Wfr

Dairy Bull 1995

Total coral cover

42%

2%

23%

M. annularis complex

7%

1%b

13%

A. cervicornis

12%

0%

0%

a The 1977 data for Discovery Bay are from Huston (1985b). The 1995 data for Discovery Bay and Dairy Bull are from Edmunds and Bruno (1996 and unpubl. data).

b Value for M. annularis cover at Discovery Bay in 1995 agrees with values for the same depth in 1993 reported by Hughes (1994).

a The 1977 data for Discovery Bay are from Huston (1985b). The 1995 data for Discovery Bay and Dairy Bull are from Edmunds and Bruno (1996 and unpubl. data).

b Value for M. annularis cover at Discovery Bay in 1995 agrees with values for the same depth in 1993 reported by Hughes (1994).

Aller, and Thomson 1974; Dustan 1975; Graus and Macintyre 1982; Highsmith, Lueptow, and Schonberg 1983; Shinn et al. 1989). Because most of the M. annularis colonies at Discovery Bay and Dairy Bull were considerably taller than 50 cm in 1995, they must have predated Hurricane Allen and the Diadema mass mortality. This suggests that much of the difference between the sites in 1995 was due to the mortality and subsequent macroalgal overgrowth of corals more susceptible to hurricane damage than M. annularis.

Prior to 1980, the most common coral at the two sites other than M. annularis was A. cervicornis, judging from (1) surveys conducted at Discovery Bay in the 1970s (Porter et al. 1981; Huston 1985b), and (2) the enormous quantity of A. cervicornis rubble at 10 m depth at both sites in the 1990s. Hurricane Allen had the main effect of subtracting A. cervicornis from Discovery Bay and (presumably) Dairy Bull, leaving M. annularis more or less intact (Woodley et al. 1981; Hughes 1989; Steneck 1994; see also Bak and Luckhurst 1980). Hurricane Gilbert in 1988 had less dramatic effects on corals at Discovery Bay and (presumably) Dairy Bull, because A. cervicornis and other corals had only begun to recover from Hurricane Allen (Woodley 1989). M. annularis colonies again were largely unaffected (Hughes and Connell 1999).

This pattern was repeated elsewhere. Stoddart (1963) reported the nearly complete destruction of A. cervicornis but only moderate to minimal destruction of M. annularis following Hurricane Hattie in Belize (see also Glynn, Almodóvar, and González 1964 on Hurricane Edith's impact in Puerto Rico). Shinn (1976; Shinn et al. 1989) commented on the volatility of A. cervicornis populations in the Florida Keys, and Curran et al. (1994) noted that A. cervicornis died but M. annularis remained intact in the Bahamas. Macroalgal overgrowth subsequent to the Diadema mass mortality reduced the cover of M. annularis in Jamaica (Hughes 1994), but this did not happen in Belize, the Florida Keys, or the Bahamas.

In extreme cases of herbivore removal, macroalgae will overgrow massive corals (Hay and Taylor 1985;Lewis 1986; but see de Ruyter van Steveninck, van Mulekom, and Breeman 1988). Although the absence of herbivores may have been largely responsible, the mortality of M. annularis at Discovery Bay can also be linked to the activities of pomacentrids. L. S. Kaufman (personal communication, 1998) noted an increase in the number of three-spot damselfish (Stegastesplanifrons) territories on M. annularis after Hurricane Allen. S. plan-ifrons apparently moved their territories to head corals after the hurricane eliminated their preferred microhabitat, A. cervicornis thickets. M. annularis heads were less preferred before the demise of A. cervicornis (Kaufman 1977; see also Williams 1978; Ebersole 1985),but those coral heads are currently the microhabitat of choice for S. planifrons in St. Croix (Tolimieri 1998). Eakin (1989) also observed that, in the absence of live branching corals, juvenile S. planifrons occupied living M. annularis colonies in Florida. By partially killing head corals, S. planifrons probably contributed to macroalgal overgrowth. Qualitative observations over the past few years indicate increasing numbers of S. planifrons territories on massive corals in Belize and the Florida Keys, again causing partial mortality (W. F. Precht, unpublished observations, 1998; S. L. Miller and A. M. Szmant, personal communications, 1998).

Differential coral mortality has also occurred between reef zones. For example, fore-reef zones at Carrie Bow Cay, Belize, experienced differential mortality in the 1980s. The shallow spur-and-groove zone (3-6 m depth) had high coral cover in the 1970s to early 1980s, dominated by the blade-shaped lettuce coral, Agaricia tenuifolia. Even though Diadema were present in this shallow zone at that time, scarids and acanthurids were the most important herbivores (Lewis and Wainwright 1985). The deeper spur-and-groove zone at intermediate depths (9-15 m) also had high coral cover, dominated by A. cervicornis (Burke 1982; Rutzler and Macintyre 1982). Diadema were rare in this deeper zone, herbivorous fishes were more abundant in the deeper zone, and herbivory was intense and macroalgal cover was low in both zones (Lewis and Wainwright 1985; Littler et al. 1987). In the 1990s, Diadema were absent from both zones, and scarids and acanthurids were abundant and not subject to fishing pressure. Coral cover remained variable but high in the shallow spur-and-groove (Aronson and Precht 1995), while the deeper spur-and-groove was largely covered by macroalgae (Aronson et al. 1994). The difference in macroalgal cover between zones in the 1990s arose because most of the A. cervicornis in the deeper zone died in the early 1980s.

These observations highlight the importance of coral mortality, specifically the mortality of A. cervicornis, as a prerequisite to macroalgal dominance at intermediate depths. Presumably the mortality of A. palmata is equally neces sary for coral-to-macroalgal transitions in reef crest and shallow fore-reef habitats. What are the important causes of coral mortality in the Caribbean?

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