摘要：1 - Transitional waters are ecotones between freshwater, marine and terrestrial ecosystems and include a number of different ecosystem types, one of which is constituted by lagoons. 2 - Lagoons are important patches in the coastal landscapes, which constitute for their ecological relevance a priority habitat (1150) in the EC Nature 2000 Directive. 3 - In the Northern part of Scotland and in the Scottish islands, lagoons are typical coastal ecosystems, while estuaries dominate the Southern Scottish coast. 4 - In this study, we investigate the phytoplankton communities in six Scottish lagoons in Orkney Islands (Kirwall, Ouse, Oyce of Isbister, Point of Backaquoy, Skaith and Loch of Stenness), describing their taxonomic composition and abundance. 5 - The taxonomic list consists of 9 phyla and 16 classes, including 160 taxa, which are evenly distributed among classes. Generally, the higher taxonomical abundance in the studied lagoons is reached by Bacillariophyceae, Chlorophyceae, Coscinodiscophyceae and Dinophyceae. The dominant groups among all the systems in terms of number of cells are: small undetermined phytoplankton and phytoflagellates, which are numerically abundant in all the six Scottish lagoons, Bacillariophyceae, Chlorophyceae, Coscinodiscophyceae, Cyanophyceae and Fragilariophyceae.
