ORIGINAL ARTICLE Biodiversity in transitional waters: steeper ecotone, lower diversity Sofia Reizopoulou 1 , Nomiki Simboura 1 , Enrico Barbone 2 , Floriana Aleffi 3 , Alberto Basset 2 & Artemis Nicolaidou 4 1 Hellenic Centre for Marine Research, Institute of Oceanography, Anavyssos, Attiki, Greece 2 DiSTeBA, University of Salento, Lecce, Italy 3 Marine Biology Laboratory, Trieste, Italy 4 Department of Zoology Marine Biology, School of Biology, University of Athens, Athens, Greece Keywords Benthos; coastal lagoons; diversity; Mediterranean. Correspondence Sofia Reizopoulou, Hellenic Centre for Marine Research, Institute of Oceanography, PO Box 712, 190 13 Anavyssos, Attiki, Greece. E-mail: sreiz@hcmr.gr Accepted: 7 September 2013 doi: 10.1111/maec.12121 Abstract Benthic communities were studied in four transitional water ecosystems in the Mediterranean, located in Albania (Narta), Greece (Logarou) and Italy (Grado- Marano and Margherita di Savoia), with different degrees of salinity ranges, in order to investigate biodiversity trends across a scale of environmental stress. The intensity of natural stress in the transition zones, from the marine-based to the land-based influence, varied from gradual to sharp. The spatial variabil- ity in the physical environment had a stronger effect on species richness than did the temporal fluctuations. The sharper the spatial variations of salinity, the lower the number of species and the diversity level of the system. The differ- ences in intensity of natural instability were also reflected by the presence of different sets of species, with the euryhaline species developing large popula- tions and dominating the more enclosed systems, whereas the marine compo- nent of the fauna plays the most important role in increasing the level of benthic diversity. Introduction Coastal lagoons are ecotones between marine and terrestrial environments, receiving variable amounts of fresh water. Mediterranean lagoons are usually shallow water bodies and, due to their geo-morphological and hydrological characteristics, environmental conditions in them undergo frequent fluctuations on a spatial and seasonal basis. Seawater renewal, depending on the degree of enclo- sure of the lagoon, affects most environmental variables and has a prominent role in the organization of biologi- cal communities. Natural instability leads to wide varia- tions in species diversity. The recurring pattern of species richness, decreasing from the marine regions to the inner parts of the lagoon, has been widely documented (Gu e- lorget & Perthuisot 1983; Lardicci et al. 1993; Koutsoubas et al. 2000; Baza € ıri et al. 2003; Reizopoulou & Nicolaidou 2004). A strong relationship is documented between diversity and confinement (sensu Gu elorget & Perthuisot 1992), as environmental instability increases in relation to the degree of isolation from the marine influence (Reizo- poulou & Nicolaidou 2004). In coastal ecosystems with poor seawater inflow, several lagoon specialist species may tolerate extreme environ- mental conditions, and potentially develop large popula- tions in a wide range of salinity gradients. Bamber et al. (1991) stated that lagoon specialists are better adapted to the environmental variability, most likely afforded by a degree of genetic plasticity; however, in salinity extremes a drop of species richness is expected (Cognetti 1992). It is essential to investigate and compare spatial and temporal patterns of biota in coastal lagoons with various degrees of natural stress, in order to distinguish and classify the principal factors structuring communities, the role of critical extremes, and the degree of instability as a predictor of species distribution. Marine Ecology (2013) 1–7 ª 2013 Blackwell Verlag GmbH 1 Marine Ecology. ISSN 0173-9565