WFD and eutrophication assessment: the role of nitrogen as a driving nutrient in shaping phytoplankton assemblages in 13 Italian water bodies

This research was carried out in the frame of the LIFE project INHABIT (Local hydro-morphology, habitat and RBMPs: new measures to improve ecological quality in South European rivers and lakes). A key action of the project was finalized to detect the relationship between nutrients on the ecological quality of water bodies and to identify management options to reduce their effects on aquatic ecosystems to be included in RBMPs, focusing, in particular, on nitrogen. The outcome of the project will serve as a basis for the implementation of Water Framework Directive river basis management plans in Italy and, possibly, in Europe. Analyses were carried out on data from 13 lakes, located in two Italian Regions, Piedmont (North-western Italy, 7 lakes; 5 natural and 2 reservoirs) and Sardinia Island (6 lakes; 1 natural, 5 reservoirs). Seasonal samples were collected in each lake, from 3 to 6, depending on environmental characteristics. The following variables were considered: water temperature, conductivity, alkalinity, pH, dissolved oxygen, SRP, TP, N-NH4, N-NO3, TN, RSi, Secchi depth and chlorophyll. Species/environment relationships were explored using multivariate statistical techniques (CCA, RDA and Generalised Additive Models). Analysis was carried out on 23 phytoplankton orders and 51 genera/species. The results pointed out an increase of cyanobacteria (Nostocales and Oscillatoriales) with the increasing nitrogen availability. Moreover, a positive response of some chlorophytes species was also observed. On the other side, diatoms declined when nitrogen concentration increased. Our findings, confirming some experimental results previously obtained, emphasize the need for a further attention towards the impact of increasing nitrogen, in particular from atmospheric inputs, when restoration plans against eutrophication are designed.