Macrophyte Meadows in Europe’s Largest Coastal Lagoon: Nutrient and Carbon Sequestration

Macrophyte-dominated benthic meadows play a key role in promoting sedimentation, carbon storage, and nutrient sequestration, especially in enclosed systems like coastal lagoons. However, these shallow ecosystems are highly susceptible to climate change and anthropogenic pressures, including nutrient inputs from agricultural runoff and urban discharges. Seagrasses and seaweed help regulate nutrient availability, reducing the risk of eutrophication by limiting nutrients accessible to opportunistic organisms.

In the Mar Menor coastal lagoon (southeastern Spain), recurrent eutrophication events since 2016 have caused severe ecological disruptions, including mass fish mortality. These events are driven by persistent nutrient inputs resulting from intensified agricultural practices. While the allochthonous seaweed Caulerpa prolifera and the native seagrass Cymodocea nodosa have demonstrated significant nutrient-sinking capabilities, the buffering potential of C. nodosa has diminished due to population declines during these episodes.

This study analyzed twelve sediment cores for total organic carbon (TOC %), δ¹³C, total nitrogen (TN %), and δ¹⁵N, with age estimations based on ²¹⁰Pb dating. Carbon sequestration rates, nutrient stocks, and nutrient origins were determined. Results revealed spatial variability in nutrient distribution, with higher organic carbon enrichment from terrestrial sources concentrated in the lagoon's central areas connected to agricultural lands. Nutrient-depth profiles indicated increased nutrient input beginning in the early 20th century.

The seasonal dynamics and potential loss of macrophyte meadows could exacerbate resuspension events, releasing nutrient-enriched sediments and triggering eutrophication, leading to ecological and socio-economic consequences. Understanding the role of macrophyte meadows in nutrient cycling within sediment-focused, enclosed systems is crucial for effectively managing and conserving these impacted habitats.