Trophic modeling of the Northern Humboldt Current Ecosystem, Part I: Comparing trophic linkages under La Niña and El Niño conditions
Introduction
The northern part of the Humboldt Current Ecosystem (HCE) off Peru has been modelled with carbon and nitrogen budget models (Dugdale and MacIsaac, 1971, Walsh and Dugdale, 1971, Walsh, 1981), mass balance models (Jarre-Teichmann, 1989, Jarre et al., 1991, Jarre and Pauly, 1993, Ballón, 2005), a size-based carbon flow model (Carr, 2003) and an empirical carbon flow model (Jahncke et al., 2004). Mass balance models have also been applied in the southern HCE off Chile (Wolff, 1994, Ortiz and Wolff, 2002, Arancibia et al., 2003, Neira et al., 2004, Neira and Arancibia, 2004). These models have permitted comparisons between the HCE and other eastern boundary current ecosystems (Jarre-Teichmann, 1998, Jarre-Teichmann and Christensen, 1998, Jarre-Teichmann et al., 1998, Moloney et al., 2005). However, none of these models have focused on the impact of the interannual variability associated with El Niño (EN).
According to Alheit and Niquen (2004), a regime shift occurred in Peruvian waters between 1968 and 1970, wherein waters warmed and zooplankton and anchovy (Engraulis ringens) biomass decreased, followed by an increase in sardine (Sardinops sagax) stocks. However, another regime shift back to cold conditions occurred during 1984–1986, in this case characterized by an increase of phytoplankton and zooplankton biomasses (see Ayón et al., 2008) associated with excellent recruitment of anchovy but decreases in sardine biomass.
Arntz and Fahrbach (1991) summarized the effects of the 1982–83 EN on the Northern Humbolt Current Ecosystem (NHCE). During EN, in the NHCE near surface temperature increases and the thermocline deepens, causing a collapse of the diatom-based trophic web, with emigration of anchovy and immigration of tropical and oceanic species. Gutiérrez, 2001, Bertrand et al., 2004 described the effects of the 1997–98 EN on anchovy distribution and abundance, confirming that anchovy move to deeper waters but finding that the main spatial effect was concentration of stocks very nearshore. These authors attribute an apparent reduction in anchovy biomass to decreased effectiveness of acoustic sampling, unfavorable environmental conditions, increase of natural mortality due to poor feeding conditions, and to a much lesser degree, to mortality due to predation and fishing. Bouchon et al. (2001) analysed the ichthyofauna fluctuations over an El Niño Southern Oscillation (ENSO) cycle and concluded that in cold years the pelagic community is characterized by a high productivity and a low diversity (abundant diatoms and anchovy), but in warm years this pattern is reversed due to the immigration of offshore and tropical species and the reduction of anchovy. While responses of the main fish resources to EN-related perturbations are relatively well known (Aguilar, 1999, Tarazona, et al., 2001), understanding the ecosystem response as a whole requires a multispecies ‘ecotrophic’ approach. Given the observed changes in biomass and species composition, it is expected that a strong EN impacts the food web, reducing or redistributing the main energy channel that flows through anchovy under La Niña (LN) conditions.
Previous models of the NHCE (Jarre et al., 1991), which described the flow of energy through the ecosystem during three decades (1953–1959, 1960–1969, 1973–1979), brought great understanding of ecosystem functioning. Now however, biological changes, new data sets, and the advancement of trophodynamic modeling permit construction of more detailed models through the inclusion of additional ‘functional groups’ of organisms (see also Guenette et al., 2008). In this paper we divided the phytoplankton compartment into two groups (diatoms and dino- and silicoflagellates) and zooplankton into three groups (micro-, meso- and macro-zooplankton) to account for the feeding preferences of different small pelagic fish. We incorporated the groups of mesopelagic fish and jumbo squid (Dosidicus gigas), which have gained in importance since the last 1997–98 EN. We also increased the detail of demersal groups and separated the hake into three different life history stages. The ecotrophic model framework is a simplified approach where species are aggregated into functional groups. Each group is represented by two linear equations, each of which must balance. One equation ensures balance between groups in the model, the other equation, balances the flows within each group.
The objective of this study is to compare such improved mass balance trophic models for a cold LN conditions (1995–96) versus a warm EN (1997–98) conditions, with the a priori hypothesis that the EN perturbation should decrease ecosystem organization. This paper also provides the basis for further explorations of ecosystem dynamics (Taylor et al., 2008a), wherein non-steady state simulations of ecosystem change during and following the 1997–98 EN are performed and evaluated. In dynamic models, biomass changes are expressed in form of coupled differential equations derived from mass balance models equations.
Section snippets
Input data
Our models of the Northern Humboldt Current Ecosystem (NHCE) extend from 4°S to 16°S, and 60 nm offshore, covering an area of approximately 165000 km2 (Fig. 1). In agreement with the ENSO cycle, data from June 1995 to May 1996 and from May 1997 to April 1998 were used as inputs for the cold LN and warm EN mass balance models, respectively, covering a full “biological year” each (i.e. starting from about the middle of a calendar year).
The models included 33 functional groups, namely: (1) diatoms,
Biomass and catch changes
Tables 5A and B presents results for LN and EN conditions, respectively. The model synthesizes available data and estimates additional parameters (bold type in Table 5) that define the relationships between the functional groups of organisms in the model. During the LN conditions diatoms, mesozooplankton, anchovy, horse mackerel, mackerel and jumbo squid dominated in biomass in their respective trophic levels. During EN conditions, biomasses of most groups decreased (anchovy, jumbo squid, horse
Conclusions
While past ecotrophic modeling efforts in the NHCE dealt with interdecadal changes (Jarre et al., 1991), this study focused on the interannual changes associated with El Niño and the Southern Oscillation (ENSO) cycle. The main finding of previous models was a decrease in relative ascendency from the 1950s to the 1970s, after the decline of the anchoveta, which led to an increase in parallel energy transfer and food web connectance, as energy flows through anchovy were channeled through other
Acknowledgements
We want to thank Renato Guevara, Miguel Ñiquen, Mariano Gutiérrez, Carmen Yamashiro and Sonia Sánchez from IMARPE, for the shared information. We also thank Arnaud Bertrand and Timothée Brochier from the Institute of Research for Development (IRD) for discussions on anchovy ecology, the staff of the Oceanographic and Fishery Biological Modelling Research Center (CIMOBP, IMARPE) and the staff of the CENSOR project for their helpful comments. We also thank Dr. Lynne Shannon, Dr. J. Timothy
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