Mesoscale primary production and bio-optical variability off Antofagasta (23-24º S) during the transition to El Niño 1997-1998

Gemita Pizarro,Vivian Montecino,José L Iriarte

doi:10.4067/s0716-078x2002000100019

Abstract

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 eu ) deepening of thermocline below Z eu , 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 Nino 1997-1998 appeared in this study area during March 1997. During the July cruise, the El Nino 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 eu . 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 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 d in comparison to 942 mg C m d in July. The attenuation coefficient k d of photosynthetic active radiation (PAR), determined a significant change in the mean depth of Z eu 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 Nino event could negatively affect primary production during July, prevailing space and seasonal variability masked this effect.

Highlights

Explaining observed variability in El Niño-Southern Oscillation (ENSO) events and their subsequent effects on near-shore ecosystems remains a difficult challenge for biological oceanographers
1997), significant changes occurred in the water column, including higher temperatures in the euphotic zone (Zeu) deepening of thermocline below Z, an increase of oxygen concentration and the intrusion of Subtropical Waters with eu potential limitation of nutrients
The results of this study support the hypothesis that the larger differences occur in coastal-oceanic gradients in biological and bio-optical properties even considering the physical anomalies associated with the development of El Niño

Summary

Introduction

Explaining observed variability in El Niño-Southern Oscillation (ENSO) events and their subsequent effects on near-shore ecosystems remains a difficult challenge for biological oceanographers. During ENSO events along the Pacific Coast of South America, there is typically a deepening of the thermo-nutricline, leading to lower primary productivity and phytoplankton biomass and increased light limitation by deepening of the mixed layer depth (e.g., Cowles et al 1977, Barber & Chávez 1983, 1986, Barber et al 1996, Bidigare & Ondrusek 1996) These oceanographic and biological changes cascade through the marine food chain, causing drastic and often long-term changes in the coastal marine system (e.g., Arntz et al 1985, Glynn 1988). Small scale events related to the intensification of mixing, produced by along shore winds greater than 1.6 km h-1, increase nitrate (Marín et al 1993) as well as nutrient regeneration (Friederich & Codiposti 1981), favouring localised pulses of high primary productivity (PP) and the recurrence of phytoplankton blooms (Avaria & Muñoz 1987, Marín et al 1993, González et al 1998) Bottom topography along this coast is characterised by an extremely narrow shelf of 5-11 nm (Morales et al 1996, Strub et al 1998). Zooplancton grazing is another important factor that influences the Chl-a concentration and González et al (1998) reported for the Antofagasta area during the same sampling periods in 1997, that these rates were at least twice higher in January than in July

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Conclusion