[OANNES Foro] Changes in trophic flow structure of Independence Bay (Peru) over an ENSO cycle

[raul sanchez]

Progress In Oceanography
Volume 79, Issues 2-4, October-December 2008, Pages 336-351 
The Northern Humboldt Current System: Ocean Dynamics, Ecosystem Processes, and Fisheries 
doi:10.1016/j.pocean.2008.10.006    

Changes in trophic flow structure of Independence Bay (Peru) over an ENSO cycle 

Marc H. Taylora, , , Matthias Wolffa, Jaime Mendob and Carmen Yamashiroc

aCenter for Tropical Marine Ecology, Fahrenheitstrasse 6, 28359 Bremen, Germany

bFacultad de Pesqueria, Universidad Agraria La Molina, Lima, Peru

cInstituto del Mar del Perú, Esq. Gamarra y Gral. Valle s/n, La Punta, Callao, Peru

Abstract
During the strong warm El Niño (EN) that occurred in 1997/98, Independence Bay (14°S, Peru) showed a ca. 10 °C increase in surface temperatures, higher oxygen concentrations, and clearer water due to decreased phytoplankton concentrations. Under these quasi-tropical conditions, many benthic species suffered (e.g. macroalgae, portunid crabs, and polychaetes) while others benefited (e.g. scallop, sea stars, and sea urchins). The most obvious change was the strong recruitment success and subsequent proliferation of the scallop Argopecten purpuratus, whose biomass increased fiftyfold. To understand these changes, steady-state models of the bay ecosystem trophic structure were constructed and compared for a normal upwelling year (1996) and during an EN (1998), and longer-term dynamics (1996–2003) were explored based on time series of catch and biomass using Ecopath with Ecosim (EwE) software. Model inputs were based on surveys and landings data collected
 by the Instituto del Mar del Perú (IMARPE). Results indicate that while ecosystem size (total throughput) is reduced by 18% during EN, mainly as a result of decreased total primary production, benthic biomass remains largely unchanged despite considerable shifts in the dominant benthic taxa (e.g. scallops replace polychaetes as secondary consumers). Under normal upwelling conditions, predation by snails and crabs utilize the production of their prey almost completely, resulting in more efficient energy flow to higher trophic levels than occurs during EN. However during EN, the proliferation of the scallop A. purpuratus combined with decreased phytoplankton increased the proportion of directly utilized primary production, while exports and flows to detritus are reduced. The simulations suggest that the main cause for the scallop outburst and for the reduction in crab and macroalgae biomass was a direct temperature effect, whereas other changes are
 partially explained by trophic interactions. The simulations suggest that bottom-up effects largely control the system.

Keywords: El Nino phenomena; Scallop fisheries; Steady state; Trophic relationships; Humboldt current; Peru; Independence Bay

Saludos,

Raúl E. Sánchez Scaglioni