Abstract

Planktonic food-web structure and carbon dynamics were studied in Mejillones Bay (23° S, off northern Chile) on 3 occasions: February, August and October 2001. Mejillones Bay was influenced by moderate upwelling events during February and October and presented a shallow (30 to 40 m depth) oxygen minimum layer (OML). On all sampling occasions, chain-forming diatoms that were grazed by small zooplankton and heterotrophic (h-)dinoflagellates comprised most of the autotrophic biomass. Heterotrophic (h-)nanoflagellates were largely bacterivorous, and responsible for a substantial removal of bacterial biomass, mostly associated with productive surface waters and the OML. Ciliate biomass was relatively low during all periods, but ciliates removed a large fraction of h-nanoflagellate production (12 to 22 % d-1). Thus, in the microbial food web bacterial carbon can be transferred to ciliates and then subsequently to zooplankton. The impact of small zooplankton on primary production (PP) was relatively low on all sampling occasions. Small copepods and appendicularians removed from 0.6 to 5%, and from 0.08 to 0.6% PP d-1 respectively. Appendicularians were the only zooplankton able to graze on bacteria, but grazing impact was not sufficient to regulate bacterial biomass. The microheterotrophic pathway could be an important link in this highly productive upwelling embayment. Our results showed that a large part of the photosynthetically fixed carbon was channeled through the microbial food web, with only a small part allocated to copepods and appendicularians. The food-web structure of Mejillones Bay can be classified as multivorous, with herbivorous and microbial grazing playing an important role in carbon export.