Seagrass meadows are key blue carbon ecosystems, sequestering atmospheric CO₂ in their biomass and eventually the sediments below, however their resilience to combined climate stressors remains poorly understood. The MULTI-CYN project responds to questions within the Horizon Europe initiative C-BLUES, and experimentally investigates how warming and ocean acidification — individually and in combination — aCect greenhouse gas emissions and carbon cycling in Cymodocea nodosa meadows, a widespread Mediterranean seagrass resilient to ocean warming. Using flow-through flume tanks under realistic hydrodynamic conditions, intact seagrass communities were exposed to a stepwise gradient of temperature (+2 to +8 °C) and pH (−0.2 to −0.8 units) scenarios. Physico-chemical parameters, biological performance, and carbon/gas fluxes (CO₂, CH₄, net community production, respiration) were measured throughout to characterize stressor-response relationships. Results will be benchmarked against existing data for Posidonia oceanica and Zostera marina, generating species-specific and generalized vulnerability thresholds. Findings will inform biogeochemical model parameterization for C-BLUES, and support evidence-based coastal ecosystem management and climate mitigation policy, while providing applied training for an early career researcher in mesocosm experimental methods.

Aim
The project (MULTI-CYN) investigates how combined warming and ocean acidification aCect greenhouse gas emissions and carbon cycling in Cymodocea nodosa seagrass meadows, using realistic mesocosm/flume conditions at the LOV (Villefranche) facility under EMBRC-IMEV access. It sits within the Horizon Europe project C-BLUES, feeding directly into Deliverables of the project.

Objectives

• Quantify independent and combined eCects of temperature (+2 to +8 °C) and pH (−0.2 to −0.8 units) on C. nodosa physiology, carbon fluxes, and community metabolism, producing response curves.
• Measure a full suite of variables: physico-chemical parameters (T, salinity, pH, DO, alkalinity, nutrients), biological performance (shoot density, growth, C/N), gas/carbon fluxes (NCP, respiration, CO₂/CH₄, DOC), and sediment processes.
• Derive stressor-response curves to parameterize C-BLUES biogeochemical models.
• Contextualize C. nodosa responses against existing data on Posidonia oceanica obtained in the same experimental flumes and Zostera marina generated by the same team to generate species-specific and generalized resilience/vulnerability thresholds.
• Provide a structured training opportunity for the PhD student (Emmanuella Orero Rubio) to independently lead a complex flume experiment, building on a prior 2025 trial at Kristineberg (UGOT).

Expected outputs

• Empirical response functions linking temperature/pH stress to seagrass carbon and GHG dynamics, to be integrated into C-BLUES predictive models (WP1/WP2, Deliverable D1.5).
• At least one peer-reviewed open-access publication.
• Conference presentations (e.g., EGU, ASLO).
• A research brief for coastal managers and policymakers.
• FAIR-compliant open datasets deposited in Zenodo/PANGAEA and the C-BLUES repository.
• Outreach/science communication contributions (social media, public talks).