Decadal phytoplankton dynamics in Lake Maggiore between trophic evolution and climatic constraints

Phytoplankton studies in Lake Maggiore date back to early 1950s, although a regular monitoring started in late seventies, in the frame of an agreement between Swiss and Italian Governments. The full long-term series (1981-2011) allows to follow the lake's recovery, since the mesotrophic period (mid 1970s), to the present oligotrophic status. Phytoplankton showed a clear response to the decrease of P load, in terms of both chlorophyll and biomass, as well as species assemblages: some remarkable changes were observed in the basic structure of the phytoplankton assemblages, such as the constant increase in the number of taxonomic units and the decrease of average phytoplankton cell size. On the other side, the variability recorded on shorter time scale, against an almost unchanged trophic status, can be regarded as a response to the variability of the local climate, sometimes interfering with the effects of the changing trophy. During the lake's oligotrophication the role of climatic constraints became more and more important in controlling the phytoplankton dynamics, affecting, through mixing and runoff, the supply of nutrients, as well as the resource ratio. These new emerging patterns could explain some recent events occurred inside the phytoplankton assemblages.