Mineralization of autochthonous particulate organic carbon is a fast channel of organic matter turnover in Germany's largest drinking water reservoir

Abstract

Abstract. Turnover of organic matter (OM) is an essential ecological function in inland water bodies and relevant for water quality. This is especially important for the potential of dissolved organic carbon (DOC) removal as well as for emissions of CO2. In this study, we investigated various phases of OM including DOC, autochthonous particulate organic carbon (auto-POC), allochthonous particulate organic carbon (allo-POC), and sedimentary matter (SED) in a temperate drinking water reservoir (Rappbode Reservoir, Germany) by means of dissolved inorganic carbon (DIC) concentrations and carbon stable isotope ratios. In order to best outline carbon turnover, we focused on the metalimnion and the hypolimnion of the reservoir, where respiration is expected to be dominant and hardly disturbed by atmospheric exchange or photosynthesis. DIC concentrations ranged between 0.30 and 0.53 mmol L−1, while δ13CDIC values ranged between −15.1 ‰ and −7.2 ‰ versus the VPDB (Vienna PeeDee Belemnite) standard. Values of δ13CDOC and δ13Cauto-POC ranged between −28.8 ‰ and −27.6 ‰ and between −35.2 ‰ and −26.8 ‰, respectively. Isotope compositions of sedimentary material and allochthonous POC were inferred from the literature and from measurements from previous studies with δ13CSED=-31.1 ‰ and δ13Callo-POC ranging from −31.8 ‰ to −28.6 ‰. Comparison of DIC concentration gains and stable isotope mass balances showed that auto-POC from primary producers was the main contributor to increases in the DIC pool. Calculated OM turnover rates (0.01 to 1.3 µmol L−1 d−1) were within the range for oligotrophic water bodies. Some higher values in the metalimnion are likely due to increased availability of settling auto-POC from the photic zone. Samples from a metalimnetic oxygen minimum (MOM) also showed dominance of respiration over photosynthesis. Our work shows that respiration in temperate lentic water bodies largely depends on auto-POC production as a major carbon source. Such dependencies can influence the vulnerabilities of these aqueous systems.

The influence of carbon cycling on oxygen depletion in north-temperate lakes

“…Similarly to what was found by Wilkinson et al (2013), the standing stock of DOC in the water column of lakes in our study was from predom-inantly allochthonous sources. However, we emphasize in our study that autochthonous OC pools have higher turnover rates than allochthonous OC pools (Dordoni et al, 2022) and often are lower in concentration than the more recalcitrant allochthonous pools (Wilkinson et al, 2013). Thus, studies based on correlative relationships between lake concentrations of organic matter and water quality metrics likely overlook the importance of more labile organic matter in driving observable ecosystem phenomena, such as gas flux and the formation of hypolimnetic anoxia (Evans et al, 2005;Feng et al, 2022).…”

Metabolic activity of deep living cyanobacteria: Photosynthesis versus respiration

“…These fluctuations are mainly related to photosynthesis and respiration, which are critical for the functioning of aquatic ecosystems. A metalimnetic bloom during intensive photosynthesis in the daylight can cause significant increases in oxygen concentrations in deeper water layers, but intensive respiration during the night, as well as during organic matter decay, can cause significant oxygen deficits leading to the so‐called metalimnetic oxygen minimum (MOM; Dordoni, Seewald, Rinke, Friese, et al., 2022). Therefore, establishing the relationship between photosynthesis and respiration in the deep environments where P. rubescens blooms is particularly important for understanding lake biogeochemistry such as the mineralization of organic matter (Dordoni, Seewald, Rinke, Friese, et al., 2022; Dordoni, Seewald, Rinke, Schmidmeier, & Barth, 2022).…”

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

“…Conversely, respiration effects on δ 18 O DO are much greater than those driven by photosynthesis and can led to increases of δ 18 O DO values (Mader et al., 2017). Dissolved oxygen dynamics in standing water bodies are strongly influenced by the occurrence of P. rubescens blooms because of potentially excessive DO production by photosynthesis during growth phases and substantial DO consumption by respiration and mineralization during night‐time or bloom‐decay (Dordoni, Seewald, Rinke, Schmidmeier, et al., 2022; Dordoni, Seewald, Rinke, van Geldern, et al., 2022; Wentzky et al., 2019).…”