Physical and ecological uncertainties in the widespread implementation of controlled upwelling in the North Pacific Subtropical Gyre

Abstract

Based on the recent hypothesis of Karl & Letelier (2008; Mar Ecol Prog Ser 364:257–268), Fennel (2008; Mar Ecol Prog Ser 371:301–303) presents a model and initiates an important discussion on the potential effect that widespread controlled upwelling of deep water in the North Pacific Subtropical Gyre (NPSG) may have on upper water column stratification, nitrogen (N2) fixation, and C sequestration. Fennel concludes that the upwelling required to support the sequestration of 1Gt C yr–1 would deepen the mixed-layer significantly, inhibiting N2 fixation and precluding the enhancement of the biological carbon pump. However, her model does not include the role that solar radiant heating plays in the maintenance of the upper ocean stratification, nor does she discuss observations suggesting that N2 fixation in the NPSG is never completely suppressed. When the solar radiance absorbed by the mixed-layer is considered, the upper ocean stratification is maintained in Fennel’s model. Nevertheless, and as Fennel suggests, the effects that a basin scale long-term implementation of controlled upwelling may have on the pelagic ecosystem are difficult to predict due to the complex nature of the system and our incomplete understanding of feedback mechanisms between ocean physics, biology and climate. For this reason, we contend that controlled upwelling of deep water in oligotrophic regions should be viewed as originally concluded by Karl & Letelier (2008): a tool to study the response of pelagic microbial assemblages to perturbations at different spatial and temporal scales, rather than a strategy to stabilize climate through the large-scale manipulation of poorly understood ecosystems.