Abstract
Abstract. The high-latitude Southern Ocean phytoplankton community is shaped by the competition between Phaeocystis and silicifying diatoms, with the relative abundance of these two groups controlling primary and export production, the production of dimethylsulfide, the ratio of silicic acid and nitrate available in the water column, and the structure of the food web. Here, we investigate this competition using a regional physical–biogeochemical–ecological model (ROMS-BEC) configured at eddy-permitting resolution for the Southern Ocean south of 35∘ S. We improved ROMS-BEC by adding an explicit parameterization of Phaeocystis colonies so that the model, together with the previous addition of an explicit coccolithophore type, now includes all biogeochemically relevant Southern Ocean phytoplankton types. We find that Phaeocystis contribute 46±21 % (1σ in space) and 40±20 % to annual net primary production (NPP) and particulate organic carbon (POC) export south of 60∘ S, respectively, making them an important contributor to high-latitude carbon cycling. In our simulation, the relative importance of Phaeocystis and diatoms is mainly controlled by spatiotemporal variability in temperature and iron availability. In addition, in more coastal areas, such as the Ross Sea, the higher light sensitivity of Phaeocystis at low irradiances promotes the succession from Phaeocystis to diatoms. Differences in the biomass loss rates, such as aggregation or grazing by zooplankton, need to be considered to explain the simulated seasonal biomass evolution and carbon export fluxes.
Highlights
Phytoplankton production in the Southern Ocean (SO) regulates the uptake of anthropogenic carbon in marine food webs and controls global primary production via the lateral export of nutrients to lower latitudes (e.g., Sarmiento et al, 2004; Palter et al, 2010)
In the 5-phytoplankton functional type (PFT) Baseline simulation of Regional Ocean Modeling System (ROMS)-BEC, total summer chlorophyll is highest close to the Antarctic continent (> 10 mg chl m−3) and decreases northwards to values < 1 mg chl m−3 close to the open northern boundary (Fig. 1a). While this south–north gradient is in broad agreement with remotely sensed chlorophyll concentrations (Fig. 1b), our model generally overestimates high-latitude chlorophyll levels, which has already been noted for the 4-PFT setup of ROMS-BEC (Nissen et al, 2018)
With Phaeocystis added, the model overestimates annual mean satellite-derived surface chlorophyll biomass estimates by 18 % (40.8 Gg chl in ROMS-BEC between 30–90◦ S compared to 34.5 Gg chl in the MODIS Aqua chlorophyll product; Table 3; NASAOBPG, 2014a; Johnson et al, 2013) and satellite-derived net primary production (NPP) by 38 %–42 % (17.2 compared to 12.1–12.5 Pg C yr−1; Table 3; Behrenfeld and Falkowski, 1997; O’Malley, 2016; Buitenhuis et al, 2013)
Summary
Phytoplankton production in the Southern Ocean (SO) regulates the uptake of anthropogenic carbon in marine food webs and controls global primary production via the lateral export of nutrients to lower latitudes (e.g., Sarmiento et al, 2004; Palter et al, 2010). The amount and stoichiometry of these laterally exported nutrients are determined by the combined action of multiple types of phytoplankton with differing ecological niches and nutrient requirements. Despite their important role, the drivers of phytoplankton biogeography and competition and the relative contribution of different phytoplankton groups to SO carbon cycling are still poorly quantified. As climate change is expected to differentially impact the competitive fitness of different phytoplankton groups and their contribution to total net primary production (NPP; IPCC, 2014; Constable et al, 2014; Deppeler and Davidson, 2017) with a likely increase in Published by Copernicus Publications on behalf of the European Geosciences Union
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