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Investigaciones marinas

On-line version ISSN 0717-7178

Investig. mar. vol.30 no.1 suppl.Symp Valparaíso Aug. 2002 


Food Web Responses to La Niña and
El Niño Events*

Gilbert T. Rowe

Department of Oceanography, Texas A&M University,
Mail Stop 3146, College Station , Texas, USA 77840,

Conceptual models of coastal ecosystem food webs are used to illustrate system level changes that occur in response to El Niño and La Niña events along the coast of South America. La Niña events, considered the norm, are characterized by intense upwelling along the coast, high primary production, healthy pelagic fisheries and anoxic bottom water, whereas El Niño events are just the opposite: diminished upwelling, low primary production, failure of the pelagic fisheries and normoxic conditions on the bottom. These shifts are accompanied by severe wet and dry periods ashore and complicated reversals in the biogeochemical cycles on the sea floor that are a function of the presence or absence of oxygen in the bottom water. As the bottom transitions from anoxic (normal or La Niña) to oxic (El Niño) and back, the biota switches from a diverse microbial assemblage to more typical invertebrate benthos and back (Figs. 1 and 2).

Fig. 1 Coastal sediment food web, shallow water, off Dichato, Chile. Boxes are standing stocks and arrows are fluxes or transformations.

Fig 2 Conceptual model of carbon cycling in sediments in the oxygen minimum layer off western S. America. Boxes are concentrations or standing stocks of organism and arrows are transformations or fluxes.

The subsystems reviewed include coupled upwelling-nitrate-primary production-zooplankton grazing rates (the U-N-P-Z paradigm), pelagic fisheries, sediment carbon seques-tration and responses of the bottom communities to a spectrum of environmental conditions, in particular low oxygen. Simulation models are developed which couple the UNPZ biological pump, the responses of bottom communities and potential carbon sequestration, to a spectrum of environmental conditions, in particular low oxygen. A simplified food web simulation of coastal benthic communities near the coast of Chile (Fig. 1) is pulsed with various levels of organic particulate carbon input to estimate the theoretical effects of alterations in biomass and secondary production as responses to La Niña/El Niño cycles. The growth patterns mimic those observed in commercially important bivalve mollusks that dominate the biomass (Fig. 3).

Fig. 3 Simulation of food web component biomass, in mg C/sq. meter, in Coliumo Bay, off Dichato, Chile. Megainfauna are the commercial bivalve molluscs studied by H.-Jörg Urban (Urban & Campos 1994, Urban 1996). Simulation run is in days ( for five years).

This `healthy' benthic community is contrasted with La Niña events and the over-riding effects of low oxygen that result in the elimination of large invertebrates in the OML. Below about 5% oxygen saturation, the invertebrates are replaced by interdependent heterotrophic and chemoautotrophic bacterial groups (Fig. 2) of extraordinary biomass, which have remarkably high rates of growth, according to model calculations. The most salient and well-studied are species of the genus Thioploca, large sheath-forming sulfide oxidizers. They have been observed to contain high concentrations of intracellular nitrate, as well as globules of elemental sulfate. It has been suggested that by utilizing sulfide as an energy source, they detoxify the sedimentary environment. The nitrate presumably is utilized to oxidize the sulfide.

A comparison is made with other coastal habitats characterized by high primary production and intermittent hypoxia to gain better understanding of the effects of climate oscillations on food web structure and function. The broad continental shelf off Louisiana, USA, under the Mississippi River plume, is subject to nitrate loading, intense stratification and hypoxic/anoxic conditions for several months each year. The flux of POC to the bottom appears to be about equal to Chile/Peru, and the sediment biota switches back and forth between invertebrate and bacteria-dominated communities during oxic/hypoxic periods. The biomass however of the invertebrates and the bacterial groups (Fig. 4) never reaches the levels encountered along the coast of South America (Figs. 1-3). The differences in the food webs of the contrasted ecosystems may be related to high sediment accumulation rates associated with rivers, different temperatures, the quality of the organic matter, or higher concentrations of reactive Fe in the sediments off the river.

Fig. 4 Biomass (boxes) in terms of mg carbon per square meter of sediment, down to a depth of 1 meter, and arrows are fluxes or transformations, in mg C m-2d-1, down to a depth of 1 meter, in the hypoxic are off the Mississippi River, Gulf of Mexico.

* Keynote speaker.

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