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Macrobenthic communities along the Magra Estuary (Ligurian Sea, NW Mediterranean) were investigated between 1988 and 1990 using different taxonomic levels to estimate which level balances the loss of information with the time gained in sorting and identification at coarser levels than species. Samples were collected by dredge in four stations (FO, SC, IG and CF) positioned along the estuarine gradient and in one station (MA) located in the marine area in front of the estuary. The data set consists of 17,200 individuals belonging to six phyla, 10 classes, 32 orders, 75 families, 98 genera and 114 species. In the ordination plot from correspondence analysis at the species level four different benthic communities could be recognised: marine (MA), mouth (FO), lower (SC) and higher estuary (IG+CF). Their relative positions reflected the topographic distribution along the marine–estuarine gradient. The four communities remained stable enough over the 3-year period. The interannual differences among station–year groups were much more significant for the community of the mouth (FO), which showed major temporal drifts. At the genus level, similar results were obtained. At the family level, there was no substantial loss of information. At the order level, the pattern changed: the communities were not clearly distinct and did not scatter in the ordination diagram according to the marine–estuarine gradient. At class level, the plot is not readable. At phylum level, correspondence analysis did not give significant results and no ecological interpretation was possible. [Copyright &y& Elsevier]

Abstract: Posidonia oceanica, the most important and abundant seagrass in the Mediterranean Sea, forms large meadows from the sea surface down to 40 m. The depth of the lower limit of the meadows marks the boundary between the infralittoral and the circalittoral zone, and is said to be normally set by light attenuation underwater, while the role of water movement has been little explored. In this paper, the position (i.e. distance from the shoreline and depth) of P. oceanica meadow lower limits along the whole Ligurian coastline (about 350 km) was related to the annual storm wave base. This depth represents the limit of interaction between waves and seafloor and corresponds to L 0/2, where L 0 is the annual offshore wave length. In all meadows, the lower limit has never been found deeper than the annual storm wave base, and its depth (Zc ) showed related to L0 according to the equation Zc  = 0.32∙L 0 + 5.62. In the coastal tracts affected by the least intense waves, the reduction of water movement with depth represents the most important constraint to the seaward development of the meadow, whereas light availability plays a major role in meadows affected by the most intense waves. The present study represents the first attempt at understanding the role of hydrodynamic factors in setting the depth limit of seagrass meadows. If corroborated by future research at other sites, this will have important implications for both basic and applied science, as it would imply rethinking about the relative importance of water movement and light in seagrass depth distribution, and could allow for a better estimate of the extent of meadow regression in anthropized areas. [Copyright &y& Elsevier]