Daniele Curiel

Distribution of introduced Japanese macroalgae Undaria pinnatifida, Sargassum muticum (Phaeophyta) and Antithamnion Pectinatum (Rhodophyta) in the Lagoon of Venice

The present distribution of invasive seaweeds Undaria pinnatifida (Phaeophyta, Laminariales) and Sargassum muticum (Phaeophyta, Fucales) in the Lagoon of Venice is updated in this work. The distribution of Antithamnion pectinatum (Rhodophyta, Ceramiales) is presented for the first time. All these species have quickly colonized the hard substrata competing with indigenous species. Competition acts differently in the two species, U. pinnatifida competing for the substratum and S. muticum for light. On tidal flats composed of silt and sand, the growth of two brown seaweeds is restricted owing to the lack of a solid substrata. While Undaria pinnatifida and Sargassum muticum have colonized only some areas of the lagoon in a short vertical range (from 0 to -3 m), Antithamnion pectinatum is present on the three portmouths, along the inner shores of the city of Venice, and in the lagoon islands with extensive vertical distribution (from +0.2 to -8 m).

The introduced alga Undaria pinnatifida (Laminariales, Alariaceae) in the lagoon of Venice

Since its appearance in 1992 in the lagoon of Venice, the brown algae Undaria pinnatifida (kelp) has gradually expanded along the banks of canals both at Chioggia and Venice, becoming the dominant species in the local algal community chiefly from February to July. In Chioggia Island, where another brown seaweed (Sargassum muticum) is present since 1992, the spreading of Undaria reaches a plateau. In Venice Island, instead, the colonisation process is still in progress. During 1999, the kelp has colonised the main canals (e.g. The Grand Canal) and, subsequently, the small inner ones. In order to evaluate the dynamics of substrate re-colonisation by Undaria, two mechanical eradications were carried out during (March) and after the fertile period (July) of the algae. Such experimental manipulations provided evidence of the fast re-colonisation potential of the algae mainly attributable to its efficient reproductive system. Eradication made during the fertile period, in fact, permitted the kelp development during the following year, while re-colonisation has started 2 years later where eradication was performed after the reproductive period. A significant decrease in the surface covered by other species has been observed both in shallow (Ulva rigida, Enteromorpha spp., Antithamnion pectinatum, Chondracanthus acicularis) and deeper areas (Rhodymenia ardissonei) during the period of maximum development of U. pinnatifida. On the basis of the results of this study, the following conclusions can be drawn: (1) the alga U. pinnatifida is continuously expanding in lagoon environments of Venice; (2) in order to limit its spreading, mechanical eradications would be done on a large spatial scale and before the zoospores release; (3) there is suggestive evidence of competition between Undaria and the remaining indigenous algae.

Seasonal pattern of Cymodocea nodosa biomass and production in the lagoon of Venice

Abstract Primary production and the biomass cycle of a shallow Cymodocea nodosa (Ucria) Ascherson meadow in the lagoon of Venice (Italy-Northern Adriatic) were measured from February 1993 to February 1994. Leaf growth was estimated by the Zieman method; rhizome production was estimated by marking rhizome terminals. All data showed a clear seasonality. In general, maximum growth was reached in July. Leaf production (a maximum of 12.3 g dry weight m−2 day−1 in July) and leaf biomass (a maximum of 810 g dry weight m−2) were much higher than in other Mediterranean localities. Cymodocea nodosa contributes significantly in stabilizing lagoon bottom sediments. The percentage of the belowground fraction of total biomass varied from 85% in winter to 58% in summer, and maximum shoot density was 2302 shoots m−2.

Sexual reproduction of Zostera noltii Hornemann in the lagoon of Venice (Italy, north Adriatic)

Abstract Flowering of Zostera noltii Hornemann in Venice lagoon (Italy, north Adriatic) has been observed in 1993 and 1994, from May to October. Measurements at three stations at different depth included vegetative and generative shoot density, number of spathes per generative shoot, length of spathes, numbers of male and female flowers and seed size.

