Ecosystem impacts of Ocean Alkalization in an oligotrophic marine plankton community: A mesocosm study

In light of the climate crisis and the necessity to meet the Paris Agreement goal of staying well below the 2°C temperature increase whilst respecting other UN sustainable development goals, an array of technologies to absorb and store atmospheric , named Carbon Dioxide Removal (CDR) technologies, are being proposed, developed and researched. One such technologies is Ocean Alkalinization, or Ocean Alkalinity Enhancement (OAE), which stems from the natural process of rock weathering. Several alkalinity sources and deployment options have been proposed, each associated with differing biological drivers. Among these, a carbonate-based, dissolved, equilibrated addition stands as the most optimistic deployment scenario, both from a carbon sequestration verification and an ecosystem impact stand point.

We decided to test this implementation in the first community-level mesocosm experiment to be done in the field. Pelagic mesocosms were deployed in Taliarte (Gran Canaria), enclosing 8 m³ of oligotrophic coastal waters with their associated natural plankton community. Nine OAE addition scenarios were simulated, increasing alkalinity (TA) in steps of 300 µeq/L from ambient levels up to its doubling, using a mixture of sodium carbonate and bicarbonate. Particular focus was placed on the impacts of enhanced TA on the ecosystem service (ES) of food production. This was addressed via zooplankton properties pertaining to ecosystem stability, food quantity and nutritional quality. Zooplankton diversity, functional composition, biomass, CN stoichiometry, population size structure, secondary production, trophic length, and a number of fatty acid nutritional indexes and trophic markers were monitored throughout the 33-day experiment.

Here, only 4 out of the over 30 different stability, food quantity and quality proxies were significantly affected by enhanced TA. Out of these, two were interpreted as negative impacts: a shorter-term halving in small copepod production, coinciding with a halving in copepod nauplii biomass, with a doubling in TA. These responses could be partly explained by the halving in large microplankton, an assumed preferred food source for copepods, detected right after treatment. However, none of these were sustained until the end of the experiment, thus suggesting no longer-term consequences. All in all, this study provides evidence for a low impact risk of enhanced TA on zooplankton, and ultimately the ES of food production. These findings set a promising stage towards the safe implementation of CO2-equilibrated OAE in oligotrophic coastal waters.