Diurnal and bathymetric changes in chlorophyll fluorescence yields of reef corals measured in situ with a fast repetition rate fluorometer

A newly developed underwater fast repetition rate fluorometer (FRRF) was used for in situ measurements of chlorophyll fluorescence yields on the reef-building corals Montastraea faveolata and Montastraea cavernosa from around Lee Stocking Island, Bahamas. Diel studies of the quantum yield of chlorophyll fluorescence (F '/F m ') in photosystem II (PSII) reveal a pattern of mid-day depression of F '/F m ' in both of these species of coral. At the same time, non-photochemical quenching (qN ) increased significantly during the day, a pattern consistent with the regulation of PSII by dynamic photoinhibition mediated by non-photochemical quenching. Despite these mid-day depressions in F '/F m ', net productivity, measured as oxygen flux, remains high, suggesting that non-photochemical quenching dissipates the majority of the absorbed photons at mid-day and protects the photosynthetic apparatus, allowing the endosymbiotic dinoflagellates (zooxanthellae) to operate at maximum rates of photosynthesis. In 1999 measurements of F '/F m ' on M. faveolata over a bathymetric range of 2 to 30 m showed an increase in F '/F m ' with increasing depth when measured at the same time of day. This suggests, although there is year-to-year variability, that changes in the underwater light field, and photoacclimation to that light field, control the degree of photoprotection attributable to non-photochemical quenching in the zooxanthellae of these corals. The fluorescence yields of M. faveolata exposed to elevated temperatures (> 32C) in the field showed a significant decrease in F '/F m ' before visible signs (e.g., paling of colonies) occurred. It was also possible to predict which colonies at the same depth and light regime would bleach first in response to elevated temperatures before any visible signs of bleaching were evident using F '/F m ' as a predictor.