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vol.30 número1  suppl.SympEl Niño 1997-98 by Means of Planktonic Foraminifera off Coquimbo, ChileStudy of the Pelagic Fish Environmental and Behavioral Adaptation to Strong Physicochemical Changes Induced by ENSO in Peruvian Waters (1997-1998) 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 

Revisiting in Situ Chlorophyll-a Data
Along the Coast in North-Central Chile
Considering Multiscale
Environmental Variability

Vivian Montecino1, Rosa Astoreca1,
M. Alejandra Paredes1, Jose Rutllant2

1Dep. Ciencias Ecológicas, Facultad de Ciencias,
Universidad de Chile, Casilla 653, Santiago, Chile,
2Dep. Geofísica, Facultad de Ciencias Físicas y
Matemáticas, Universidad de Chile.

Phytoplankton distribution in the Humboldt Current System (HCS) is often characterised by high temporal and low spatial variability. Temporal changes include: the response to quasi-weekly upwelling favourable wind events, in which the rapid "aging" of newly upwelled water is due to phytoplankton nutrient utilisation; intraseasonal changes in the nutricline depth associated with Coastal Trapped Waves (CTWs) and alongshore wind variability related to the subtropical anticyclone; the seasonal cycle and interannual changes associated with the El Niño/Southern Oscillation (ENSO) cycle. Weak seasonal variability patterns in phytoplankton concentration have been reported between 18-30°S through remote sensing of ocean colour, with maxima (minima) centred on austral summer (winter) coinciding with the season of maximum (minimum) upwelling favourable winds along the coast. Seasonal changes are moderate in strength off Perú and stronger at mid- and high latitudes off Chile. Along the coast in north-central Chile, environmental variability on interannual (ENSO) scales is important, particularly during strong El Niño/La Niña events.

Results from studies of spatial variability off Chile differ from those reported for California. They indicate that the region of higher phytoplankton biomass (Chlorophyll-a pigment from ship measurements) is restricted to within 37-50 km of the coast, although high pigment concentrations associated with filaments extending several hundred kilometres from coastal upwelling centres have been detected in satellite pigment and SST imagery.

The objective of this study is to examine published/unpublished Chl-a ship data along the Chilean coast influenced by upwelling, to quantitatively assess differences in chlorophyll distribution patterns from both extremes of the ENSO cycle and during transition periods in the last decade.

Recently, we have proposed that the phytoplankton response to the cold/warm ENSO signal during a complete cycle of wind-driven upwelling may be offset by opposite seasonal and/or intraseasonal thermal conditions. Intraseasonal fluctuations in the thermo/nutricline depth are mainly caused by CTWs monitored through the adjusted sea-level (ASL) time series: SST is lower when adjusted sea level is rising and SST is higher when adjusted sea level is falling.

To examine the average phytoplankton response to the ENSO cycle considering these multiscale environmental variability relationships, water column-integrated Chl-a data collected in different cruises and seasons were ranked according to the phase of the local upwelling cycle (AC: active, RX: relaxed) and the phase of intraseasonal oscillations in the ASL. For statistical analysis of Chl-a variability, results from coastal stations were separated from the more oceanic station. Along the coast, data concentrate off Antofagasta (23 °S) and Coquimbo (29-30 °S) with some sparse information off Caldera (27 °S) and Valparaíso (33 °S). For the sake of comparison, additional data off Concepción (36 °S) were also used. Light penetration behaviour (depth of 1% surface light) and other bio-optical parameters were also examined.

The greatest difficulty during the analysis was the unevenness in sampling efforts at all sites, resulting in under-sampling of the cold periods. Data from Valparaíso in late-springs of 1996-1997 show that, during the active phase of the upwelling cycle with rising sea-level, significant differences were found between the ENSO warm (1997) and cold (1996) periods (F=10.2; p=0.01). During falling sea-level and active upwelling, differences between ENSO warm/cold periods were marginally significant (F=4.8; p= 0.052). Chl-a integrated mean values during the relaxed phase of the upwelling cycle with El Niño/La Niña conditions were comparatively higher (63.2 ± 30.1 / 75.9 ± 27.6 mg m-2) with rising sea-level than during the opposite ASL phase (41.6 ± 38.0 / 60.3 ± 21.8 mg m-2). Chl-a ranges were 8-126 mg m-2 and 36-122 mg m-2 during El Niño and La Niña, respectively.

In the case of Antofagasta, the ANOVA results indicate that the combination of ENSO (El Niño and transitions) with the ASL variability was marginally significant (F=3.77 p= 0.055). Within this period, significant interannual and intraseasonal differences were found only in winter, during relaxation and rising sea-level (28.4 ± 9.4 / 111.9 ± 119.8 mg m-2 for El Niño/ transitions; p=0.006). Summer mean Chl-a concentrations ranged from 58.0 to 76.5 mg m-2 for El Niño and transition periods respectively.

At 29-30 °S (Coquimbo), only the combination of seasonal and interannual variability was significant (p= 0.006). In summer, during active upwelling and falling sea-level, significant differences were found (14.0 ± 7.7 / 38.6 ± 13.8 mg m-2 for El Niño/ transitions; F=9.62, p=0.015).

No latitudinal patterns could be found in the available cruise data. This is attributed to the very uneven sampling distribution in time. Even in the areas off Antofagasta and Coquimbo, where the largest amount of data and longest time series exist, only a few La Niña conditions could be sampled due to the prevailing warm conditions during the 1986-1997 period.

Phytoplankton growth can be positive in coastal regions during warm anomalies, provided that nutrients and light are available. If processes that enhance phytoplankton growth near the coast are maintained interannually, the increase in temperature associated with the ENSO warm signal could interact with changes in the euphotic zone depth, photo-acclimation, phytoplankton cell size and export processes. Although in oceanic areas interannual Chl-a changes have been more discernible, differences in integrated Chl-a concentrations at coastal stations were subtle and at most locations not significantly different between El Niño and La Niña periods when higher-frequency environmental variability was taken into account.

Acknowledgements: J.L. Blanco, V. Marín, C. Morales, G. Pizarro & H. Sievers. JGOFS-Chile, FONDECYT, FONDAP-Humboldt & U. de Chile-Programa en Dinámica de la Atmósfera y el Clima (PRODAC).

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