Light absorption and utilization by colonies of the congeneric hermatypic corals Montastraea faveolata and Montastraea cavernosa

The congeneric species Montastraea faveolata and Montastraea cavernosa are important hermatypic corals on reefs throughout the Bahamas, Caribbean, and the Florida reef tract that have overlapping bathymetric distributions. Ho wever, these congeners differ in their respective abundance at similar dept hs. The underlying mechanism for these patterns may partly be because of th eir relative dependence on photoautotrophy versus heterotrophy. The depende nce of these two corals on photoautotrophy was examined quantifying the opt ical properties and productivity of these two species of corals at two diff erent depths in the Dry Tortugas. Maximum surface irradiances in the Dry To rtugas during this study varied from 1,900 to 2,100 mu mol quanta m(-2) s(- 1). Spectral attenuation coefficients calculated from the 1995 and 1996 irr adiance data differed by as little as 10% within the visible wavelengths (p hotosynthetically active radiation [PAR], 400-700 nm), suggesting year-to-y ear similarities in the optical properties of the overlying water column. U nderwater irradiances of PAR were similar to 400 mu mol quanta m(-2) s(-1) and 25 mu mol quanta m(-2) s(-1) at 10 m and 18 m, respectively. Significan tly lower rates of maximum photosynthesis were observed for samples of M. c avernosa compared with M. faveolata at 10 m and 18 m. For samples of M. fav eolata from both depths, the mean chlorophyll-specific absorption (a*) acro ss all PAR wavelengths was greater than that of M. cavernosa. When spectral ly corrected for the underwater light field and used to calculate the minim um quantum requirements (1/phi(m)) of these corals at each depth, we observ ed that M. faveolata always had higher 1/phi(m) than M. cavernosa (50 versu s 18 quanta O-2(-1) and 39 versus 15 quanta O-2(-1) at 10 m and 18 m, respe ctively). M. cavernosa, with its greater pigment concentrations and lower a *, exhibits a significant package effect that results in a smaller function al optical cross section and lower maximum photosynthetic capacities, where as M. faveolata at the same depths, despite the greater minimum quantum req uirements, has a larger functional optical cross section and enhanced absor ption of available visible radiation, resulting in a greater maximum photos ynthetic capacity. Based on polyp size, corallite structure, and surface ar ea considerations, M. faveolata appears to depend on photoautotrophy versus heterotrophy to a greater extent than its congener, M. cavernosa. Recent d ata suggest, however, that polyp size alone may not be a good indicator for differences in trophic strategies and that coordinated studies on feeding and productivity in corals are needed to better understand their ecological distributions.