Along the Land-Sea aquatic continuum, dissolved matter may undergo various biogeochemical processes according to the compartment. While these dynamics have been extensively studied, knowledge is limited for Mediterranean small temporary rivers and estuaries, for which the transfer of matter from a reactive catchment to the oligotrophic Mediterranean Sea is primordial. Sampling of key dissolved matter species took place in a flowing and an entire ebb tide, accounting for their spatio-temporal variability. Water δ 2 H and δ 18 O, and δ 13 C-DIC were used to perform Bayesian mixing models and to calculate catchment contributions to estuarine water and matter, and their fluxes along the continuum. Continental water and matter mixed with marine ones only in the lower estuary, and this transition was faster for the mixing of matter. Markedly low concentrations of matter (e.g., river DIN = 15.9 μM; DIP = 0.4 μM) were measured across the continuum, associated with modest fluxes flowing into the coastal sea. A linear decrease along the salinity gradient was exhibited for almost all dissolved species, except close to intertidal flats for reactive species. This highlights a conservative behaviour and an essentially transport role for the estuary, conducting riverine nutrients following a flood event to the sea, joined by riparian matter input at high tide. These significant inputs weakly supported autochthonous biological processes, underlying the oligotrophic status of the whole continuum. These findings open up broad environmental research perspectives regarding the role of such estuaries in the Mediterranean Sea along an annual hydrological cycle and among contrasted biogeochemical characteristics. • Dissolved matter was measured along a tide and across a temporarily closed estuary. • In a humid winter setting, the land-sea continuum was oligotrophic and conservative. • According to mixing models, estuarine mixture differed depending on water or matter. • Riparian zones were an active supplier of nutrients along the estuary. • Estuarine exports are determinant for the coastal Mediterranean biogeochemistry.
Ferchiche et al. (Sun,) studied this question.