Revista Chilena de Historia Natural 75 (1): 201-215, 2002
RESEARCH ARTICLE
Mesoscale primary production and bio-optical variability off Antofagasta (23-24° S) during the
transition to El Niño 1997-1998
The spatial variability of primary production (PP), chlorophyll-a (Chl-a) and the
photosynthetic parameters were studied off Antofagasta, Chile (23-24° S, 70-72° W) during austral summer and winter. Between
cruises (January and July 1997), significant changes occurred in the water column, including higher temperatures in the euphotic zone
(Z<sub>eu</sub>) deepening of thermocline below Z<sub>eu</sub>, an increase of oxygen concentration and the
intrusion of Subtropical Waters with potential limitation of nutrients. These strong physical anomalies due to the transition period of El
Niño 1997-1998 appeared in this study area during March 1997. During the July cruise, the El Niño event 1997-1998 was in the middle
of its development (IOS–2). The hypothesis that chlorophyll-a concentrations and primary production differ significantly in the coastal
areas in the Antofagasta region due to year round coastal upwelling was tested in this study. Photosynthesis versus irradiance (P-E)
experiments were performed daily, using simulated in situ incubations with samples collected within the Z<sub>eu</sub>.
Also in vitro incubations were done at several selected stations. For results analyses, stations were pooled in coastal and oceanic sites
according to distance from the narrow shelf and differential influence of local upwelling. Integrated Chl-a values during both cruises
were significantly higher at the coastal stations, and since between cruises no differences were found, a mean value of 44 mg Chl-a
m<sub>-2</sub> can be reported for the coastal area. Daily PP values were significantly different in space and time (P <
0.001), and at the coast also between cruises (P < 0.004) as a result of the high mean coastal value in January, 3,129 mg C
m<sub>-2</sub> d<sub>-1</sub> in comparison to 942 mg C m<sub>-2</sub> d-1 in July. The
attenuation coefficient kd of photosynthetic active radiation (PAR), determined a significant change in the mean depth of
Z<sub>eu</sub> between coastal and oceanic stations (44 ± 20 and 80 ± 17 m, respectively) during both sampling periods.
Notwithstanding the spatial differences in chlorophyll-a concentrations and primary production, the observed weaker upwelling
favourable winds during both cruises, the increase in depth of the mixing layer and light limitation in July, and the higher mean values of
zooplankton grazing rate during January contributed to the similar abundance of chlorophyll-a in time. Although the El Niño event could
negatively affect primary production during July, prevailing space and seasonal variability masked this effect.
Key words:
primary production,
photosynthetic parameters, attenuation coefficient, euphotic zone, mixing depth, El Niño event 1997-1998
