The freshwater diatom Didymosphenia geminata (Didymo) is considered an invasive species in Iceland. The main objective of the Icelandic case study within the ESENIAS-TOOLS project Case study 2: Comparative study on the effect of hydrological regime on the distribution of the invasive diatom Didymosphenia geminata in extreme environments (Icelandic rivers and lakes, and Bulgarian high-mountain lakes) is to identify the current distribution of this invasive diatom and compare it with historic observation from the 1980s and 2000s. By assessing changes in the distribution of Didymo and changes in the hydrological regime (water flow/ current/ water level) and water chemistry, interconnections between physical factors and spreading of Didymo will be investigated. The case study focuses on the River Elliðaár, Lake Elliðavatn and the tributaries Suðurá and Hólmsá.

In the framework of the Case study 2, Prof. Dr. Nadja Ognjanova from the Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences (IBER-BAS), visited Iceland between 19 and 25 July 2015. The purpose of the visit was that project leader Asst. Prof. Dr. David Finger and Prof. Ognjanova coordinate and compile a work plan for the Case study 2. During this visit several meetings and discussions were organised in order to come up with a coherent working plan. The meetings were attended by project partners from Reykjavik University (Asst. Prof. Finger and Vaiva Čypaitė), the Institute of Freshwater Fisheries (Dr. Ingi Rúnar Jónsson and Dr. Jón S. Ólafsson) and IBER-BAS (Prof. Ognjanova). Furthermore, a field trip to collect new data on Didymo was carried out to the River Elliðaár, Lake Elliðavatn, and the tributaries Suðurá and Hólmsá (seven locations were sampled). A complementary field trip to the Hengill area, was also organised to visit research sites of the Institute of Freshwater Fisheries. Microscopical investigations of collected samples were conducted at the laboratory of the Institute of Freshwater Fisheries.

Within this task the following activities are planned:

- Collecting available data on the existence of Didymo in the Elliðaár River and tributaries, as well as data on other diatoms/ algae in the rivers;

- Collecting existing chemical and hydrological data;

- Putting the data into database and linking the data;

- Conducting a complementary field trip in order to validate the existence/ abundance of Didymo in Elliðaár River and the tributaries Suðurá and Hólmsá;

- Identifying the difference in abundance in the three streams and identifying factors that may explain this difference;

- Harmonisation of methods with Bulgarian partners.