Annual Variability of Phytoplankton and Bacteria in the Subtropical North Pacific Ocean at Station ALOHA during the 1991-1994 ENSO Event

Lisa Campbell¹, Hongbin Liu², Hector Nolla² and Daniel Vaulot³

¹Department of Oceanography, Texas A&M University, College Station, TX 77843-3146

²School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822

³Station Biologique, CAWS, INSU, Universite' Pierre et Marie Curie, BP 74, Roscoff Ce'dex, France 29682

Abstract

Time-series data on community structure in the upper 200 m at Station ALOHA in the subtropical North Pacific were collected at approximately monthly intervals from December 1990 through March 1994 during an extended El Niño-Southem Oscillation (ENSO) event. Samples were analyzed by flow cytometry to enumerate Prochlorococcus, Synechococcus, picoeucaryotes, 3-20 µm algae and heterotrophic bacteria, as well as to quantify cellular chlorophyll fluorescence for the autotrophic components. A significant seasonal cycle was evident in cellular chlorophyll fluorescence for each of the autotrophic components, with maxima occurring each winter as a consequence of photoacclimation. Contour plots of abundance for each picophytoplankton component suggested temporal variability on both seasonal and interannual scales. Although the magnitude of the seasonal cycles in the abundance was relatively small, the cycles appeared to be out of phase: Synechococcus abundance maximum occurred in winter, picoeucaryotes in spring, and Prochlorococcus during summer/fall. The difference in these cycles may explain why the presence of a seasonal pattern in total phytoplankton biomass has been difficult to establish. Abundance of the larger 3-20 µm algae varied over 2 orders of magnitude during the time series, with no obvious seasonal pattern. The 3-20 µm algae were a small percentage of the total estimated carbon biomass (approximately 8 %). Heterotrophic bacteria were the most numerous of the picoplankton and the seasonal pattern in their 200-m integrated abundance paralleled Prochlorococcus over the time series. Together, these two procaryotes contributed 60-90% of the total estimated microbial carbon. Significant itnerannual variation in the total 200-m integrated microbial carbon estimates may be related to the effects of the extended ENSO event which began in 1991.