Forms of Spontaneous Coagulation and Recycling of Dissolved Organic Carbon in a Monimolimnetic Water Body

The dissolved organic carbon (DOC) depth profiles and the morphology of in-vitro coagulated dissolved organic matter (DOM) were investigated in a steady-state mostly anoxic aquatic system (Lac Pavin monimolimnion). High-resolution cryo-scanning electron microscopy (Cryo-HRSEM) was used to reveal the ultrastructure of the coagulated matter. The study focused on differences in DOC concentrations and coagulation morphotypes between microfiltered and nanofiltered waters. Two types of molecular weight (MW) DOC profiles were identified: although all MW DOC concentrations increased across the chemocline, high MW DOC (> 1 kDa) continued to increase significantly below, while low MW DOC (< 1 kDa) had a quasi-constant value downward the monimolimnion. Cryo-HRSEM images showed organic material composed on the one hand of various distinct shapes such as sheets, nanobelts, tubes, rings, platelets and globules entangled with each other, and on the other hand of fragments of netted threads, trapping Fe-phosphate grains. MW DOC depth profiles and the coagulation morphotype distribution suggest that the distinct organic shapes (sheets, tubes, rings, etc.) correspond to lower MW DOM equally distributed through the monimolimnion, while netted material should be attributed to higher MW DOM scavenged during its upward turbulent diffusion through the monimolimnion. It is concluded that recycling by sedimentation in the monimolimnion does not involve the total DOC but specifically the higher MW DOC imaged by meshed threads coagulated and aggregated in flocculation networks. Ultrastructural 3D morphological observation through Cryo-HRSEM is proposed as a tool to characterize DOM.

• Dissolved organic carbon molecular weights are fractionated through the water column.

• 3D Cryo-HRSEM images distinguish different coagulate morphotypes.

• Flocculation limits high molecular weight dissolved organic carbon diffusion.

• Differential nanostructural gelation controls diffusion, flocculation, and sedimentation.