| Summary: | Predications about the ecological consequences of oceanic uptake of CO 2 have been 20 preoccupied with the effects of ocean acidification on calcifying organisms, particularly those 21 critical to the formation of habitats (e.g. coral reefs) or their maintenance (e.g. grazing 22 echinoderms). This focus overlooks the direct effects of CO 2 on non-calcareous taxa, 23 particularly those that play critical roles in ecosystem-shifts. We used two experiments to 24 investigate whether increased CO 2 could exacerbate kelp loss by facilitating non-calcareous 25 alga that we hypothesised: (1) inhibit the recovery of kelp forests on an urbanised coast; and 26 (2) form more extensive covers and greater biomass under moderate future CO 2 and 27 associated temperature increases. Our experimental removal of turfs from a phase-shifted 28 system (i.e. kelp to turf-dominated), revealed that the number of kelp recruits increased, 29 thereby indicating that turfs can inhibit kelp recruitment. Future CO 2 and temperature 30 interacted synergistically to have a positive effect on the abundance of algal turfs, whereby 31 they had twice the biomass and occupied over four times more available space than under 32 current conditions. We suggest that the current preoccupation with the negative effects of 33 ocean acidification on marine calcifiers overlooks potentially profound effects of increasing 34 CO 2 and temperature on non-calcifying organisms. 35 36 3
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