Size matters: bleaching dynamics of the coral Oculina patagonica

Abstract

A 2-yr continuous photographic monitoring of a tagged population of the encrusting coral Oculina patagonica in the Mediterranean was conducted to study intra-colonial bleaching dynamics and the relationship between bleaching, mortality, and colony size. Surveys of non-tagged colonies showed that during the peak bleaching season (August, sea surface temperature = 31°C), non-bleached colonies were frequently found to be small colonies averaging 4.6 ± 2.3 cm in diameter. Within tagged colonies, percent bleached surface area was correlated to water temperature. In colonies that underwent bleaching, the perimeter of the colony was affected first, and, as water tem- peratures increased, bleaching progressed toward the colony center. During the summer months, partial mortality occurred in the perimeter region of bleached colonies in 22% of the tagged colonies and 25% of the tagged colonies died; 40% of the colonies that died belonged to the largest size group. This partial mortality caused an average decline of 46 ± 27% in the average colony size, resulting in a shift to a smaller size group within the monitored population. Since in this species, colonies as small as 2 cm in diameter are reproductive, bleaching may have a less significant effect on the reproductive fitness of the small size groups in the population. The high mortality of large colonies, high survivorship of the small colonies, and the decline in colony size, due to partial mortality, suggest that, in the case of bleaching in populations of O. patagonica, small colony size is advantageous.