TEMPORAL VARIABILITY OF PHYTOPLANKTON COMMUNITY STRUCTURE-BASED ON PIGMENT ANALYSIS

Algal chlorophyll and carotenoid distributions were measured periodica lly in the euphotic zone of Sta. ALOHA (22-degrees-45'N, 158-degrees-0 0'W) bet-en February 1989 and October 1991 to document the variability in phytoplankton abundance and composition. The annual mean depth-int egrated (0-200 db) concentration of Chl a displayed significant intera nnual variability. Seasonal patterns in Chl a concentration were found to be depth-dependent. Elevated Chl a in the mixed layer is the resul t of photoadaptation as the mixed layer deepens in winter. Increases i n Chl a at the deep chlorophyll maximum layer (DCML) in spring are exp lained by increased nutrient availability caused by a deepening of the DCML relative to the sigma(theta) = 24.25 density surface. An algorit hm based on the ratios of Chl a to diagnostic pigments present in spec ific algal taxa was used to estimate the contribution to total Chl a b y the major algal groups represented within the DCML. Results indicate the presence of a phytoplankton community at the DCML with the follow ing mean composition: Prochlorococcus spp. (39%), cyanobacteria (24%), prymnesiophytes (22%),and chrysophytes (13%). No single taxon is resp onsible for the springtime increase in Chl a observed in this habitat. Results from size fractionation and normal-phase high performance liq uid chromatography confirm that Prochlorococcus spp. are the principal contributors of Chl a to the DCML.