Early life stages of fish under ocean alkalinity enhancement in coastal plankton communities

Abstract. Ocean alkalinity enhancement (OAE) stands as a promising carbon dioxide removal technology. Yet, this solution to climate change entails shifts in environmental drivers with unknown consequences for marine fish that are critical to ecosystem health and food security. Fish and their supporting food webs may be stressed by the novel carbonate chemistry or the nutrients contained in the deployed minerals. With a mesocosm experiment on natural plankton communities, we studied early life stages of fish under alkalinity (+600 µmol kg−1) and silicate (+75 µmol L−1) addition. Larvae and young juveniles of temperate coastal species, including Atlantic herring (Clupea harengus) and cod (Gadus morhua), were exposed to direct physiological and indirect food-web-mediated effects of OAE for 49 d. Neither in the shorter nor in the longer term did we find an impairment of fish growth and survival. Alkalization even led to an increase in fish biomass. This resistance to OAE was despite using non-CO2-equilibrated deployment that induces more severe perturbations in carbonate chemistry (ΔpH =+0.7, pCO2=75 µatm) compared to alternative scenarios. Overall, our community-level study suggests that some fish populations, including key fisheries' species, may be resilient to the water chemistry changes under OAE. Whilst these results give cause for optimism regarding the large-scale application of OAE, other life history stages (embryos) and habitats (open ocean) may prove more vulnerable.