Temporal variations in phytoplankton community structure in the
 North Pacific subtropical gyre

Robert R. Bidigare, Ph.D.
University of Hawaii, SOEST

Wednesday, August 15, 2001
3:00 p.m.–Pacific Forum

Phytoplankton composition and productivity are thought to be important factors in determining the rates of carbon cycling and export in marine ecosystems. Since the regime shift of 1989, the subtropical North Pacific Ocean has been in a "warming" phase. This has produced an increase in upper-ocean stratification and a decrease in the availability of macro-nutrients. To investigate potential biological responses to basin-scale climate forcing in the Pacific, temporal variations in phytoplankton community structure and primary productivity were investigated at Station ALOHA (22.75N, 158W) during 1989-1999. Changes in phytoplankton populations were monitored using taxon-specific pigment analyses and flow cytometry. Discrete samples were collected from the euphotic zone (0-175 m) at (approximate) monthly intervals. HPLC-determined fucoxanthin, zeaxanthin, 19’-hexanoyloxyfucoxanthin, and 19’-butanoyloxyfucoxanthin concentrations were used as biomass proxies for diatoms, cyanobacteria (including Prochlorococcus and Trichodesmium), haptophytes, and pelagophytes, respectively. Flow cytometry provided direct abundance estimates for Prochlorococcus spp., Synechococcus spp., and photosynthetic picoeukaryotes. Pigment analyses revealed distinct temporal patterns, with highest diatom/pelagophyte abundances during the periods 1990-1992 and 1996-1999. For other key groups, such as the haptophytes and cyanobacteria, there appears to be a recent post-1996 enhancement in their biomass relative to the previous 7-year period of observation. Despite significant changes in phytoplankton community structure, the annual rate of optically modeled and ¹⁴C-measured primary productivity at Station ALOHA has remained relatively constant during the study period. Rates of DOC accumulation and POC flux, however, were much more variable and co-varied with the abundances of Prochlorococcus and diatoms, respectively. The results of this decade-long study suggest that new production rates in the subtropical North Pacific Ocean are modulated by phytoplankton community structure rather than the absolute rate of carbon fixation by photoautotrophs.

Next: 2001 Internship Symposium