Effects of elevated seawater temperature and phosphate enrichment on the crustose coralline alga Porolithon onkodes (Rhodophyta)

Both global and local environmental changes threaten coral reef ecosystems. To evaluate the effects of high seawater temperature and phosphate enrichment on reef‐building crustose coralline algae, fragments of Porolithon onkodes were cultured for 1 month under laboratory conditions. The calcification rate of the coralline algae was not affected at 30°C, but it decreased to the negatives at 32°C in comparison to the control treatment of 27°C, indicating that the temperature threshold for maintaining positive production of calcium carbonate lies between 30 and 32°C. Phosphate enrichment of 1–2 μmol L ^?1 did not affect the calcification rate. The net oxygen production rate was enhanced by phosphate enrichment, suggesting that the photosynthetic rate was limited by the availability of phosphate. It was concluded that moderate phosphate enrichment does not directly deteriorate algal calcification but instead ameliorates the negative effects of high seawater temperature on algal photosynthesis.