Lake Maggiore (N. Italy) trophic history: fossil diatom, plant pigments, and chironomids, and comparison with long-term limnological data

Abstract

The availability of long-term series of chemical and biological data and the eutrophication/oligotrophication history of Lake Maggiore allows an attempt to correlate the registered changes with sedimentary records in several sediment cores. Documentary and palaeolimnological data were used to calibrate two important suites of sedimentary indicators of phytoplankton, diatoms and algal pigments. Diatom assemblages in the sediment cores precisely reflect the pelagic diatom development for approximately the last century. Prediction of total algal biomass from the profile of the ubiquitous β-carotene and some algal groups in certain period of lake development (e.g. diatoms, Cryptophyta, cyanobacteria) was good, whereas the comparison of taxa-specific carotenoids and algal biomass standing stocks (as cell biovolume) in some case revealed poor correspondence. Selective carotenoid losses, taxa production, and mechanisms controlling pigment sedimentation are factors that biased the comparison. However, pigment concentrations and algal biovolumes are different units but equally valid. The use of fossil pigments complements other studies and provides more detailed information on algal development. A sub-fossil chironomid profile agrees well with the general trophic reconstruction as inferred from the pigment and diatom data, adding more details on changes in littoral substratum, water-level fluctuation and flood events. Models to infer primary productivity and total phosphorus concentration in lake water from sedimentary pigments and diatom assemblages are tested: in the case of the TP reconstruction, reliable results were obtained in this case for the period of high trophic state and for the last decade. During the recovery phase of the 1980s, unexpected high abundance of Stephanodiscus minutulus leads to strong overestimation of TP concentrations. Similarly, the reconstructed primary productivity only disagrees with the experimental data for some years in the last decade. Sensitivity of the sedimentary pigment model as well as the relatively reduced sampling dates likely explains this discrepancy. Similarities are also evident in the temporal diatom assemblage variations of an additional three sub-alpine Italian lakes. As well, the palaeolimnological reconstruction for Lake Maggiore parallels that for Lake Constance, another large sub-alphine lake located north of the Alps.