Temporal variability of phytoplankton community structure based on pigment analysis

R. M. Letelier, R. R. Bidigare, D. V. Hebel, M. Ondrusek, C D. Winn, and D. M. Karl

Abstract

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 alpha-theta = 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.