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

• Published in: Geophysical research letters 2019-11, Vol.46 (21), p.12183-12191
• Main Authors: Organelli, Emanuele, Claustre, Hervé
• Format: Article
• Language: English
• Subjects: Aquatic ecosystems; BGC‐Argo floats; Biogeochemical Cycles, Processes, and Modeling; Biogeochemical Kinetics and Reaction Modeling; Biogeochemistry; Biogeosciences; bio‐optics; Carbon cycle; Carbon Cycling; Chlorophyll; Chlorophylls; Climate change; Colour; Coloured dissolved organic matter; Correlation; Cryosphere; Dissolved organic matter; Drifters; Dynamics; Electromagnetics; Environmental Sciences; Floats; Global Change; Global warming; Gyres; Marine ecosystems; Metabolism; Ocean Observing Systems; Ocean Optics; Ocean temperature; Ocean warming; Oceanography: Biological and Chemical; Oceanography: General; Oceans; Optics; Organic matter; Paleoceanography; Phytoplankton; pico‐phytoplankton; Plankton; Research Letter; Research Letters; Subtropical climates; subtropical gyres
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL084699

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. Plain Language Summary Colored dissolved organic matter (CDOM) is ubiquitous in aquatic ecosystems. CDOM absorbs sunlight and, ultimately, colors the oceans. In the blue and clear subtropical gyre of the North Atlantic Ocean, the temporal dynamics of CDOM are strongly correlated with the concentration of chlorophyll contained into tiny plants called phytoplankton. The reasons of such a correlation are unexplained. Here, we use field data collected by autonomous robotic platforms and show that CDOM is a fresh product of phytoplankton metabolism in the sampled area. More importantly, we observe that this production is driven by the presence of the smallest phytoplankton on the planet. The role of picophytoplankton (i.e., all phytoplankton with size smaller than 2 μm) as a producer of CDOM will thus become critical for the ocean carbon cycle in the future ocean, as climate change allows subtropical gyres expanding. Key Points Colored dissolved organic matter is produced via phytoplankton metabolism in the North Atlantic subtropical gyre Picophytoplankton shapes the dynamics of colored dissolved organic matter within the mixed layer of the North Atlantic subtropical gyre Picophytoplankton may become a critical source of colored dissolved organic matter under ocean warming