Info

18° 20" 22° 24° 26° 28" 30° 32° 34"C

I I I I I I I I I I I I I 1 |~T O Severe bleaching • Low to medium bleaching

18° 20" 22° 24° 26° 28" 30° 32° 34"C

I I I I I I I I I I I I I 1 |~T O Severe bleaching • Low to medium bleaching

Figure 6.2. Maximum monthly mean sea surface temperature for 1998, 2002 and 2005, and locations of reported coral bleaching (data source, NOAA Coral Reef Watch (coralreefwatch.noaa.gov) and Reefbase (www.reefbase.org)).

Global climate model results imply that thermal thresholds will be exceeded more frequently with the consequence that bleaching will recur more often than reefs can sustain (Hoegh-Guldberg, 1999, 2004; Donner et al., 2005), perhaps almost annually on some reefs in the next few decades (Sheppard, 2003; Hoegh-Guldberg, 2005). If the threshold remains unchanged, more frequent bleaching and mortality seems inevitable (see Figure 6.3a), but with local variations due to different susceptibilities to factors such as water depth. Recent preliminary studies lend some support to the adaptive bleaching hypothesis, indicating that the coral host may be able to adapt or acclimatise as a result of expelling one clade1 of symbiotic algae but recovering with a new one (termed shuffling, see Box 4.4), creating 'new' ecospecies with different temperature tolerances (Coles and Brown, 2003; Buddemeier et al., 2004; Little et al., 2004; Obura, 2005; Rowan, 2004). Adaptation or acclimatisation might result in an increase in the threshold temperature at which bleaching occurs (Figure 6.3b). The extent to which the thermal threshold could increase with warming of more than a couple of degrees remains very uncertain, as are the effects of additional stresses, such as reduced carbonate supersaturation in surface waters (see Box 4.4) and non-climate stresses (see Box 16.2). Corals and other calcifying organisms (e.g., molluscs, foraminifers) remain extremely susceptible to increases in SST. Bleaching events reported in recent years have already impacted many reefs, and their more frequent recurrence is very likely to further reduce both coral cover and diversity on reefs over the next few decades.

Figure 6.3. Alternative hypotheses concerning the threshold SST at which coral bleaching occurs; a) invariant threshold for coral bleaching (red line) which occurs when SST exceeds usual seasonal maximum threshold (by ~1°C) and mortality (dashed red line, threshold of2°C), with local variation due to different species or water depth; b) elevated threshold for bleaching (green line) and mortality (dashed green line) where corals adapt or acclimatise to increased SST (based on Hughes et al., 2003).

0 0

Post a comment