Acropora size-frequency distributions reflect spatially variable conditions on coral reefs of Palau

The extreme thermal stress event of 1998 caused extensive coral mortality on palau's reefs. While impact varied in accordance with coral species (Scleractinia) and habitat, the acroporid corals were the most vulnerable, and there was much concern about how long it would take the reefs to recover. Seven years after the thermal stress, long-term monitoring has shown rapid recovery of coral coverage, which was highest on the western reefs of palau, although higher Acropora recruitment favored the east coast. Differential recovery may have resulted from higher remnant regrowth, higher post-settlement survival, or more rapid growth of recruits on the western reefs. Here we test the latter two hypotheses to examine which processes were favored on the western outer reefs. If corymbose Acropora colony densities on western reefs are the same or higher than eastern reefs, we can infer favorable post-settlement survival on western slopes, because Acropora recruitment was higher on eastern reefs. In contrast, differential growth capacity would show larger colonies on the western reefs with similar colony densities. We undertook an extensive, depth-stratified spatial survey of corymbose Acropora size-frequency distributions. The western reefs had significantly larger Acropora digitifera (Dana, 1846) at 3 m, and larger Acropora nasuta (Dana, 1846), Acropora cerealis (Dana, 1846), and Acropora selago (Studer, 1878) at 10 m, while colony densities did not differ spatially. We conclude that for the dominant Acropora colonies post-recruitment survival is higher and coral-colony growth is more favorable on the western reef slopes of palau.