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vol.30 número1  suppl.SympEffects of the 1997-98 El Niño on the Oceanographic Conditions and Zooplankton Community Structure in the Coastal Upwelling System Off Northern ChileInfluence of the El Niño - La Niña cycle on satellite-derived primary production in the California Current índice de autoresíndice de materiabúsqueda de artículos
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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002

http://dx.doi.org/10.4067/S0717-71782002030100027 

Primary Productivity and Biomass of
Size-Fractionated Phytoplankton in an
Upwelling Area, Bahía de Mejillones
(23ºS, Chile), during and after the
1997-1998 El Niño*

J.L. Iriarte1, 2, H. E.González3

1 Instituto de Acuicultura, Universidad Austral de
Chile, P.O. Box 1327, Puerto Montt, Chile,
E-mail: jiriarte@uach.cl
2 Programa Doctorado, Depto. Oceanografía,
Universidad de Concepción, Concepción, Chile
3 Instituto de Biología Marina, Universidad Austral de
Chile, P.O. Box 567, Campus Isla Teja, Valdivia, Chile

Temporal observations are useful for documenting the consequences of climatic-oceanographic events (e.g. El Niño phenomena) on primary productivity and also for establishing baseline conditions for long-term studies in coastal ecosystems.

The aim of this paper is to assess the changes in size-fractionated primary productivity and biomass in a coastal area, Bay of Mejillones (Chile, 23ºS), resulting from El Niño of 1997-1998. Data were obtained during six visits to the area over a 4-year period that included El Niño of 1997-1998.

The mean values of primary productivity (integrated over the photic layer) estimated during the six cruises are shown in Table 1. During Winter 1997 and Summer 1998, when El Niño took place, the observed primary productivity, 1800 mg C m-2 d-1, was only one third of that estimated during Summer-Winter 2001. Primary productivity during Spring 2000 increased to a mean of 8000 mg C m-2 d-1, an increase of 400% over the productivity during the Winter 1997 El Niño. Time series observations taken during the 1992 El Niño warm event along the equatorial Pacific by Barber et al. (1996) indicated a 50% decrease in primary productivity compared with a normal cold water period. The satellite-measured chlorophyll and temperature figures in Thomas et al. (2001) indicate that the temperature patterns normally associated with upwelling were absent and chlorophyll concentrations were lower during the period from Winter 1997 to Summer 1998.

Table 1. Mean values of primary productivity (integrated over the photic layer)
estimated during the six cruises.

Cruise

Date

Season

Primary Productivity

Chlorophyll

0

Month/Year

0

mg C m-2 d-1

mgChl. m-2

Sectorial

Jan, 1997

Summer

2262

65

Sectorial

July, 1997

Winter

1996

19

Reclan

Jan, 1998

Summer

1597

55

Geminis I

Oct, 2000

Spring

8184

89

Geminis II

Feb, 2001

Summer

4622

585

Geminis III

Aug, 2001

Winter

6479

695

The temporal variation in chlorophyll concentration showed a similar pattern compared with primary productivity; values estimated were lowest during Summer and Winter 1997 (less than 3 mg Chl. a m-3) and highest during Winter 2001 (ranging between 10 and 80 mg Chl.a m-3).


Fig. 1

Microphytoplankton biomass and primary productivity is usually high in the study area (Marín & Olivares, 1999). However, during El Niño, the pico and nanophytoplankton (< 20 um) were the dominant size classes, contributing more than 50% of chlorophyll a and primary productivity in coastal waters. After re-establishment of cold, nutrient rich upwelled water, microphytoplankton (> 20 um) productivity and biomass was enhanced. There was a significant positive relationship between pico, nano and microphytoplankton biomass and total chl. a (rpico = 0.85; rnano = 0.89; rmicro = 0.95; n = 54, p < 0.05), indicating that biomass in all three phytoplankton size classes increased with the return from El Niño (oligotrophic) to more "normal" (eutrophic) conditions. The relative contribution of picophytoplankton decreased as phytoplankton biomass (r = -0.73, n = 54, p < 0.05) and primary productivity (r = - 0.81, n = 37, p < 0.05) increased. During El Niño, water with low chlorophyll a and low primary productivity tended to have a high proportion of picophytoplankton.


* This study was supported by FONDECYT Grant number 1000419 and FONDECYT-SECTORIAL Grant number 5960002 - 1996.

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