A comparative isotopic study of the biogeochemical cycle of carbon in modern stratified lakes: the hidden role of DOC

Abstract

Abstract. The carbon cycle is central to the evolution of biogeochemical processes at the surface of the Earth. In the ocean, which has been redox-stratified through most of the Earth’s history, the dissolved organic carbon (DOC) reservoir holds a critical role in these processes because of its large size and involvement in many biogeochemical reactions. However, it is rarely measured and examined in modern stratified analogs and yet commonly invoked in past C cycle studies. Here, we characterized the C cycles of four redox-stratified alkaline crater lakes from Mexico. For this purpose, we analyzed the concentrations and isotopic compositions of DOC together with dissolved inorganic and particulate organic C (DIC and POC). In parallel we measured physico-chemical parameters of the water columns and surficial bottom sediments. The four lakes have high DOC concentrations (from ~ 15 to 160 times the amount of POC, averaging 2 ± 4 mM; 1SD, n=28) with an important variability between and within the lakes. All lakes exhibit prominent DOC peaks (up to 21 mM), found in the oxic and/or anoxic zones. δ13CDOC signatures also span a broad range of values from -29.3 to -8.7 ‰ (with as much as 12.5 ‰ variation within a single lake), while δ13CPOC and δ13CDIC varied from -29.0 to -23.5 ‰ and -4.1 to +2.0 ‰, respectively. The DOC peaks in the water columns and associated isotopic variability seem mostly related to oxygenic and/or anoxygenic primary productivity through the release of excess fixed C in three of the lakes (Atexcac, La Preciosa and La Alberca de los Espinos). By contrast, the variability of [DOC] and δ13CDOC in Lake Alchichica could be mainly explained by partial degradation and accumulation in anoxic waters. Overall, DOC records metabolic reactions that would not have been clearly detected if only DIC and POC reservoirs had been analyzed. For example, DOC analyses evidence an active DIC-uptake and use of a DIC-concentrating mechanism by part of the photosynthetic plankton. Despite the prominent role of DOC in the C cycle of these lakes, variations of [DOC]/δ13CDOC and associated reactions are not reflected in the sedimentary organic carbon record, hence calling for special care when considering sediments as reliable archives of metabolic activities in stratified water columns. Overall, this study brings to light the need of further investigating the role of DOC in the C cycles of modern stratified analogs.