Spatial environmental variation causes populations to adapt to local conditions. Understanding how adaptation occurs at the molecular level is surprisingly rudimentary and it remains a challenge to identify the causative mutations. This research project will study the genetic variation that causes the green seaweed sea lettuce (Ulva) to from large macroalgal blooms (“green tides”). Using the morphologically variable species complex U. compressa/mutabilis, we will generate F2 populations resulting from crosses between compatible but morphologically and physiologically different strains. High-throughput automated phenotyping under high and low nitrogen conditions will allow to quantify growth parameters associated with bloom-formation, followed by SLAF-seq and QTL analysis. In parallel, six natural populations will be sampled at non-eutrophic or bloom-associated sites for phenotyping and whole-genome resequencing. Both approaches will allow us to identify genomic regions associated with bloom-formation. Our major outstanding question is whether adaptations result from genetic trade-offs or from conditionally neutral alleles. Our expertise in developing molecular tools for Ulva allows to functionally characterize regulators of growth and development using mutant studies. This project will link QTL studies of local adaptation with population genomics and reverse genetics to understand why certain genotypes of the green seaweed Ulva thrive in disturbed, nutrient-rich habitats.