Ongoing ecological shifts in a large lake are driven by climate change and eutrophication: evidences from a three decades study in Lake Garda

The long-term research carried out since the 1970s in the deep perialpine Lake Garda documented more than a threefold increase of total phosphorus (TP) in the whole water column until mid-2000s and a continuous warming. We evaluated the impact of these changes on phytoplankton, assessing similarities and peculiarities compared with other lakes. The increase of TP favoured cyanobacteria (microcystins producer Planktothrix rubescens (De Candolle ex Gomont) Anagnostidis & Komárek) and large diatoms. The warming of the lake caused a decrease in the frequency of full mixing episodes, which stopped completely after 2006, and a lower supply of nutrients to the upper layers. This “climate warming-induced oligotrophication” was mediated by the physiographic characteristics typical of deep lakes. The decrease of nutrients in the upper layers induced by the long period of incomplete mixing caused a decline of the mesotrophic P. rubescens, which was partially replaced by anatoxins producer Tychonema bourrellyi (J.W.G. Lund) Anagnostidis & Komárek. Lake warming favoured a higher development of mixotrophic dinoflagellates and cryptophytes. Many dinoflagellates are adapted to grow in warm and low nutrient lakes, therefore it is likely that the development of selected species will increase in large (meso-)oligotrophic lakes because of climate change.