[Coral-List] Bermuda bleaching event

[Samantha de Putron]

Dear Coral List colleagues

The following is a report of the bleaching event witnessed in Bermuda this 
summer.

Coral bleaching was witnessed this summer on the outer rim reefs and 
lagoonal patch reefs of Bermuda (32 20N, 65 50W). Surveys of 2 sites at 
each reef type were conducted by the Benthic Ecology Research Program of 
the Bermuda Biological Station for Research (BBSR) at the end of 
August/beginning of September (28/08/03  03/09/03) and again at the end of 
September/beginning of October (19/09/03  06/10/03), looking at the 
occurrence of bleaching and disease. The Rim Reef sites are at the edge of 
the Bermuda platform approximately 17 km from shore, at a depth of 2-8m. 
Inshore of the Rim Reef is the enclosed lagoon. The lagoonal patch reefs 
monitored in this study are approximately 9km from land, at a depth of 
1-4m.  Each survey consisted of 12 x 20m long 2 m wide transects. The 
surveys recorded the percentage cover as well as the occurrence of 
bleaching and disease of the primary reef building and most abundant coral 
species in Bermuda: Diploria labyrinthiformis, Diploria strigosa, 
Montastraea cavernosa, Montastraea franksii and Porites astreoides.  The 
fire coral Millepora alcicornis was included as well as the gorgonians 
Pseudoplexaura porosa Gorgonia ventalina.  General observational notes were 
made for other species, which are not included in this report. Please 
contact us if you would like information on a particular species not 
included here.

Of all the coral colonies recorded on the initial survey, 21% were bleached 
at the rim reef sites and 19% were bleached at the lagoonal sites.  Water 
temperatures were at a maximum and had remained so for longer periods of 
time than is typical (see below for temperature information). The second 
surveys, when water temperatures had decreased, showed a decrease in total 
bleaching to 13% at both sites. The fire coral, Millepora alcicornis was 
the coral species most affected by bleaching this summer, with 94% of the 
colonies being bleached to some degree on the rim reefs in early 
September.  On the lagoonal reefs at this time, 77% of M. alcicornis 
colonies showed some sort of bleaching. Bleaching of M. alcicornis reduced 
to 70% occurrence on the rim reefs, and to 49% on the lagoonal reefs, one 
month later by the time of the second survey. In this species, the 
bleaching on the rim reefs was mainly composed of completely bleached 
(white) colonies or recently dead colonies (white skeleton with no fouling 
of algae). At the time of the initial survey 35% of the total M. alcicornis 
colonies on the rim reef were recently dead. This increased to 43% by the 
time of the second survey. On the lagoonal reefs, the majority of bleached 
M. alcicornis were classed as ‘pale’ or ‘blotchy’ and only 4% of the total 
colonies were initially recorded as recently dead. This reduced to 3% by 
the time of the follow up survey.  A total of 60% of M. alcicornis colonies 
were classed as ‘pale’ or ‘blotchy’ initially; this reduced to 43% by the 
time of the second survey.  Other colonies that were bleached at both sites 
were (in order of prevalence) Montastrea franksi, Diploria strigosa, M. 
cavernosa, D. labyrinthiformis and Porites asteroides.  It should be noted 
that many Agaricia fragilis colonies were extensively bleached in areas on 
the outer rim reef, although this species was not included in these surveys.

Average temperature over the week in which the initial survey was performed 
(28/08/03  03/09/03) was 28.9 ºC at the rim reef and 29.5ºC at the lagoonal 
reefs. The maximum temperatures recorded from the sites were 30ºC at the 
rim reef, and 30.7ºC at the lagoonal patch reef. The average temperature 
during the week in which the second surveys were performed 
(19/09/03  06/10/03) was 26.7ºC at the rim reef and 26.9ºC at the lagoonal 
reefs. Bleaching was therefore more prevalent at the Rim Reef sites 
compared to the Lagoonal sites even though the temperature maximum was 
greater at the latter. This is similar to past observations in Bermuda 
(Cook et al., 1990: Coral Reefs 9:45-49; McKinney et al, 1998: BBSR 
unpublished data) and is understood to relate to a combination of increased 
turbidity in the lagoon reducing irradiance, and/or the prior thermal 
history at each site causing the adaptation (and therefore sensitivity) of 
the coral populations to local conditions.
The incidence of disease was very low, with a total of only 1.5% of the rim 
reef colonies, and 0.1% of the lagoonal corals being diseased on the 
initial survey. Disease, however, increased slightly by the time of the 
second survey at both sites, to 1.9% at the rim reefs, and 0.9% at the 
lagoonal sites. The most commonly witnessed diseases on the scleractinians 
were a “White Plague” type disease observed on Diploria labyrinthiformis, 
Diploria strigosa, Montastrea cavernosa, Montastrea franksi, and Porites 
asteroides on the rim reefs, and on all these colonies with the exception 
of Montastrea cavernosa on the lagoonal reefs. Black Band Disease was 
observed only on D. strigosa on the rim reefs, and on D. strigosa, M. 
cavernosa and M. franksi on the lagoonal reefs. Aspergillosis was observed 
on a low percentage of the sea fan Gorgonia ventalina (0% and 1.5% 
initially at the rim and lagoonal reefs respectively, and 10% and 1.2% at 
the rim and lagoonal reefs by the time of the second survey). We also 
collected data on the incidence of hyperplasmic tumours (seen in the 
Diploria species only) and fish biting (primarily of P. astreoides). Algal 
galls were sporadically seen on the sea rod Pseudoplexaura porosa.


Please contact us if you require any further information.

Regards,

Helen Brylewska
Samantha de Putron Ph.D.
Graham Webster M.Sc.
Matt Strong



Dr Samantha de Putron
-----------------------------------------------------------
Bermuda Biological Station for Research
Ferry Reach,  St Georges  GE 01,
Bermuda
Tel: (441) 297 1880 x 261
Fax: (441) 297 8143
www.bbsr.edu