Green gold rush in the Baltic Sea: Investigating sea lettuce's performance by distribution mapping of valuable compounds in a fluctuating environment.

The growing prominence of Ulva in aquaculture is driven by its adaptability, rapid growth, nutritional advantages, and diverse biochemical composition. These green macroalgae have gained increasing attention for cultivation in lower salinity areas, expanding aquaculture beyond traditional euhaline environments. Our study investigated the distribution of economically valuable compounds in the most prominent Ulva crop species of the wider Baltic Sea region. We included 249 populations across the full Atlantic-Baltic Sea transect (>3000 km) and took into account prevailing fluctuating abiotic factors like salinity and nutrient regimes during the vegetation peak season. We revealed an overall trend of increasing crude protein content with increasing salinity in species with occurrence along the whole Atlantic-Baltic Sea transect (U. intestinalis: slope = 0.29; U. linza: slope = 0.09) and those only present in higher salinity regions (U. compressa: slope = 0.36; U. fenestrata: slope = 0.18) and confirmed an increase of pigments with increasing nitrogen tissue levels for most species (U. compressa: r = 0.43; U. fenestrata: r = 0.01; U. intestinalis: r = 0.13; U. linza: r = 0.21). With this mapping of the resource availability of valuable compounds in natural Ulva biomass of the Baltic Sea region, we have contributed to the understanding of the potential of Ulva species as key players in sustainable aquaculture practices evolving in the Baltic Sea region. Further, we have underscored the necessity of habitat-dependent crop selection and the importance of horticulture methodology for establishing Ulva as a viable future crop in the wider Baltic Sea.