Origin, accumulation and fate of dissolved organic matter in an extreme hypersaline shallow lake.

Hyper-Saline Endorheic Aquatic Systems (H-SEAS) are shallow lakes in arid and semiarid climatic zones that undergo to extreme oscillation in salinities and large drought episodes. Although their geochemical uniqueness and microbiome are deeply studied, very little is known about availability, transformation and fate of dissolved organic matter (DOM) in water column, interstitial waters and in salts that precipitate under driest conditions. To advance in this direction, a small hypersaline shallow lake from Monegros desert (Zaragoza, NE, Spain) has been studied during a complete hydrological wet-drough-rewetting transition. DOM analysis includes: i) a dissolved organic carbon (DOC) mass balance;  ii) optical spectroscopy (absorbance and fluorescence) characterization and; iii) molecular description by negative electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS).

The studied system stored large amount of DOC and the mass balance revealed that under highest salinity conditions, salt-saturated waters (i.e. brines, salinity >30%) accumulated a disproportionate quantity of DOC indicating a significant net in-situ DOM production. Simultaneously, during the hydrological transition from wet to drought, the DOM pool changed drastically its qualitative properties: thus, aromatic and humified moieties were rapidly replaced by fresher, relatively small size and microbial derived moieties with large C/N ratio. Further FT-ICR-MS highlight the accumulation of small-size, saturated and, highly oxidized molecules (O/C molar ratio >0.5) with a remarkable increase of relative contribution of sugar-like molecules and decrease of aliphatic and carboxyl-rich alicyclic like molecules. Overall, there results highlight that H-SEAS are extremely active in accumulating and processing DOM and, the observed patterns pointed to a notable release of organic solutes from decaying microplankton probably triggered by the osmotic stress under extremely high salinities.