Pulse amplitude modulated (PAM) fluorometry measures the photosynthetic efficiency of photosystem II (PS II) within the endosymbiotic Symbiodinium spp. that may be used as a quantitative measure of photoinactivation during coral bleaching (Warner et al. 1999). Chronic photoinhibition is the irreversible photodamage to PS II and is hypothesized to be one of the primary mechanisms of thermally induced bleaching (Warner et al. 1999; Gorbunov et al. 2001). It is expected that a decrease in the maximal quantum yield of PS II fluorescence will precede any visible signs of coral bleaching (Fitt and Warner 1995), improving the early detection of bleaching stress and leading to a better identification of threshold values for timely feedback of real-time conditions in situ.
Zooxanthellae fluorescence was measured in situ, remotely, and in near real-time for a colony of Siderastrea siderea and Agaricia tenuifolia at Lee Stocking Island, Bahamas during the Caribbean-wide 2005 bleaching event. These colonies displayed evidence of PS II inactivation coincident with thermal stress and seasonally high doses of solar radiation. Hurricane-associated declines in temperature and light appear to have facilitated the recovery of maximum quantum yield of PS II within these two colonies, although both corals responded differently to individual storms. PAM fluorometry, coupled with long-term measurement of in situ light and temperature, provides much more detail of coral photobiology on a seasonal time scale and during possible bleaching conditions than sporadic, subjective, and qualitative observations. S. siderea displayed evidence of PS II inactivation over a month prior to the issuing of a satellite-based, sea surface temperature (SST) bleaching alert by the National Oceanic and Atmospheric Administration (NOAA). In fact, recovery had already begun in S. siderea when the bleaching alert was issued. Fluorescence data for A. tenuifolia were difficult to interpret because the shaded parts of a colony were monitored and thus did not perfectly coincide with thermal stress and seasonally high doses of solar radiation as in S. siderea. These results further emphasize the limitations of solely monitoring SST (satellite or in situ) as a bleaching indicator without considering the physiological status of coral-zooxanthellae symbioses (e.g., Dunne and Brown 2001; Fitt et al. 2001). For full study see Manzello et al. (2009).
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