Bacterial diversity and hydrography of Etoliko, an anoxic semi-enclosed coastal basin in Western Greece

Etoliko, an anoxic semi-enclosed basin, is part of a complex wetland in Western Greece extremely rich in biodiversity. It covers an area of 1,700 ha with an atypical orientation that has been formed tectonically. In order to identify the main factors influencing the bacterial profile at the Etoliko basin, 48 samples were collected, representing seasonal variation at four sampling stations. Physico-chemical analysis of the samples indicates the presence of three layers in the Etoliko basin: (1) low-density surface layer, (2) a layer with a steep density gradient, and (3) dense water below a depth of 20 m. A permanent halocline, whose thickness is varying seasonally, has been identified in the Etoliko basin water column, while the spatiotemporal salinity distribution was highly affected by the basin's interaction with the nearby Messolonghi lagoon. The anoxic zone extends from 20 m below the surface to the bottom of the Etoliko basin in summer, while the bottom layer was hypoxic during winter. Bacterial populations were analyzed by Automated Ribosomal Intergenic Spacer Analysis (ARISA). Bacterial richness and diversity were calculated and compared across samples. Hierarchical analysis showed that ARISA clustered the surface water samples according to seasonal variation, while sediment and near-to-bottom water samples appear to be stable and to cluster together. Non-metric multidimensional scaling (MDS) indicates that bacterial composition depends on dissolved oxygen and salinity. Increase in salinity of the ecosystem leads to a significant reduction of the microbial diversity.