During the strong El Niño (EN) 1997/98, nutrient and phytoplankton concentrations in waters surrounding the Galápagos archipelago greatly decreased, while Sea Surface Temperature (SST) increased (> 7° C). Under these conditions many species populations suffered (eg. macro-algae, marine iguanas, sea lions and sea birds) while other benefited (eg: sea cucumbers, sea urchins and crabs), and in addition new species appeared. To understand the mechanisms behind the observed changes, a steady-state (trophic) model of the Bolivar Channel Ecosystem (BCE) was constructed and longer-term dynamics (1994-2009) were explored using time series of observed compartment biomasses and applying the software Ecopath with Ecosim 6.0 (EwE). The model integrates historical data sets from the subtidal ecological monitoring and the marine vertebrate population monitoring (2004 to 2008) carried out by the Charles Darwin Foundation (CDF), and consists of 30 compartments groups, which are trophically linked by a diet matrix. The objectives of the present study were (1) to summarise and integrate available ecological and fisheries information to create a reference model for the BCE, (2) to identify keystone groups of the system in terms of compartment biomasses and energy cycling and to explore-through simulation modeling, if and how biomass changes of some of these groups, as observed during the EN impact, may trophically affect other groups of the system.
Results/Conclusions Preliminary results revealed that ecosystem size (total throughput) was reduced during EN, mainly as a result of a decrease in primary production (phytoplankton and macro-algae), which also affected exports and flows to detritus as well as trophic interactions within the ecosystem. Although detritus production decreased during EN, accumulated detritus in the semi-enclosed BCE system from the preceding normal conditions provided the energy sources for the lower trophic groups such as sea cucumbers and sea urchins, thereby maintaining the functionality of the ecosystem, and providing nutrients even when upwelling stopped. Simulations suggest that bottom-up effects largely control the system during EN events, while the role of top-down control increases during normal conditions.
