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Sources of particulate organic carbon (POC) play important roles in aqueous carbon cycling because internal production can provide labile material that can easily be turned into CO 2 . On the other hand, more recalcitrant external POC inputs can cause increased loads to sedimentary organic matter that may ultimately cause CH 4  release. In order to differentiate sources, stable isotopes offer a useful tool. We present a study on the Itupararanga Reservoir (Brazil) where origins of POC were explored by comparing its isotope ratios (δ 13 C POC ) to those of dissolved inorganic carbon (δ 13 C DIC ). The δ 13 C POC  averaged around − 25.1‰ in near-surface waters, which indicates higher primary production inferred from a fractionation model that takes into account carbon transfer with a combined evaluation of δ 13 C POC,  δ 13 C DIC  and aqueous CO 2 . However, δ 13 C POC  values for water depths from 3 to 15 m decreased to − 35.6‰ and indicated different carbon sources. Accordingly, the δ 13 C DIC  values of the reservoir averaged around + 0.6‰ in the top 3 m of the water column. This indicates CO 2  degassing and photosynthesis. Below this depth, DIC isotope values of as low as − 10.1‰ showed stronger influences of respiration. A fractionation model with both isotope parameters revealed that 24% of the POC in the reservoir originated from detritus outside the reservoir and 76% of it was produced internally by aqueous CO 2  fixation.

Dominance of in situ produced particulate organic carbon in a subtropical reservoir inferred from carbon stable isotopes