Supratidal foraminifera as ecological indicators in anthropically modified wetlands (Lagoon of Venice, Italy)

Abstract

In transitional environments, the intertidal zones represent a peculiar case characterized by halophile vegetation and by a low diversity benthic community. On these areas just a few particular foraminiferal species, a class of Protoctista secreting a shell called test, can survive for a certain time out of water. They are distributed in well-defined vertical zonations with respect to mean sea level and they correspond to analogous marsh floral zonations. In particular, the Trochammina macrescens Brady+Trochammina inflata (Montagu) association characterizes the salt marsh zone above mean high water level. The potential of these taxa as bioindicators is tested, since their presence–absence–dominance differentiates the subtidal/supratidal environments. Over the last few centuries, various engineering works generated major physical changes in the Venetian Lagoon. These changes affected the natural evolution of the intertidal morphologies, the surface of which is decreasing. In an attempt to reverse this tendency, numerous artificial salt marshes have been constructed and more are under construction. In this study, the Mazzorbo artificial salt marsh, built during the second half of 1999, is considered. On its surface, 16 samples were collected along a transect line in May 2008 to verify the ecological role of this salting within the lagoon ecosystem. The sediment grain size distribution of the salt marsh reflects the dissipative role of the tide and the effect of sediment transport due to the wave and tidal action. However, the presence of only a few Trochammina individuals shows that the foraminiferal fauna did not recognise this morphology as a salt marsh. The lack of Trochammina colonisation can be related to the excessive elevation of the salt marsh surface. This hypothesis is confirmed by the lack of the salt-tolerant plant Spartina. The unsuccessful colonisation by the foraminifera seems to indicate that this artificial salting does not have the natural dynamism of the intertidal morphologies and it may only be classified as land recovery. The supratidal foraminiferal taxa can act as an ecological indicator: through their observation it is possible to verify whether an artificial salt marsh accomplishes its task of functioning as an ecological unit with the community of organisms.