Role of the riparian groundwater in a pristine Mediterranean micro-estuary catchment to quantify nutrient fluxes

Catchment fluxes are the main vector of nutrients to marine ecosystems. Most of the time research focuses on regional catchments and their nutrient contribution toward estuaries, deltas or lagoons. Fluxes coming from coastal watersheds with smaller size are under-investigated, especially micro-estuaries, in spite of their predominance in many coastal areas and particularly in the Mediterranean.

The catchment-micro-estuary continuum has a strong seasonal variation dynamic controlled by transversal water and nutrients fluxes through the watershed, estuary and coastal zone.

Identifying and quantifying the water, inorganic and organic fluxes is unavoidable to understand the hydro-ecological behaviour of the micro-estuaries. For this purpose, the Fiume Santu watershed located on the Mediterranean Island of Corsica (France) has been investigated with a hydro-ecological approach based on the crossing-use of isotopic, chemical and nutrient tracers.

The study was carried out in a humid period in February 2018 and investigations involved a multi-tracer methodology, namely major ions and traces elements, stable isotopes of water, carbon isotope ratios and concentrations of DIC as well as measurements of DIM and DOM. Eight sampling sites were chosen along the estuary from top to bottom: four along the river, three for groundwater and one in open-sea water.

Results highlight hydrological processes in the down part of the catchment which are constrained by the tidal phenomena stopping temporarily the freshwater and nutrient circulation. The catchment fluxes, the chemical and biological intrinsic processes (remineralization, sediment uptake and release) and the marsh inputs constitute sources of nutrients along the stream and induce non-conservative budgets. The role of groundwater and subsurface water is significant in terms of nutrient fluxes even their discharge contribution is neglected.

This first mutual tracer-approach in the hydrological functioning and the ecological behaviour is unique in terms of the quantification of nutrient fluxes in a pristine coastal site and is the first step toward a transposition of the methodology to other Mediterranean sites.