Abstract

Phytoplankton emit fluorescent red light that can be detected by sensors on earth-orbiting satellites. This fluorescence signal has the potential to reveal information about the physiological status of phytoplankton on a global scale. Realisation of this potential is however limited because the controls governing fluorescence signatures are not well constrained. In order to better understand these controls we conducted a suite of measurements and experiments on cruises over a period of 4 months in the South Atlantic and Southern Ocean (via the UK-GEOTRACES and UK-DIMES projects). We found physiological changes in phytoplankton over broad oceanographic regions that mean a dynamic correction needs to be added to existing methods attempting to deconvolve satellite-retrieved fluorescence. We have developed and applied such a correction, revealing fluorescence signatures that correlate well with anticipated, and experimentally confirmed, regions of iron stress.

In austral summer, phytoplankton in the ‘high-nitrate low-chlorophyll’ (HNLC) regions of the Southern Ocean are widely assumed to be limited by iron only. I will present evidence from a suite of iron and volcanic ash enrichment experiments that suggest (i) phytoplankton in these waters respond strongly to supply of volcanic ash, and (ii) manganese could potentially be limiting phytoplankton growth in the most oligotrophic areas of the Southern Ocean.