Plankton study investigates how marine food webs respond to increasing alkalinity

The ocean naturally absorbs between a quarter and a third of the CO2 produced by humans.₂ emissions, but this process also leads to the acidification of seawater. By increasing the alkalinity of seawater through the addition of certain minerals (e.g. carbonates and silicates), the ocean can chemically bind more CO₂ without further acidification.

However, there is still little research on the environmental effects of increasing ocean alkalinity (OAE). Scientists from Professor Ulf Riebesell’s group at the GEOMAR Helmholtz Center for Ocean Research in Kiel, as part of the European OceanNETs project, have studied the response of zooplankton and potential impacts on the food web for the first time. from an experiment carried out off the coast of Gran Canaria.

The results of their study are published today in the journal Scientific advances.

Experimenting in giant test tubes

The study took an approach with moderate disruptions to seawater chemistry: improving CO₂-balanced ocean alkalinity. With this approach, the alkalized water has already absorbed CO₂ intended for carbon dioxide removal (CDR) before being released into the marine environment.

For their experiment, the scientists used KOSMOS mesocosms (Kiel Off-Shore Mesocosms for Ocean Simulations), large test tubes immersed directly in seawater, isolating eight cubic meters of water column.

Different concentrations of sodium carbonate and bicarbonate were added to obtain different CO intensities₂-Balanced OAE, ranging from no increase in alkalinity to doubling of natural alkalinity.

For 33 days, researchers monitored the effects of alkalization on zooplankton, which play a key role in transferring energy to fish through the food web. A range of responses have been studied in zooplankton, from biomass and production to diversity and fatty acids.

Overall, the researchers found that plankton communities remained stable and zooplankton largely tolerated the moderate chemical changes associated with CO.₂-Balanced OAE. During the experiment, the nutritional quality of the particles on which zooplankton can feed potentially deteriorated, but this does not seem to affect consumers.

The researchers argue that food limitation, resulting from the oligotrophic conditions in which this experiment took place and which characterize subtropical waters, could have dampened these possible indirect responses of zooplankton to OAE.

“Our study shows that increasing alkalinity has minor impacts on zooplankton and that the food web as a whole remains stable,” explains Nicolás Sánchez, Ph.D. student and first author of the study.

Potential for climate protection and need for further research

Improving ocean alkalinity could become an important ally in CO reduction₂ emissions to fight climate change. By allowing the ocean to absorb more CO₂ without becoming more acidic, this approach could strengthen the ocean’s role as a buffer against global warming.

This could help ease the transition to a future in which fossil fuels are replaced by renewable energy, where emissions from industries that cannot be decarbonized are neutralized, and where historic carbon emissions are removed and safely stored. However, in-depth research is urgently needed to determine the impact of OAE on the entire marine environment.

“Our experience has shown that CO₂“Balanced OAE does not have a lasting impact on zooplankton and the food web in the nutrient-poor subtropical zone we studied,” says Sánchez, “but it says nothing about how it will affect other populations. other marine environments, nor on the safety of other, more technically feasible forms of OAE that result in greater changes in seawater chemistry.”

The scientists recommend further research into the method and into different ecosystems, as there will not be a single OAE approach that can be applied everywhere. Sánchez says: “Our study represents a promising first step toward defining a responsible framework for the application of alkalinity improvement. »