Biogenic sinking particle fluxes and sediment trap collection efficiency at Ocean Station Papa

Abstract

Comprehensive field observations characterizing the biological carbon pump (BCP) provide the foundation needed to constrain mechanistic models of downward particulate organic carbon (POC) flux in the ocean. Sediment traps were deployed three times during the EXport Processes in the Ocean from RemoTe Sensing campaign at Ocean Station Papa in August–September 2018. We propose a new method to correct sediment trap sample contamination by zooplankton “swimmers.” We consider the advantages of polyacrylamide gel collectors to constrain swimmer influence and estimate the magnitude of possible trap biases. Measured sediment trap fluxes of thorium-234 are compared to water column measurements to assess trap performance and estimate the possible magnitude of fluxes by vertically migrating zooplankton that bypassed traps. We found generally low fluxes of sinking POC (1.38 ± 0.77 mmol C m–2 d–1 at 100 m, n = 9) that included high and variable contributions by rare, large particles. Sinking particle sizes generally decreased between 100 and 335 m. Measured 234Th fluxes were smaller than water column 234Th fluxes by a factor of approximately 3. Much of this difference was consistent with trap undersampling of both small (<32 μm) and rare, large particles (>1 mm) and with zooplankton active migrant fluxes. The fraction of net primary production exported below the euphotic zone (0.1% light level; Ez-ratio = 0.10 ± 0.06; ratio uncertainties are propagated from measurements with n = 7–9) was consistent with prior, late summer studies at Station P, as was the fraction of material exported to 100 m below the base of the euphotic zone (T100, 0.55 ± 0.35). While both the Ez-ratio and T100 parameters varied weekly, their product, which we interpret as overall BCP efficiency, was remarkably stable (0.055 ± 0.010), suggesting a tight coupling between production and recycling at Station P.

A Visual Tour of Carbon Export by Sinking Particles

“…The modeled POC flux collected by the surface‐tethered trap was greater than that collected by the neutrally buoyant trap on 8 out of 12 occasions and was never lower than the neutrally buoyant trap. This apparent collection bias is consistent with measured differences in POC fluxes collected by these two platform types (Estapa et al., 2021 ). Differences between the modeled POC fluxes collected by the surface‐tethered and neutrally buoyant traps were comparable to differences caused by small scale spatial variation (Figure S6b in Supporting Information S1 ) (0.17–7.9 mmol C m −2 d −1 , or 31%–114% of mean flux).…”

“…Modeled carbon flux in the subarctic North Pacific was an order of magnitude lower than at the New England shelf break at all observed depths (95–510 m), and matched the scale and general depth profile of measured fluxes (Estapa et al., 2021 ) even though carbon modeling parameters were not fit from this location (Figures 5c–5e ). Particle composition varied drastically with depth and across the three deployment periods.…”

“…Although measured POC fluxes in the subarctic N. Pacific depended on particle surface area, the POC fluxes modeled from particle images at this location did not depend on measured POC because particle type‐specific parameters of shape, volume, and carbon content were determined independently. The agreement between the measured (Estapa et al., 2021 ) and modeled POC fluxes can therefore be viewed as a validation of the model estimates. The gel particle sample collected at one subtropical station (location 4, Table 1 ) did not have a corresponding bulk POC flux measurement.…”

“…Samples were also collected at two locations in the subtropical North Pacific and one location in the California Current on a transit between Honolulu, Hawaii and Portland, Oregon aboard the R/V Falkor between January 24 and February 20, 2017 (FK170124). Additional samples were collected during the NASA EXPORTS field campaign near Ocean Station PAPA in the subarctic North Pacific during three successive collection periods ("epochs") between August 10 and September 12, 2018 (Figure S1 in Supporting Information S1 , Table 1 ; Estapa et al., 2021 ; Siegel et al., 2021 ). Sediment trap tubes were deployed on various platform designs, including neutrally buoyant sediment traps (Estapa et al., 2020 ), surface tethered sediment traps (Knauer et al., 1979 ) and on the base of a Wirewalker profiler (Rainville & Pinkel, 2001 ).…”

“…In total, three parameters were fit to 13 observations from trap platforms that carried both a polyacrylamide gel layer and poisoned brine (Figure 3 ), and included samples from location 1, 2, 3 and 5 in Table 1 . Observations made during the EXPORTS cruise in the subarctic N. Pacific were not included in fitting parameters because gel images were used to correct POC fluxes from that cruise (Estapa et al., 2021 ). Although measured POC fluxes in the subarctic N. Pacific depended on particle surface area, the POC fluxes modeled from particle images at this location did not depend on measured POC because particle type‐specific parameters of shape, volume, and carbon content were determined independently.…”