Historical phytoplankton data from Helgoland Roads: Can they be linked to modern time series data?

• Helgoland Roads, initiated in 1962, is one of the most detailed and longest time series in Europe
• This short note describes a historic diatom data set from Helgoland preceeding 1962
• Many species in the historic data are still found in Helgoland. Exceptions are the diatoms Bacteriastrum furcatum and Pseudosolenia calcar-avis.
• Pseudosolenia was present in the Helgoland and other investigated historic data from the beginning of the 20th century but has not been observed since
• Pseudosolenia appears to be a very rare example for the local extinction of a diatom

The Helgoland Roads phytoplankton data set is one of the longest and most detailed phytoplankton time series data sets in the world. The time series has been initiated in 1962 and based on the resulting counts a taxon list of 261 taxa has been assembled, 93 of which are diatoms. However, plankton research has been carried out at the site since the 19th century although many old records were lost during the World War II. Here we describe a small historic data set that was recovered from the Wadden Sea Station Sylt and placed in the AWI's own archive, the Archive for German Polar Research (Archiv für deutsche Polarforschung — AdP). This data set is unique in that it contains species specific diatom data from the period 1906–1908. As the data set itself contained only little direct metadata linking it to the phytoplankton monitoring site Helgoland Roads (‘Helgoland Reede’ in German), further research of the available literature and additional searches of material in the AWI archive were carried out before it could be ascertained that the documents recovered from Sylt really contained data from the Biological Institute on the island of Helgoland. Following this the data in the original data set underwent taxonomic checks to establish that all species were valid species or older synonyms of currently accepted taxon names. The resulting data set was then compared to the Helgoland Roads taxon list and the most current published checklist for Helgoland Roads. Of the 6 pennate and 59 centric diatoms recorded between 1906 and 1908 most are also found in the current taxon lists, only 7 were not found in either the checklist or Helgoland Roads taxon list. However, there are exceptions, most notably the centric diatom Pseudosolenia calcar-avis, which only occurred in the historic records of 1906–1908 and has not been reported from the North Sea in any modern records. While only semi-quantitative, these data are nevertheless an important complement to the Helgoland Roads phytoplankton data set, providing a historic basis for documenting long-term changes in the diatom community at Helgoland Roads.