Decoupling of ΔO 2 ∕Ar and particulate organic carbon dynamics in nearshore surface ocean waters
2020
Abstract. We report results from two Lagrangian drifter surveys off the
Oregon coast, using continuous shipboard sensors to estimate mixed-layer
gross primary productivity (GPP), community respiration (CR), and net
community production (NCP) from variations in biological oxygen saturation
( ΔO2∕Ar ) and optically derived particulate organic carbon
(POC). At the first drifter survey, conducted in a nearshore upwelling zone
during the development of a microplankton bloom, net changes in ΔO2∕Ar and [POC] were significantly decoupled. Differences in GPP and
NCP derived from ΔO2∕Ar (NCP O 2 / Ar ) and POC (NCP POC)
time series suggest the presence of large POC losses from the mixed layer.
At this site, we utilized the discrepancy between NCP O 2 / Ar and
NCP POC , and additional constraints derived from surface water excess
nitrous oxide ( N2O ), to evaluate POC loss through particle export, DOC
production, and vertical mixing fluxes. At the second drifter survey,
conducted in lower-productivity, density-stratified offshore waters, we also
observed offsets between ΔO2∕Ar and POC-derived GPP and CR
rates. At this site, however, net [POC] and ΔO2∕Ar changes
yielded closer agreement in NCP estimates, suggesting a tighter relationship
between production and community respiration, as well as lower POC loss rates.
These results provide insight into the possibilities and limitations of
estimating productivity from continuous underway POC and ΔO2∕Ar
data in contrasting oceanic waters. Our observations support the use of diel
POC measurements to estimate NCP in lower-productivity waters with limited
vertical carbon export and the potential utility of coupled O2 and
optical measurements to estimate the fate of POC in high-productivity
regions with significant POC export.
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