Species‐specific separation of lake plankton reveals divergent food assimilation patterns in rotifers

Summary The analysis of functional groups with a resolution to the individual species level is a basic requirement to better understand complex interactions in aquatic food webs. Species‐specific stable isotope analyses are currently applied to analyse the trophic role of large zooplankton or fish species, but technical constraints complicate their application to smaller‐sized plankton. We investigated rotifer food assimilation during a short‐term microzooplankton bloom in the E ast A frican soda lake N akuru by developing a method for species‐specific sampling of rotifers. The two dominant rotifers, B rachionus plicatilis and B rachionus dimidiatus , were separated to single‐species samples (purity >95%) and significantly differed in their isotopic values (4.1‰ in δ 13 C and 1.5‰ in δ 15 N). Bayesian mixing models indicated that isotopic differences were caused by different assimilation of filamentous cyanobacteria and particles <2 μm and underlined the importance of species‐specific sampling of smaller plankton compartments. A main difference was that the filamentous cyanobacterium A rthrospira fusiformis , which frequently forms blooms in A frican soda lakes, was an important food source for the larger‐sized B . plicatilis (48%), whereas it was hardly ingested by B . dimidiatus . Overall, A . fusiformis was, relative to its biomass, assimilated to small extents, demonstrating a high grazing resistance of this species. In combination with high population densities, these results demonstrate a strong potential of rotifer blooms to shape phytoplankton communities and are the first in situ demonstration of a quantitatively important direct trophic link between rotifers and filamentous cyanobacteria.