Diel variations ofin vivo fluorescence in the eastern equatorial Pacific: an unvarying pattern

Records of in vivo chlorophyll a fluorescence obtained at various periods in upwelled waters of the equatorial Pacific appeared to be strongly dominated by diel variations. High pass filtering of the records by a 24 h running mean, followed by normalization (division by the standard deviation in the −12 h to +12 h interval) produce a normalized diel cycle ofin vivo fluorescence, which was almost exactly the same in January, August and December 1991, October 1992 and January 1996. This cycle was characterized by a minimum at 12:00 h, mainly caused by nonphotochemical quenching, and a maximum from 19:00 h to midnight. Fluorescence decreased during the second half of the night in the absence of forcing by solar energy, probably in response to a circadian physiological rhythm. Vertical profiles of fluorescence made during an oceanographic cruise at the equator showed that this physiological rhythm explained the fluorescence cycle at depth, where it was characterized by a minimum at 7:00 h and a maximum at 19:00 h. Knowledge of this diel cycle of fluorescence can help to estimate the chlorophyll content of surface seawater in the equatorial Pacific using under-way records ofin vivo fluorescence with a limited number of filtration-extraction based chlorophyll determinations.