Spatial, temporal and taxonomic variation in the incidence of partial mortality among reef-building corals
To establish baseline levels of partial mortality among common reef-building corals, I first quantified prevalence (proportion of colonies with injuries) and severity (areal extent of injuries on individual colonies) of injuries across four common coral taxa (massive Porites, encrusting Montipora, Acropora hyacinthus and branching Pocillopora) on the Great Barrier Reef, Australia. A total of 2,276 adult colonies were surveyed annually over three years across three latitudinal sectors, nine reefs and 27 sites along the Great Barrier Reef. The prevalence of injuries was very high (>83%) for all coral taxa, but especially high for Porites (91%) and Montipora (85%). Within individual taxa, there was significant temporal or spatial variation in prevalence of partial mortality in Montipora and Pocillopora. Severity of partial mortality on injured colonies ranged from 5% for A. hyacinthus up to 21% for Montipora, and varied both spatially and temporally. These findings confirm that background levels of partial mortality are high even in the absence of major disturbances, and are likely to significant influence differential vulnerability of colonies, populations and coral species to other more acute disturbances.
Building on my initial studies on the Great Barrier Reef, I then compared rates of background mortality (including both partial and whole colony mortality) between Lhaviyani Atoll, Maldives and the northern Great Barrier Reef, Australia. Comparisons were made for four dominant and widespread coral taxa (Porites, Montipora, Acropora hyacinthus and Pocillopora), which were surveyed across multiple reefs and sites at each location. Prevalence of partial mortality was consistently higher on the GBR (99.4% of colonies in Porites, 66% in Acropora hyacinthus and 64% in Pocillopora) than at Lhaviyani Atoll (92.4% in Porites, 47.5% in A. hyacinthus and 44% in Pocillopora). Conversely, severity of partial mortality was higher for A. hyacinthus and Porites (9.6% and 12.2%, respectively) at Lhaviyani Atoll than on the GBR (7% in A. hyacinthus and 9.6% in Porites). However, marked differences in severity of partial mortality were most apparent at the smallest spatial scale (e.g., among colonies located on the same transect within the same habitat). This suggests that corals in different geographical locations are consistently subject to high levels of background mortality, but the specific effects are highly patchy and likely contribute to significant intercolony variation in susceptibility of corals to major disturbances (e.g., bleaching).
After investigating spatial variation in tissue loss, I tested the specific effects of injuries on colony condition. Intraspecific variation in physiological condition was measured, based on total lipid content and zooxanthellae density, in adult colonies of two common and widespread coral species (Acropora spathulata and Pocillopora damicornis). Importantly, these corals were subject to different levels of biological and physical disturbances and marked intraspecific variation in the physiological condition of A. spathulata was clearly linked to differences in local disturbance regimes. Conversely, P. damicornis exhibited very limited intraspecific variation in physiological condition, despite marked differences in the severity of partial mortality. This study shows that the physiological condition of individual coral colonies is influenced by differences in injury regimes, at least for some coral taxa. Moreover, these differences in physiological condition are likely to have important effects on the fate and fitness of individual colonies, and especially their vulnerability to subsequent disturbances.
Overall, my PhD research shows that the prevalence of coral injuries is consistently high (>80% of colonies have visible injuries) across different coral taxa and among reef locations spanning 10 degrees of latitude and in different ocean basins. The severity of injuries meanwhile, varies greatly among individual colonies within the same habitat and site, which may have important ramifications for susceptibility to acute disturbances and environmental change. Much more research is needed to understand the major causes of routine injuries to reef building corals, but studies on the effects of acute disturbances on coral populations and/ or communities should not discount background levels of mortality, and the extent to which this might further increase susceptibility to, and prevent recovery from, acute disturbances.