The macrophyte vegetation of running waters in the North-East of Italy: a study of the influence of morphological variables and chemical parameters on the aquatic plant community

Abstract

The WFD requires the assessment of the running waters based on various biotic elements, one of them being macrophytes. In Italy there is no official bioindication method based on aquatic vegetation and the data about it are still few. The present work focuses on the relationships between macrophyte biocenosis and environmental variables of the river ecosystem, in order to understand which are the most important factors, in determining the presence and kind of aquatic plant community. The detected variables are then used to characterize and describe different river types, with reference to their aquatic community. Particular attention is devoted to the relation between plant species and nutrient concentration in water that is the base of many trophic macrophyte indexes.

The study was conducted on 54 sites localised along different water courses in the North-East of Italy (Trentino and Veneto). The macrophyte vegetation has been mapped according to Kohler’s method (Kohler, 1978). We mapped not only the vascular plants, but also the bryophytes and the filamentous algae. The main river characters, like flow velocity, river width and depth, substrate composition, degree of shade and many others, were surveyed according to a field data sheet, mainly in a half-quantitative way. The chemical analyses about nutrient concentrations in water were acquired for every river site.

Through the application of cluster analyses based on different methods and similarity indexes, we established that the assessed sites can be grouped according to their macrophyte vegetation and this division largely corresponds to the one obtained through the clustering of sites, on the basis of river morphological and hydrological variables. The macrophyte community is therefore mainly conditioned by some important variables, like flow speed and flow kind, substrate granule dimension, river width and depth and altitude of the site.

We described the composition of the community not only with the calculation of the Shannon-Weaver diversity index and the Evenness, but also through the percentage abundance and the taxa number of different components, such as hydrophytes, helophytes, amphiphytes, bryophytes and filamentous algae. Then we looked for a correlation between this metrics and the variables of the sites and the results confirmed what previously obtained. Moreover we found that nitrate concentration and hardness are the most important water chemical parameters in determining the composition and abundance of the macrophyte biocenosis. All these result were further validated by two different kinds of matching-two table analyses, procrustean rotation and co-inertia analysis, applied on macrophyte species array and on site variable array. At the same time, these analyses proved the relatively little importance of water nutrient concentrations (phosphorus and ammonium) in determining the aquatic plant community, for the assessed environments. This fact seriously questions the application of trophic indexes on running waters, at least for the typologies that we have studied.

The calculation of correlation coefficients between species presence and abundance and trophic and saprobic indicators in water (phosphorus, nitrogen and BOD5) showed that only few among the recorded taxa have indicator value of trophic conditions. The IBMR (Indice Biologique Macrophytique en Rivière; AFNOR, 2003) has been applied to our dataset, since it will be the official macrophyte method for Italy, according to the decision of the Ministry of Environment. The French index resulted to be quite good in classifying the river sites, for what concerns the trophic level, but in some cases it gave false outputs.

Finally we described 9 different river types, into which the running water environments of the studied area can be classified. Every type is characterized by a certain kind of macrophyte community, in terms of coverage and biological forms, while the species composition can vary in dependence on site specific conditions, one of them being trophy. From the dataset it is evident that there are no real reference sites inside the considered area and this has prevented us from establishing the reference species composition of the macrophyte community for each type. Anyway the future step for the setting of an Italian macrophyte ecological method has necessarily to be that of finding at least the best available sites for each fluvial type in order to describe the “reference” vegetation.