The Northern Humboldt Current Ecosystem and its resource dynamics: Insights from a trophic modeling and time series analysis

Abstract

The Northern Humboldt Current Ecosystem (NHCE) is one of the most productive in the world. Wind-driven coastal upwelling brings cool, nutrient-rich water to the photic zone where rich plankton communities develop. This high productivity supports large populations of small plankton-grazing pelagic fish, which are an important food source for many higher predators and support a large fish meal industry. The NHCE is subject to strong interannual environmental variability from the El Nino Southern Oscillation (ENSO), which has direct and indirect effects on the biotic components of the ecosystem. This complex mixture of environmental, trophic, and human influences calls for a holistic approach for management. This thesis contributes to our understanding of the NHCE by shedding light on the changes in energy flow that occur during strong "El Nino" events (warm phase of ENSO) as well as differentiating between the relative importance of environmental, trophic, and human influences in reproducing historical changes in fisheries resources. Methods included the application of time-series and trophic modeling analyses for two NHCE subsystems of different scale: i) the nearshore benthic ecosystems of Independencia Bay and Sechura Bay (2 models, include the area of <30m depth), and ii) the larger coastal upwelling system (4à ��à �à °-16à ��à �à °S with a 110km extension from the coast). Comparisons of steady-state trophic models between a normal upwelling year (~1995/96) and a strong El Nino year (1997/98) describe the changes in energy flow structure. Longer-term dynamics were explored with multivariate analyses for the scallop species Argopecten purpuratus, and with dynamic simulations of the trophic model. Results show that during normal upwelling the NHCE is quite efficient on a large scale, with most energy utilized (3.6% of total flows are exports). The nearshore subsystem is much less efficient (~28% of total flows are exports) due to higher primary production and low oxygen concentrations, preventing efficient utilization by primary consumers. During El Nino, both subsystems show increased overall efficiency due primarily to improved usage of the (reduced) primary production. El Nino appears to negatively affect flows at higher trophic levels most, as observed through statistics of flow organization and development (relative ascendancy, A/C) and cycling of energy (predatory cycling index, PCI). Explorations of dynamics revealed a dominance of bottom-up control among predator-prey interactions. The larger coastal upwelling subsystem showed a higher importance of trophic interactions on dynamics than the nearshore benthic system. Fishing-related changes are also more important in the larger upwelling system, while dynamics appear to be dominated by environmental effects in the nearshore subsystem. The spatial scale of the models affected the ability to reproduce dynamics, as the larger scale of the coastal upwelling model contained a higher degree of closure of flows. Recommendations are given for improving the models for future explorations of management strategies; including the extension and standardization of historical time-series data for a more robust analysis, and further research on the underlying mechanisms of population dynamics for species showing strong environmental mediation.