The Bolivar Channel Ecosystem of the Galapagos Marine Reserve: Energy flow structure and role of keystone groups

The Bolivar Channel Ecosystem (BCE) is among the most productive zones in the Galapagos Marine Reserve (GMR). It is exposed to relatively cool, nutrient-rich waters of the Cromwell current, which are brought to the photic zone through topographic upwelling. The BCE is characterized by a heterogeneous rocky reef habitat covered by dense algae beds and inhabited by numerous invertebrate and fish species, which represent the food for higher predators including seals and sharks and exploited fish species. In addition, plankton and detritus based food chains channel large amounts of energy through the complex food web. Important emblematic species of the Galapagos archipelagos reside in this area such as the flightless cormorant, the Galapagos penguin and the marine iguanas. A trophic model of BCE was constructed for the habitats <30m depth that fringe the west coast of Isabela and east coast of Fernandina islands covering 14% of the total BCE area (44km²). The model integrates data sets from sub tidal ecological monitoring and marine vertebrate population monitoring (2004 to 2008) programs of the Charles Darwin Foundation and consists of 30 compartments, which are trophically linked through a diet matrix. Results reveal that the BCE is a large system in terms of flows (38695tkm⁻²yr⁻¹) comparable to Peruvian Bay Systems of the Humboldt upwelling system. A very large proportion of energy flows from the primary producers (phytoplankton and macro-algae) to the second level and to the detritus pool. Catches are high (54.3tkm⁻²yr⁻¹) and are mainly derived from the second and third trophic levels (mean TL of catch=2.45) making the fisheries gross efficiency high (0.3%). The system's degree of development seems rather low as indicated by a P/R ratio of 4.19, a low ascendency (37.4%) and a very low Finn's cycling index (1.29%). This is explained by the system's exposure to irregular changes in oceanographic conditions as related to the EL Niño Southern Oscillation. Most important keystone groups of large relative impact over other system compartments are sharks and marine mammals. In addition, the important role of macro-algae, sea stars and urchins, phytoplankton and barracudas should be emphasized for their great contribution to the trophic flows and biomass of the system.