Letizia Sabetta

Species-area patterns of benthic macro-invertebrates in Italian lagoons

The selection of adequate descriptors of the ecological status in aquatic ecosystem is a major requirement for the implementation of monitoring tools. It requires an analysis of the stress-independent sources of variation of potential descriptors, which need to be taken into account in the definition of ecosystem Typology and Classification. Here, we investigate at what extent the surface area of lagoons accounted for species richness of communities and body size abundance components. To this aim, the species–area and body size–area patterns of benthic macro-invertebrates were investigated in Italian lagoons. The analysis was based on a literature survey carried out considering a 30-year period from 1975 to 2004. Overall, 168 papers were selected, reporting taxonomic lists of benthic macro-invertebrates for 26 Italian lagoons, whose surface area ranged from few hectares to some hundreds of square kilometres. The analysis of published taxonomic lists recorded 1,055 taxa, belonging to 13 phyla, 106 orders and 351 families. Measures of standard body size for each of the 1,055 taxa were also obtained from published data as standard individual body length. Significant species–area relationships were observed at each level of taxonomic resolution considered, within dominant phyla (i.e. Arthropoda and Mollusca). Slopes of the power regressions were in the range of 0.11–0.24, increasing consistently with taxonomic resolution. Significant relationships were also observed by comparing the upper limit of the body size spectra to the surface area of the considered lagoons. Maximum body size–area relationships were described by power regressions with slopes in the range of 0.10–0.28. The observed species–area relationships underline the importance of physiographic characteristics of transitional water ecosystems in the identification of transitional water types, as required by the WFD. Moreover, the scaling of average taxonomic richness with lagoon surface, could also represent a standardisation tool for classifying the ecological status of transitional ecosystems. Finally, the patterns of body size area relationships would also have an important application to the field of monitoring transitional ecosystem health.

Colonization of ephemeral detrital patches by vagile macroinvertebrates in a brackish lake: a body size-related process?

The mechanisms regulating the build-up of invertebrate assemblages on ephemeral detritus patches are still poorly understood. Here, the daily colonization of decaying reed leaves by vagile macroinvertebrates was monitored in an brackish lake in Italy. The highly variable abundance patterns of dominant taxa were analysed by spectral and geostatistical techniques to test for nonrandomness and to further determine whether they were related to body size. Comparisons between two contrasting sites allowed an assessment of the generality of our observations. At both sites, the macroinvertebrate assemblage was dominated by three detritivorous taxa, i.e. the isopod Lekanesphaera monodi, the amphipod Microdeutopus gryllotalpa and the polychaete Neanthes caudata. Overall, their abundance patterns were characterised by short-term fluctuations of a nonrandom, autocorrelated nature. In addition, a significant covariation was observed between the average body mass of each taxon and the complexity of the respective abundance pattern, expressed by the fractal dimension D. The covariation was observed at both study sites, notwithstanding the diverging outcomes of bivariate pattern comparisons for similar-sized taxa. Our findings indicate that the size of macroinvertebrates is strongly related to the short-term dynamics of their abundance patterns on reed detritus, suggesting that the interaction between vagile consumers and ephemeral resource patches might be influenced by individual energetics. The implications of size-related constraints for the coexistence of species on decaying detrital patches are discussed.

Influence of Drought and Abiotic Factors on Phragmites australis Leaf Decomposition in the River Pula, Sardinia, Italy

ABSTRACT Temporal and spatial patterns of Phragmites australis leaf decomposition and the relative influence of summer droughts and abiotic features were studied in a Mediterranean-type river basin, the River Pula. Reed processing rates varied among seasons and sites, being on average significantly faster in the cold seasons (i.e., winter-spring) than in the warm seasons (i. e., summer-fall) and in low rather than in high stream order reaches. On the other hand, at sites protected from summer drought events (i.e., wet sites), decay rates were faster in the warm than in the cold season. Along stream order, leaf decay rates were significantly faster at wet sites than at sites suffering from summer droughts. Summer drought events explained 44% of reed decay rate spatial variability, and major water abiotic parameters explained an additional 8%.

Factors Affecting Macroinvertebrate Distribution in a Mediterranean Intermittent Stream

ABSTRACT The relative influence of water regime and other environmental factors on the organization of benthic macroinvertebrates was studied over three years in a small intermittent river basin (River Pula) in Sardinia. The intermittent nature of the led to high spatial and temporal variability of abiotic characteristics. Twenty-eight invertebrate taxa accounted for 90% of the sampled individuals; and of there four (Chironominae, Caenis luctuosa, Diamesinae, Protonemura ichnusae) accounted for more than 50% of the total density. Canonical correspondence analysis showed that 34.6% of the biological variance was accounted for by abiotic factors and that different environmental features, including sediment structure and stream order, had a larger influence on benthic macroinvertebrate distribution and abundance than the intermittent water regime, even though local drought events completely disrupted the macroinvertebrate community.

Phytoplankton size structure and environmental forcing within the euphotic zone in the Southern Adriatic–Ionian Coastal Area

Here, we analysed the variation of size–abundance distributions of marine nano- and micro-phytoplankton guilds in relation to main environmental forcing factors and taxonomic composition. The study was carried out in the Southern Adriatic–Ionian Region and was based on physical, chemical and biological data collected during four cruises at 21 stations on seven transects. Biological data included density, individual cell size and species composition of nano- and micro-phytoplankton guilds and total and size‐fractionated biomass of the entire phytoplankton community. We used canonical correspondence analysis to relate variation in nano- and micro-phytoplankton size–abundance distributions to variation of spatial, abiotic and biotic environmental forcing factors. Results showed that environmental forcing factors explained up to 75% of the size–abundance distribution variation in the study area. Variations in size–abundance distributions of the nano- and micro-phytoplankton guilds were relatively independent of taxonomic composition. Therefore, the obtained results suggested a relevant role of individual body size as an organizing factor of phytoplankton guilds.

Body-Size Patterns

Body size is a highly informative, ‘taxon-free’ trait of individuals, linking their phenotypes to their overall ecological success and general performance. Population and community-level characteristics – mainly related to the individual organism’s perception, use, and transfer of available energy – are body-size dependent, scaling with body size according to well-defined models of variation. The deterministic variations of population and community-level characteristics with individual body size, consistently observed across a range of spatial scales from local guilds and communities to the entire biosphere, are defined as ‘body-size patterns’.

Equilibrium Concept in Phytoplankton Communities

Phytoplankton species are collectively the most widespread and quantitatively relevant primary producers in the aquatic ecosystems. They colonize aquatic habitats as different as freshwater springs and stream branches, lakes, lagoons, coastal marine areas and the open ocean, showing deterministic patterns of biomass variation on temporal and spatial scales. At the same time, biomass density of every single local population is extremely variable and unpredictable in time and species richness is commonly much higher than factors limiting phytoplankton fitness at every time and space. The dichotomy between (1) the balance between phytoplankton as a whole and limiting factors at large spatial and temporal scales and (2) the apparent unbalance and lack of competitive equilibria among species at local scales and fixed times has long been the subject of debate on the ‘equilibrium concept’ in phytoplankton communities.