The artificial creeks in Sant'Erasmo island (Venice lagoon): present status and suggestion for management based on two indicator species

Abstract

Estuaries and coastal lagoons show high levels of habitat heterogeneity and support a large fish production (Elliott & Hemingway, 2002). Hosting a wide variety of biodiversity, the Venice lagoon is the biggest lagoon in the Mediterranean basin, and Sant'Erasmo island is one of the largest of the lagoon. The salt marshes represent one of the main habitats for many fish species of the Venice lagoon, characterized by seagrass meadows, bare sand areas, intertidal flats, sandy and muddy subtidal beds, and tidal marshes. The ecological importance of these marginal habitats is mainly due to the high level of trophic resources available and to the refuge function from predation deriving from a complex morphological structure Land claim, erosion, pollution, aquaculture and relative sea level rise determined the alteration or the complete destruction of these habitats. An important role is played by the artificial creeks, present in many islands of the lagoon, once used for aquaculture and transport means. Nowadays, even though the use has changed in many cases, these artificial sites gained an ecological value as alternative refuge habitats to natural salt marsh creeks.

The Aphanius fasciatus, as an autochthonous species, belongs to the ecosystem of the lagoon of Venice, in particular to the shallow waters and subsequently, also to the artifical creeks of some islands of the lagoon. Considering that many studies have been carried out on the Aphanius, and knowing the chemical and physical characteristics of its habitat, one of the objectives of this work is to determine what is the cause of absence or presence of the Aphanius in Sant'Erasmo creeks (Venice lagoon). As the salt marshes, the artificial creeks offer favorable conditions and protection from predators for the Aphanius, but recently this species in Sant'Erasmo island, started to decrease. Despite both species can survive in a wide range of salinty conditions, the Aphanius shows a more aggressive behavior at higher salinity conditions compared to the Gambusia, thus we expect to find a higher concentration of Aphanius in sea water conditions (Alcaraz, Carles, et al. 2008). Defining the Gambusia (Porte, C., Barceló, D., & Albaigés, J., 1992) and the Aphanius (Messaoudi, Imed, et al 2009) as indicator species and knowing their habitat, then it will be possible to indicate which watercourses have faced a change by observing their presence or absence. Aiming to confirm the previous studies on the habitats and ecological niches of the two species, this study will allow us to identify which creeks are suitable both for the aphanius and gambusia populations, and therefore, suitable for other species with the same ecological preferences. The intrinsic objective of the work, is to give more importance to the potential role of biological reservoir of the artificial creeks of Sant'Erasmo, which could possibly host part of the salt marshes population in view of sea level rise. Therefore, the artificial creeks are supposed to be mostly brackish water.