Predicting the impact of future climate on ecologically important macroalgae

Macroalgae play an important role in coastal reef systems and are often referred to as ecosystem engineers. They serve as primary producers, supporting a diverse range of organisms, and are a sink for atmospheric CO2. Water acidification and ocean warming caused by anthropogenic activities are affecting many marine flora and fauna, potentially impacting the physical and chemical performance of macroalgae and the consumption rates of associated herbivores. Many studies have focused on ocean acidification or ocean warming individually but there is an overall lack of research investigating the combined effects and the ensuing repercussions on consumer-prey relationships. Three species of ecologically important macroalgae (Ecklonia radiata, Sargassum linearifolium and Laurencia brongniartii) were subjected to elevated temperature and increased pCO2 conditions and observed for alterations in algae physiology and chemical production, in terms of growth, toughness, bleaching, density, blade mass, quantum efficiency yields, carbon: nitrogen (C:N) ratios and phenolic content. A series of feeding assays were conducted with two abundant marine herbivores, an amphipod (Allorchestes compressa) and a gastropod (Family Trochidae), to examine the indirect impact of climatic stressors on the palatability of the algae. The overall impact of climate change on macroalgae was species-specific, with each algal species having distinct physical and chemical responses to the changes in environmental conditions. S. linearifolium functioned poorly at high temperatures, exhibiting high levels of bleaching, lower quantum efficiency yields and, when ground, was less palatable to Trochidae. Overall, E. radiata was less affected by the projected climate change conditions, with only the C:N ratios being impacted in the combined increased temperature and increased pCO2 treatment. The palatability of E. radiata was also altered with the gastropod consuming a greater amount of the ground algae exposed to the combined temperature and pCO2 conditions. Finally, L. brongniartii was impacted in all the performance tests measured across all treatments, showing increases in levels of bleaching, density, and C:N ratios and decreases in growth, quantum efficiency yields, blade toughness and total phenolics. Uniquely, this study shows the vulnerability of understory red algal species, such as L. brongniartii to changes in climatic conditions. Surprisingly, these alterations in algal performance for L. brongniartii did not change the consumption rates of either herbivore. This study indicates that extreme climatic events have the potential to affect the performance and health of three abundant habitat-forming temperate algal species. The loss in health and performance seen in each species could have key implications for benthic communities in temperate Australian reefs, through processes such as changes in herbivory rates, competition with invasive species or simply through algal death. A possible implication of these stressors is the facilitation of range shifts along the west coast which could lead to the retraction of distribution ranges for many temperate Australian species.