Future coral reefs are expected to be subject to higher pCO2 and temperature due to anthropogenic greenhouse gas emissions. Such global stressors are often paired with local stressors thereby potentially modifying the response of organisms. Benthic macroalgae are strong competitors to corals and are assumed to do well under future conditions. The present study aimed to assess the impact of past and future CO2 emission scenarios as well as nutrient enrichment on the growth, productivity, pigment and tissue nutrient content of the common tropical brown alga Chnoospora implexa. Two experiments were conducted to assess the differential impacts of the manipulated conditions in winter and spring. C. implexa‘s growth rate averaged over winter and spring declined with increasing pCO2 and temperature. Furthermore, nutrient enrichment did not affect growth. Highest growth was observed under spring pre-industrial conditions, while slightly reduced growth was observed under winter A1FI (‘business-as-usual’) scenarios. Productivity was not a good proxy for growth, as net O2 flux increased under A1FI conditions. Nutrient enrichment, whilst not affecting growth, led to luxury nutrient uptake that was greater in winter than in spring. The findings suggest that in contrast to previous work, C. implexa is not likely to show enhanced growth under future conditions in isolation or in conjunction with nutrient enrichment. Instead, the results suggest that greatest growth rates for this species appear to be a feature of the pre-industrial past, with A1FI winter conditions leading to potential decreases in the abundance of this species from present day levels.
Bender B., Diaz-Pulido G. & Dove S., in press. The impact of CO2 emission scenarios and nutrient enrichment on a common coral reef macroalga is modified by temporal effects. Journal of Phycology. Article (subscription required).