Abstract
The North Atlantic bloom corresponds to a strong seasonal increase in phytoplankton that produces organic carbon through photosynthesis. It is still debated what physical and biological conditions trigger the bloom, because comprehensive time series of the vertical distribution of phytoplankton biomass are lacking. Vertical profiles from nine floats that sampled the waters of the North Atlantic every few days for a couple of years reveal that phytoplankton populations start growing in early winter at very weak rates. A proper bloom with rapidly accelerating population growth rates instead starts only in spring when atmospheric cooling subsides and the mixed layer rapidly shoals. While the weak accumulation of phytoplankton in winter is crucial to maintaining a viable population, the spring bloom dominates the overall seasonal production of organic carbon.
Highlights
The bloom in the sub-polar North Atlantic represents the most dramatic seasonal increase in phytoplankton biomass for the global ocean
In order to explain this winter phytoplankton biomass accumulation, Behrenfeld argued that the net population growth was triggered by a decrease in loss rates rather than an increase in division rates, which were still close to their minimum in early winter
For each yearly time series, we focus on the period from September to August to fully cover the seasons of interest, winter through spring
Summary
The bloom in the sub-polar North Atlantic represents the most dramatic seasonal increase in phytoplankton biomass for the global ocean. In order to explain the lack of acceleration in spring population growth, it was presumed that any increase in phytoplankton cell division was matched by a corresponding increase in grazing rates According to these observations, the bloom appeared to start in winter and it developed over many months with low rates of accumulation and without any acceleration in spring. Satellite remote sensing of ocean color measures only the surface phytoplankton concentration and returns incomplete maps due to cloud cover, especially at high latitudes These limitations have only recently been overcome through the development of miniaturized biooptical sensors installed on profiling floats, the so-called Biogeochemical-Argo (BGC-Argo) floats, which acquire time series of key variables whatever the sea conditions and over several full annual cycles[11]. Our main conclusion is that phytoplankton populations start increasing in winter, but at very weak rates, while the explosive acceleration in these rates, typical of blooms, is not observed until spring
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