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vol.30 número1  suppl.SympComparing the Effects of El Niño/La Niña Phases of ENSO on the Continental Margin Benthos of the California and the Humboldt Current SystemsImpacts of El Niño Events on the Coastal Benthos of the Eastern Pacific Upwelling Systems 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 

Effects of El Niño on Nearshore
Stratification and Larval Settlement*

Jesus Pineda1, Manuel López2

1MS 34, Biology Department, Woods Hole
Oceanographic Institution, Woods Hole,
Massachusetts 02543, USA,
2Departamento de Oceanografía Física, Centro de
Investigación Científica y de Educación Superior de
Ensenada, Km, 107, Ensenada, Baja California, Mexico

Many nearshore benthic populations have a two-phase life cycle, where the adults are relatively large and live associated to the sea floor, while the juveniles are small, pelagic, and drift at the mercy of the ocean currents. Larvae are produced in the nearshore, and then drift and accumulate in an offshore larval pool. The temporal and spatial dynamics of these populations is influenced by both pelagic and benthic factors. In particular, the population dynamics of many nearshore invertebrates is strongly influenced by recruitment, which is often dependent on the rate at which larvae are transported from the offshore larval pool to the nearshore adult habitats. Internal motions can transport onshore pelagic larvae, and it is expected that the factors that modulate the internal motions would also modulate the onshore larval transport. Stratification strongly influences internal motions, and a recent study suggests that spatial and temporal variability in stratification results in spatial and temporal variability in settlement rate a proxy variable for onshore larval transport. We found that a site with stronger stratification (La Jolla, California) had much higher settlement rates that a site 100 km to the South (La Salina, Baja California) where stratification was weaker. In La Jolla, changes in stratification were positively correlated with changes in settlement rate. Furthermore, the position of the thermocline modulated the manifestation of the internal tide in the nearshore and the onshore larval transport, with nearshore internal tidal bores occurring only when the thermocline shallowed.

We consider that the position of the nearshore seasonal thermocline must be regulated by the larger-scale circulation field, and that both stratification and circulation are influenced by El Niño. In California, for example, El Niño induces deepening of the seasonal thermocline. We also consider that the internal tidal bores and larval transport occur as a result of the shallowing of the thermocline. For example, if the thermocline is relatively deep, internal tidal bores would not propagate all the way into the inner-shelf and surf-zone. This would translate into fewer larvae transported to adult habitats.

We have started to evaluate this hypothesis using nearshore-temperature time series and barnacle settlement data. We obtained weekly time series of barnacle settlement rate from February 1997 to June 1999 at La Jolla, and from August 1997 to May 1999 in two southern stations, Medio Camino and La Salina. Barnacle settlement was moderately high from February to July 1997, and then it decreased sharply. There was a large, three-week long settlement event in April 1998 at La Jolla, coinciding with the decay of ENSO conditions. However, settlement increased only modestly in the two stations 80 and 100 km to the South. The main effects of El Niño occurred during the winter, a period with relatively low settlement rates, and therefore the effects of El Niño might not have been apparent in this nearshore system.

Studies in a strong upwelling region suggest increased recruitment and onshore larval transport along the Pacific coast of North America during El Niño, a result opposite to the one found here. We discuss these results in light of the effects of El Niño on regimes with strong and weak coastal upwelling.

* Keynote presentation.

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