Sustainable Management of Invasive Algal Waste (

Composting seaweed biomass reduces environmental impacts while supporting circular-economy strategies in coastal areas, where seaweed removed for recreational management is commonly treated as waste. This approach aligns with regional and EU policies on circular bioeconomy and coastal ecosystem restoration. This study evaluated the effects of Mar Menor seaweed compost applied at 0%, 15% and 35% on lettuce cultivation. Two nitrogen supply levels (100% and 60%) were also used to assess interactive effects on plant growth and nutrient dynamics. The optimal rate of 15% compost enhanced lettuce growth by 25.1% under 100% N irrigation and by 32.2% under 60% N irrigation, indicating that reduced nitrogen availability did not limit biomass accumulation. Indeed, irrigation nitrogen level did not affect total biomass. Compost addition also improved nutrient content and increased phenolic compounds in leaves. When nitrogen was reduced, the combination with compost further boosted phenolic accumulation, by 39.6% with 15% compost and 34.7% with 35%, suggesting a synergistic response. Overall, seaweed compost improves crop performance and nutritional quality while lowering dependence on synthetic fertilisers. Environmentally and economically, it provides coastal municipalities a sustainable option for managing excess seaweed by converting waste into valuable agricultural inputs and mitigating impacts of algal overgrowth.