Subsurface ocean is blooming

Phytoplankton, ubiquitous microorganisms in the world’s oceans, play a fundamental role in marine ecosystems. These organisms synthesize organic matter from sunlight, supporting the entire marine food web and driving oceanic life. Through photosynthesis, phytoplankton absorb atmospheric carbon dioxide, making them integral to the biological carbon pump—a natural mechanism sequestering carbon in the ocean’s depths. In the context of climate change, understanding phytoplankton’s contribution to carbon cycling is critical, as their rate of carbon assimilation, or net primary production (NPP), is a key metric for assessing ecosystem health and carbon sequestration potential.

Traditionally, global NPP estimates have relied on satellite observations that detect chlorophyll-a—a green pigment essential for photosynthesis. However, satellite imagery can only capture surface-level data, assuming most NPP occurs in the upper ocean layers. This approach overlooks subsurface processes, potentially underestimating the true extent of phytoplankton productivity. In response, the Biogeochemical Argo floats network—a global fleet of autonomous profiling floats—now provides unprecedented, depth-resolved measurements of chlorophyll, carbon proxies, and light availability. These observations enable estimates of NPP at greater depths, offering a more comprehensive view of phytoplankton’s role across the ocean.

In this study, we leverage Biogeochemical Argo Argo float data and a modified carbon-based productivity model to quantify subsurface phytoplankton contributions to global NPP. Our findings reveal a significant portion of NPP occurring below the ocean’s surface, extending down to depths exceeding 200 meters. This subsurface productivity is particularly pronounced in regions spanning from the equator to 45 degrees latitude—areas previously considered low in productivity. Our results highlight an overlooked reservoir of organic matter, available for deeper marine organisms and representing an unquantified carbon export pathway.

These insights redefine our understanding of oceanic carbon cycling, emphasizing the critical role of subsurface phytoplankton in sustaining deeper ecosystems. Our findings also stress the importance of modern observation systems, like Biogeochemical Argo floats, in capturing the full depth and complexity of ocean productivity. This research not only contributes to a deeper understanding of carbon pathways but also has implications for the broader marine food web, influencing the availability of resources for higher trophic levels, including commercially important fish species.