Phytoplankton productivity in the North Pacific ocean since 1900 and implications for absorption of anthropogenic CO₂

Abstract

THE world's carbon budget has not been in steady state since the beginning of the Industrial Revolution¹. At present, carbon dioxide released by anthropogenic activities adds about 7±1.2 gigatonnes (Gt)Cyr⁻¹ to the atmosphere, of which about 2Gt Cyr⁻¹ is thought to be sequestered in the oceans². In the steady state, phytoplankton fix about 35–50 Gt C yr⁻¹, representing a significant component of the natural carbon cycle¹. If ocean productivity were changing, these biological processes could have a significant influence on anthropogenic CO₂ levels by drawing down the CO₂ concentration in surface waters and increasing the concentration gradient across the air–sea interface^1,3,4. The question of productivity changes is unresolved, however^2,5,6. Venrick et al.⁷ reported that phytoplankton chlorophyll concentrations had roughly doubled in the central North Pacific gyre between 1965 and 1985. Here we use historical records of Secchi depth data to investigate whether such dramatic changes in phytoplankton biomass have occurred throughout the North Pacific ocean during this century. We find that, although very minor changes may have occurred in this basin over the past 70 years, they are too small to have a significant effect on the rise in atmospheric CO₂ concentrations.