A Comparative Analysis of the
Community Structure of the Central
Chile Marine Ecosystem During an
El Niño (1992) and La Niña (1998)
Conditions

Sergio Neira¹, Hugo Arancibia²

¹Programa de Magister en Ciencias Mención
Pesquerías, Universidad de Concepción, P.O. Box 160-C,
Concepción, Chile, E-mail: seneira@udec.cl
²Departamento de Oceanografía, Universiad de
Concepción, P.O. Box 160-C, Concepción, Chile,
E-mail: harancib@udec.cl

Objective

To analyse the community structure of the Central Chile marine eosystem under El Niño (1992) and La Niña (1998) conditions, using system attributes calculated through ecotrophic modelling.

Results & Discussion

It is well known that El Niño events cause long-term changes in marine ecosystems in the Eastern Pacific Ocean. In this contribution, we analyse the community structure of the Central Chile marine ecosystem (CChME) under El Niño and la Niña conditions.

Table 1 summarises global attributes of the CChME derived from two independent steady-state trophic models built for 1992 (El Niño) and 1998 (La Niña) periods using biological, fishery and ecological data for 23 functional groups as input data for the Ecopath software. Attributes were selected that describe changes at the ecosystem level rather than changes in isolated components.

Table 1 Summary statistics describing the Central Chile marine ecosystem,
1992 and 1998. Parameters are expressed in t km^-2.year^-1.

As ecosystems develop to maturity both Primary Production/Respiration ratio (PP/R) and Primary Production/Biomass (PP/B) are expected to decrease. In CChME, PP/R was higher in 1992 than in 1998, indicating a lower system maturity in 1998 than in 1992. Accordingly, PP/B was also higher in 1998 than in 1992.

Food chain structure changes from linear (developing systems) to weblike (mature systems). In addition, higher cycling rates and longer trophic path length are characteristics of mature systems. In CChME both the Finn's cycling index (FCI, a measure of matter cycling) and the mean trophic path length (TPL) have the same value in 1992 and 1998, while the connectance index (CI, a measure of the food chain structure) was lower in 1998 compared to 1992, suggesting a lower system maturity, i.e. in 1998 the system was less efficient at retaining energy. Total system biomass/production ratio (B/P) represents the average size of the organisms in a system, with mature systems presenting higher B/P ratios. The results indicate that B/P was lower in 1998 than in 1992.

The primary production required to sustain fishery landings (PPR) in Central Chile was 16% of total primary production (PP), while in 1998 PPR was only 10%. In addition, the mean trophic level of fishery landings (TLm) was lower in 1992 (TLm=3.06) than in 1998 (TLm=3.427). The above results can be explained by the collapse in the fishery of horse mackerel (Trachurus symmetricus). In fact, in 1992 that fishery was abundant while in 1998 it was heavily overexploited, and landings were based mainly on small-sized phytophagous pellagic fishes such as sardine (Strangomera bentincki) and anchovy (Engraulis ringens), resulting in a lower PPR and TLm.

Under normal conditions, ecosystems are expected to develop towards maturity. However, this process can be disrupted by anomalous conditions such as those related to ENSO events. Therefore, it is expected that CChME should be more mature under normal conditions (even during a moderate El Niño event) than after an El Niño or during La Niña events. Our results indicate that during modrate El Niño conditions (1992) the CChME seems to have been more mature than during La Niña conditions (1998). The El Niño 1997 event could have produced a disruption driving the system to a lower level of maturity.