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

Thursday, 18 December 2014
Gerianne J Terlouw, Patrick S Drupp, Eric Heinen De Carlo and Michael Tomlinson, University of Hawaii at Manoa, Honolulu, HI, United States
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.