Biological production, export efficiency, and phytoplankton communities across 8000 km of the South Atlantic

Abstract

In situ oxygen tracers (triple oxygen isotope and oxygen/argon ratios) were used to evaluate meridional trends in surface biological production and export efficiency across~8000 km of the tropical and subtropical South Atlantic in March-May 2013. We used observations of picophytoplankton, nanophytoplankton, and microphytoplankton to evaluate community structure and diversity and assessed the relationships of these characteristics with production, export efficiency, and particulate organic carbon (POC) fluxes. Rates of productivity were relatively uniform along most of the transect with net community production (NCP) between 0 and 10 mmol O 2 m À2 d À1, gross primary production (GPP) between 40 and 100 mmol O 2 m À2 d À1 , and NCP/GPP, a measure of export efficiency, ranging from 0.1 to 0.2 (0.05-0.1 in carbon units). However, notable exceptions to this basin-scale homogeneity included two locations with highly enhanced NCP and export efficiency compared to surrounding regions. Export of POC and particulate nitrogen, derived from sediment traps, correlated with GPP across the transect, over which the surface community was dominated numerically by picophytoplankton. NCP, however, did not correlate with POC flux; the mean difference between NCP and POC flux was similar to published estimates of dissolved organic carbon export from the surface ocean. The interrelated rates of production presented in this work contribute to the understanding, building on the framework of better-studied ocean basins, of how carbon is biologically transported between the atmosphere and the deep ocean.Plain Language Summary Photosynthesis in the ocean results in the drawdown of atmospheric carbon dioxide. In this study, we use biological and chemical data from a cruise transiting 8000 km of the South Atlantic Ocean in order to study how biological communities in the surface ocean produced oxygen and took up carbon dioxide from the atmosphere in order to grow and how that carbon was consumed or transferred to greater depths away from the atmosphere. Specifically, we used dissolved gases to measure phytoplankton respiration and photosynthesis and sediment traps to collect falling particles; we also determined phytoplankton community structure at 5 m of water depth. This combination of information about the biological community, production, and export across a large region provides insights into the relationships underlying carbon cycling in the South Atlantic.

Organic sulfur: A spatially variable and understudied component of marine organic matter

“…Our cruise track spanned a broad segment of the western Atlantic Ocean and included samples from the surface ocean down to just above the seafloor. There were no significant differences in the flux of particulate organic carbon, measured at 125 m, along this transect (Durkin et al ), and net community production and gross primary production were relatively uniform (Howard et al ). We measured the concentrations of TOC and TON in our samples using well‐established protocols.…”

Insights into the controls on metabolite distributions along a latitudinal transect of the western Atlantic Ocean

“…Prochlorococcus was numerically more abundant in the surface waters of the south and equatorial Atlantic than Synechococcus, picoeukaryotes, and nanoeukaryotes (Howard et al 2017). In the southern and equatorial Atlantic, phytoplankton data were complemented by the prokaryotic community composition based on 16S rRNA gene sequences (Figure S4).…”

Triple Oxygen Isotopic Compositions of Ocean Water from the Mariana Trench

“…In recent years, it is observed that the mixed layer has a component of 17 O-enriched O 2 accumulated in the less vented thermocline by either entrainment of water or vertical turbulent flux from the thermocline, which results in an overestimation of mixed layer GOP. − Wurgaft et al added a term for GOP contribution from beneath the mixed layer to a simplified form of eq . Haskell et al divided the euphotic regions into the mixed layer and below mixed layer (BML) and constructed similar equations with GOP of the mixed layer influenced by the BML, and GOP of BML influenced by the deep water column below the euphotic depth.…”