Abstract

Trophic interactions between microbial and copepod communities were studied during winter and under spring-bloom conditions in the Reloncaví Fjord, Chile, and adjacent channels. Grazing by heterotrophic nanoflagellates (HNF) and microzooplankton was estimated using the size-fractionation method. Simultaneously, copepod grazing experiments using naturally occurring plankton assemblages were performed. Contrasting food environments for planktonic consumers were found between winter and spring, with biomasses of prey organisms < 1 μg C l-1 during winter and ca. 150 μg C l-1 during spring. The highest bacterial and phytoplankton biomasses were observed during spring, when autotrophic biomass in the fjord and adjacent channels mostly consisted of diatoms. Most grazers in the <20 μm filtered fraction belonged to the HNF, which exhibited maximum ingestion during winter, whereas the highest grazing by microzooplankton occurred in spring. Grazing experiments showed contrasting trophic interactions between copepods and their prey. In winter, copepod grazing rates were among the lowest reported for oligotrophic areas (< 0.2 μg C ind.-1 d-1), while HNF and dinoflagellates contributed significantly to the total average daily ingestion of prey items (> 50% d-1). During spring, small and large copepods exhibited prey ingestion rates of 2 to 3 and ca. 6 to 10 μg C ind. -1 d-1, respectively. In such a contrasting food environment, copepods have to alleviate the effects of food scarcity either by modifying their metabolic demands or by switching their diet to microbial organisms that are available during periods of low diatom biomass. The present study reveals that, even under productive spring-bloom conditions, the less-abundant heterotrophic protists constitute a substantial proportion (ca. 30% of the daily consumption) of the copepod diet in fjord ecosystems. © Inter-Research 2008.