CO2 Fertilisation Counteracts the Negative Effect of Poor Water Quality on the Growth and Photosynthesis of a Great Barrier Reef Coralline Alga

Abstract

The global problem of ocean acidification and localised decline in water quality are major threats to coral reefs worldwide. This study examined the individual and interactive impacts of global and local stressors by investigating the effects of increased seawater pCO₂, elevated nutrient concentrations and reduced light levels on linear growth and metabolic rates of the common branching crustose coralline alga Lithophyllum cf. pygmaeum. We found complex interactions between factors on algal growth and photosynthetic rates, but overall, growth was significantly enhanced by pCO₂ enrichment under all light and nutrient combinations. This is the first study to report a positive growth response in coralline algae to elevated pCO₂ using linear extension methods. In contrast, the combination of reduced light levels and high nutrient concentrations simulating poor water quality conditions reduced algal growth rates by up to 67% (compared to individuals exposed to high light, low nutrients and elevated pCO₂). Decreased light levels reduced linear growth, P_gross and P_net rates by 33%, 18% and 24%, respectively, highlighting the critical role of light in coralline algal physiology. We suggest that poor water quality may counteract any CO₂ fertilisation effect under ocean acidification conditions on the growth of coralline algae, and this has implications for coral reef conservation as it emphasises the importance of improving water quality to maintaining coral reef functions. These results further highlight the need for multifactorial experiments to better understand the interplay between global and local processes on coralline algae growth.