Phytoplankton not only determine the potential biomass an ocean can support but also play a pivotal role in regulating atmospheric carbon dioxide, influencing our planet's climate. To flourish, these microorganisms require sunlight and essential nutrients, including iron and nitrogen. These elements are transported to the ocean's surface through currents and upwelling.
Historically, researchers would embark on marine expeditions to conduct in-situ studies of phytoplankton nutrient restrictions. However, this method captures a minimal portion of the marine environment. Addressing this limitation, an international research group explored the potential of satellite data to offer a more extensive view of nutrient restrictions.
In their recent investigation, the team focused on phytoplankton fluorescence signals in the Equatorial Pacific, which were recorded by satellites. This region was of particular interest due to the natural variation in nutrient availability and phytoplankton productivity caused by the El Nino Southern Oscillation (ENSO). The results of this study have been detailed in the renowned scientific journal, Nature.
Dr. Thomas Browning, a marine biologist and chemist at GEOMAR Helmholtz Centre for Ocean Research Kiel, coordinated this research. He revealed, "Although satellites have been making fluorescence measurements for two decades, we do not know yet how to properly interpret them". Under the "Ocean Glow" project, funded by the European Research Foundation, Browning and his team are attempting to decode these satellite readings to determine which nutrients restrict phytoplankton growth.
During a 2019 expedition on the German research vessel SONNE, the scientists examined phytoplankton proteins and conducted experiments to understand which nutrients were inhibiting their growth. These studies also included an assessment of the variations in phytoplankton fluorescence - red light emitted by these microorganisms, the intensity of which is believed to be influenced by the nutrients available to them. They discovered that iron restriction led phytoplankton to produce highly fluorescent pigment-protein complexes, while a lack of nitrogen did not produce such a response.
The team also employed instruments similar to those used by NASA's MODIS satellites. These devices, fitted at the ship's forefront, provided a downward view of the sea surface. The data collected during this expedition was then juxtaposed with satellite data to understand historical nutrient restriction trends in the Equatorial Pacific over the past two decades.
Dr. Browning elaborated on their findings, "We found that phytoplankton were either limited by either iron or by nitrogen, resulting in distinctly different fluorescence properties detectable by satellites." Furthermore, they observed that fluorescence patterns from satellites aligned with iron supply variations due to ENSO cycles.
A notable discrepancy arose when the team compared their satellite observations with predictions from a global biogeochemical model. Although the model's changes in iron restriction aligned with ENSO dynamics, it overestimated the impact of iron restriction on phytoplankton by double compared to the field observations. Such satellite-derived data could be pivotal in refining these models, enabling more accurate predictions about the repercussions of climate change on marine ecosystems.
In conclusion, Dr. Browning highlighted the potential of satellite technology in marine research. He said, "These initial results underscore the significance of satellite data in evaluating the effect of nutrient restrictions on phytoplankton and their overarching role in the global ocean and our climate system." The team aims to further validate their findings across all oceanic regions through the ongoing 'Ocean Glow' project.
Research Report:Persistent equatorial Pacific iron limitation under ENSO forcing
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Helmholtz Centre for Ocean Research
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