Reefs at Risk

[Thaddeus J. Murdoch]

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
>toward [forced-recruitment]?

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
           -            x
           -         x  
           -                          ++++++++++++++++
           -         ++++++++++++++++-----------------
           0___________________________________________
            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 
cover.

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 
environmental condition.


Thad Murdoch
--------------------------------------------------------------
Project Manager
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