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

Joseph Tamborski; Pieter van Beek; Pascal Conan; Mireille Pujo-Pay; Charlene Odobel; Jean-François Ghiglione; Jean-Luc Seidel; Bruno Arfib; Marc Diego-Feliu; Jordi Garcia-Orellana; Armand Szafran; Marc Souhaut

doi:10.1016/j.scitotenv.2020.139106

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

Sci Total Environ. 2020 Aug 25:732:139106. doi: 10.1016/j.scitotenv.2020.139106. Epub 2020 May 5.

Affiliations

¹ LEGOS, Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, Université de Toulouse, CNES, CNRS, IRD, UPS, Observatoire Midi Pyrénées, 14 Ave Edouard Belin, 31400 Toulouse, France. Electronic address: jtamborski@whoi.edu.
² LEGOS, Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, Université de Toulouse, CNES, CNRS, IRD, UPS, Observatoire Midi Pyrénées, 14 Ave Edouard Belin, 31400 Toulouse, France.
³ LOMIC, Laboratoire d'Océanographie Microbienne, Observatoire Océanologique, Sorbonne Université, CNRS, UPMC Univ Paris 06, UMR7621, 66650 Banyuls/Mer, France.
⁴ HydroSciences Montpellier, UMR 5569 - UM2 - CNRS - IRD - UM1 Place Eugène Bataillon -CC MSE, 34095 Montpellier Cedex 5, France.
⁵ Aix Marseille Université, CNRS, IRD, INRAE, Coll France, CEREGE, Aix-en-Provence, France.
⁶ Institut de Ciència i Tecnologia Ambientals (ICTA-UAB), Universitat Autònoma de Barcelona, Bellaterra, Catalunya, Spain.
⁷ Institut de Ciència i Tecnologia Ambientals (ICTA-UAB), Universitat Autònoma de Barcelona, Bellaterra, Catalunya, Spain; Department de Física, Universitat Autònoma de Barcelona, Bellaterra, Catalunya, Spain.

PMID: 32422479
DOI: 10.1016/j.scitotenv.2020.139106

Abstract

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 PO₄^3-, 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.

Keywords: GEOTRACES; Karst; Nutrients; Radium isotopes; Springs; Submarine groundwater discharge.