Microplanktonic respiration off northern Chile during El Nino 1997-1998

Microplanktonic respiration rates were estimated in waters off the coast of northern Chile (Antofagasta, 23 degrees S) during El Nino and pre-El Nino conditions. Three cruises were conducted during pre-El Nino summer (January /February 1997), El Nino winter (July 1997) and El Nino summer (January:199 8). Oxygen consumption was estimated by the Winkler method using a semi-aut omatic photometric end-point detector. The ranges of microplanktonic respir ation rates found were 0.11-21.15, 0.03-6.25 and 0.06-9.01 mu mol O-2 1(-1) day(-1) during pre-El Nino summer, El Nino winter and El Nino summer, resp ectively. Significant differences were found between winter and summer resp iration rates (non-integrated and integrated). The mean integrated respirat ion (mixed layer) for pre El Nino summer, El Nino winter and El Nino summer was 95 +/- 51 (SD) mmol O-2 m(-2) day(-1) 50 +/- 23 (SD) mmol O-2 m(-2) da y(-1) and 63 +/- 32 (SD) mmol O-2 m(-2) day(-1), respectively. The strong s easonal signal detected in microplanktonic integrated respiration in the ar ea seems to be characteristic of the pre-El Nino/El Nino 1997-98 period. Th e integrated respiration rates found off Antofagasta are similar to reporte d values for the upwelling area off Peru despite methodological differences . A positive significant correlation was found between respiration and wate r temperature (r = 0.76, P less than or equal to 0.001, pre-El Nino summer; r = 0.61, P less than or equal to 0.001, El Nino winter), as well as betwe en respiration and dissolved oxygen concentration (r = 0.78, P less than or equal to 0.001, pre-El Nino summer; r = 0.23, P less than or equal to 0.02 , EI Nino winter). The Humboldt Current System (HCS) presents an extensive oxygen minimum layer composed of Equatorial Subsurface Water. Our results i ndicate that oxygen levels lower than 177.3 and 136.0 mmol O-2 m(-3) acted as a physical barrier for aerobic respiration during pre-El Nino and El Nin o conditions, respectively.