DIEL VARIATIONS OF IN-VIVO FLUORESCENCE IN THE EASTERN EQUATORIAL PACIFIC - AN UNVARYING PATTERN

Records of in vivo chlorophyll a fluorescence obtained at various peri ods in upwelled waters of the equatorial Pacific appeared to be strong ly dominated by diel variations. High pass filtering of the records by a 24 h running mean, followed by normalization (division by the stand ard deviation in the -12 h to +12 h interval) produce a normalized die l cycle of in 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 non photochemical quenching, and a maximum from 19:00 h to midnight. Fluor escence decreased during the second half of the night in the absence o f forcing by solar energy, probably in response to a circadian physiol ogical rhythm. Vertical profiles of fluorescence made during an oceano graphic cruise at the equator showed that this physiological rhythm ex plained 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 of in vivo fluorescence with a limited number of filtration-extraction ba sed chlorophyll determinations. (C) 1998 Elsevier Science Ltd.