The ocean waters are changing color: they are becoming greener in the poles and bluer in the tropics, a sign that phytoplankton —the basis of the marine food chain— is shifting towards higher latitudes.
This was revealed by a study published in Science, which analyzed two decades of satellite data (2003-2022) to assess chlorophyll concentration, an indirect indicator of phytoplankton biomass.
“The tropics are losing chlorophyll, while polar regions are greening,” explained Haipeng Zhao, lead author of the study. This pattern suggests a latitudinal redistribution of phytoplankton, driven by changes in sea surface temperature, light, wind, and ocean mixing dynamics.
Impact on trophic chains and global fishing
The phenomenon could affect the structure of marine trophic networks, as a persistent decrease in phytoplankton in equatorial zones —where over 50% of global fish catches are concentrated— threatens the stability of coastal fisheries, on which many countries depend for their food and economic development.
“If phytoplankton decreases in certain regions, it also affects higher species that depend on it, which could imply a redistribution of fishing resources on a global scale,” noted Nicolas Cassar, an oceanographer at Duke University.
Possible implications for the carbon cycle
Phytoplankton also plays a crucial role in carbon sequestration, as when it dies, it transports carbon to the ocean depths. If this process occurs in deep waters that do not resurface frequently, the carbon remains stored for longer. But if the activity is concentrated in surface waters, the carbon could return to the atmosphere faster, limiting the ocean’s role as a carbon sink.
A trend not definitively attributed to climate change
Although researchers describe a clear trend, they do not directly attribute these changes to global warming. “The analyzed period is still too short to rule out the influence of natural phenomena such as El Niño,” warned Susan Lozier, co-author of the article and scientist at the Georgia Institute of Technology.
Therefore, they emphasize the need to extend observations in the coming decades to determine if these transformations in the marine biosphere are due to lasting climate changes or natural variability.
An ocean transforming before our eyes
The color of the oceans —green, blue, or a transition between both— is more than an aesthetic matter: it is a biological and ecological thermometer anticipating profound changes in global marine dynamics, with consequences for fishing, carbon storage, and global food security.
