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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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environments exposed to large temperature fluctuations on
daily or even hourly scales due to local-scale oceanographic
factors such as internal waves. In these cases, the local
oceanography could reduce the vulnerability of mesophotic
corals to bleaching both through mitigating maximum
temperatures and exposing corals to large fluctuations in
temperature to which they have become adapted (e.g. Buerger
et al. 2015). Water temperatures on MCEs may vary with
changes in the depth of the thermocline, particularly due to the
El Niño Southern Oscillation, upwelling and internal waves.
Thermocline depth appears to exert a significant influence on
MCEs, and a deepening of the thermocline associated with El
Niño events has been implicated in bleaching of corals in Palau
(Colin pers. obs.).
6.3.2.
Impact of highly variable temperature
regimes on mesophotic coral ecosystems
The El Niño Southern Oscillation is a naturally occurring
phenomenon that produces varying ocean temperatures
in the equatorial Pacific. It generally involves fluctuations
between two phases (El Niño and La Niña) that can last for
several seasons. For equatorial reefs in the Western Pacific,
El Niño events are associated with cool surface waters, and
shallow thermoclines and nutriclines. La Niña events cause
the opposite effect, with warm surface waters, and deep
thermoclines and nutriclines. This is in contrast to conditions
in the Central and Eastern Pacific, where warmer than normal
ocean temperatures are associated with El Niño and cooler
than normal temperatures are associated with La Niña events.
The oscillation between El Niño and La Niña conditions can
lead to rapidly fluctuating conditions that may pose a serious
threat to MCEs (Glynn and D’Croz 1990).
During a bleaching event in Palau in August 2010, constantly
high temperatures to a depth of 90 m were observed for periods
of hours to days (Figure 6.5). Overall, the 2010 bleaching event
was moderate and limited in duration, so coral mortality was low
(unlike the sustained bleaching event in 1998; Bruno et al. 2001).
6.3.3.
Disease
Disease in corals and other organisms has increased in
shallow coral reef ecosystems in the past decade along with
increasing seawater temperatures (Burge et al. 2014). Such
temperatures render organisms more susceptible to disease
outbreaks, as well as bleaching events. Recent surveys of
MCEs in Puerto Rico and the U.S. Virgin Islands indicate
that mesophotic coral communities are indeed susceptible
to biotic diseases (Smith et al. 2010, Weil unpubl. data — see
Puerto Rico Case Study in Chapter 3) down to at least 100 m
in depth. Coral bleaching and disease were observed in at
least six of the 23 mesophotic scleractinian coral species in
Puerto Rico (Figure 6.6) —
Agaricia undata
,
A. lamarcki
,
Undaria agaricites
,
Mycetophyllia aliciae
,
Montastraea
cavernosa
and
Stephanocoenia intersepta
. Most of the
“disease signs” observed fall into a general category known
as “white syndromes” due to their characteristic white area
of recent tissue-cleared skeletal material (Raymundo et al.
2008, Weil and Hooten 2008), with some signs resembling
typical white plague disease (Figure 6.6). In 2014 at Pulley
Ridge, an MCE in the eastern Gulf of Mexico, from a total
of 7,329 individual plate corals (
Agaricia
spp. and
Helioseris
cucullata
) counted from transect photos, 247 were noted to
be bleached, partially bleached, totally bleached, partly dead,
recently dead or diseased, resulting in 4 per cent morbidity
of the total population measured (Reed et al. 2015).
A good example of white plague disease-like signs was
observed in a single medium-sized (832 cm
2
) coral colony
of
Mycetophyllia aliciae
at 55 m off La Parguera, Puerto
Rico. This colony was healthy in October 2009, but showed
white plague disease-like signs in December 2009, and was
dead by March 2010 (Figure 6.7). Rate of tissue mortality
varied approximately between 6–10 cm/month, one-third
slower than the maximum tissue rate mortality reported
for this disease type in shallower waters, but still faster than
all other shallow coral diseases except black band disease
(Weil 2004).
Figure 6.7.
A time-series showing a colony of
Mycetophyllia aliciae
at 50 m off La Parguera, Puerto Rico that was (a) healthy in October
2009, (b) developed signs of white-plague-like disease and suffered rapid tissue mortality (white skeleton without tissue and secondary
macroalgae colonization) and (c) was dead by March of 2010 (photos Héctor Ruiz).
October 2009
December 2009
March 2010
October 2009
December 2009
March 2010
October 2009
Decemb r 2009
March 2010
(a)
(b)
(c)