Role of plankton communities in sea–air variations in pCO2 in the SW Atlantic Ocean

摘要

ABSTRACT: The influence of the plankton community structure on carbon dynamics was studied in the surface waters of the Argentinean continental shelf (SW Atlantic Ocean) in summer and fall 2002, 2003 and 2004. The horizontal changes in plankton community respiration (R), net community production (NCP) and gross primary production (GPP) were (1) compared with the difference in the partial pressure of CO₂ (pCO₂) between the sea surface and the atmosphere (ΔpCO₂), (2) compared with oxygen saturation and (3) related to the microscopic phytoplankton assemblages. This area, which has recently been shown to be a CO₂ sink, had an average surface oxygen saturation of 108.1%, indicating that net photosynthesis could have played a dominant role in the CO₂ dynamics. At most stations, the production:respiration (GPP:R) ratio was greater than 1, indicating that planktonic communities were autotrophic; the average GPP:R ratio for the whole study was 2.99. Phytoplankton biomass (chlorophyll a) and NCP showed an inverse relationship with ΔpCO₂ and a direct relationship with %O₂ saturation when phytoplankton assemblages were dominated by diatoms (30% of the stations). This was not the case when small (≤5 µm) flagellates were the most abundant organisms. Although NCP was mostly positive for both groups of stations (i.e. diatom-dominated or small flagellate-dominated), other physical and biological processes are thought to modify the CO₂ dynamics when small flagellates are the prevailing phytoplankton group.