Phytoplankton are microscopic, plant-like organisms that form the base of the marine food web, sustaining a wide range of marine life from tiny fish to massive whales. Additionally, they play a significant role in sequestering carbon dioxide from the atmosphere. Monitoring phytoplankton biomass is crucial for understanding the impacts of climate change and evaluating potential carbon-removal strategies. Traditionally, satellite technology has been used to observe phytoplankton on the sea surface, but this method falls short as it cannot detect phytoplankton below the surface. This limitation leaves a substantial gap in our knowledge of phytoplankton biomass, which influences assessments of ocean health and global carbon cycles.
Researchers at Dalhousie University are addressing this challenge with the help of Biogeochemical-Argo (BGC-Argo) floats—an expanding network of underwater robots that measure phytoplankton at various depths. These floats are part of an international effort to monitor the biological, geological, and chemical components of the ocean. A recent study led by graduate student Adam Stoer, in collaboration with Dalhousie’s Department of Oceanography, utilized data from these floats to estimate the global phytoplankton biomass at approximately 343 million tons—equivalent to the combined weight of 250 million elephants. Remarkably, their findings reveal that at least half of this biomass is not visible to satellites, underscoring the importance of these innovative underwater robots.
Stoer emphasized that the deployment of BGC-Argo floats is a significant advancement in marine research. These floats enable year-round monitoring across the entire ocean, an achievement that was previously impossible. This comprehensive approach allows scientists to quantify phytoplankton biomass more accurately and monitor changes over time, which is essential as ocean warming accelerates. The research highlights the importance of understanding how phytoplankton respond to climate shifts, as these organisms are vital to marine ecosystems and the global carbon cycle.
The study used around 100,000 water-column profiles collected by BGC-Argo floats to detail the distribution and seasonal variations in phytoplankton biomass. This data revealed a substantial portion of biomass exists below the depth that satellites can observe, challenging the reliance on surface chlorophyll-a as a proxy for carbon biomass. Dr. Blair Greenan from the Department of Fisheries and Oceans pointed out that this study demonstrates the limitations of satellite data, emphasizing the need for deeper and more comprehensive methods of tracking oceanic carbon levels.
Collecting seawater samples via research ships has long been a traditional method for studying marine ecosystems. However, logistical constraints such as the limited number of ships, personnel, and funding make global-scale sampling impractical. Satellite imagery has provided broader coverage, but it has inherent limitations. The study notes that surface chlorophyll-a, commonly used as a proxy for phytoplankton biomass, does not accurately reflect true biomass due to variable growth conditions influenced by sunlight and other factors. Dr. Katja Fennel, senior author and chair of Dalhousie’s Department of Oceanography, highlighted a significant outcome of the research: a mismatch between the seasonal cycles of carbon biomass and chlorophyll-a in two-thirds of the global ocean. This finding underscores the need for improved techniques to measure the actual carbon biomass and better track how it fluctuates with seasonal and climatic changes.
This study by Dalhousie University showcases the transformative role of BGC-Argo floats in expanding the capability to monitor phytoplankton biomass globally and year-round, bridging the critical knowledge gap left by satellite-only observations and enhancing our understanding of the ocean’s response to climate change.
https://phys.org/news/2024-10-global-fleet-undersea-robots-reveals.html