THE DISTRIBUTION OF POLYCHAETES ALONG ENVIRONMENTAL GRADIENTS - AN EXAMPLE FROM THE ORBETELLO LAGOON, ITALY

Biological zonation in brackish habitats is usually related to the gra dients of several factors, of which salinity is often taken as the mos t important. In contrast, recent papers, mainly on Mediterranean lagoo ns, have emphasized the role of other factors such as water movement a nd trophic status. In this paper, the distribution of polychaetes in t he Orbetello lagoon (a polyhyperhaline pond on the northern Tyrrhenian coast) is compared with the gradients of a number of environmental pa rameters. Polychaete distribution turned out to be under the control o f a variety of highly interrelated factors. The study of biotic zonati on as a function of a single driving factor is thus overly simplistic even when dealing with 'simple' environments, as lagoons are often def ined. Several factors and their synergistic effects must always be tak en into account.