Abstract

Understanding the mechanisms that influence variability in zooplankton biomass in eastern boundary upwelling systems (EBUS) is a critical step in estimating secondary production in the ocean, particularly in the context of biogeochemical modelling and assessing ecosystem productivity. In this study, we utilised a physicalbiogeochemical hindcast simulation (2002-2008) to investigate the sources of seasonal and interannual variability of mesozooplankton biomass within the upwelling zone of central-southern Chile. Monthly observations were obtained from an oceanographic time series conducted at Station 18 (36 degrees 30 ' S) off the coast of Concepci ' on, Chile. The first step was to assess the coherence between model outputs and field observations of physical and biogeochemical variables, including the biomass of mesozooplankton. The factors accounting for the variation in biomass at Station 18 were then assessed. In the model, advective flows along the shore and across the shelf control the majority of biomass. The advection effect appears to be more pronounced during the upwelling season (austral spring-summer) than in winter, although it still significantly contributes to biomass variation during the latter. Our physical-biogeochemical model ROMS-PISCES applied on this EBUS showed that alongshore and cross-shelf advection play a crucial role in driving spatial and temporal variability in zooplankton biomass within the upwelling zone. Therefore, it is essential to consider these physical processes when interpreting observational data from fixed time series stations.