Temporal variability of phytoplankton community structure based on pigment analysis

Algal chlorophyll and carotenoid distributions were measured periodically in the euphotic zone of Sta. ALOHA (22°45'N, 158°00'W) between February 1989 and October 1991 to document the variability in phytoplankton abundance and composition. The annual mean depth‐integrated (0–200 db) concentration of Chl a displayed significant interannual variability. Seasonal patterns in Chl a concentration were found to be depth‐dependent. Elevated Chl a in the mixed layer is the result of photoadaptation as the mixed layer deepens in winter. Increases in Chl a at the deep chlorophyll maximum layer (DCML) in spring are explained by increased nutrient availability caused by a deepening of the DCML relative to the σθ = 24.25 density surface. An algorithm based on the ratios of Chl a to diagnostic pigments present in specific algal taxa was used to estimate the contribution to total Chl a by the major algal groups represented within the DCML. Results indicate the presence of a phytoplankton community at the DCML with the following mean composition: Prochlorococcus spp. (39%), cyanobacteria (24%), prymnesiophytes (22%),and chrysophytes (13%). No single taxon is responsible for the springtime increase in Chl a observed in this habitat. Results from size fractionation and normal‐phase high performance liquid chromatography confirm that Prochlorococcus spp. are the principal contributors of Chl a to the DCML.