Impacts of the 1998-1999 El Niño and La Niña events on the role of heterotrophic protists in the microbial food web of the northeast equatorial Pacific Ocean

Abstract

The distribution and grazing of heterotrophic protists (HP) were examined to understand the response of HP to the climate events in the northeast equatorial Pacific (5-11ºN). This study area experienced El Ni&ntilde;o in 1998, La Ni&ntilde;a in 1999 and relaxation of La Ni&ntilde;a in 2000. The changes of water mass physicochemical properties affected the biological features during these climate events. Chl-a concentration was higher in 1999 than other years with the dominance of larger size phytoplankton. The biomass of HP was the highest in 1999, but the relative contribution of nanoprotists was the greatest in 2000. The smaller size (<20 &micro;m) of the heterotrophic dinoflagellates was dominant in 1998 and 2000, whereas the larger size (>20 &micro;m) was dominant in 1999. The size distribution of the HP well coincided with that of phytoplankton. This relationship suggests that the community structure of HP could be influenced by the size structure of their prey sources, namely, phytoplankton, whose community structure can be changed by the climate events. But, the grazing efficiency and impact of HP on the microbial food web seemed less affected by the climate events in this area.