Control mechanisms in nutrient dynamics and biological productivity in the eastern equatorial Pacific Ocean

摘要

The equatorial Pacific has been recognized for decades as a major region of enhanced biological production which is due to persistently cool waters of equatorial upwelling. Early observations of unused nitrate and relatively low phytoplankton biomass in the equatorial Pacific led to the question of "Why isn't the equatorial Pacific greener?" The main objective of my dissertation work is to unravel the roles and interactions of regulatory mechanisms in the eastern equatorial Pacific Ocean, challenged and inspired by the progress represented by the JGOFS EqPac (1992) and the Equatorial Biocomplexity (2004 and 2005) projects, as well as some recent development of modeling capabilities. In this study, I use a box model approach to construct a complete iron (Fe) budget in the equatorial Pacific, which reveals the importance of upwelling in supplying this growth-limiting nutrient. Moreover, through coupled physical and biological model simulations for 2004 and 2005, I reveal the important role that tropical instability waves play in regulating supply and diatom uptake of another limiting nutrient: silicon (Si). Extending the model simulations from 1991 to 2009, I then examine the role of El Nino Southern Oscillation in controlling interannual to decadal variability in nutrient dynamics and biological productivity in the eastern equatorial Pacific. The results of this dissertation work point at a surprising resilience of the biological system to long-term changes in physical environment in the Pacific Ocean. I speculate that Si and Fe co-limitation and a strong grazing pressure create a well-balanced ecosystem where the diatom population cannot thrive, thus explaining why the equatorial Pacific is not greener.