Primary production and biomass of size-fractionated phytoplankton off Antofagasta, Chile (23–24°S) during pre-El Niño and El Niño 1997

During a major study focusing on the fate of primary production through the food web in an upwelling system located in the Humboldt Current System, Antofagasta, Chile (23–24°S), we examined the spatial variation of phytoplankton size structure. One of the main objectives was to estimate primary production (14C) and chlorophyll-a biomass for three phytoplankton size fractions; pico-, nano- and micro-phytoplankton, at coastal (<10 nmi) and oceanic (20–200 nmi) stations during two research cruises, January (pre-El Niño) and July 1997 (El Niño). During January, primary production ranged from 0.5 to 9.5 mg C m−3 h−1 (mean=3.1 mg m−3 h−1). Close inshore, pico- and nanoplankton size fractions accounted for 79% of total primary production and 63% of total chlorophyll-a, while these fractions accounted for 88% of total production and 70% of total chlorophyll-a in offshore oceanic waters. The micro-phytoplankton fraction (>23 μm) increased its relative contribution to primary production from oceanic (12%) to coastal (21%) stations and was related to an upwelling process which was restricted to a narrow inshore band (<15 nmi). During July (El Niño conditions), primary productivity ranged from 0.39 to 10.7 mg C m−3 h−1 (mean=3.4 mg C m−3 h−1). The pico- and nanoplankton fractions dominated both oceanic and coastal stations and both fractions accounted for 67% of the primary production and chlorophyll-a biomass. We suggest that an oceanographic anomaly observed during July (1997 El Niño), associated with the intrusion of warmer oceanic waters to the coastal border, reduced the upwelling of nutrient-rich waters in the upper 200 m to a narrow coastal band (<6 nmi) leading to a higher dominance of pico- and nanoplankton populations in inshore waters. These observations show that pico- and nanoplankton size fractions make a significant contribution to the production and biomass, and may thus represent an alternative energy flow pathway within this upwelling area.