Composition of pelagic microbial communities in Mediterranean coastal aquatic ecosystems under extreme drought conditions

Abstract

Mediterranean coastal aquatic ecosystems are heterogeneous systems covering a wide range of hydrological characteristics. Differences in the hydrological pattern determine the nutrient dynamics and microbial community composition of these habitats. We analysed the composition of the microbial community composition and the factors affecting the predominance of autotrophic (A) or heterotrophic (H) microorganisms in 17 coastal water bodies of the Empordà and Baix Ter wetlands (NE Iberian Peninsula) during mid-summer under severe drought conditions. We sampled three types of water bodies with differences in their water residence time: 6 isolated euhaline basins, 8 oligohaline basins and the estuaries of the three rivers that flood into the area. Our aim was to determine if differences exist in the microbial community structure in Mediterranean coastal water bodies with different residence time under conditions of severe drought or, on the other hand, if the lack of flooding during the summer makes isolated, choked and leaky ecosystems more similar. Isolated basins showed higher values than estuaries for temperature, conductivity and the proportion of nutrients in organic forms. Regarding microbial community composition, microphytoplankton (mainly dinoflagellates) dominated in isolated lagoons, while picoplankton dominated in estuaries. We found no significant differences between oligohaline basins and both estuaries and isolated basins for either physical and chemical variables or microbial community composition. Only the proportion between autotrophic and heterotrophic organisms was lower in oligohaline basins than in isolated ones. We conclude that the estuaries of rivers with measurable flow during the summer differ from lagoons long isolated from any surface water input; similarities that we expected to find between leaky and isolated ecosystems under drought conditions arise in only oligohaline waters with intermediate residence times.