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

Ir Schloss,Me Ferrario,Aa Bianchi,Cf Balestrini,D Ruiz Pino,A Poisson,Ha Ochoa,R Codina,Ga Ferreyra,Go Almandoz

doi:10.3354/meps332093

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 pro- duction (NCP) and gross primary production (GPP) were (1) compared with the difference in the par- tial 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%, indi- cating that net photosynthesis could have played a dominant role in the CO2 dynamics. At most sta- tions, the production:respiration (GPP:R) ratio was greater than 1, indicating that planktonic commu- nities 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 relation- ship 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 flagel- late-dominated), other physical and biological processes are thought to modify the CO2 dynamics when small flagellates are the prevailing phytoplankton group.

Highlights

Oceans play a key role in modulating CO2 dynamics, storing nearly 48% of the anthropogenic CO2 emitted into the atmosphere (Sabine et al 2004, Takahashi 2004)
The aim of the present study was to determine whether there is a relationship between the plankton community composition and (1) its activity, (2) the distribution of ΔpCO2 levels and (3) the distribution of O2 saturation levels in the surface waters of the SW Atlantic Ocean
The simultaneous presence of the lowest seawater pressure of CO2 (pCO2) and the highest chl a concentrations on the stratified side of tidal fronts led to the conclusion that the combined physical–biological processes control pCO2 dynamics in surface waters over almost all the Argentinean shelf

Summary

Introduction

Oceans play a key role in modulating CO2 dynamics, storing nearly 48% of the anthropogenic CO2 emitted into the atmosphere (Sabine et al 2004, Takahashi 2004). Some latitudinal differences have been observed in natural CO2 concentrations (Taylor & Orr 2000), the seasonal and geographical variations in the partial pressure of CO2 (pCO2) in surface water are much greater than those of atmospheric pCO2 (Takahashi et al 2002). Seawater pCO2 depends on physical factors and biogeochemical processes (Takahashi et al 1993). The main factors controlling seawater pCO2 are temperature, affecting CO2 solubility and acid constants; winds, influencing the gas transfer velocity; carbonate dissolution in deep water, eventually neutralizing the effect of CO2 on ocean acidity; calcification, producing a net pCO2 increase; and organic carbon production (photosynthesis) and consumption (respiration), decreasing and increasing seawater pCO2, respectively. A pCO2 undersaturation together with an O2 supersaturation relative to atmospheric concentrations of both gases has been shown to be related to active photosynthesis (Carrillo et al 2004). The opposite could indicate net respiration by the plankton community

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