Annual variability of phytoplankton and bacteria in the subtropical North Pacific Ocean at Station ALOHA during the 1991–1994 ENSO event

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 to March 1994 during an extended El Niiio-Southern 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. Abundance of each picophytoplankton component exhibited 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. Typically, abundance maxima of Synechococcus occurred in winter, of picoeucaryotes in spring, and of Prochlorococcus during summer/fall. The different timing 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 two 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 (∼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, the procaryotes contributed 60–90% of the total estimated microbial carbon. Significant interannual 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.