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

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 NO3was likely derived from atmosphere-aquifer interactions, while DOC:DON ratios reveal that NO2and 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 m 3 s -1 for the entire investigated coastline, determined from simultaneous 224 Ra and 226 Ra 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.

Improving hydrogeological understanding through well-test interpretation by diagnostic plot and modelling: a case study in an alluvial aquifer in France

“…The plain is crossed by the Huveaune River, flowing from North to South and then turning westward downstream to the city of Marseille where it discharges into the Mediterranean Sea. At the regional scale, the hydrology is split in two main outflows: surface water in the Huveaune River and groundwater flow supplying submarine karstic springs to the south of the case study area (Arfib and Charlier 2016;Tamborski et al 2020). At the local scale, the alluvial plain is mainly composed of three kinds of outcropping Quaternary sediments (Fig.…”

RaDeCC Reader: Fast, accurate and automated data processing for Radium Delayed Coincidence Counting systems

“…Open ocean samples (106 raw data files, 19 seawater samples) were collected from 60-100 litres of seawater using MnO2 impregnated fibres, during the along southwest Greenland during the ICY-LAB expedition aboard RRS Discovery in 2017 (Hendry et al, 2019). Coastal surface seawater samples (~0.5 m depth) (40 raw data files, 9 surface samples) and karstic spring-water samples (62 raw data files, 16 samples) were collected offshore of the Calanques of Marseille-Cassis on 27-28 March 2018 aboard the R/V Antédon II, by trace-metal clean submersible pump and scuba-divers respectively (Tamborski et al, 2020).…”

“…The development of Radium Delayed Coincidence Counting (RaDeCC) systems has made radium-based studies in aqueous environments more feasible (Moore and Arnold, 1996). For example, radium isotopes are increasingly used to trace, quantify and advance understanding of many fundamental ocean processes in coastal (Moore, 2000;Tamborski et al, 2020), shelf sea (Hendry et al, 2019), open ocean surface (Charette et al, 2007) and deep water settings (Kipp et al, 2018).…”

Applicability of Landsat 8 Thermal Infrared Sensor to Identify Submarine Groundwater Discharge Springs in the Mediterranean Sea Basin

“…Another aspect that influences SGD thermal plume visualization is the coastal morphology; depending on its characteristics, the seawater residence time in the study zone may allow the formation of a thermal plume. In semi-enclosed areas, such as bays or coves, where the residence time of discharging water in the sea is in the range of one or several days (Tamborski et al, 2020), the formation of thermal plumes from SGD can be easily detected by satellite in comparison with open sea areas, where the seawater residence time is shorter (less than a day), due to the effect of marine or weather factors such as waves, coastal currents or fetch. Groundwater discharge can occur below sea level, either at shallow (< 10 m) or at greater depths, right on the shoreline or a few meters from it (Zektser & Dzhamalov, 2006).…”