Seawater pH as a Controlling Factor in Macroalgal Calcification and Photosynthesis

Sammanfattning: Biological calcification and photosynthesis are important processes with a great influence on both structure and function of oceanic ecosystems. The pH of the seawater has a strong influence on both these processes and therefore the impacts of different pH levels on calcareous macroalgae were investigated in laboratory and field experiments at Chwaka and Fumba Bays in Zanzibar, Tanzania. The levels of pH were manipulated, first in the laboratory by adding HCl, NaOH or by bubbling seawater with CO2 enriched air. Secondly, pH was allowed to fluctuate naturally as a consequence of marine photosynthetic carbon uptake or release through respiration by mussels. The effects on both photosynthesis and calcification were then analyzed on a seagrass (Thalassia hemprichii), and the calcareous red and green algae Mesophyllum sp., Hydrolithon sp., Amphiroa fragilissima and Halimeda renschii, as well as on the mussel Pinna muricata. The laboratory studies revealed a significant decrease in calcification rates in Hydrolithon sp. with decreasing pH, while photosynthesis showed an opposite trend. Also, increased dissolved CO2 lowered pH from 8.1 to 7.8 and caused a ~20% decline in calcification rates. In the field, seagrasses raised pH to ~9, increasing calcification rates significantly in the calcareous algae while photosynthetic rates showed no significant differences with changes in pH expect in Mesophyllum sp., in which rates increased at elevated pH caused by the presence of seagrasses. Conversely, seagrass photosynthesis increased significantly in the presence of mussels. Based on these findings, we conclude that pH is important in shaping biological processes that determines ecological interactions within shallow tidal areas by modifying seawater carbon composition and, thus, influencing calcification and photosynthesis processes.