First Report of the Genus Sorocarpus Pringsheim (Fucophyceae, Ectocarpaceae) in the Mediterranean

Abstract The genus Sorocarpus (Fucophyceae, Ectocarpaceae) is reported for the first time in the Mediterranean. The genus is characterized by the clustered plurilocular sporangia, often several on a cell, true phaeophycean hairs and several disc-shaped chloroplasts in each cell. The alga was only collected at one site in the city of Venice, growing on the hard substrata, close to the lower limit of the midlittoral zone (−0.4 m). It appears difficult to assign the collected specimens to any well defined species, owing to the lack of definite morphological characteristics.

Fuzziness and Heterogeneity of Benthic Metacommunities in a Complex Transitional System

We propose an extension to the metacommunity (MC) concept and a novel operational methodology that has the potential to refine the analysis of MC structure at different hierarchical levels. We show that assemblages of species can also be seen as assemblages of abstract subregional habitat-related metacommunities (habMCs). This intrinsically fuzzy concept recognizes the existence of habMCs that are typically associated with given habitats, while allowing for the mixing and superposition of different habMCs in all sites and for boundaries among subregions that are neither spatially sharp nor temporally constant. The combination of fuzzy clustering and direct gradient analysis permits us to 1) objectively identify the number of habMCs that are present in a region as well as their spatial distributions and relative weights at different sites; 2) associate different subregions with different biological communities; and 3) quantitatively assess the affinities between habMCs and physical, morphological, biogeochemical, and environmental properties, thereby enabling an analysis of the roles and relative importance of various environmental parameters in shaping the spatial structure of a metacommunity. This concept and methodology offer the possibility of integrating the continuum and community unit concepts and of developing the concept of a habMC ecological niche. This approach also facilitates the practical application of the MC concept, which are not currently in common use. Applying these methods to macrophytobenthic and macrozoobenthic hard-substrate assemblages in the Venetian Lagoon, we identified a hierarchical organization of macrobenthic communities that associated different habMCs with different habitats. Our results demonstrate that different reference terms should be applied to different subregions to assess the ecological status of a waterbody and show that a combination of several environmental parameters describes the spatial heterogeneity of benthic communities much better than any single property can. Our results also emphasize the importance of considering heterogeneity and fuzziness when working in natural systems.

Species composition and spatial variability of macroalgal assemblages on biogenic reefs in the northern Adriatic Sea

Abstract We sampled macroalgal assemblages on 37 rocky outcrops in the northern Adriatic over the last 2 decades by SCUBA. Macroalgal assemblages were rich (173 taxa), but there was high variability in the number of species and coverage. The morphology of the outcrops, the distance from the coast and the depth were identified as the main factors accounting for this variability. Both the mean total algal coverage (14.8%) and encrusting layer coverage (8.0%) were low when compared to typical Mediterranean coralligenous habitats. Four main groups of algal assemblages were distinguished on the basis of their species composition and coverage. Groups 1 and 3 were located inshore off the Lido inlet and south of it, and had, respectively, low or intermediate total algal cover. Group 2 included outcrops situated offshore from the Venice lagoon along with all outcrops off the Grado-Marano lagoon; these had the highest total algal cover. Finally, group 4 included assemblages comprising algae that were widespread on outcrops in the inshore habitats of the Venice lagoon, between Malamocco and Chioggia inlets. Offshore outcrops subject to low turbidity and eutrophication levels had several characteristic Mediterranean coralligenous taxa.

FLOWERING OF ZOSTERA MARINA IN THE LAGOON OF VENICE (NORTH ADRIATIC, ITALY)

The flowering of Zostera marina was studied in a shallow meadow in the Venice lagoon (North Adriatic Sea) in spring and summer 1995. Vegetative and fertile shoot density and size were recorded during the flowering period. Spathe maturation was divided in six phases according to different stages of anthesis. New spathes were first observed in February and anther and pistil maturation proceeded until March. The beginning of anthesis, with styles erecting, occurred in April while fruits appeared in May and June. A statistically significant difference between fertile and vegetative shoot height was found for all months. The start of flowering (February—March) appears to be early in the Venice lagoon in comparison to Atlantic European sites, where it starts from June onward. A six-month long flowering period is one of the longest observed. The estimated potential seed production seems limited if compared with the values relative to North European and North American coasts.