Phytoplankton size structure during and after the 1997/98 El Nino in a coastal upwelling area of the northern Humboldt Current System

Primary production (PP) and phytoplankton biomass changes in an intense upwelling area off northern Chile (Antofagasta, 23degrees S) associated with the strong El Nino of 1997/98 are described over a 5 yr sampling period. The oceanographic anomalies observed during July 1997 and January 1998, associated with the intrusion of warmer oligotrophic waters to the coast, reduced the upwelling of cold, nutrient-rich waters in the upper 100 m. An oligotrophic regime seems to have resulted in a higher dominance (45 to 70 %) of pico- and nanoplankton in inshore areas during summer/winter 1997 and summer 1998, with values of biomass and PP of 5 mg chl a m(-3) and 2.0 g C m(-2) d(-1), respectively. After that period, when the frequent upwelling of cold, nutrient rich water was re-established along the coast off Antofagasta, biomass and PP estimates increased up to 80 mg chl a m(-3) (mean = 25.5 mg chl a m(-3)) and 12 g C m(-2) d(-1) (mean = 6.5 g C m(-2) d(-1)), respectively. During that period, the microphytoplankton size fraction accounted for >50% of the biomass and productivity. Sedimentation trap studies showed that the sedimentation rate of diatoms was very low during El Nino conditions (January 1997 and 1998) with values between 0.02 and 0.2 Mg C m(-2) d(-1), increasing by 2 orders of magnitude during winter and spring 2001 (mean = 28 mg C m(-2) d(-1)). The data showed that during 1997/98 El Nino event, pico-and nanophytoplankton size classes made a significant contribution to the production and may thus represent an alternative energy-flow pathway within this upwelling area.