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

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.
