MEPS 338:107-117 (2007)  -  doi:10.3354/meps338107

ABSTRACT: The aim of this study was to evaluate seasonal and diurnal variability in pH and inorganic carbon in shallow-water macroalgal habitats and to evaluate the importance of high pH for macroalgal photosynthesis. Seasonal variations in pH, oxygen saturation and inorganic carbon concentration were measured at an exposed and a sheltered shallow-water (0 to 1 m) macroalgal habitat. Daytime pH was significantly higher in spring, summer and autumn than in winter at both study sites. Diurnal measurements at the most exposed site showed significantly higher pH during the day than during the night. The diurnal variations were largest in shallow water and decreased with increasing water depth. High pH resulted in periodically low concentrations of available inorganic carbon in summer (as low as 1.3 mmol [CO₂ + HCO₃^–] l^–1). Photosynthesis as a function of inorganic carbon concentrations was measured at pH 8 and 9.3 for 4 common macroalgal species (Fucus vesiculosus, F. serratus, Ceramium rubrum, Ulva sp.). Photosynthesis in these species was not limited by natural concentrations of inorganic carbon, but maximum photosynthesis and inorganic carbon concentrations at saturation were lower when measured at pH 9.3 than at pH 8. Our results suggest that pH is higher in natural shallow-water habitats than previously thought, and that high pH has a direct effect on photosynthesis that cannot be accounted for by low availability of inorganic carbon.

KEY WORDS: pH · Seasonal variations · Diurnal variations · Macroalgae · Photosynthesis