Understanding the seasonal dynamics of phytoplankton biomass and the deep chlorophyll maximum in oligotrophic environments: A Bio‐Argo float investigation

Abstract

We deployed four Bio-Argo profiling floats in various oligotrophic locations of the Pacific subtropical gyres and Mediterranean Sea to address the seasonal phytoplankton dynamics in the euphotic layer and explore its dependence on light regime dynamics. Results show that there is a similar phytoplankton biomass seasonal pattern in the four observed oceanic regions. In the lower part of the euphotic layer, the seasonal displacement of the deep chlorophyll maximum (DCM) is light driven. During winter, the chlorophyll a concentration ([Chl a]) always increases in the upper euphotic mixed layer. This increase always results from a photoacclimation to the reduced irradiance. Depending on the location, however, the concentration can also be associated with an actual increase in biomass. The winter increase in [Chl a] results in an increase in irradiance attenuation that impacts the position of the isolume (level where the daily integrated photon flux is constant) and DCM, which becomes shallower. In summer when the [Chl a] in the upper layer decreases along with light attenuation, the DCM deepens and becomes closer to (and sometimes reaches) the nitracline, which enhances the phytoplankton biomass at the DCM. The bio-optical mechanisms and their relationship to light regimes that are revealed by the time series appear to be generic and potentially characteristic of all of the areas where a DCM forms, which is 50% of the open ocean.

A Consumer's Guide to Satellite Remote Sensing of Multiple Phytoplankton Groups in the Global Ocean

“…In addition, coincident optical [i.e., R rs (λ) and IOPs] observations will be highly important to connect to the signals observed by satellite radiometers. The expanding optical sensors on Bio-Argo floats may also provide a valuable data stream for PFT development, particularly for vertical structure of phytoplankton communities (Mignot et al, 2014). It is important to expand the capability to measure phytoplankton carbon in situ (Graff et al, 2012(Graff et al, , 2015 so future definitions of PFTs can be more carbon-relevant (Kostadinov et al, 2016a).…”

Seasonal development of iron limitation in the sub-Antarctic zone

“…S3a), indicative of iron limitation within the mixed layer and a supply mechanism (seasonal, sub-seasonal, remineralized or stormdriven) that is not sufficient to meet mixed-layer phytoplankton demands. Precaution must, however, be taken when investigating Chl a concentration as a proxy for phytoplankton biomass Bellacicco et al, 2016;Mignot et al, 2014;Westberry et al, 2008Westberry et al, , 2016, as a higher average concentration over the euphotic zone (0.8 mg m −3 ) relative to the shallower mixed layer (0.4 mg m −3 ) may represent a Chl a packaging effect due to lower light levels at depth (rather than an increase in biomass). As such, particulate backscatter (b bp ) (Fig.…”

Interannual variability of the Mediterranean trophic regimes from ocean color satellites

“…However recent results from profiling floats measuring the [Chl] and the particle mass concentration, suggest also that in this region the photoacclimatation process could contribute to the change in the [Chl] surf observed (up to 70 %, Mignot et al, 2014). Other recent results from profiling floats measuring nitrate concentration suggest that, more than the deep convection events, the permanent cyclonic circulation in this region was the primary factor inducing favorable conditions for phytoplankton bloom, by bringing the nitracline depths close to surface.…”