Contrasting trophic interactions of microbial and copepod communities in a fjord ecosystem, Chilean Patagonia

Abstract

Trophic interactions between microbial and copepod communities were studied during winter and under spring-bloom conditions in the Reloncavi 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 plank- ton assemblages were performed. Contrasting food environments for planktonic consumers were found between winter and spring, with biomasses of prey organisms 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 pre- sent 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.