Microplanktonic respiration off northern Chile during El Nino 1997-1998

Abstract

Microplanktonic respiration rates were estimated in waters off the coast of northern Chile (Antofagasta, 23°S) during El Niño and pre-El Niño conditions. Three cruises were conducted during pre-El Niño summer (January/February 1997), El Niño winter (July 1997) and El Niño summer (January 1998). Oxygen consumption was estimated by the Winkler method using a semi-automatic photometric end-point detector. The ranges of microplanktonic respiration rates found were 0.11–21.15, 0.03–6.25 and 0.06–9.01 μmol O2 l–1 day–1 during pre-El Niño summer, El Niño winter and El Niño summer, respectively. Significant differences were found between winter and summer respiration rates (non-integrated and integrated). The mean integrated respiration (mixed layer) for pre-El Niño summer, El Niño winter and El Niño summer was 95 ± 51 (SD) mmol O2 m–2 day–1, 50 ± 23 (SD) mmol O2 m–2 day–1 and 63 ± 32 (SD) mmol O2 m–2 day–1, respectively. The strong seasonal signal detected in microplanktonic integrated respiration in the area seems to be characteristic of the pre-El Niño/El Niño 1997–98 period. The integrated respiration rates found off Antofagasta are similar to reported values for the upwelling area off Peru despite methodological differences. A positive significant correlation was found between respiration and water temperature (r = 0.76, P ≤ 0.001, pre-El Niño summer; r = 0.61, P ≤ 0.001, El Niño winter), as well as between respiration and dissolved oxygen concentration (r = 0.78, P ≤ 0.001, pre-El Niño summer; r = 0.23, P ≤ 0.02, El Niño winter). The Humboldt Current System (HCS) presents an extensive oxygen minimum layer composed of Equatorial Subsurface Water. Our results indicate that oxygen levels lower than 177.3 and 136.0 mmol O2 m–3 acted as a physical barrier for aerobic respiration during pre- El Niño and El Niño conditions, respectively.