In situ measured seasonal variations in F_v/F_m of two common Red Sea corals

Pulse amplitude modulated (PAM) fluorometry has been suggested as a tool for estimating environmental stresses on corals. However, information regarding natural changes in maximal quantum yields (F_v/F_m) of corals during "normal" (i.e. non-bleaching) years has been limited. In this study, seasonal variations in F_v/F_m for Stylophora pistillata and Favia favus, measured in situ, correlated with seasonal changes in solar irradiance but not in sea temperature. Interactions between sea temperature and irradiance were further studied by growing these corals and Pocillopora damicornis under controlled conditions. Exposure to high light with normal or high temperatures resulted in lower F_v/F_m values than exposure to low light at both temperatures. Thus, high irradiances may cause decreased F_v/F_m values in corals at least as much as, if not more than, high temperatures. Such seasonal variations should be taken into account when using PAM fluorometry as a diagnostic tool for predicting coral bleaching.