Algal class abundances, estimated from chlorophyll and carotenoid pigments, in the western Equatorial Pacific under El Nino and non-El Nino conditions

Phytoplankton samples were collected from the equatorial Pacific (10 degree s S to 10 degrees N along 155 degrees E) in June 1992 as part of the Austra lian contribution to the JGOFS program. Chlorophyll and carotenoid pigments were measured by HPLC, and a PC-based computer program (CHEMTAX) was used to estimate the contribution of 9 algal classes to the total chlorophyll a concentration in 9 separate depth bands at each location. This cruise occur red in the middle of the prolonged 1991/1993 El Nino, and the results are c ompared with similar data from a cruise in October 1990 which occurred befo re this El Nino but after the 1988/1989 La Nina. Changes in the pigment:chlorophyll a ratios appeared consistent across alga l classes and, apart from some minor exceptions, consistent between cruises . Pigments involved in light-harvesting generally increased relative to chl orophyll a with increasing depth, whereas the ratio for photoprotective pig ments (e.g. diadinoxanthin) usually decreased with depth. The zeaxanthin co ncentration per cell for cyanobacteria decreased with depth in the surface 75 m during 1992 as would be expected for a photoprotective Figment. Based on their contribution to the total chlorophyll a concentration, hapto phytes, prochlorophytes, cyanobacteria (Synechococcus) and chlorophytes wer e the dominant algal classes in 1992, The chlorophyte contribution to chlor ophyll a in 1992 (14.8%) was almost double that in 1990 (7.8%). This increa se was largely at the expense of the cyanobacteria and haptophytes, which b oth decreased significantly. The increase in chlorophytes in 1992 was parti cularly noticeable in the surface waters south of the equator at about 4 de grees S, where there was evidence of upwelling. (C) 2000 Elsevier Science L td. All rights reserved.