Tiny sea algae form the basis of all marine food webs and are an important CO2 sink in the climate system. But how much of this phytoplankton is there in the oceans? So far, this has been determined using satellite measurements of the photosynthetic pigment chlorophyll. But this only covers the phytoplankton on the surface. Now a good 99,000 additional measurements with floating measuring buoys have enabled a closer look at the phytoplankton at greater depths for the first time. According to these measurements, the total biomass of phytoplankton living in the open ocean corresponds to 314 million tons of carbon. However, around half of them are hidden at depths that are not accessible to satellite measurements. The team reports that the seasonal distribution of single-celled sea algae was also only inadequately recorded by previous measurement data.
They are the most important oxygen producers on our planet: The microscopic sea algae of the phytoplankton produce a good 50 percent of all oxygen in the earth’s atmosphere and are responsible for around half of the earth’s net primary production. At the same time, they bind enormous amounts of CO2 through their photosynthesis and thus act as a buffer in the climate system. “The sinking of photosynthetically produced organic material into the depths of the ocean keeps atmospheric CO2 concentrations around 200 parts per million (ppm) lower than would be the case in a phytoplankton-free ocean,” explain Adam Stoer and Katja Fennel from Dalhousie University in Halifax, Canada. It is all the more important to know as precisely as possible the quantity and productivity of these tiny climate helpers and their long-term development trends. But this is not easy given the enormous size of the world’s oceans.
Diving buoys look beneath the surface of the sea
To date, measurements of large-scale and global phytoplankton density have mostly relied on data from satellites, which can record the density of chlorophyll-a in surface water of the oceans based on absorbed and reflected visible and near-infrared light. “However, these satellite measurements have two known limitations: first, they are limited to the surface layer of water and second, chlorophyll-a is not an ideal measure of phytoplankton biomass,” write Stoer and Fennel. The reason for this: Single-celled sea algae also live in deeper layers of water and contribute to CO2 binding and biomass production. In addition, the amount of chlorophyll-a in individual phytoplankton cells can differ depending on the type of algae and growth conditions, the researchers explain. Therefore, previous studies could only estimate the phytoplankton biomass in the oceans: their results varied between 250 and 2400 million tons of phytoplankton carbon.
However, there is now a way to record the density and distribution of single-celled sea algae even below the visual depth of satellite measurements – using so-called Argo floats. These sensor-equipped, autonomous measuring buoys are programmed to carry out measurements at different ocean depths up to 6,000 meters at specific time intervals. They appear at regular intervals and transmit their measurement data via satellites to central data centers. There are currently almost 4,000 such Argo floats floating in the world’s oceans. Stoer and Fennel have now evaluated the data from 903 Argo buoys in the Pacific, Atlantic and Indian Oceans that were specially designed for biogeochemical measurements, including almost 99,350 so-called biooptical profiles, which provide information about phytoplankton density and properties. Stoer and Fennel used this data to determine the phytoplankton biomass at weekly intervals for different depths and latitudes and compared the values with satellite-based chlorophyll-a measurements.
True distribution of phytoplankton biomass revealed
The evaluations revealed a phytoplankton biomass in the three major world oceans of around 314 million tons of phytoplankton carbon. This value is therefore at the lower end of the previous range of estimates. “But unlike previous approaches, our estimate is based on depth-resolved, carbon-centered measurements from the entire euphotic zone,” emphasize Stoer and Fennel. The euphotic zone is the depth area of a body of water in which photosynthesis is possible. “According to our measurements, oceanic phytoplankton only makes up around 0.06 percent of the total biomass of the terrestrial biosphere. “This is a tiny fraction compared to their enormous importance for the preservation of marine ecosystems and their important role in the carbon cycle,” the researchers write. This makes it all the more important to know the distribution of phytoplankton in the deeper ocean layers. The new measurements revealed that around half of all phytoplankton carbon and chlorophyll-a can be found beneath the mixed surface layer of the oceans. “An even larger proportion lies below the depth recorded by the satellite measurements – it corresponds to 85 percent of the phytoplankton carbon and 88 percent of the chlorophyll-a,” the scientists report.
The buoy measurements also showed that the satellite measurements of marine chlorophyll also do not correctly record the seasonal development of sea algae blooms. “We found that surface chlorophyll-a did not correctly identify the time of peak annual biomass in two-thirds of the ocean,” report Stoer and Fennel. The surface chlorophyll suggests a particularly extensive and long algal bloom in subtropical regions, with durations and intensities decreasing sharply towards the poles. However, if you look at the entire phytoplankton biomass below the sea surface, there are significantly smaller differences and regional fluctuations, as the team found. “The misjudgment of the satellite data has potential implications for how to link algal blooms to the productivity and survival of higher trophic levels such as zooplankton or fish larvae,” the researchers explain. Possible decoupling of predator-prey cycles due to climate change would therefore be difficult to accurately capture using satellite data alone.
According to Stoer and Fennel, it will therefore be important in the future to take data from both satellites and Argo floats into account. “The combination of both technologies is an important step for long-term monitoring of terrestrial phytoplankton,” they wrote. “It will help understand how marine algae are affected by anthropogenic climate change and potential marine geoengineering projects.”
Source: Adam C. Stoer and Katja Fennel (University of Dalhousie, Halifax), Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2405354121