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 $(22^\circ 45\prime N, 158^\circ 00\prime W)$ 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 dep chlorophyll maximum layer (DCML) in spring are explained by increased nutrient availability caused by a deepening of the DCML relative to the $\sigma_0 = 24.25$ 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.