How Does Seasonal Seedling Outplant Influence Sea-Based Ulva Aquaculture? Controlling Performance and Biochemical Composition of a Temperate Seaweed Crop

Optimizing biomass yield and biochemical composition in sea-based cultivation of Ulva fenestrata requires balancing environmental conditions with cultivation timing. This study examined how outplant and harvest timing influence growth and biochemical traits in a temperate sea-based aquaculture system. Outplant timing had a pronounced effect on biomass yield, while biochemical composition was more strongly influenced by harvest timing and associated environmental factors. Seedlings deployed in autumn (September-November) achieved up to eight-fold higher biomass than those outplanted in late winter or spring (February-March). Despite low temperature and light during early winter, autumn outplants maintained slow but steady growth, indicating physiological resilience and effective nutrient utilization under suboptimal conditions. Biochemical composition - including crude protein, chlorophyll, carotenoids and total fatty acids - peaked during late winter to early spring harvests when ambient nitrate levels were highest. As temperature and light increased in spring, tissue nitrogen and polyunsaturated fatty acid contents declined, reflecting nitrogen limitation and temperature-driven lipid remodelling. This trade-off between maximizing biomass under warmer conditions and maintaining nutritional quality during cooler, nutrient-rich periods highlights the need for strategic cultivation management. We conclude that optimal production of U. fenestrata in temperate waters is achieved by autumn outplanting and harvest before the spring nutrient decline when aimed at food purposes. Continuous environmental monitoring, adaptive harvest timing and strain selection could further enhance both biomass yield and quality, supporting sustainable and high-value Ulva cultivation. HIGHLIGHTS circle Autumn outplanting significantly enhances biomass accumulation in Ulva fenestrata. circle Harvest timing exerts stronger control on biochemical composition than deployment: peak protein, pigment and fatty acid levels coincide with high ambient nitrate while elevated temperature and irradiance reduce tissue nitrogen and PUFA content. circle Optimized production requires autumn seeding and pre-spring nutrient decline harvest.