Interactions between Chironomus plumosus (L.) and the microbial community in surficial sediments of a shallow, eutrophic lake

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Abstract
Feeding by Chironomus plumosus (L.) larvae on sedimentary microbiota was studied in eutrophic Vallentunasjön, where microbial biomass constitutes a large portion of sedimentary organic matter. The life history of C. plumosus consisted of two univoltine cohorts with an overall production of 1.4 g ash‐free dry wt m−2. Gut concentrations of bacteria, Microcystis, and Melosira were several times higher than sedimentary concentrations, indicating that the larvae are foraging selectively. Assimilation of bacteria and newly sedimented Melosira cells on average accounted for 11 and 49% of the estimated carbon requirement (production + respiration) of the larvae. Digestive efficiencies for bacteria and Melosira seasonally averaged 43 and 34%. “Overwintering” Microcystis colonies, which constitute the dominant proportion of microbial biomass in the sediments, were used to a small extent ifat all as a carbon source by the chironomids. Ifa decrease in autofluorescence intensity of cells during gut passage is indicative of partial digestion of the cells, Microcystis was used with a digestive efficiency of 9% and accounted for ~ 10% of the carbon requirements of the larvae. On the basis of ambient Chironomus densities, gut passage time, enrichment factor (gut vs. sedimentary density of bacteria), and digestive efficiency, chironomid grazing is not a dominant sink for sedimentary bacteria in Vallentunasjön. The feeding activity of Chironomus larvae decreased bacterial densities but increased the cell‐specific production in laboratory experiments. The decrease was not proportional to grazer density, indicating that indirect effects of the chironomids (on sedimentary structure and chemistry) also affect the abundance of bacteria.

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