Global network of undersea robots unveils hidden depths of phytoplankton biomass

Global network of undersea robots unveils hidden depths of phytoplankton biomass
by Clarence Oxford
Los Angeles CA (SPX) Oct 29, 2024

Marine phytoplankton play a critical role in Earth's ecosystems, driving primary production and anchoring the global carbon cycle. These microscopic organisms, which thrive in the sunlit layers of the ocean, use photosynthesis to capture carbon dioxide, forming the base of the marine food web and influencing climate regulation. Historically, scientists have relied on satellite imaging to monitor phytoplankton biomass through the measurement of chlorophyll-a (Chla) concentrations, a pigment directly involved in photosynthesis and frequently used as an indicator of phytoplankton abundance. However, these satellite observations only capture the ocean's surface, leaving much of the subsurface phytoplankton biomass out of reach. Additionally, Chla is a limited proxy for phytoplankton biomass, as environmental factors like nutrient availability and cellular conditions can impact the Chla-to-carbon ratio, complicating carbon biomass estimates.

To overcome these observational limitations, a global network of Biogeochemical-Argo (BGC-Argo) floats has recently been deployed. Developed as an enhancement to the Argo program - an international project that has been measuring ocean temperature and salinity for over two decades - BGC-Argo floats are equipped with sensors to monitor additional biogeochemical parameters, including oxygen, nitrate, pH, and Chla, at varying ocean depths. With nearly 100,000 profiles collected, these floats now provide unprecedented, three-dimensional data on oceanic carbon dynamics, tracking phytoplankton abundance and distribution far beyond the surface and offering a comprehensive view of biomass fluctuations over time and depth.

The latest research led by Dalhousie University scientists takes advantage of this global BGC-Argo dataset to map the total carbon biomass of open ocean phytoplankton and reveal significant findings about its spatiotemporal variability. The study estimates that the total phytoplankton biomass in the open ocean reaches approximately 314 teragrams of carbon (Tg C), with nearly half of this biomass located at depths inaccessible to satellites. This depth-resolved perspective on phytoplankton distribution is a first of its kind on a global scale, illuminating previously hidden aspects of marine ecology.

The study's findings reveal striking seasonal discrepancies between satellite-detected surface Chla and the actual peak cycles of carbon biomass deeper in the ocean. In nearly two-thirds of the world's oceans, the timing of peak phytoplankton biomass does not coincide with peak surface Chla levels observed from space. This suggests that surface-level measurements alone are insufficient to capture the full dynamics of phytoplankton biomass cycles, highlighting the importance of depth-resolved data. As oceans warm and stratify due to climate change, understanding these vertical biomass distributions will be increasingly important for predicting how marine ecosystems respond to environmental shifts.

Beyond climate implications, phytoplankton dynamics have critical ramifications for human societies. Phytoplankton contribute to oxygen production, support fisheries, and influence global carbon storage - all factors integral to ocean health and food security. Enhanced monitoring through BGC-Argo provides crucial data that could aid in assessing the potential impacts of geoengineering interventions aimed at mitigating climate change. For instance, proposals to enhance oceanic carbon sequestration through iron fertilization - adding iron to stimulate phytoplankton growth - depend on a thorough understanding of existing carbon cycles and phytoplankton productivity. The insights gained from the BGC-Argo network will improve our ability to evaluate the effectiveness and potential ecological consequences of such interventions.

By building a more complete picture of phytoplankton distribution and its response to seasonal and environmental changes, this study underscores the value of combining satellite and in-situ data for global ecological assessments. The BGC-Argo floats represent an important advancement in ocean science, offering a robust tool for understanding the ocean's biological pump and its role in global climate regulation.

Research Report:Carbon-centric dynamics of Earth's marine phytoplankton