Abstract
In 2014, a sediment trap mooring was deployed adjacent to station Kuroshio Extension Observatory (KEO)’s National Ocean and Atmosphere Administration (NOAA) surface mooring. These data, from July 2014 to July 2016, are used here to investigate nutrient supply mechanisms that support ocean productivity in this oligotrophic region of the subtropical western North Pacific. Both years of sediment trap data show that biogenic material fluxes at ~ 5000 m increased between late winter (March) and late spring (June). Based on sea surface temperature and upper ocean water temperature profiles, from the NOAA surface buoy, and satellite-based surface chlorophyll-a , this increase was likely due to an increase of ocean productivity in early spring (March) that was supported by nutrients supplied by winter mixing. On the other hand, biogenic material fluxes also increased in October 2014, and between late December 2014 and January 2015 when concentrations of nutrients near the surface typically are low. Sea surface height anomalies and vertical profiles of water temperature in the upper 500 m showed cyclonic eddies passing station KEO and causing upwelling in late July–early August 2014 and November 2014. It appears that these events supplied nutrients to the upper layer, which then caused ocean productivity in the subsurface layer to temporally increase, resulting in increased deep biogenic material fluxes in autumn and winter. This interpretation of the data is consistent with a simple 3D physical-biological model simulation that shows meso-scale cyclonic eddies can supply nutrient to support new production at KEO. During the 2-year-long sediment trap deployments, several typhoons also passed near station KEO and near-inertial internal waves were observed near the nitracline depth after the typhoons passed. Although turbulent mixing caused by near-inertial internal wave could have possibly supplied nutrient to upper oligotrophic euphotic layer, numerical simulations of the turbulent nutrient supply indicate that enhanced turbulent diffusion across the nutrient concentration gradient did not supply enough nitrate to support the increase in biogenic material flux in autumn.
Highlights
Recent increases of atmospheric CO2 and associated global warming have caused warming, stratification, deoxygenation, and acidification of the ocean, collectively called “multiple stressors” (e.g., Bopp et al 2013 and references in their paper)
It is noted that if total mass flux (TMF) or organic carbon flux (OCF) increase observed in early October 2014 was due to increase of subsurface phytoplankton following an increase in nutrient supplied by CE1 in late July 2014, sinking velocity of the particulate biogenic materials is estimated to be less than 100 m day− 1 (4900 m/60 days)
Time-series observation of settling particle from a sediment trap deployed adjacent to a surface buoy, instrumented for meteorology, physical oceanography, and biogeochemistry, revealed that cyclonic eddies are likely an important mechanism for nutrient supply
Summary
Recent increases of atmospheric CO2 and associated global warming have caused warming, stratification, deoxygenation, and acidification of the ocean, collectively called “multiple stressors” (e.g., Bopp et al 2013 and references in their paper). Consistent with the K2S1 results, recently, using data from the National Oceanic and Atmospheric Administration (NOAA) Kuroshio Extension Observatory (KEO) surface mooring, Fassbender et al (2017) estimated the annual net community productivity (aNCP) in the surface mixed layer of oligotrophic North Pacific subtropical region to be 7 ± 3 mol-C m− 2 year− 1. In order to study nutrient supply mechanisms that support ocean productivity in the western North Pacific oligotrophic subtropical region, since July 2014, a deep-sea (~ 5000 m) sediment trap mooring has been maintained at station KEO, adjacent to the NOAA surface mooring. This study reports on results from the first 2 years, between July 2014 and June 2016, while drawing upon the longer meteorological and physical oceanographic time-series from the adjacent NOAA surface buoy in order to interpret seasonal variability in sediment trap data. The biological fields at the end of the 5 years were used as the initial condition for the physical-biological model, driven by the JRA25 from 1995 to 2012
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