Intermittent Upwelling Events Trigger Delayed, Major, and Reproducible Pico-Nanophytoplankton Responses in Coastal Oligotrophic Waters

Pico-nanophytoplankton organisms are dominant in oceanic oligotrophic areas but their adaptive growth rates make their contribution to the carbon cycle difficult to estimate. Here we address their response capacities after sporadic wind gusts causing upwelling events in a coastal Mediterranean station. When the water column is stratified, corresponding to oligotrophic conditions, these events generate intense short-lived nutrient pulses and seawater temperature drops lasting 6 days on average with decreases up to 10 degrees C. Using an automated flow cytometer and statistical rupture-detection methods, we characterize the responses of five pico-nanophytoplankton functional groups at a two-hour frequency from September 2019 to November 2021. These events trigger delayed increases in both abundances and biomasses following similar patterns for most groups that can overpass spring bloom values, and are immediately followed by an overall decrease, suggesting a clear physical driver. These submesoscale events, due to their short duration, are poorly represented in coastal carbon budgets.