Intermittency in phytoplankton bloom triggered by modulations in vertical stability

Madhavan Girijakumari Keerthi,Marina Lévy,Olivier Aumont

doi:10.1038/s41598-020-80331-z

Abstract

Seasonal surface chlorophyll (SChl) blooms are very chaotic in nature, but traditional bloom paradigms have climbed out of these subseasonal variations. Here we highlight the leading order role of wind bursts, by conjoining two decades of satellite SChl with atmospheric reanalysis in the Northwestern Mediterranean Sea. We demonstrate that weekly SChl fluctuations are in phase with weekly changes in wind stress and net heat flux during the intial state of the bloom in winter and early spring, thus expanding the convection shutdown hypothesis of bloom onset to subseasonal timescales. We postulate that the mechanism reflected by this link is intermittency in vertical stability due to short-term episodes of calm weather in winter or to stormy conditions in early spring, leading to short-term variations in light exposure or to events of vertical dilution. This strong intermittency in phytoplankton bloom may probably have important consequences on carbon export and trophic web structure and should not be overlooked.

Highlights

Seasonal surface chlorophyll (SChl) blooms are very chaotic in nature, but traditional bloom paradigms have climbed out of these subseasonal variations
We investigated the hypothesis that subseasonal modulations in vertical stability triggered by wind bursts explain subseasonal SChl variations during the initial states of the bloom in the Northwestern Mediterranean Sea
With our initial assumption that during winter and early spring, the rate of change in Net Heat Flux (NHF) measures the change in vertical stability, these results show that during the onset phase of the bloom, subseasonal fluctuations in SChl are driven by the intermittency in vertical stability: when NHF decrease, vertical stability decreases and SChl decreases, and vice versa

Summary

Introduction

Seasonal surface chlorophyll (SChl) blooms are very chaotic in nature, but traditional bloom paradigms have climbed out of these subseasonal variations. They relate the period of rapid growth of SChl between winter and spring to the change in vertical stability, and to the increased light exposure of the phytoplankton population associated with it.

Results

Conclusion