versão On-line ISSN 0717-7178
Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002
Impact of ENSO on Near-Shore
Dynamics of two Peruvian Upwelling
Centres between 1993 - 2001
Carmen Grados, Luis Vásquez,
Dimitri Gutiérrez, Luis Quipúzcoa,
José Tenorio, Robert Marquina
Instituto del Mar del Perú, IMARPE, P.O. Box 22,
Callao-PERU, E-mail: firstname.lastname@example.org
Introduction & Objectives
Environmental and fishery variability in the Southeastern Pacific occurs over a wide spectrum of time-scales, ranging from seasonal to interdecadal scales. Interannual variations of primary production, plankton and benthos, as well as variations in abundance and/or availability of small pelagics and demersal fishes are associated with alternate ocurrences of cold (La Niña), normal, and warm (El Niño) periods. Fluctuations in hydrographic and biochemical parameters, as well as in benthos biomass and diversity, have been studied in the last eight years through intensive near-shore monitoring in two coastal upwelling centres off Peru: Paita (05°00' S) and Callao (12°05' S). In the Paita upwelling centre the local forcing interacts with a recurrent extraregional forcing (e.g. pulses of advective transport of equatorial surface waters and of oxygenated subsurface waters), while the dynamics of the Callao upwelling centre respond mainly to the local wind forcing.
The objectives of this work are: i) To assess the seasonal to interannual physical and biochemical variability of the near-shore vertical structure in the two upwelling centres, ii) To compare the timing and intensity of the oceanographic changes associated with El Niño (EN) in both upwelling centres; and iii) To determine the responses of benthic communities to environmental variability in both upwelling centres.
Results & Discussion
The Multivariate ENSO Index showed a close relationship with the coastal SST anomalies in both upwelling centres. Strong SST anomalies were recorded during the 1997/98 EN. In both areas the maximum positive anomalies were recorded in January 1998 (+8°C and +7°C, for Paita and Callao respectively). The ENSO-related SST anomalies were observed nearly simultaneously in both areas.
Fig. 1 Location of sampling stations.
Fig. 2 a) Monthly time series of MEI and SST anomalies off Paita and Callao; b) temporal variability of thermal structure off Paita; and c) temporal variability of thermal structure off Callao.
A seasonal variation of the depth of the 15°C isotherm (and of the 1 ml L-1 iso-oxygen) occurred in both areas. Off Paita, the 15°C isotherm deepened in autumn-winter and shoaled in spring-summer, probably associated with the dynamics of the southern branch of the Cromwell Undercurrent. Off Callao, the 15°C isotherm deepened in summer-autumm and shoaled in winter-spring according to the seasonal variation of upwelling. Strong ENSO-driven interannual variations were associated with anomalies of +6°C in the whole column during the El Niño mature phase at both sites, and a deepening of the 1 ml L-1 iso-oxygen down to >100 m depth.
The temporal variability of macrobenthos mostly reflected the interannual physical and chemical changes associated with the ENSO in both upwelling centres. Off Callao, under mature `normal', hypoxic oceanographic conditions, macrofauna was not able to colonize the bottom, but Thioploca mats were developed. During El Niño, Thioploca tended to disappear and macrofauna responded positively in terms of biomass and species richness. Return to `normal' bottom conditions took at least a couple of years after EN. In contrast, off Paita, mature `normal' conditions favoured higher abundances and species richness of macrofauna (here Thioploca is never found); reduction of these parameters during the EN 1997/98 may be related to the significant increase of terrigenous load, due to enhanced river discharge.
Fig. 3 a) Macrofaunal species richness at 65 m off Paita; b) Biomass of Thioploca and macrofauna at 90 m (Callao); and c) Macrofaunal species richness at 90 m (Callao).