Quantifying net community production and calcification at Station ALOHA
near Hawai’i: Insights and limitations from a dual tracer carbon budget
approach
Abstract
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
North Pacific Subtropical Gyre (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 (DIC) in the NPSG has a relatively small amplitude, larger fluxes
offset each other over an average year, with major supply from physical
transport, especially lateral eddy transport throughout the year and
entrainment in the winter, and 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- 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.