Submarine karstic springs as a source of nutrients and bioactive trace metals for the oligotrophic Northwest Mediterranean Sea

Joseph Tamborski (1) , Pieter van Beek (1) , Pascal Conan (2) , Mireille Pujo-Pay (2) , Charlène Odobel (2) , Jean-François Ghiglione (2) , Jean-Luc Seidel (3) , Bruno Arfib (4, 5) , Marc Diego-Feliu (6) , Jordi Garcia-Orellana (6) , Armand Szafran (1) , Marc Souhaut (1)

1 LEGOS - Laboratoire d'études en Géophysique et océanographie spatiales
2 LOMIC - Laboratoire d'Océanographie Microbienne
3 HSM - Hydrosciences Montpellier
4 SNO Karst - Service National d'Observation sur le KARST
5 CEREGE - Centre européen de recherche et d'enseignement des géosciences de l'environnement
6 UAB - Universitat Autònoma de Barcelona = Autonomous University of Barcelona = Universidad Autónoma de Barcelona

Groundwater springs in karstified carbonate aquifers are known to transport carbon, nutrients and trace elements to the coastal ocean. The biogeochemical significance of submarine karstic springs and their impact on coastal primary production are often difficult to quantify. We investigated several karstic springs, including the first-order Port-Miou spring, in an urbanized watershed that is also severely impacted by sewage effluent (Calanques of Marseille-Cassis, France). Karstic springs were elevated in major nutrients and bioactive trace metals over Mediterranean seawater, with relatively low concentration ranges. Groundwater NO3- was likely derived from atmosphere-aquifer interactions, while DOC:DON ratios reveal that NO2- and NH4+ was autochthonously produced during mixing between karst groundwater and seawater. Submarine groundwater discharge (SGD) during March 2018 (wet season, baseflow conditions) was 6.7 ± 2.0 m3 s-1 for the entire investigated coastline, determined from simultaneous 224Ra and 226Ra mass balances. The contribution of groundwater PO43-, the major limiting nutrient of the Mediterranean Sea, sustained only 1% of primary production adjacent to sewage outfall, but between 7 and 100% of the local primary production in areas that were not impacted by sewage. Groundwater and seawater Fe:DIN and Fe:DIP ratios suggest that Fe was not a limiting micro-nutrient during the period of study, where bioactive trace metal fluxes were dominated by sewage and atmospheric deposition, although excess Fe from groundwater may locally enhance N fixation. Groundwater solute fluxes may easily vary by a factor of two or more over time because karst aquifers are sensitive to precipitation, as is the case of the regional carbonate karstified aquifer of Port-Miou, highlighting the critical importance of properly characterizing nutrient and trace metal inputs in these coastal environments.

Mots clés: GEOTRACES; Karst; Nutrients; Radium isotopes; Springs; Submarine groundwater discharge; GEOTRACES

Domaines: Océanographie; Géochimie

Joseph Tamborski, Pieter van Beek, Pascal Conan, Mireille Pujo-Pay, Charlène Odobel, et al.. Submarine karstic springs as a source of nutrients and bioactive trace metals for the oligotrophic Northwest Mediterranean Sea. Science of the Total Environment, 2020, 732, pp.139106. ⟨10.1016/j.scitotenv.2020.139106⟩. ⟨hal-02891620v2⟩