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

A newly developed underwater fast repetition rate fluorometer (FRRF) was us ed for in situ measurements of chlorophyll fluorescence yields on the reef- building corals Montastraea faveositu and h Montastraea cavernosa from arou nd Lee Stocking Island, Bahamas. Diel studies of the quantum yield of chlor ophyll fluorescence (DeltaF ' /F-m') in photosystem II (PSII) reveal a patt ern of mid-day depression of DeltaF ' /F-m' in both of these species of cor al. At the same time, non-photochemical quenching (qN) increased significan tly during the day, a pattern consistent with the regulation of PSII by dyn amic photoinhibition mediated by non-photochemical quenching. Despite these mid-day depressions in DeltaF ' /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 photosynt hetic apparatus, allowing the endosymbiotic dinoflagellates (zooxanthellae) to operate at maximum rates of photosynthesis. In 1999 measurements of Del taF ' /F-m' on M faveolata over a bathymetric range of 2 to 30 m showed an increase in range of 2 to 30 m showed an increase in DeltaF ' /F-m' with in creasing depth when measured at the same time of day. This suggests, althou gh there is year-to-year variability, that changes in the underwater light field, and photoacclimation to that Light field, control the degree of phot oprotection attributable to non-photochemical quenching in the zooxanthella e of these corals. The fluorescence yields of M. faveolata exposed to eleva ted temperatures (>32 degreesC) in the field showed a significant decrease in DeltaF ' /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 DeltaF ' /F-m' as a predictor .