Ocean’s shifting hues: Satellite data reveals declining phytoplankton

An analysis of satellite data reveals ocean waters are becoming greener at the poles and bluer toward the equator. The color change reflects shifting concentrations of a green pigment called chlorophyll made by phytoplankton, which are photosynthetic marine organisms found at the base of the ocean food chain.

“In the ocean, what we see based on satellite measurements is that the tropics and the subtropics are generally losing chlorophyll, whereas the polar regions — the high-latitude regions — are greening,” details first author Haipeng Zhao, a postdoctoral researcher of the study.

If the trend continues, marine food webs could be impacted, with potential repercussions for global fisheries, warn the study authors. In the nutrition sector, the omega-3 content of commercial fish species is linked to levels of phytoplankton, which is a primary producer of this nutrient.

A persistent decline in phytoplankton in equatorial regions could alter fisheries that many low- and middle-income nations, such as those in the Pacific Islands, rely on for food and economic development — especially if that decline spreads to coastal regions, according to the authors.

Ocean greening

Since the 1990s, scientists have observed an increase in vegetation on land, a phenomenon known as global greening, largely attributed to rising temperatures. However, tracking photosynthesis in the ocean has been more challenging. While satellites can detect chlorophyll at the ocean’s surface, this only provides a partial view.

To address this, the researchers examined NASA satellite data collected between 2003 and 2022. This instrument scans the entire Earth every two days, measuring light wavelengths to identify changes in chlorophyll concentration, which indicates phytoplankton biomass.

For accuracy, the study focused exclusively on open ocean data, excluding coastal areas. “There are more suspended sediments in coastal waters, so optical properties are different than in the open ocean,” Zhao explains.

The analysis revealed a widespread shift in ocean color, indicating that chlorophyll is decreasing in subtropical and tropical regions and increasing toward the poles. Building on that finding, the team examined how chlorophyll concentration changes at specific latitudes.

To manage background noise and gaps in data, they had to work creatively.

“We borrowed concepts from economics called the Lorenz curve and the Gini index, which together show how wealth is distributed in a society. So, we thought, let’s apply these to see whether the proportion of the ocean that holds the most chlorophyll has changed over time,” details co-author Nicolas Cassar, the Lee Hill Snowdon Bass Chair at Duke University’s Nicholas School of the Environment.

The omega-3 content of commercial fish species is linked to levels of phytoplankton, which is a primary producer of this nutrient.The team discovered similar but opposing trends in chlorophyll concentration over the two-decade period. Green areas became greener, particularly in the northern hemisphere, while blue regions got even bluer.

“It’s like rich people getting richer and the poor getting poorer,” Zhao says.

Impact on carbon storage

Next, the team looked at how several variables impacted the observed patterns, including sea surface temperature, wind speed, light availability, and mixed layer depth — a measure that reflects mixing in the ocean’s top layer by wind, waves, and surface currents.

Warming seas correlated with changes in chlorophyll concentration, but the other variables showed no significant associations, they note.

The authors caution that their findings cannot be attributed to climate change yet.

“The study period was too short to rule out the influence of recurring climate phenomena such as El Niño,” explains Susan Lozier, dean of the College of Sciences.

“Having measurements for the next several decades will be important for determining influences beyond climate oscillations.”

However, if poleward shifts in phytoplankton continue, they could affect the global carbon cycle, warn the authors. During photosynthesis, phytoplankton act like sponges, soaking up carbon dioxide from the atmosphere.

When these organisms die and sink to the ocean bottom, carbon goes down with them. The location and depth of that stored carbon can influence climate warming, note the scientists.

“If carbon sinks deeper or in places where water doesn’t resurface for a long time, it stays stored much longer,” explains Cassar. “In contrast, shallow carbon can return to the atmosphere more quickly, reducing the effect of phytoplankton on carbon storage.”

“Phytoplankton are at the base of the marine food chain. If they are reduced, then the upper levels of the food chain could also be impacted, which could mean a potential redistribution of fisheries.”

Marine conservation efforts came into focus with the recent third UN Ocean Conference in Nice, France. Nutrition Insight dove into insights from Aker BioMarine and the Marine Stewardship Council about evolving industry practices and rising global standards, while looking deeper into the ecological impacts of krill trawling and regulatory changes.