Incorporating historic specimens to reassess <i>Ulva</i> diversity in the Netherlands and Belgium

Abstract While DNA‐assisted identifications have significantly enhanced our understanding of Ulva diversity, they have also widened the gap between historical taxonomic and floristic studies based solely on morphology and contemporary studies utilizing DNA sequences. To bridge this gap, we compared historical assessments of Ulva sensu lato diversity in the Netherlands and Belgium with molecular‐based identifications. By combining DNA barcoding of recent field collections with NGS data from historical herbarium voucher specimens, we confirmed the presence of 17 Ulva species using tufA barcoding, including three unidentified clades. Additionally, we identified Blidingia marginata , B. minima , Monostroma grevillei , Umbraulva dangeardii , and a taxonomic entity that matched reference sequences of both Gayralia oxysperma and Kornmannia leptoderma . Of the 24 species previously reported in the southern North Sea based on published literature, only 13 species were molecularly confirmed, highlighting the substantial discrepancy between traditional and DNA‐based species identifications in this group of algae. Integrating historical herbarium vouchers into our biodiversity assessment revealed that this mismatch was largely due to misinterpretation of names and to a lesser extent changing species concepts. Plastid genome and nuclear ribosomal DNA assemblies of Ulva intestinaloides and U. pseudolinza type material showed that their ITS, rbcL , and tufA sequences were identical or highly similar to reference sequences of U. compressa and U. intestinalis , respectively, indicating that they are synonyms of these species. Finally, we report four new species records for the southern North Sea: Ulva capillata and Umbraulva dangeardii that are native to the Northeast Atlantic, and the non‐indigenous Ulva californica and U. shanxiensis. The latter is a freshwater species that has not been recorded outside China until now. The discovery of unidentified clades and non‐native species in the southern North Sea emphasizes the importance of molecular monitoring in uncovering cryptic diversity and improving our understanding of Ulva biodiversity.