Reefs at Risk
Thaddeus J. Murdoch
tmurdoch at jaguar1.usouthal.edu
Fri Jun 26 16:25:04 EDT 1998
Recently, several comments have been made regarding the decline and
artificial regeneration of the coral assemblages of the Florida Keys:
Phil Dustan - Wed. 24 June 1998:
>...the reefs [of the Keys]...will be gone soon...
Les Kaufman - Wed. 24 June 1998:
>Since acroporids and agariciids are the principal corals with
>response times and growth rates commensurate with human
>intervention, should they not be the principle focus of efforts
Given the fact of coral decline in the Keys: The null hypothesis is that
all coral species are declining, and are all doing so at an equal rate.
This would imply that all coral species are equally affected by whatever
large-scale disturbances are perturbating the Florida reef tract.
In 1995, as part the Keyswide Coral Reef Expedition, I sampled the coral
assemblage structure of 16 spur and groove reefs (15-20 m depth). The
sampled reefs were located over the entire Florida reef tract, from
Biscayne Bay to the Dry Tortugas.
Coral cover and abundance varied dramatically from reef to reef in a way
that suggests that meso-scale gradients in water quality are the primary
factors affecting the community composition from reef to reef.
Particularily interesting patterns emerge when the coral cover data are
segregated into guilds, with each guild composed of species that share
morphology and reproductive mode. The guilds I examined are:
- Mound-like corals that broadcast spawn gametes,
- Mound-like corals that release brooded planula,
- Plating and Foliose Corals (which are mostly brooding
corals in the Caribbean),
- Branched corals that broadcast spawn gametes and
- Branched corals that release brooded larvae.
When the % cover of each guild is examined relative to total coral cover
on each reef, the following pattern emerges (simplified below):
20- . Total % coral cover .
- x % Massive Spawners .
- + % Massive Brooders .
- - % Branching and Plating . x
- . x
15- . x
- . x
- . x
- . x
Guild - . x
% cover 10- . x
per reef - . x
- . x
- . x
- . x
5- . x
0 5 10 15 20
Total % coral cover per reef
Massive corals that spawn gametes track changes in total coral cover
consistently. Massive corals that brood demonstrate only slightly
increased cover on reefs with high total coral cover. All other guilds do not
change in cover at all relative to reef to reef differences in total coral
It appears that only the massive spawners are affected by the
environmental gradients which control total coral cover on each reef,
while all other groups are relatively unaffected (at present). Note:
branching corals that spawn gametes (the acroporids) were all but absent
from the 20-m deep reefs in 1995, demonstrating the powerful effects of
region-wide disturbances such as disease.
In terms of artificially increasing coral cover in the Keys - the only
steps that are likely to work, based on the above, should be improving
water quality (locally and regionally) and the forced-recruitment of the
massive spawners and the branching spawners. These corals employ rapid,
indeterminate growth and high fragmentation rates as a means of
dominating a reef. The forced-recruitent of massive brooding corals or
the other guilds (which include agariciids) is not likely to lead to an
increase in total coral cover on reefs of good environmental quality. These
corals are more adaped to utilize patches that are at an early successional
state, and generally maintain low cover on these reefs regardless of
No-Take Zones of the Florida Keys - Benthic Monitoring Project
Dauphin Island Sea Lab,
PO Box 369, Dauphin Island, Alabama, 36528, USA.
Tel: (334) 861-7532 Fax: (334) 861-7540
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