Groundwater-driven nutrient inputs to coastal lagoons: The relevance of lagoon water recirculation as a conveyor of dissolved nutrients

Abstract

Evaluating the sources of nutrient inputs to coastal lagoons is required to understand the functioning of these ecosystems and their vulnerability to eutrophication. Whereas terrestrial groundwater processes are increasingly recognized as relevant sources of nutrients to coastal lagoons, there are still limited studies evaluating separately nutrient fluxes driven by terrestrial groundwater discharge and lagoon water recirculation through sediments. In this study, we assess the relative significance of these sources in conveying dissolved inorganic nutrients (NO, NH and PO) to a coastal lagoon (La Palme lagoon; France, Mediterranean Sea) using concurrent water and radon mass balances. The recirculation of lagoon water through sediments represents a source of NH (1900-5500 mol d) and PO (22-71 mol d), but acts as a sink of NO. Estimated karstic groundwater-driven inputs of NO, NH and PO to the lagoon are on the order of 200-1200, 1-12 and 1.5-8.7 mol d, respectively. A comparison between the main nutrient sources to the lagoon (karstic groundwater, recirculation, diffusion from sediments, inputs from a sewage treatment plant and atmospheric deposition) reveals that the recirculation of lagoon water through sediments is the main source of both dissolved inorganic nitrogen (DIN) and phosphorous (DIP) to La Palme lagoon. These results are in contrast with several studies conducted in systems influenced by terrestrial groundwater inputs, where groundwater is often assumed to be the main pathway for dissolved inorganic nutrient loads. This work highlights the important role of lagoon water recirculation through permeable sediments as a major conveyor of dissolved nutrients to coastal lagoons and, thus, the need for a sound understanding of the recirculation-driven nutrient fluxes and their ecological implications to sustainably manage lagoonal ecosystems.

Enhanced Growth Rates of the Mediterranean Mussel in a Coastal Lagoon Driven by Groundwater Inflow

“…Salses-Leucate lagoon is located on the southwestern French Mediterranean coast. The region experiences rainfall during fall and spring (500 mm per year) with little rain during summer, and is characterized by strong northwesterly winds, regularly exceeding 10 m s −1 , which play an important role in the hydrodynamics of the lagoon (e.g., Stieglitz et al, 2013;Rodellas et al, 2018).…”

“…For instance, southeasterly winds favor the input of seawater in the lagoon while northwesterly winds reduce its input. Furthermore, wind-driven circulation of lagoon water through sediments is recognized as an important source of nutrient in coastal lagoons (Rodellas et al, 2018). This "winddriven" nutrient supply may increase phytoplankton abundance, which in turn may control mussel growth, albeit with a small temporal lag governed by primary production timescales.…”

Rare earth elements in oyster shells: provenance discrimination and potential vital effects

“…The average water depth is 1.7 m, and the hydrology is a balance between the entrance of marine waters from the Mediterranean Sea, a supply of groundwater discharge from two main karstic springs with flows of 3 × 10 5 and 2 × 10 5 m 3 d −1 , respectively (Fleury et al, 2007), and rainfall of approximately 500 mm yr −1 restricted to the fall and spring periods. The superficial watershed covers 162 km 2 , but the total area including the karstic waters is not yet known accurately, but it is likely extended to 60 km inland far from the shore (Salvayre, 1989;Ladagnous and Le Bec, 1997), with karstic waters penetrating Jurassic and Cretaceaous limestone and dolomite. While the tidal range is restricted, the seawater level changes in the lagoon are controlled by strong northwesterly winds regularly exceeding 10 m s −1 (Rodellas et al, 2018).…”

“…The superficial watershed covers 162 km 2 , but the total area including the karstic waters is not yet known accurately, but it is likely extended to 60 km inland far from the shore (Salvayre, 1989;Ladagnous and Le Bec, 1997), with karstic waters penetrating Jurassic and Cretaceaous limestone and dolomite. While the tidal range is restricted, the seawater level changes in the lagoon are controlled by strong northwesterly winds regularly exceeding 10 m s −1 (Rodellas et al, 2018).…”

Temporal variations in porewater fluxes to a coastal lagoon driven by wind waves and changes in lagoon water depths

“…This study is aimed at characterizing porewater fluxes in a shallow coastal lagoon (La Palme Lagoon, France), where the circulation of significant volumes of surface water through sediments have been previously documented (Cook et al, 2018a;Rodellas et al, 2018;Stieglitz et al, 2013;Tamborski et al, 2018). These previous studies have estimated the average magnitude of porewater fluxes to the lagoon, but they provided little insight into their temporal variations and the mechanisms driving these fluxes.…”