LAUSR

Atanasovsko Lake is a coastal lagoon used for salt extraction for many years. Its salinity varies within a wide range (from 6.3 to > 72‰). Despite these extreme conditions, phytoplankton blooms are often observed. Therefore, our aim was to study the key abiotic drivers and ecological optimum of the picoplankton in a 5-year period (2013-2017). The taxonomic composition, long-term dynamics and abiotic drivers of the autotrophic picoplankton (APP) has been investigated in North Saltern of the coastal lagoon in Atanasovsko Lake. The APP species were identified by using a polyphasic approach based on molecular and phylogenetic analyses of 16S/18S rDNA in combination with cytomorphological and ecological characterization. Picochlorum oklahomense (Trebouxiophyceae, Chlorophyta) and marine Synechococcus sp. (Synechococcales, Cyanobacteria) were the dominant genotypes within the APP. The relative biovolume of the APP to the total phytoplankton biovolume was strongly variable (from 36 to 99%) as the salinity and temperature were found to be the main abiotic drivers of its dynamics. Salinity between 30-59‰ and mid-summer temperature above 24°С provide an ecological optimum for the APP development at a relative biovolume of 93%. This is the first record of Picochlorum оklahomense/Synechococcus sp. assemblage within the APP composition of a European coastal lagoon.