Benthic algae are an important part of coral reef ecosystems, providing food and shelter for many organisms. However, algae are also powerful competitors for space. Their interactions with reef-building corals are of particular interest, as coral reef health is influenced by both herbivores and the dynamics of coral-algal interactions. With increasing anthropogenic influence on coral reefs, local factors, such as nutrient input, and global factors, e.g. ocean acidification and elevated temperatures, and their effects on organisms need to be considered. Algae are expected to maintain or increase productivity in the face of higher seawater CO2 concentrations. However, little research has been conducted on the combined effects of temperature and acidity on algal performance, with even less information available for tropical species that live close to their thermal maxima. In order to understand the effects of ocean acidification and climate change on coral reef algae, four separate experiments were conducted.
During the experiments, algae were subjected to different seawater temperature and pCO2 treatments associated with specific CO2 emission scenarios predicted for the end of the century. These scenarios were based on the Special Report on Emissions Scenarios of the Intergovernmental Panel on Climate Change (IPCC), which outlines different trajectories dependent on carbon use strategies and other factors.
The first two experiments focused on turf assemblages. During a study conducted at Lizard Island, the composition of turf algal communities was found to change significantly due to an interaction of pH and temperature: the communities grown under A1FI- acidification were found to be the most distinct. The interactive effect was, at least in part, driven by 5 eukaryotic species that were affected by temperature, CO2 or a combination of both. There was also a very strong shift towards the prokaryote cyanobacterial genus Lyngbya under elevated temperature and A1FI-acidification. This genus reportedly encompasses toxic species, which could lead to the deterioration of the ecosystem under future conditions.
The effect of IPCC CO2 emission scenarios on turf algal communities farmed by territorial damselfish was investigated in an experiment conducted in winter at Heron Island. The species composition of algal territories was dominated by the presence of the fish, and only in samples not subjected to fish grazing, a small scenario treatment effect on algal composition was observed. Biomass was reduced under the mid range (B1) scenario while productivity in terms of oxygen production was greatest under this scenario. The results indicated, as did the results of the Lizard Island experiment, that algal productivity will not increase by end-of-century under a high CO2/temperature (A1FI) scenario.
Two additional experiments focused on macroalgae and the combined effects of CO2 emission scenarios as well as elevated ammonium and phosphate concentrations on their performance. Single species responses allowed for a more in-depth assessment of the treatment effects. Comparing the findings for three algal species during the season of their peak abundance suggested that the response to scenarios and nutrient enrichment is species specific. Turbinaria ornata (Phaeophyceae) showed a trend for reduced growth rates under the A1FI scenario and Chnoospora implexa (Phaeophyceae) had higher growth rates under pre-industrial conditions. Laurencia intricata (Rhodophyta) showed the greatest growth rates under pre-industrial and the lowest under A1FI conditions, while nutrient addition led to increased mortality. Overall, nutrient enrichment led to luxury nutrient uptake and modified photosynthetic pigment content. None of the three species is likely to show increased growth rates under future scenarios, nutrient enrichment or a combination of both in seasons that are currently associated with maximum abundance.
Lastly, we assessed how the effects of CO2 emission scenarios and nutrient enrichment on C. implexa were affected by the timing of the experiment (spring versus winter). Nutrient enrichment did not stimulate oxygen production or growth in either season. Instead, lower photosynthetic pigment levels were observed under nutrient addition, despite luxury nutrient uptake. This relatively unpalatable and coral-smothering species showed greatest growth under spring pre-industrial scenarios, while its growth rate was slightly reduced under winter A1FI scenarios, irrespective of nutrient addition.
The results of this research suggest that climate change and ocean acidification effects are species specific and that the responses of algae to such changed conditions have the potential to vary throughout the year. It cannot be inferred from this research, if the conditions that are projected to occur by the turn of the next century will enhance eukaryotic algal growth. This includes the effect of eutrophication, which did not lead to growth stimulating responses in the species tested. Furthermore, mixed turf algal assemblages may be dominated by the farming activity of fish rather than by future conditions. However, in the absence of farming fish, shifts in species composition are more likely. This leads to the conclusion that coral reef algal communities have the potential to change during the course of the century and the evidence suggests that from the group of algae investigated prokaryotic species such as Lyngbya are most likely to prosper.