The role of omnivory in calanoid copepods and the effects of mixotrophy on microbial food webs were investigated in complex artificial food webs. Mixotrophy was manipulated by the presence/absence of mixotrophic flagellates, omnivory in calanoid copepods by the absence/presence of microzooplankton in the food webs, making the copeopds either herbivorous or omnivorous.
The presence of mixotrophic flagellates lead in all experiments to a marked reduction of picophytoplankton and bacteria, though HNFs were present in all food webs. Effects on other food web compartments and on overall productivity were context dependent. Seston biomass and biomass per limiting nutrient unit (nitrogen in all experiments) were enhanced by the mixotrophs in experiments where mesozooplankton was present or where bacterial productivity was enhanced by addition of DOC. In absence of mesozooplankton and glucose addition, seston biomass was reduced by the mixotrophs. It is concluded, that the mixotrophs potentially may enhance primary production, provided that bacterial productivity is relatively high, either due to external DOC input or due to DOC production by grazers. Effects of mixotrophs on secondary production were species-dependent: The chrysophyte Ochromonas minima reduced productivity of the rotifer Brachionus plicatilis, while the haptophyte Chrysochromulina polylepis enhanced reproduction in calanoid copepods.
In a complex food web with calanoid copepods as terminal consumers, the presence of the heterotrophic dinoflagellate Oxyrrhis marina clearly enhanced copepod reproduction. In addition to that, the presence of the dinoflagellates drastically reduced the nanophytoplankton, the major food of the copepods in the absence of the dinoflagellates. Hence, the intermediate consumer Oxyrrhis enhanced copepod nutrition in spite of reducing the phytoplankton prey of the copepods. In another experiment, calanoid copepods were fed either with the diatom Skeletonema costatum, or with the diatom and the heterotrophic dinoflagellate Gyrodinium dominans, that was feeding itself on the diatom (intermediate consumer). Again, reproduction of the copepods was clearly enhanced by the dinoflagellates, though abundances of the dinoflagellates were low compared to the diatoms (appr. 200 and 20,000 cells per ml, respectively). Fast disappearance of the dinoflagellates in the mixed treatments indicated strong selective feeding by the copepods. The results of the latter experiment may explain, why negative effects of diatoms on calanoid copepods, that were found in several laboratory studies, are not found in the field during diatom blooms (Irigoien et al. 2002): Due to selective feeding by copepods, even low relative abundances of microzooplankton may significantly enhance nutritional quality of the diet for the copepods.