Algal class abundances in the western equatorial Pacific: Estimation from HPLC measurements of chloroplast pigments using CHEMTAX

Samplels for the analysis of phytoplankton photosynthesis pigments were collected from the equatorial Pacific (5°N to 15°S along 155°E) in October 1990 as part of the Australian contribution to the JGOFS program. Chlorophyll and carotenoid pigments were measured by HPLC using a diode-array detector. A PC-based computer program was used to optimise the pigment ratios and to estimate the contributions of 10 algal classes to the total chlorophyll a concentration at each location and in 7 separate depth bands. For the pigments that occur in more than one algal class, the pigment: chlorophyll a ratios for 19′-butanoyloxyfucoxanthin and 19′-hexanoyloxyfucoxanthin (chrysophytes and haptophytes), neoxanthin (prasinophytes, euglenophytes and chlorophytes) and chlorophyll b (prasinophytes, euglenophytes, prochlorophytes and chlorophytes) increase with depth, while those of violaxanthin (prasinophytes and chlorophytes), diadinoxanthin (dinoflagellates, chrysophytes, haptophytes, euglenophytes and diatoms), lutein (prasinophytes and chlorophytes) and, zeaxanthin (prasinophytes, cyanobacteria, prochlorophytes and chlorophytes) decrease with depth. Peridinin: chlorophyll 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 fucoxanthin, which increases with depth in chrysophytes and haptophytes but decreases in diatoms. Based on their contribution to the total chlorophyll a , cyanobacteria ( Synechococcus ) were dominant in the nutrient depleted surface waters, haptophytes were dominant at mid depth (70 m), and prochlorophytes were dominant at depths of 100–125 m. These three algal classes were by far the most important, and each contributed up to 30–40% of the total chloro-phyll a at some depth within the water column. Chlorophytes and chrysophytes contributed up to a maximum of about 12% of the total chlorophyll a , while cryptophytes, diatoms, dinoflagellates, prasinophytes and (possibly) euglenophytes generally contributed up to 4–8% of the chlorophyll a .