Sinking phytoplankton associated with carbon flux in the Atlantic Ocean

Abstract

The composition of sinking particles and the mechanisms leading to their transport ultimately control how much carbon is naturally sequestered in the deep ocean by the "biological pump." While detrital particles often contain much of the sinking carbon, sinking of intact phytoplankton cells can also contribute to carbon export, which represents a direct flux of carbon from the atmosphere to the deep ocean by circumventing the surface ocean food web. Phytoplankton that contributed to carbon flux were identified in sinking material collected by short-term sediment trap deployments conducted along a transect off the eastern shore of South America. Particulate organic carbon flux at 125 m depth did not change significantly along the transect. Instead, changes occurred in the composition and association of phytoplankton with detrital particles. The fluxes of diatoms, coccolithophores, dinoflagellates, and nano-sized cells at 125 m were unrelated to the overlying surface population abundances, indicating that functional-group specific transport mechanisms were variable across locations. The dominant export mechanism of phytoplankton at each station was putatively identified by principal component analysis and fell into one of three categories; (1) transport and sinking of individual, viable diatom cells, (2) transport by aggregates and fecal pellets, or (3) enhanced export of coccolithophores through direct settling and/or aggregation.

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“…At three stations (locations 2, 9, and 20) the Knorr remained stationary for one to two days. At three other stations, the ship followed drifting particle traps [Durkin et al 2016] in a quasi-Lagrangian manner for one to three days (locations 7, 25, 31). At all of these stations with longer sampling durations (Table 1), multiple discrete gas tracer samples were collected either at arrival and departure from the station, or as a time-series over the duration of the stay.…”

“…Gas tracer estimates of production across the transect were compared with concurrent underway observations of surface cyanobacterial and picoeukaryote populations [Hennon 2015, Hennon et al in prep]. Data were also compared with concurrent observations of surface eukaryotic phytoplankton abundance and the phytoplankton and carbon fluxes captured in free-floating particle interceptor traps at 125 m reported by Durkin et al [2016]. Particulate nitrogen fluxes analyzed simultaneously with particulate carbon were not previously reported, but analyzed as in Durkin et al [2016].…”

“…Production rates are tabulated in the supplemental information (Table S2) along with all other results from this work and the previously reported results of Durkin et al [2016] and Hennon et al [in prep]. All results are averaged by sample location, including over multiday periods.…”

“…The concentration (cells L -1 ) of surface phytoplankton communities of picoplankton (0.5-2 µm) populations identified by continuous underway flow cytometry (reported by Hennon [2015] and Hennon et al [in prep]) and nano-and microplankton (~5 µm and 20-200 µm) identified by light microscopy (reported in Durkin et al [2016]) were used to assess community structure and diversity. In brief, Prochlorococcus, Synechococcus, and picoeukaroyte abundances were identified based on characteristic fluorescence and light scattering [Ribalet et al 2010, Swallwell et al 2011.…”

“…For this study, continuous data and associated statistics were averaged over the 30 minutes surrounding discrete underway sampling, or, if stationary, over the duration of particle trap deployment, in order to integrate over a similar time and space resolution as the underway gas sampling. Surface phytoplankton abundances reported by Durkin et al [2016] at the same locations as trap deployments were quantified by light microscopy from 0.5 L samples. Microphytoplankton morphotypes were identified to the genus level where possible.…”