Effect of El Niño Southern Oscillation events on the distribution and abundance of phytoplankton in the Western Pacific Tropical Ocean along 165°E

Abstract

The distribution of physical and chemical parameters and their impact on the biomass and abundance of phytoplankton in the Western Pacific Ocean were compared in two opposing situations: the El Niño Southern Oscillation (ENSO) event of 1987 and the non-ENSO period of 1988. During El Niño conditions (September 1987), maximum cell abundance was recorded at 10°S at the boundary between the South Equatorial Current (SEC) and the South Equatorial Countercurrent (SECC). In September 1988, after the return of non-ENSO conditions, a well-established equatorial upwelling produced an increase in the surface layer nutrient supply over 7° of latitude. This in turn caused an increase in phytoplankton populations in the upper layer, with chlorophyll concentrations >0.2 mg m −3 and cyanobacteria and microalgae populations >8.0×10 6 l −1 and >1.2×10 6 l −1 respectively. Integrated over 120 m, the cyanobacteria and microalgae populations were respectively 4.7 and 3.2 times larger than the year before. On the other hand, transient nutrient inputs such as those observed at 10°S in September 1987 caused a large increase in cyanobacteria populations (4.4 times), compared with those in neighbouring zones, and a somewhat smaller increase in microalgae populations (1.3 times). Cyanobacteria populations were much larger than those of microalgae in the 80–100 m upper layer, whereas the latter were more numerous at that depth and below the chorophyll maximum. Population variations in cyanobacteria were accompanied by changes in form, size and fluorescence of the cells. The analysis of the 52 profiles of depth distribution of cyanobacteria and microalgae shows how the community structure is related to the depth and gradient of the nitracline.