ALGAL CLASS ABUNDANCES IN THE WESTERN EQUATORIAL PACIFIC - ESTIMATIONFROM HPLC MEASUREMENTS OF CHLOROPLAST PIGMENTS USING CHEMTAX

Samples for the analysis of phytoplankton photosynthetic pigments were collected from the equatorial Pacific (5 degrees N to 15 degrees S al ong 155 degrees E) in October 1990 as part of the Australian contribut ion to the JGOFS program. Chlorophyll and carotenoid pigments were mea sured by HPLC using a diode-array detector. A PC-based computer progra m was used to optimise the pigment ratios and to estimate the contribu tions of 10 algal classes to the total chlorophyll a concentration at each location and in 7 separate depth bands. For the pigments that occ ur in more than one algal class, the pigment: chlorophyll a ratios for 19'-butanoyloxyfucoxanthin and 19'-hexanoyloxyfucoxanthin (chrysophyt es and haptophytes), neoxanthin (prasinophytes, euglenophytes and chlo rophytes) and chlorophyll b (prasinophytes, euglenophytes, prochloroph ytes and chlorophytes) increase with depth, while those of violaxanthi n (prasinophytes and chlorophytes), diadinoxanthin (dinoflagellates, c hrysophytes, haptophytes, euglenophytes and diatoms), lutein (prasinop hytes and chlorophytes) and, zeaxanthin (prasinophytes, cyanobacteria, prochlorophytes and chlorophytes) decrease with depth. Peridinin: chl orophyll a increases with depth in dinoflagellates, while alloxanthin: chlorophyll a decreases with depth in cryptomonads. The only pigment ratio that does not change consistently with depth is that of fucoxant hin, which increases with depth in chrysophytes and haptophytes but de creases in diatoms. Based on their contribution to the total chlorophy ll a, cyanobacteria (Synechococcus) were dominant in the nutrient depl eted surface waters, haptophytes were dominant at mid depth (70 m), an d prochlorophytes were dominant at depths of 100-125 m. These three al gal classes were by far the most important, and each contributed up to 30-40% of the total chlorophyll a at some depth within the water colu mn. Chlorophytes and chrysophytes contributed up to a maximum of about 12% of the total chlorophyll a, while cryptophytes, diatoms, dinoflag ellates, prasinophytes and (possibly) euglenophytes generally contribu ted up to 4-8% of the chlorophyll a. (C) 1998 Elsevier Science Ltd. Al l rights reserved.