Recirculated submarine groundwater discharge dominates nutrient inputs and enhances eutrophication risk in a coastal lagoon

Submarine groundwater discharge (SGD) is a widely recognized pathway for nutrient transport to coastal systems. Prior studies overlook the unique biogeochemical signatures of different SGD pathways, neglecting their differences in biogeochemical transformations and spatiotemporal factors. Here, we present an integrated assessment of nutrient delivery through three SGD pathways (fresh SGD, long‐scale recirculated SGD, and short‐scale porewater exchange), alongside surface water inputs, to evaluate their seasonality and impact on nutrient dynamics in a coastal lagoon. We conducted five field surveys in March, July, and November 2021, July 2024, and March 2025 in the Mar Menor lagoon, an ecosystem facing severe ecological degradation, to analyze nutrient concentrations in stream water, groundwater, and lagoon water. Concentrations in discharging waters were shaped by watershed‐scale land use and local redox‐driven biogeochemical processes. Recirculated SGD (long‐scale and short‐scale), often overlooked, emerged as the dominant source of ammonium (; 100–160 kmol d −1 ), dissolved organic nitrogen (DON; 270–900 kmol d −1 ), dissolved inorganic phosphorus (DIP; 4–30 kmol d −1 ), and dissolved silica (DSi; 200–270 kmol d −1 ) across seasons, while continental discharge (streams and fresh SGD) was the main source of nitrate and nitrite (NO x ; 30–210 kmol d −1 ). Short‐scale recirculation flows controlled seasonal nutrient limitation by delivering additional DIP inputs (24 kmol d −1 ) in summer, increasing eutrophication risk. Identifying preferential transport pathways for each nutrient into coastal systems is crucial for predicting and managing ecosystem vulnerability.