Biodiversity in transitional waters: steeper ecotone, lower diversity

Abstract

Benthic communities were studied in four transitional water ecosystems in the Mediterranean, located in Albania (Narta), Greece (Logarou) and Italy (GradoMarano 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 variability 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 differences in intensity of natural instability were also reflected by the presence of different sets of species, with the euryhaline species developing large populations and dominating the more enclosed systems, whereas the marine component of the fauna plays the most important role in increasing the level of benthic diversity. IntroductionCoastal 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 enclosure of the lagoon, affects most environmental variables and has a prominent role in the organization of biological communities. Natural instability leads to wide variations 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 elorget & 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 (Reizopoulou & Nicolaidou 2004).In coastal ecosystems with poor seawater inflow, several lagoon specialist species may tolerate extreme environmental conditions, and potentially develop large populations 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.

Assessing the ecological status of Mediterranean coastal lagoons using macroinvertebrates. Comparison of the most commonly used methods

“…Due to the natural dominance of the tolerant and opportunistic species, the transitional waters present strong analogies with the polluted coastal waters, while the biodiversity trends are mainly related to the degree of natural stress (Reizopoulou et al, 2014). In testing the functional diversity in order to discrim-inate the pollution phenomena, further research is required to detect which trait or set of traits can better evaluate the human impact and the relationships of these traits to the environmental constraints of these particular ecosystems.…”

Resistance of polychaete species and trait patterns to simulated species loss in coastal lagoons

“…The surface area of the six lagoons ranges from N100 km 2 (Grado Marano) to b 1 km 2 (Grado Valle di Pesca). Details on lagoonal characteristics and environmental parameters can be found elsewhere (Barbone et al, 2007;Basset et al, 2008;Reizopoulou et al, 2014;Sangiorgio et al, 2008;Sigala et al, 2012).…”

Application of the Benthic Ecosystem Quality Index 2 to benthos in Dutch transitional and coastal waters

“…Similarly as for S, Shannon index (H′) reference values in the Westerschelde polyhaline ecotopes are also higher than for the mesohaline zone, but this effect is numerically less pronounced probably because other environmental factors than salinity also play a role in determining H′ such as enrichment, disturbance frequency (Widdicombe and Austen, 2001), heterogeneity and steepness of gradients (e.g. Reizopoulou et al, 2014). Moreover, Shannon index is particularly sensitive to rare species (Peet, 1974).…”