The tissue composition of Montastraea franksi during a natural bleaching event in the Florida Keys

Abstract

In this study, we used a correlative approach to (1) test for an association between bleaching and host tissue composition during a natural bleaching event, and (2) assess whether bleaching susceptibility varies between years. In August 1997, Montastraea franksi at 15-m depth on Conch Reef, Florida, bleached and the severity of the response varied among individuals. Seventy-five randomly selected colonies were quantified for bleaching using both an ordinal scale, assigned by eye, and a continuous scale, assessed using red, green, and blue (RGB) spectral analysis of photographs. Zooxanthella density and chlorophyll a content were evaluated as measures of bleaching, and coral tissue was analyzed for glycerol, free amino acids (FAA), protein, and mycosporine-like amino acids (MAAs); collectively, these are described as the "tissue composition." In 1998, most of the same coral colonies were analyzed for color and zooxanthella density to determine whether colony color in 1997 was correlated with color in 1998. In 1997, colonies of M. franksi that were ranked by color differed significantly in RGB brightness, zooxanthella density, and chlorophyll a content, but not in tissue composition. Similarly, a multivariate test for a linear relationship between color and tissue composition did not reveal a significant association. Analyses of corals in 1997 and 1998 revealed a significant positive relationship between color in both years (i.e., the same colonies were similarly colored in each year). These results are discussed in the context of the temporal scale of the sampling regime, the nature of the measured traits, and the adaptive bleaching hypothesis.