Over the last ten years, satellite and geographically constrained in
situ observations largely focused on the northern hemisphere have
suggested that annual phytoplankton biomass cycles in bloom-forming
ocean regions cannot be fully understood from environmental properties
controlling phytoplankton division rates (e.g., nutrients and light).
Here, we use multi-year observations from a very large array of robotic
drifting buoys in the Southern Ocean to determine key factors governing
phytoplankton biomass dynamics over the annual cycle. Our analysis
reveals phytoplankton blooming events occurring during periods of
declining division rates, an observation that clearly highlights the
importance of changing loss processes in dictating the evolution of the
bloom. Bloom magnitude is found to be greatest in areas with high
dissolved iron concentrations, consistent with iron being a
well-established primary limiting nutrient in the Southern Ocean.
Projections for expected future seasonal variations in nutrient and
light availability indicate a 10% change in phytoplankton division rate
may be associated with a 50% reduction in mean bloom magnitude and
annual primary productivity in the Southern Ocean. Our results highlight
the importance of quantifying and accounting for both changing
phytoplankton division and loss processes when modeling future changes
in phytoplankton bloom cycles.