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

Abstract

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. Reservoirs and their tributaries are active components of landscapes. They receive, transport, process and store inorganic and organic carbon 1. Although lakes and reservoirs cover only 2.2% of the global continental area, they may play so far poorly accounted roles in cycling continental carbon 2-4. In addition, regional investigations of carbon budgets become increasingly important for managing water resources because they define ecosystem functions and services 5-8. Spatiotemporal changes of physicochemical and biological parameters in subtropical reservoirs, can increase our knowledge of carbon turnover in water bodies. For instance, excessive precipitation, can increase terrestrial nutrients, contaminants, soil leaching and additions of untreated sewage input 9. Rainfall events can also cause near-surface turbulences in lakes that has been shown to enhance gas exchange by increasing transfer velocities of CO 2 10,11. Precipitation can also lead to increased input from external sources (i.e. allochthonous matter for instance from plant debris or soils) that may become trapped in lakes and reservoirs. This can, in turn, change biogeochemical dynamics of the water column 12. Such processes include carbon uptake and release of carbon within the water column that may also be controlled by seasonal changes of temperature, weather patterns or light availability. These may in turn have important influences on carbon fluxes inside open water bodies such as for instance sedimentation, or gross primary production, ecosystem respiration and external carbon inputs 13. For the latter, reservoir catchments ma...

Metabolic activity of Planktothrix rubescens and its consequences on oxygen dynamics in laboratory experiment: A stable isotope study

“…Field studies in lotic and lentic water bodies with variable spatial and temporal resolution can assess the status of entire systems with simultaneous occurrence of algal photosynthesis and bacterial mineralization by means of δ 18 O DO . However, this research setup can address the metabolism of communities only indirectly because of atmospheric exchange (Barth et al., 1998; De Castro Bueno et al., 2020; Dordoni, Seewald, Rinke, Schmidmeier, et al., 2022). Specific laboratory experiments may overcome this lack of detail when algae populations can be analyzed separately and in the absence of air.…”