Abstract
1. 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.2. We investigated rotifer food assimilation during a short-term microzooplankton bloom in the East African soda lake Nakuru by developing a method for species-specific sampling of rotifers.3. The two dominant rotifers, Brachionus plicatilis and Brachionus dimidiatus, were separated to single-species samples (purity >95%) and significantly differed in their isotopic values (4.1‰ in δ13C and 1.5‰ in δ15N). 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.4. A main difference was that the filamentous cyanobacterium Arthrospira fusiformis, which frequently forms blooms in African 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.5. 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.
Highlights
Microzooplankton are the main consumers in marine systems, ingesting more than half of the daily primary production across all major oceanic biomes (Calbet and Landry, 2004)
We investigated rotifer food assimilation during a short-term microzooplankton bloom in the East African soda lake Nakuru by developing a method for species-specific sampling of rotifers
Bayesian mixing models indicated that isotopic differences were caused by different assimilation of filamentous cyanobacteria and particles
Summary
Microzooplankton are the main consumers in marine systems, ingesting more than half of the daily primary production across all major oceanic biomes (Calbet and Landry, 2004). Rotifers are often a substantial part of microzooplankton. Despite their ecological importance, they are rarely incorporated into in situ food-web studies (Chick et al, 2010), and rotifer grazing rates and feeding behaviour are typically studied only under laboratory conditions In natural systems, examining the broad variety of available food types, ranging from algae and heterotrophic protists to bacteria and detritus, remains a fundamental challenge for food-web ecologists
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