Mesophotic Coral Ecosystems

6.3.3. Disease

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).

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).

(a)

(b)

(c)

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

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 73