Vertical and tidal variability of dissolved organic matter in mangrove soils: influence on dissolved trace metal dynamics

ABSTRACT: Mangrove ecosystems are dynamic ecotones that play a key role in organic carbon and trace metals (TM) cycling along tropical coastlines. Tidal pumping is a key process that allows dissolved organic matter (DOM) and dissolved trace metals export towards adjacent ecosystems. While this process is well characterized, DOM-TM relationships and variability within mangrove soils remain poorly studied. In the present study, we investigated the vertical distribution of DOM and TM in a Rhizophora stylosa mangrove stand in New Caledonia over 12-hour tidal cycles during both spring and neap tides. DOM quantity and quality were assessed through UV-visible absorbance, fluorescence spectroscopy, total and dissolved organic carbon analysis, and physicochemical measurements. Dissolved TM were quantified using ICP-MS. Our results show a vertical gradient in DOM, with deeper layers concentrated in aromatic and humified compounds, attributed to vertical migration through potential convection process. This gradient (DOM and salinity) is accentuated during neap tide due to lower hydrodynamics. Dissolved TM exhibited distinct vertical trends with depth: Cr and Mn followed DOM trends, suggesting either common vertical transport mechanisms or complexation with humic substances. In contrast, Mo and Ni decreased with depth, suggesting stronger control by physicochemical or mineralogical conditions, and potential precipitation within pyrite in the most anoxic layers. This study highlights the combined effect of vertical soil gradients and tidal fluctuations on DOM-TM interactions, contributing to a better understanding of the role of mangroves as biogeochemical filters and as both sources and sinks of trace elements at the land-ocean interface. Highlights • Dissolved organic carbon content is higher in deeper soil layers • The deep dissolved organic matter is older and more humified than at the surface • The vertical gradients studied are amplified during neap tides • Cr and Mn follow the dissolved organic carbon vertical gradient • Ni and Mo follow an opposite vertical gradient to dissolved organic carbon