Abstract
Intensive sampling at the coastal waters of the central Red Sea during a period of thermal stratification, prior to the main seasonal bloom during winter, showed that vertical patches of prokaryotes and microplankton developed and persisted for several days within the apparently density uniform upper layer. These vertical structures were most likely the result of in situ growth and mortality (e.g., grazing) rather than physical or behavioural aggregation. Simulating a mixing event by adding nutrient-rich deep water abruptly triggered dense phytoplankton blooms in the nutrient-poor environment of the upper layer. These findings suggest that vertical structures within the mixed layer provide critical seeding stocks that can rapidly exploit nutrient influx during mixing, leading to winter bloom formation.
Highlights
We hypothesise that phytoplankton patchiness occurs and persists within the mixed layer (ML), but it is important for the onset of seasonal bloom, especially in warm oligotrophic seas where relatively calm weather conditions are common even in the blooming season, allowing for biological heterogeneity within the ML
Using the temporal succession of depth profiles we have illustrated the presence of patchiness in the vertical distribution of planktonic organisms within the ML
We have shown how different mechanisms may act to shape the structure of planktonic communities at different layers within the ML, and how the appropriate conditions may trigger a bloom in these nutrient-poor waters within a few days
Summary
We hypothesise that phytoplankton patchiness occurs and persists within the ML, but it is important for the onset of seasonal bloom, especially in warm oligotrophic seas where relatively calm weather conditions are common even in the blooming season, allowing for biological heterogeneity within the ML. In these areas, light is seldom limiting for phytoplankton growth, but nutrients are found at very low concentrations in surface waters most of the year. We argue that vertical patchiness of small plankton within the ML (here defined as the depth with a change of 0.125 in sigma-t from the surface15) was the result of a trade-off between nutrient use and grazing mortality
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