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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002 

Upwelling is the Disturbance, not El Niño:
Insights from Modelling Community
Organization and Flow Structure*

Matthias Wolff,
Jaime Mendo

University of Bremen, Zentrum für Marine
Tropenökologie (ZMT), Bremen, Germany,

During "normal" upwelling - and El Niño conditions, benthic community structure was analysed and trophic flows were modelled in Independence bay, Peru's centre of the diving fishery of invertebrates. During both warming (El Nino) events 1983/84 and 1997, the bay's scallop population proliferated enormously (by almost two orders of magnitude compared to normal years) and mortalities of cold water adapted mussels, crabs and other species occurred simultaneously. It was hypothesized that the living community of the benthos would loose its structure and would decrease in species richness and evenness during the El Niño impact. As phytoplankton primary production decreased dramatically, total energy throughput was also expected to decrease simultaneously during El Nino affecting system production characteristics.

Community structure was analysed by the use of data of benthic surveys conducted during normal and El Nino years. A rank order of species (in numbers and biomass) was established and the log-series model was applied, which allows for a graphical representation of species richness and evenness and of the relative importance of each species of the assemblage.

Trophic flows of the system were modelled using the ECOPATH programme of Christensen and Pauly (1995) which combines an approach for estimating biomass and food consumption of the various system elements (species or species groups) with an analysis of the flows between the ecosystem elements and for the calculation of ecosystem indices such as Throughput Ascendancy, Transfer efficiency and others. Biomass production of a compartment is balanced by predation, non-predation losses, and export. Important input parameters are (for each compartment): biomass (B), production per unit of biomass (P/B), consumption per unit of biomass (Q/B) and export (EX).

Contrary to expectation, benthic diversity and evenness increased during El Niño, paralleled by a change in the rank order of species. A picture emerged of a rather tropical community of a higher degree of complexity. In terms of compartment biomass and trophic flows interesting changes were observed as well: while total system biomass seemed to decrease somewhat during El Niño, most biomass decrease of the cold water adapted species was compensated by increased biomass of the remaining and immigrating species. One single filter feeder (the scallop) contributed about 50% to total system biomass, thus very efficiently occupying the niche previously occupied by other filter feeders (i.e. mussels, clams and others) under upwelling conditions. The biomass production of this scallop species appeared higher than the combined production of all other resources of the bay in normal years, showing that - despite a clear drop in primary production in the bay - energy was very efficiently used and converted into somatic production It seems that this enormous scallop biomass production during El Niño (about 800t /km2/year) was at least in part due to a shift from plankton to detritus (bacteria) as primary food source of this filter feeder. Thus from the points of view of a diving fisher but also of a benthic ecologists, El Nino should not be considered as a disturbing event, that disrupts the benthic community. It rather seems to lead to an increase in system complexity, transfer efficiency and resource production.

The fact that the benthic system rapidly adjusts to the abiotic changes introduced by El Niño (through a change of some species at different functional spots of the system) without a disruption of the system flow structure, suggests that El Niño could be regarded as a system condition to which the benthic community has well adapted during the course of evolution. It could further be speculated that with the onset of intense upwelling along the Peruvian coast some thousand years ago (after a period of seemingly warmer conditions according to recent literature), a formerly more tropical fauna was disturbed and many warm water species were replaced by some (fewer) cold water species, thus creating a new somewhat impoverished benthic community, which is rapidly enriched when El Niño brings (the original) tropical conditions.

* Keynote presentation

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