The relation of mixed‐layer net community production to phytoplankton community composition in the Southern Ocean

Abstract

Surface ocean productivity mediates the transfer of carbon to the deep ocean and in the process regulates atmospheric CO 2 levels. A common axiom in oceanography is that large phytoplankton contribute disproportionally to the transfer of carbon to the deep ocean because of their greater ability to escape grazing pressure, build biomass, and sink. In the present study, we assessed the relationship of net community production to phytoplankton assemblages and plankton size distribution in the Sub-Antarctic Zone and northern reaches of the Polar Frontal Zone in the Australian sector of the Southern Ocean. We reanalyzed and synthesized previously published estimates of O 2 /Ar net community oxygen production (NCP) and triple-O 2 isotopes gross primary oxygen production (GPP) along with microscopic and pigment analyses of the microbial community. Overall, we found that the axiom that large phytoplankton drive carbon export was not supported in this region. Mixed-layer-depth-integrated NCP was correlated to particulate organic carbon (POC) concentration in the mixed layer. While lower NCP/GPP and NCP/POC values were generally associated with communities dominated by smaller plankton size (as would be expected), these communities did not preclude high values for both properties. Vigorous NCP in some regions occurred in the virtual absence of large phytoplankton (and specifically diatoms) and in communities dominated by nanoplankton and picoplankton. We also observed a positive correlation between NCP and the proportion of the phytoplankton community grazed by microheterotrophs, supporting the mediating role of grazers in carbon export. The novel combination of techniques allowed us to determine how NCP relates to upper ocean ecosystem characteristics and may lead to improved models of carbon export.

Ecosystem metabolism in salt marsh tidal creeks and ponds : applying triple oxygen isotopes and other gas tracers to novel environments

“…There was no significant correlation (KruskalWallis test,section 2.6) between these community structures and GPP, NCP, NCP/GPP, or POC flux (Figure 7d-g). As in a similar Southern Ocean dataset [Cassar et al 2015], diatom influenced community structures did not necessarily have greater production or export as might be expected.…”

“…Plankton community structure is variable across space (tens of meters to ocean basins) and time (hours to decadal time scales), and may be a major factor determining the magnitudes of production rates and carbon export, and thus the efficiency of the biological carbon pump in sequestering carbon to the deep ocean [Karl et al 2001, 2012, Vaillancourt et al 2003, Richardson and Jackson 2007, Henson et al 2012. Investigators have described correlations of community structures with patterns of primary production and carbon export in a variety of nutrient regimes [Pollard et al 2009, Moreno-Ostos et al 2011, Cassar et al 2015. Transition regions at gyre edges between higher and lower nutrient regimes can be zones of high biomass and intensified production , Ostle et al 2015 with unique phytoplankton communities distinct from the regions to either side [Kavanaugh et al 2014a, Palevsky et al 2013, Ribalet et al 2010.…”

Interannual variability in net community production at the Western Antarctic Peninsula region (1997–2014)

“…Cassar et al . [] recently highlighted a strong relationship between NCP and [POC] in the Australian sector of the Southern Ocean. In the supporting information, we present a satellite‐[POC] algorithm specific to the Pal‐LTER region.…”

“…The spatial and temporal resolution provided by the various in situ methods presented above is however not sufficient to characterize the seasonal cycle of carbon export production over the entire WAP region. On these spatial and temporal scales, NCP can be estimated based on empirical or mechanistic relationships between NCP and ocean‐color parameters such as chlorophyll a concentration ([Chl]) and particulate organic carbon concentration ([POC]) [ Cassar et al ., ; Huang et al ., ; Reuer et al ., ]. For example, Chang et al .…”

Southern Ocean Phytoplankton in a Changing Climate

“…The SAZ is more productive in the Atlantic sector and around 170 • W where iron concentrations are higher due to the proximity of land (Figure 2) (Comiso et al, 1993;de Baar et al, 1995;Moore and Abbott, 2000). Despite the low levels of primary productivity, export efficiency is high in HNLC waters of the SAZ, suggesting that small taxa contribute to a high proportion of carbon export (Trull et al, 2001b;Lam and Bishop, 2007;Cassar et al, 2015;Laurenceau-Cornec et al, 2015).…”