Data from: Microplastics reduce eelgrass tolerance to heat stress with implications for restoration and blue carbon

We experimentally tested how these stressors, alone and combined, affect the seagrass Zostera marina (eelgrass) using a controlled mesocosm experiment grounded in multiple-stressor and trait-based ecological theory. Plants were grown for 43 days in sediments with or without polyethylene/polypropylene MPs and a simulated MHW, (+5ºC for 15 days) was imposed in the final phase. MPs exposure markedly reduced rhizome elongation (-35%), total root length (-65%), and below-ground biomass, and depleted non-structural carbohydrates (NSC) in leaves and rhizomes (-35% to -40%). Warming alone stimulated leaf growth but further reduced NSC, while the MP × MHW interaction produced the lowest below-ground growth and carbohydrate reserves, consistent with synergistic stress predicted by multiple-stressor theory. MP exposure also reshaped the microbiome enriching putative sulfur-cycling taxa in the rhizosphere and indicating more reducing sediment conditions. With a carbon-balance and holobiont framework, MPs appears to constrain resource supply (oxygen and nutrients) and increase maintenance costs, whereas warming amplifies metabolic demand. The resulting carbon deficit limits below-ground growth, traits that underpin restoration success and blue-carbon function. These findings show the importance to incorporate microplastic monitoring into seagrass management to anticipate cumulative stress under a warming ocean.