FY 2023 Quantifying net community production and calcification at Station ALOHA near Hawai’i: Insights and limitations from a dual tracer carbon budget approach Knor, L.A.C.M., C.L. Sabine, A.J. Sutton, A.E. White, J. Potemra, and R.A. Weller Global Biogeochem. Cycles, 37(7), e2022GB007672, doi: 10.1029/2022GB007672, View open access article at AGU/Wiley (external link) (2023) A budget approach is used to disentangle drivers of the seasonal mixed layer carbon cycle at Station ALOHA (A Long-term Oligotrophic Habitat Assessment) in the North Pacific Subtropical Gyre (NPSG). The budget utilizes data from the WHOTS (Woods Hole—Hawaii Ocean Time-series Site) mooring, and the ship-based Hawai'i Ocean Time-series (HOT) in the NPSG, a region of significant oceanic carbon uptake. Parsing the carbon variations into process components allows an assessment of both the proportional contributions of mixed layer carbon drivers and the seasonal interplay of drawdown and supply from different processes. Annual net community production reported here is at the lower end of previously published data, while net community calcification estimates are 4- to 7-fold higher than available sediment trap data, the only other estimate of calcium carbonate export at this location. Although the observed seasonal cycle in dissolved inorganic carbon in the NPSG has a relatively small amplitude, larger fluxes offset each other over an average year. Major supply comes from physical transport, especially lateral eddy transport throughout the year and entrainment in the winter, offset by biological carbon uptake in the spring. Gas exchange plays a smaller role, supplying carbon to the surface ocean between Dec-May and outgassing in Jul-Oct. Evaporation-precipitation (E-P) is variable with precipitation prevailing in the first half and evaporation in the second half of the year. The observed total alkalinity signal is largely governed by E-P with a somewhat stronger net calcification signal in the wintertime. Plain Language Summary. The ocean carbon cycle is a complicated system where chemical compounds react, are moved by ocean physics, altered by organisms, and exchange with CO2 in the atmosphere. To explore how the ocean will continue to take up CO2 from the atmosphere and how much will be removed into the deep ocean, we need to know how these processes influence ocean carbon. Here, we investigate them over a year. We create a model from observations of two carbon compounds, together with calculated estimates of processes (evaporation and precipitation, transport through the water, and air-sea exchange) to back out the influence of two important reaction pairs executed by organisms: Photosynthesis and respiration, and calcification and dissolution. Over a year, the surface community at this location near Hawai'i in the Pacific photosynthesizes more than it respires, removing 53 g of CO2 per square meter. Also, marine calcifiers perform calcification, and our estimates are much higher than previous measurements from sediment traps. Gas exchange and evaporation-precipitation vary with the seasons in opposite directions, and there are carbon inputs from horizontal transport throughout the year and from water column mixing in the winter. Feature Publications | Outstanding Scientific Publications Contact Sandra Bigley | Help
