Influence of Ocean Alkalinity Enhancement with Olivine or Steel Slag on a Coastal Plankton Community in Tasmania
Authors: Guo Jiaying A. , Strzepek Robert F. , Swadling Kerrie M. , Townsend Ashley T. , Bach Lennart T. 
Published: September 2023

Abstract. Ocean alkalinity enhancement (OAE) aims to increase atmospheric CO2 sequestration in the oceans through the acceleration of chemical rock weathering. This could be achieved by grinding rocks containing alkaline minerals and adding the rock powder to the surface ocean where it dissolves and chemically locks CO2 in seawater as bicarbonate. However, CO2 sequestration during dissolution coincides with the release of potentially bio-active chemicals and may induce side effects. Here, we used 53 L microcosms to test how coastal plankton communities from Tasmania respond to OAE with olivine (mainly Mg2SiO4) or steel slag (mainly CaO and Ca(OH)2) as alkalinity sources. Three microcosms were left unperturbed and served as a control, three were enriched with olivine powder (1.9 g L−1), and three with steel slag powder (0.038 g L−1). Phytoplankton and zooplankton community responses as well as some biogeochemical parameters were monitored for 21 days. Olivine and steel slag additions increased total alkalinity by 29 µmol kg−1 and 361 µmol kg−1 respectively, which corresponds to a theoretical increase of 0.9 % and 14.8 % of the seawater storage capacity for atmospheric CO2. Olivine and steel slag released silicate nutrients into the water column, but steel slag released considerably more and also significant amounts of phosphate. Both minerals released dissolved aluminium (> 400 nmol L−1). The slag addition increased dissolved manganese concentrations (784 nmol L−1), while olivine increased dissolved nickel concentrations (38 nmol L−1). The slag treatment increased the total particulate manganese concentrations (22 nmol L−1), while olivine increased the total particulate nickel (5 nmol L−1), which was consistent with the increase in the dissolved concentrations of these trace metals in seawater. There was no significant difference in total chlorophyll a concentrations between the treatments and the control, likely due to nitrogen limitation of the phytoplankton community. However, flow cytometry results indicated an increase in the cellular abundance of several smaller (~<20 µm) phytoplankton groups in the olivine treatment compared to the slag treatment and the control. The abundance of larger phytoplankton (~>20 µm) decreased much more in the control than in the mineral addition treatments after day 10. Furthermore, the maximum quantum yields of photosystem II (Fv/Fm) were higher in slag and olivine treatments, suggesting that mineral additions increased photosynthetic performance. The zooplankton community composition was also affected with the most notable changes being observed in the dinoflagellate Noctiluca scintillans and the appendicularian Oikopleura sp. Overall, steel slag is much more efficient for CO2 removal with OAE than olivine and appears to be induce less changes in the plankton community when relating the CO2 removal potential to the level of environmental impact that was observed here.