Krill larvae, copepods and the microbial food web: interactions during the Antarctic fall

Abstract

Experiments were run in the Antarctic Bellinghausen Sea during the austral fall to ascertain the interactive effects of krill larvae (furcilia) and copepods on the microbial food web. A cross-classi- fied design with addition of the small cyclopoid cope- pod Oithona similis and either the furcilia larvae of Euphausia superba or the calanoid copepod Metridia gerlachei to the natural planktonic community was used, as well as a single experiment with varying den- sities of O. similis. The experiments showed furcilia to have much higher grazing rates on copepods and cili- ates than on algae, with positive selection for ciliates that was not influenced by the presence or absence of O. similis. M. gerlachei had a grazing impact on cili- ates that was much less than that of furcilia larvae, but due to a high taxonomic resolution of the ciliate community it could be shown that there were groups of grazing-vulnerable and -resistant forms. Contrary to expectations, O. similis had very low grazing rates on both ciliates and algae, but here too grazing-vul- nerable and -resistant groups of ciliates could be dis- tinguished. Neither the very strong impact of furcilia on ciliates, copepods and algae nor the more moder- ate impact of M. gerlachei cascaded down to hetero- trophic and phototrophic nanoplankton or to bacteria. At least at the time of our experiments, ciliates were tightly bound to the 'classic' aquatic food web, but do not serve to transmit a trophic cascade from the clas- sic and microbial food webs.