The fate of diatom carbon within a freshwater benthic community-a microcosm study

Partitioning of 14C from labeled diatoms (Stephanodiscus hantzschii Grun v. pusillus) within a freshwater benthic community was determined in a microcosm study for three different treatments: sediment with only natural, ambient meio- and microfauna, and microbes, addition of 30 Chydorus sphaericus Müller per microcosm, and addition of four Chironomus riparius Meigen larvae per microcosm. Assimilation of 14C label in Chironomus, Chydorus, ambient meiofauna, respired 14C, and [14C]DOC in both overlying and interstitial waters was measured, and [14C]POC was calculated. The carbon budget clearly showed a rapid assimilation of diatom C by surface deposit-feeding Chironomus larvae, more than 20% of the total added label was used after 8 d. Community respiration in the Chironomus treatment increased from 6% on day 2 to 18% on day 8, or 92% higher than that of the treatments with ambient meiofauna only and added Chydorus. Chironomus respiration was low relative to assimilation, indicating that larvae efficiently used the easily accessible and digestible food resource. Sediment microheterotrophs (microbes and microfauna) were also found to be important quantitatively for C turnover. Respiration by microheterotrophs was ∼10% of the total added label after 8 d.