Regional variation in δ¹³C of coral reef macroalgae

Macroalgae are important components on coral reefs that underpin food webs but are also indicators of declines in coral reef condition. The sensitivity of macroalgal communities to environmental factors, including climate and pollutants from the land, is important for the future state of coral reefs. We assessed regional variation in the photosynthetic physiology of macroalgae on the Great Barrier Reef and Moreton Bay, Queensland, Australia, using δ¹³C, tissue chemistry, and metabolic indicators over broad environmental gradients from six sites spanning 13° of latitude and varying distance from shore. Our data set included 568 samples from 54 genera and 19 macroalgal orders. The δ¹³C of tissues had a strong taxonomic basis (explaining 66% of the variation in δ¹³C), varying significantly among algal orders. Particularly low values of δ¹³C, indicating direct use of CO_2(aq) in photosynthesis, were mainly associated with lineages within the Rhodophyta. We observed declines in δ¹³C with increasing depth (inshore only) and latitude, from offshore to inshore sites and with increasing tissue N and N : P ratio. Variation in δ¹³C of tissues among macroalgal lineages suggests evolution of a range of CO₂ acquisition pathways. Patterns in δ¹³C availability are most consistent with higher availability of CO_2(aq) in cooler water and inshore sites. δ¹³C in macroalgae could provide an important tool for monitoring changes in CO_2(aq) with increasing atmospheric CO₂, with changes in ocean circulation and with changes to runoff from the land to the coastal oceans.