Factors controlling the competition between <i>Phaeocystis</i> and diatoms in the Southern Ocean and implications for carbon export fluxes

• Main Authors: C. Nissen, M. Vogt
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-01-01
• Series: Biogeosciences
• Online Access: https://bg.copernicus.org/articles/18/251/2021/bg-18-251-2021.pdf

<p>The high-latitude Southern Ocean phytoplankton community is shaped by the competition between <i>Phaeocystis</i> 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<span class="inline-formula">^∘</span> S. We improved ROMS-BEC by adding an explicit parameterization of <i>Phaeocystis</i> 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 <i>Phaeocystis</i> contribute <span class="inline-formula">46±21</span> <span class="inline-formula">%</span> (1<span class="inline-formula"><i>σ</i></span> in space) and <span class="inline-formula">40±20</span> <span class="inline-formula">%</span> to annual net primary production (NPP) and particulate organic carbon (POC) export south of 60<span class="inline-formula">^∘</span> S, respectively, making them an important contributor to high-latitude carbon cycling. In our simulation, the relative importance of <i>Phaeocystis</i> 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 <i>Phaeocystis</i> at low irradiances promotes the succession from <i>Phaeocystis</i> 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.</p>