Seagrass response to CO₂ contingent on epiphytic algae: indirect effects can overwhelm direct effects.

Increased availability of dissolved CO2 in the ocean can enhance the productivity and growth of marine plants such as seagrasses and algae, but realised benefits may be contingent on additional conditions (e.g. light) that modify biotic interactions between these plant groups. The combined effects of future CO2 and differing light on the growth of seagrass and their algal epiphytes were tested by maintaining juvenile seagrasses Amphibolis antarctica under three different CO2 concentrations representing ambient, moderate future and high future forecasts (i.e. 390, 650 vs. 900 µl l(-1)) and two light levels representing low and high PAR (i.e. 43 vs. 167 µmol m(-2) s(-1)). Aboveground and belowground biomass, leaf growth, epiphyte cover, tissue chemistry and photosynthetic parameters of seagrasses were measured. At low light, there was a neutral to positive effect of elevated CO2 on seagrass biomass and growth; at high light, this effect of CO2 switched toward negative, as growth and biomass decreased at the highest CO2 level. These opposing responses to CO2 appeared to be closely linked to the overgrowth of seagrass by filamentous algal epiphytes when high light and CO2 were combined. Importantly, all seagrass plants maintained positive leaf growth throughout the experiment, indicating that growth was inhibited by some experimental conditions but not arrested entirely. Therefore, while greater light or elevated CO2 provided direct physiological benefits for seagrasses, such benefits were likely negated by overgrowth of epiphytic algae when greater light and CO2 were combined. This result demonstrates how indirect ecological effects from epiphytes can modify independent physiological predictions for seagrass associated with global change.