We present an analysis of seasonal variations in the trophic pathways of carbon in a highly productive coastal upwelling region in the Humboldt current system off Chile. Seasonal changes in phytoplankton, protozooplankton, and bacteria biomass, along with rates of primary production (PP), bacterial growth, secondary production, vertical particle fluxes, and feeding by protozooplankton, omnivorous mesozooplankton, and carnivorous gelatinous zooplankton were determined from July 2004 to June 2005. Phytoplankton biomass and PP were maximal during spring/summer months, associated with upwelling episodes. Heterotrophic nanoflagellates (HNF) were the principal consumers of bacteria, removing .100% of their biomass daily. During autumn/winter, the protozooplankton grazed down a large fraction of HNF production (56% to 96% d 21 ). The mesozooplankton consumed 1-6% of the PP d 21 ; the different size fractions of copepods were omnivorous mostly during autumn/winter months, and ctenophores preyed most strongly on small copepods (0.5% to 5% d 21 ). A large part of the PP was channeled through the microbial food web, and only a small part AcknowledgmentsWe thank the captains and crew of the RV Kay Kay (Universidad de Concepció n, Chile) and the many undergraduate and graduate students who participated in our cruises (L. Lizá rraga, V. Aguilera, C. Aparicio, E. Menschel, and A. Araneda). We also thank José Luis Acuñ a and Albert Calbet for their valuable suggestions that substantially improved an earlier version of the manuscript and two anonymous reviewers for their critical and helpful comments.

), respectively, from winter to spring. In addition, the bacterial secondary production to primary production (BSP:PP) ratio decreased from 3.7 to 0.2 in Reloncaví Fjord, suggesting a transition from microbial to classical pelagic food webs. The higher solar radiation and extended photoperiod of springtime promoted the growth of diatoms in a nutrient-replete water column. Allochthonous (river discharge) and autochthonous (phytoplankton exudates) organic matter maintained high year-round bacteria biomass and secondary production. In spring, grazing pressure from zooplankton on the microplankton (largely diatoms) resulted in the relative dominance of the classical food web, with increased export production of zooplankton faecal pellets and ungrazed diatoms. Conversely, in winter, zooplankton grazing, mainly on nanoplankton, resulted in a relative dominance of the microbial loop with lower export production than found in spring. Carbon fluxes and fjord-system functioning are highly variable on a seasonal basis, and both the multivorous trophic webs and the carbon export were more uncoupled from local PP than coastal areas.

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