Trophic modeling of the Northern Humboldt Current Ecosystem, Part II: Elucidating ecosystem dynamics from 1995 to 2004 with a focus on the impact of ENSO
Introduction
Eastern Boundary Current Systems (EBCSs), including the Humboldt, Canary, Benguela, and California Currents, and in particular their nearshore upwelling zones, are among the most productive fishing areas in the world. High primary and secondary productivity support large biomasses of small planktivorous pelagic fish, or “small pelagics”, which through predator/prey interactions can influence both higher and lower trophic levels (i.e. “wasp-waist” ecosystem control, Cury et al., 2000).
The Humboldt Current, and specifically, the Peruvian upwelling system, produces more fish landings than the other EBCSs (both total and on a per area basis). However, remote sensing-based estimates of primary production rank the Peruvian upwelling system only third, behind the Benguela and Canary Current systems (Carr, 2002). In a way, this apparent paradox supports early fishery hypotheses that emphasize quantity and quality of upwelling. The Peruvian upwelling system’s proximity to the equator and large Rossby radius results in strong and sustained upwelling under relatively mild wind forcing (Cury and Roy, 1989, Bakun, 1996). These conditions create a “particularly rich, non-turbulent, benign environment” by which rich coastal plankton communities develop and persist, in turn supporting abundant populations of small pelagics (Bakun and Weeks, 2008).
Peru’s proximity to the equator also means that Kelvin waves traveling eastward along the equator during El Niño (EN) impact Peru almost directly. During EN, the “basin-wide ecosystem” of the Pacific, which normally maintains a slope in sea level, thermal structure, and nutrient structure due to trade winds, is lost or reversed (Chavez et al., 2003, Pennington et al., 2006). While upwelling-favorable wind may continue along the Peruvian coast, water is upwelled from above the now deep thermocline and nutricline. As a result, primary production of the Peruvian upwelling system is reduced, and the “active zone” of high primary production can be 1/10th the size of normal conditions (Nixon and Thomas, 2001).
Under normal conditions diatoms dominate the nearshore phytoplankton community. Diatoms are particularly adapted to upwelling conditions through high doubling rates and their ability to form resting spores, which sink and are subsequently returned to the surface via upwelling (Pitcher et al., 1992). In the Humboldt Current system, EN reduces the upwelling of nutrient-rich water, which results in a reduction of the larger size fraction of the phytoplankton community (e.g. diatoms) (Bidigare and Ondrusek, 1996, Landry et al., 1996, González et al., 1998, Iriarte and González, 2004) and are replaced by subtropical phytoplankton normally found further offshore in nutrient poor waters (Rojas de Mendiola, 1981, Ochoa et al., 1985, Avaria and Muñoz, 1987). These changes in the phytoplankton produce changes throughout the ecosystem, with energy likely passing through different pathways before reaching a particle size suitable for grazing by small pelagics (Sommer et al., 2002, González et al., 2004, Iriarte and González, 2004, Tam et al., 2008).
This straightforward, bottom-up perspective becomes complicated when one considers the effects and interactions of top-down processes such as predation and fishing. Fortunately, trophic modeling of EBCSs has a long history from which to draw upon; including steady-state models of the Peruvian (Walsh, 1981, Baird et al., 1991, Jarre et al., 1991, Jarre-Teichmann, 1992) and other upwelling systems (Shannon et al., 2003, Heymans et al., 2004, Neira and Arancibia, 2004, Neira et al., 2004, Moloney et al., 2005). The development of the program Ecopath with Ecosim (EwE) (Walters et al., 1997) further allows for temporal explorations of dynamics, and has been previously applied to the southern Benguela system (Shannon et al., 2004a, Shannon et al., 2004b). A review of these advances (Taylor and Wolff, 2007) has assisted in the construction of new steady-state models for the Peruvian system as presented by Tam et al. (2008), which form the basis for the dynamic simulations presented here.
Our objectives are to elucidate the mechanisms of ecosystem dynamics in the Peruvian upwelling system over an ENSO cycle. We evaluate the importance of three external drivers: (i) changes in phytoplankton biomass and composition, (ii) immigration of mesopelagic fish into the model area, and (iii) changes in fishing rates. We also explore internal predator-prey control settings between functional groups of organisms (e.g. bottom-up, top-down control). We speculate that the degree of upwelling and resulting primary productivity may similarly affect ecosystem dynamics across seasonal, inter-annual (EN), and multi-decadal temporal scales, but use the data-rich period of 1995–2004 as a starting point for model exploration.
Section snippets
Methods
Using the temporal dynamic routine of Ecosim within the EwE package (Walters et al., 1997, Walters et al., 2000) we explored the relative importance of external and internal ecosystem drivers in the Northern Humboldt Current Ecosystem from 1995 to 2004. External, non-trophically-mediated drivers considered were changes in phytoplankton biomass, fishing rate (effort and mortality), and oceanic immigrant biomass (mesopelagic fish). Internal, trophically-mediated, factors concerned an exploration
External drivers
The driver to phytoplankton biomass and composition improved the overall fit of the simulation, reducing SS by 2.7% (Fig. 2b) with greatest improvement during EN and the subsequent 3 year period (Fig. 2a). The driver to immigrant biomass (mesopelagics) reduced SS by 9.2% (Fig. 2b) with the greatest improvement in later years when biomasses were highest (Fig. 2a). SS for the EN year 1997–98 alone was not improved by the immigrant driver (Fig. 2a). Fishing rate changes proved to be the most
Discussion
We use the model for the 1995–96 year as a starting point for several reasons: (i) reliable, periodic sampling conducted by IMARPE began in 1995; (ii) 1995–96 was a fairly typical, “normal” upwelling year, several years after the recovery of anchovy; and (iii) 1995–96 preceded the strong EN of 1997–98, offering insight into subsequent dynamics. We asked the question whether this EN event has been a principal perturbation over 1995–2004 and to what degree trophic interactions played a role in
Acknowledgements
The authors acknowledge assistance from the following people (in alphabetical): Milena Arias-Schreiber, Arnaud Bertrand, David Correa, Michelle Graco, Renato Guevara, Mariano Gutierrez, Kristen Kaschner, Miguel Ñiquen, Ralf Schwamborn, Sonia Sánchez, and Carmen Yamashiro. We also thank Carl Walters for the use of the Ecosim software, and Dr. Lynne Shannon and an anonymous referee for their critical suggestions on the manuscript. This study was financed and conducted in the frame of the
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