Dynamic Ocean Currents Ignite Vibrant Phytoplankton Blooms in the Equatorial Atlantic

The ocean plays a crucial role in oxygen production, food supply, and carbon storage. At the heart of this system is phytoplankton—tiny algae that generate biomass via photosynthesis, utilizing nutrients, CO₂, and sunlight.

Scientists from GEOMAR Helmholtz Center for Ocean Research Kiel, in association with Oregon State University and Lamont-Doherty Earth Observatory, have discerned how a complex interaction of wind and current systems influences nutrient supply and phytoplankton proliferation in the equatorial Atlantic.

Their research, published today in Nature Geoscience, uncovers the seasonal nutrient dynamics essential for marine productivity and the possible consequences of climate change.

The eastern equatorial Atlantic harbors a highly productive marine ecosystem reliant on the influx of nitrate-rich waters. Maximum productivity takes place during the Northern Hemisphere summer, propelled by intensified easterly breezes. These winds push warm surface waters westward, facilitating the rise of nutrient-rich deep waters in the east.

The Equatorial Undercurrent (EUC), a vigorous subsurface movement, carries cool, nutrient-rich waters eastward across the Atlantic. Seasonal wind patterns induce vertical fluctuations of this current, while daily solar heating affects wind-induced mixing, further enhancing nutrient transfer.

“Our findings illuminate the intricate interaction of wind dynamics, currents, and mixing in this marine area,” states Professor Dr. Peter Brandt, Professor of Experimental Oceanography at GEOMAR Helmholtz Center for Ocean Research Kiel and lead author.

“Three distinct mechanisms drive the nutrient influx at the equator: upwelling in the east instigated by zonal winds, the vertical ascent of the EUC, and wind-driven mixing influenced by the daily solar radiation rhythm. Each mechanism, activated by various components of the wind field, propels the upward transport of nutrients to the surface capable of instigating plankton blooms at the equator.”

Measurements and long-term data

To explore these relationships, comprehensive measurements were conducted during two research voyages aboard the German RV METEOR (M158 and M181). Data regarding temperature, salinity, nitrate levels, and current velocities were gathered at multiple depths. Long-term observational datasets from equatorial moorings and Argo floats were also employed.

“Turbulence observations in the ocean are vital for comprehending nutrient-supply mechanisms,” elucidates Dr. Mareike Körner, a former researcher at GEOMAR now situated at Oregon State University. “The turbulence data amassed during our voyages, paired with similar information from moorings collected by our U.S. colleagues, offered essential insights into the seasonal fluctuations in nutrient mixing from the deeper ocean to the surface.”

“The dynamics of the equatorial ocean represent a finely tuned system of wind-driven processes,” remarks Dr. Brandt. Even minor alterations, he cautions, could disrupt this equilibrium and produce significant repercussions. “Climate change could profoundly modify this balance, affecting nutrient provision to this critical marine ecosystem and its productivity.”