Benthic macrofauna changes in areas of Venice lagoon populated by seagrasses or seaweeds

Two areas of the Venice lagoon populated by seagrasses (three stations cove red by Cymodocea nodosa (Ucria) Asherson, Zostera marina Linnaeus, Zostera noltii Hornemann) or seaweeds (two stations: one covered by Ulva rigida C. Agardh and another at present without seaweed biomass) were monitored by me ans of six surveys over a year in order to study macrofaunal composition an d seasonal changes. The seagrass stations showed a mean species richness (2 8-30 S m(-2)), individual abundance (1854-4018 N m(-2)) and biomass (22.3-3 7.7 g m(-2) ash-free-dry-weight, AFDW) ca. 3-8 times higher than those popu lated by seaweeds (10-15 S m(-1), 494-1395 N m(-2) and 5.6-13.7 g m(-2) AFD W). Differences among seagrass or seaweed stations were much lower. The Ulv a-dominated station showed a macrofauna completely different both from the other stations and the communities recorded ca. 30 years ago, before the pr olific growth of Ulva. In this station, frequent biomass decompositions and anoxic crises created critical conditions for life favouring organisms wit h reduced life cycles, younger individuals and the epifaunal species instea d of the infaunal ones. In particular, Ulva grazers and scrapers such as Ga mmarus aequicauda Stock and Gibbula adriatica Philippi were found to be by far the most abundant species, whereas the taxa characteristic of the assoc iations found in the past, in the presence of seagrasses or seaweeds and ty pical of low eutrophicated environments, appear strongly reduced. Marked di fferences in the macrophyte dominance and in the bio-physico-chemical varia bles which characterise the main environmental conditions of the Venice lag oon support the different distribution and composition of macrofaunal commu nities. Seaweed stations appear mainly governed by the seasonal cycles of t hese un-rooted macrophytes which, by alternating periods of production and decomposition, are responsible for the drastic reduction of macrofauna biod iversity and biomass. Conversely, seagrass stations exhibit a better oxidis ation of the environment and show conditions more favourable for macrofauna colonisation, especially in the presence, of macrophytes which are charact erised by very well developed below-ground systems such as Cymodocea nodosa . (C) 2001 Elsevier Science Ltd. All rights reserved.