Evolution and biological effects of the 1997-98 El Nino in the upwelling ecosystem off northern Chile

During the 1997-98 El Nino event, the temperature and oxygen contents of th e coastal waters off northern Chile were noticeably higher than during non- El Nino conditions, but phytoplankton and zooplankton biomasses were not dr amatically altered. In contrast, the herbivorous copepod Calanus chilensis, which showed a limited tolerance to low-oxygen conditions, exhibited great er abundances, higher growth rates, and a significant reduction in adult bo dy size. These results indicate a positive effect of the changes in oceanog raphic conditions due to El Nino on zooplankton growth and production, and provide evidence for lack of food limitation on secondary production in the coastal region during this period.