Potential ecosystem regime shift resulting from elevated CO2 and inhibition of macroalgal recruitment by turf algae

Rising carbon dioxide (CO2) concentrations are predicted to cause an undesirable transition from macroalgae-dominant to turf algae-dominant ecosystems due to its effect on community structuring processes. As turf algae are more likely to proliferate due to the CO2 fertilization effect than macroalgae and often inhibit macroalgal recruitment, increased CO2 beyond certain levels may produce novel positive feedback loops that promote turf algae growth and thus can stabilize turf algae-dominant ecosystems. In this study, we built a simple competition model between macroalgae and turf algae in a homogeneous space to investigate the steady-state response of the ecosystem to changes in the partial pressure of CO2 (pCO(2)). We found that discontinuous regime shifts in response to pCO(2) change can occur once turf algae coverage reaches a critical level capable of inhibiting macroalgal recruitment. The effect of localized turf algae density on the success rate of macroalgae recruitment was also investigated using a patch model that simulated a two-dimensional heterogeneous space. This suggested that in addition to the inhibitory effect by turf algae, a self-enhancing effect by macroalgae could also be important in predicting the potential discontinuous regime shifts in response to future pCO(2) changes.

Automated summary

The rise in carbon dioxide concentrations is expected to lead to a shift from macroalgae-dominated to turf algae-dominated ecosystems, as higher CO2 levels favor turf algae proliferation and inhibit the recruitment of macroalgae. This study found that discontinuous regime shifts occur when turf algae coverage reaches a critical level that impedes macroalgal recruitment. The results also suggest that both the inhibitory effect of turf algae and the self-enhancing effect of macroalgae play a role in predicting potential regime shifts in response to future CO2 changes.