolution time-series data

Abstract:

Six years of high resolution pCO₂, water quality, and meteorological data were used to calculate air-sea CO₂ fluxes on yearly, seasonal and monthly timescales, and relate the temporal and spatial variation in CO₂ fluxes to meteorological events and land derived inputs.

Three MAPCO₂ buoys are deployed in coastal waters of Oahu as part of the NOAA/PMEL Carbon Program, that autonomously collects CO₂ and water quality data at 3-hour intervals. The buoys are located on a backreef in Kaneohe Bay and two fringing reef sites on Oahu's south shore, the latter two in open ocean like conditions but with one also influenced by fluvial inputs.

Data for this study were collected from June 2008 to July 2014. Mean pCO2 values at the Ala Wai, Kilo Nalu and CRIMP2 buoys were 396, 381 and 447µatm, respectively, with mean daily ranges of 51, 32 and 190 µatm, respectively. The daily range in pCO2 is largest at CRIMP2, reflecting a combination of higher primary production and respiration, vigorous calcification and longer water residence time within the barrier reef environment.

Net annualized air-sea CO₂ fluxes of the entire study period were 0.083, -0.014 and 1.167 mol C m^-2 year^-1 for Ala Wai, Kilo Nalu and CRIMP2, respectively. Positive values indicate a CO₂ flux from the water to the atmosphere (source behavior), and negative values from the atmosphere to the water (sink behavior).

This presentation will also discuss the effects physical and biogeochemical processes on the magnitude and variability of air-sea CO₂ fluxes. We observe a negative correlation between CO₂ flux and rainfall over monthly, seasonal, and annual timescales. This correlation however, can partly be explained by temperature, because increased rainfall is more common during the colder winter months. Nevertheless, rainfall affects CO₂ fluxes, both by rain-induced nutrient and organic matter runoff, as well as the physical effect of raindrops on air-sea gas exchange and the dilution of the air-sea boundary layer. We report that air-sea CO₂ fluxes are highly variable on short time scales, and that Kilo Nalu and Ala Wai are weak sources of CO₂ to the atmosphere, although after rainstorms and certain months in the winter season, the flux direction can reverse they become weak sinks for CO₂.