Abstract. Coastal benthic communities in soft‐bottom deposits of a restricted area were studied by seasonal sampling at nine stations. Faunal assemblages at three different depths were consistently different and exhibited a seasonal pattern of abundance and diversity. Multivariate statistical analyses suggest that each community structure is very similar during the three summer samplings; in autumn and winter many causes of disturbance can disrupt this structure, which is reconstituted the following summer. The stability of benthic communities in this physically variable environment is discussed and a “cyclic” adjustment mechanism of stability is proposed to explain the dynamics of the benthos in this area.

The annual somatic production of Prionospio caspersi Laubier was estimated, between July 1986 and, at a sandy shallow-bottom station in the area facing the Po River Delta, Italy, where this species is one of the most abundant. Primary recruitment occurs in summer, and maturation of gametes the following spring. Body sizes were estimated from the surface area of the anterior end of worms, projected onto a monitor and recorded using a digitizer tablet. Measurements, to the tenth setiger, were correlated with total length, total body surface area and AFDW. The annual production was 8.06 g m -2 yr-1 (AFDW) and the production:biomass (P:B) ratio 4.09. The results are discussed in relation to some physical and chemical features of the environment, stressing differences with the life cycle of other coexisting populations.

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-Experiments with plants grown in filtered and unfiltered open-top chambers have been conducted for a decade at two sites in the Po Valley (Northern Italy). Substantial yield losses were often recorded, showing a varying sensitivity between plants (forage, horticultural and semi-natural plants). The response to ambient air ozone doses seems moreover connected to air temperature and humidity. The need for incorporating selected modifying factors in level-II ozone critical levels is advocated for regions, like the Po Valley, where peculiar climatic conditions and high concentrations of ozone are usually present. ozone / crops / yield / Italy / open-top chambersRésumé -Réponse en relation à la productivité de plantes exposées à l'ozone dans la Vallée du Pô (Italie). Pendant une dizaine d'années on a effectué des essais sur les plantes cultivées en chambres à ciel ouvert avec air filtré et non filtré, installées dans deux postes situés dans la Vallée du Pô (Italie du Nord). On a souvent observé des pertes importantes sur la récolte des plantes exposées à des taux élevés d'ozone (air ambiant non filtré) malgré une diversité entre les espèces utilisées (plantes fourragères, horticoles et demi-naturelles). La réponse aux doses d'ozone troposphérique peut être aussi modifiée par la température et l'humidité de l'air. Il est nécessaire d'incorporer plusieurs facteurs différents de modification dans la formulation des niveaux critiques pour l'ozone (niveau II). Cela est surtout important pour des régions géographiques, telles la Vallée du Pô, où les conditions climatiques particulières et de hautes concentrations d'ozone sont une caractéristique durant l'été. ozone / plantes cultivées / production agricole / Italie / chambres à ciel ouvert

The secondary production of Spisula subtruncata (somatic production) is calculated in the framework of a benthic monitoring study in a coastal area. The temporal trends of density, biomass and growth increments are examined in the three year-classes present. These are compared to certain physical and chemical parameters.