[OANNES Foro] Trophic modeling of the Northern Humboldt Current Ecosystem, Part I: Comparing trophic linkages under La Niña and El Niño conditions

[raul sanchez]

Progress In Oceanography
Volume 79, Issues 2-4, October-December 2008, Pages 352-365 
The Northern Humboldt Current System: Ocean Dynamics, Ecosystem Processes, and Fisheries 
doi:10.1016/j.pocean.2008.10.007    

Trophic modeling of the Northern Humboldt Current Ecosystem, Part I: Comparing trophic linkages under La Niña and El Niño conditions 

Jorge Tama, , , Marc H. Taylorb, Verónica Blaskovica, Pepe Espinozaa, R. Michael Ballónc, Erich Díaza, Claudia Wosnitza-Mendoa, Juan Argüellesa, Sara Purcaa, Patricia Ayóna, Luis Quipuzcoaa, Dimitri Gutiérreza, Elisa Goyaa, Noemí Ochoad and Matthias Wolffb

aInstituto del Mar del Perú (IMARPE), Apdo. 22, Callao, Peru

bCenter for Tropical Marine Ecology, Fahrenheitstr. 6, 28359 Bremen, Germany

cIRD, UR097, CRH, Avenue Jean Monnet, 34203 Sète, France

dUniversidad Nacional Mayor de San Marcos, Apdo. 12, Lima, Peru

Abstract
The El Niño of 1997–98 was one of the strongest warming events of the past century; among many other effects, it impacted phytoplankton along the Peruvian coast by changing species composition and reducing biomass. While responses of the main fish resources to this natural perturbation are relatively well known, understanding the ecosystem response as a whole requires an ecotrophic multispecies approach. In this work, we construct trophic models of the Northern Humboldt Current Ecosystem (NHCE) and compare the La Niña (LN) years in 1995–96 with the El Niño (EN) years in 1997–98. The model area extends from 4°S–16°S and to 60 nm from the coast. The model consists of 32 functional groups of organisms and differs from previous trophic models of the Peruvian system through: (i) division of plankton into size classes to account for EN-associated changes and feeding preferences of small pelagic fish, (ii) increased division of demersal groups and
 separation of life history stages of hake, (iii) inclusion of mesopelagic fish, and (iv) incorporation of the jumbo squid (Dosidicus gigas), which became abundant following EN. Results show that EN reduced the size and organization of energy flows of the NHCE, but the overall functioning (proportion of energy flows used for respiration, consumption by predators, detritus and export) of the ecosystem was maintained. The reduction of diatom biomass during EN forced omnivorous planktivorous fish to switch to a more zooplankton-dominated diet, raising their trophic level. Consequently, in the EN model the trophic level increased for several predatory groups (mackerel, other large pelagics, sea birds, pinnipeds) and for fishery catch. A high modeled biomass of macrozooplankton was needed to balance the consumption by planktivores, especially during EN condition when observed diatoms biomass diminished dramatically. Despite overall lower planktivorous fish
 catches, the higher primary production required-to-catch ratio implied a stronger ecological impact of the fishery and stresses the need for precautionary management of fisheries during and after EN. During EN energetic indicators such as the lower primary production/total biomass ratio suggest a more energetically efficient ecosystem, while reduced network indicators such as the cycling index and relative ascendency indicate of a less organized state of the ecosystem. Compared to previous trophic models of the NHCE we observed: (i) a shrinking of ecosystem size in term of energy flows, (ii) slight changes in overall functioning (proportion of energy flows used for respiration, consumption by predators and detritus), and (iii) the use of alternate pathways leading to a higher ecological impact of the fishery for planktivorous fish.

Keywords: Ecotrophic model; Network analysis; El Niño; Peru; Humboldt Current; Trophic energy flow; Upwelling

Saludos,

Raúl E. Sánchez Scaglioni