Scleractinian or stony corals play an important role in tropical coral reef communities by creating the framework that provides habitat for a high abundance and diversity of reef organisms (Buddemeier 1976, Idjadi 2002, Brooks et al. 2007). Healthy reefs also support important coastal fisheries and provide an important source of income through tourism (Moberg and Folke 1999). However, global climate change (i.e. increasing temperature and pCO2) is associated with multiple negative effects on reef corals. High temperatures (1-2°C over seasonal maxima) contribute to mass bleaching events (i.e. large-scale loss of coloration in corals and/or the dissociation of the symbiotic relationship between photosynthetic dinoflagellates) (Smith and Buddemeier 1992, Glynn 1996). Decreases in the pH of seawater due to increasing pCO2 have been associated with reduced calcification rates ranging on average from 8% to 40%. The first part of this study (Chapter 1) tested for changes in the mass and linear components of skeletogenesis in response to increasing pCO2 by measuring the linear and mass components of skeletal formation in two species in the genus Acropora. In 14-day incubations at pC02 of ~700 ppm, the mass deposition of Acropora pulchra and A. hyacinthus decreased significantly by an average of 27%. Importantly, rates of linear extension significantly decreased 2 to 4 time more than mass deposition, and displayed average decreases of 46% and 169% for A. pulchra and A. hyacinthus respectively. The rates of mass deposition and linear extension were converted into a unit-less index to test if the mass and linear components of growth were differentially affected by high pCO2. Mass deposition and linear extension displayed a significantly different response to high pCO2 (~700 ppm) This study suggests for the first time that there may be additional skeletal effects exhibited in coral in response to increasing pCO2 due to the disproportionate effects on the mass versus the linear components of skeletogenesis. The second part of this study tested for the interactive effects of pCO2 and temperature on photosynthesis and coral calcification to test how changing abiotic conditions due to climate change may affect corals. A factorial experimental design was used with pCO2 levels that simulated present day, and year 2100 conditions, and temperature levels of ~27°C and ~29°C. In 14-day incubations with Pocillopora meandrina and Porites rus, calcification rates in high pCO2 conditions (~700ppm) decreased significantly (50% and 70%, respectively) at ~27°C however, this effect was dependent on temperature and did not occur at ~29°C. In contrast to calcification, dark-adapted maximum quantum yield displayed no significant effects of temperature or pCO2, meaning that photosynthetic efficiency was unaffected by temperature, pCO2 or an interaction of the two. Thus, while photosynthetic efficiency is robust to slight increases in temperature (1-2°C) and reduced pH, calcification rates in P. meandrina and P. rus show a sensitivity to reduced pH that is temperature dependent. In summary, this research establishes that increasing atmospheric pCO2 will differentially affect the components of skeletogenesis that determine coral morphology, and that the combined effects of slight increases in temperature along with increased pCO2 may not decrease calcification extensively for some coral species.
Thesis or Dissertation
Department of Biology, CSU Northridge