Plankton community structure and carbon cycling in a coastal upwelling system.: I.: Bacteria, microprotozoans and phytoplankton in the diet of copepods and appendicularians

Copepod and appendicularian grazing experiments using naturally occurring planktonic assemblages from a coastal embayment (Mejillones Bay, northern Chile upwelling system at 23degrees S) were conducted between October 2000 and October 2001. Total carbon ingestion rates based on size-fractioned chlorophyll data showed that dominant copepods (Acartia tonsa, Centropages brachiatus, Oithona similis and Paracalanus parvus) ingested between 2 and 8 mug C ind.(-1) d(-1), while appendicularians (Oikopleura dioica and O. longicauda) ingested similar to3 to 4 mug C ind.(-1) d(-1). Even when most copepods were feeding on larger cells (> 23 pm) at high rates, the smaller copepods also grazed at similar rates on nanoplankton (5 to 23 mum) and picoplankton (<5 mum). In contrast, chain-forming diatoms were cleared at very low rates by copepods. Bacteria were cleared only by appendicularians (similar to170 to similar to400 ml ind.(-1) d(-1)) but not by any copepod, while heterotrophic protists constituted a substantial proportion in the diet of both copepods and appendicularians (similar to10 to 100% body carbon d(-1)), particularly during austral spring. Occasionally, copepod C-specific ingestion on heterotrophs was similar to that on autotrophic cells. Large ciliates and dinoflagellates were cleared but not ingested by the appendicularian O. dioica, suggesting a mechanism of trapping large cells in their houses and implying a rapid export of fresh material. Since heterotrophs are a common component in the diet of these 2 groups (omnivory by copepods and bacterophagy by appendicularians), they can potentially affect microbial food webs in this upwelling system and thus carbon export.