Abstract
The evolution of the near-surface phytoplankton bloom towards a Deep Chlorophyll Maximum (DCM) in mid-latitudes and subpolar regions of the global ocean is a well-known biological feature. However, our knowledge about the exact mechanism that determines the end of the bloom and its irreversible evolution towards a DCM is still limited. In this work, combining satellite and in-situ oceanographic data together with reanalysis data, we investigate why and when this transition between the near-surface phytoplankton bloom and the development of a DCM occurs. For this aim, we investigate the links between changes in air-sea heat exchanges, the near-surface signature of phytoplankton bloom, and the water column vertical structure by calculating the mixed layer depth (MLD) and depth of the DCM on hydrographic and chlorophyll profiles. We find that the occurrence of the last convective mixing event (heat loss by the ocean surface) at the end of the spring which is able to reach the base of the MLD and inject new nutrients into the mixed layer marks the end of the near-surface bloom and its transition towards a DCM. Identified in this way, the spring bloom duration and the start of the transition towards a DCM can be systematically and objectively determined, providing sensitive indexes of climate and ecosystem variability.
Highlights
In mid-latitude, subpolar, and polar regions, phytoplankton growth and biomass exhibit a marked seasonal cycle
The onset of the spring bloom can be identified when air-sea heat fluxes become positive, allowing incipient stratification to be established at the ocean surface along with phytoplankton growth [2,3], this classical approach has been challenged by Behrenfeld [4]
(days–weeks), phytoplankton depletes nutrients in the well-illuminated mixed layer, becoming less abundant in the near-surface layer and accumulating at depth [5]. This process leads to the formation of persistent layers of elevated chlorophyll a concentrations, coinciding with the nutricline, which are nearly ubiquitous in stratified surface waters. These deep layers of elevated Chl are known as the Deep Chlorophyll Maximum (DCM) and the typical water column vertical structure of these regions have been characterized as the “Typical Tropical Structure” (TTS) by Herbland & Voituriez [6]
Summary
In mid-latitude, subpolar, and polar regions, phytoplankton growth and biomass exhibit a marked seasonal cycle. (days–weeks), phytoplankton depletes nutrients in the well-illuminated mixed layer, becoming less abundant in the near-surface layer and accumulating at depth [5]. This process leads to the formation of persistent layers of elevated chlorophyll a concentrations, coinciding with the nutricline, which are nearly ubiquitous in stratified surface waters. These deep layers of elevated Chl are known as the Deep Chlorophyll Maximum (DCM) and the typical water column vertical structure of these regions have been characterized as the “Typical Tropical Structure” (TTS) by Herbland & Voituriez [6].
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