Deposit-feeding animals form important links between sediment organic matter (SOM) and carnivorous fish in aquatic ecosystem food webs. They inevitably assimilate food from different origins in a selective manner from SOM, including benthic and planktonic microalgae. Lake Katanuma has a very simple community structure due to its strong acidity. It includes chironomid larvae (Chironomus acerbiphilus) as the consumer, and two primary producers: a benthic diatom (Pinnularia acidojaponica) and a planktonic green alga (Chlamydomonas acidophila). We collected the deposit feeder C. acerbiphilus, its potential food (phytoplankton, benthic diatoms, and terrestrial plant litter), and the sediments forming its diet from Lake Katanuma, and conducted feeding experiments to determine the required parameters for the mixing model. The results of the isotope-mixing model clearly showed that C. acerbiphilus larvae selectively assimilated phytoplankton and benthic diatoms as fresh deposits from bulk sediments, and assimilated phytoplankton more readily than benthic diatoms. Moreover, the assimilation rate of benthic diatoms from sediments by C. acerbiphilus larvae tended to decrease with increasing water depth due to the decrease in benthic primary production, while the rate of phytoplankton tended to increase with depth.
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