Small Phytoplankton Shapes Colored Dissolved Organic Matter Dynamics in the North Atlantic Subtropical Gyre

Abstract

The North Atlantic subtropical gyre (NASTG) is a model of the future ocean under climate change. Ocean warming signals are hidden within the blue color of these clear waters and can be tracked by understanding the dynamics among phytoplankton chlorophyll ([Chl]) and colored dissolved organic matter (CDOM). In NASTG, [Chl] and CDOM are strongly correlated. Yet, this unusual correlation for open oceans remains unexplained. Here, we test main hypotheses by analyzing high spatiotemporal resolution data collected by Biogeochemical‐Argo floats between 2012 and 2018. The direct production of CDOM via phytoplankton metabolism is the main occurring mechanism. More importantly, CDOM dynamics strongly depend on the abundance of picophytoplankton. Our findings thus highlight the critical role of these small organisms under the ocean warming scenario. Picophytoplankton will enhance the production of colored dissolved compounds and, ultimately, impact on the ocean carbon cycle.

Hydrography-Driven Variability of Optically Active Constituents of Water in the South Brazilian Bight: Biogeochemical Implications

“…This is evidenced from the increase in a CDOM (350) values at the DCM and it is supported by the fact that it is known that many phytoplankton species are able to produce CDOM and DOC (Myklestad, 1995;Romera Castillo et al, 2010, Romera-Castillo et al, 2011Fukuzaki et al, 2014;Asmala et al, 2018a) with nutrients as a main driver on the production of different DOM compounds (Myklestad, 1995;Asmala et al, 2018a;Haraguchi et al, 2019). Thus, in this study, CDOM was supplied not only by the continental runoff (given the increase toward the coast) but also by the phytoplankton community (as evidenced by the strong correlation with Chla), as recently reported for the oligotrophic North Atlantic subtropical gyre (Organelli and Claustre, 2019).…”

“…DOM production by phytoplankton has been shown to occur through multiple mechanisms such as exudation, excretion and cell lysis (Thornton, 2014). The autochthonous production of DOM by phytoplankton is one of the major sources to the marine environment, especially when with limited continental influence (Stedmon and Markager, 2005;Jørgensen et al, 2014;Organelli and Claustre, 2019). As a CDOM (350) was positively correlated with Chl-a (see Figure 4), one can infer that the distribution and consequent source/production of DOM in the region was related to the autotrophic community.…”

CDOM Spatiotemporal Variability in the Mediterranean Sea: A Modelling Study

“…Synchronization between chlorophyll and CDOM production has been documented in the North Atlantic subtropical gyre [30], where characteristics of the area could present similarities to the oligotrophic regions of the Mediterranean Sea.…”

“…These instruments can be very useful for data driven models [55] and for validation [23,32]. The advantage of these tools is that they provide information along the water column and this information could be useful to understand CDOM dynamics below the sea surface [9,30]. BGC-Argo floats are equipped with sensors measuring planar downward irradiance at 412 and 490 nm, and in particular at 380 nm where CDOM absorption is sharply dominant with respect to phytoplankton.…”

“…• EXP-1: the CDOM is partitioned as 2% of DOC in all the fluxes [28]. The formulation includes a carbon correction formulation than in nutrient stressed conditions implies that the excess carbon synthesized is excreted in the form of semi-labile DOC [29,30]; • EXP-2: the CDOM is partitioned as 2% of DOC in all the fluxes but the nutrient stress limitation is in part accounted as non-realized production [31] and in part channeled to the production of semi-labile DOC. In this experiment we chose to remove the flux to semi-labile DOC to explore an extreme condition; • EXP-3: the CDOM production is related to activity exudation and is considered colored, the semi-refractory component of CDOM in not considered.…”