Ecological effects of level fluctuations in Lake Maggiore (Northern Italy)

In the oligotrophic Lake Maggiore, a deep subalpine lake in Northern Italy, blooms of Anabaena lemmermannii appeared in recent years despite in-lake TP concentration always being <10µg L-1. In the same years, the regular summer lake-level fluctuation was altered due to the interaction between climate change and human exploitation of water resource. Seeking a causal relation linking level fluctuations and cyanobacterial blooms, we investigated the appearance of pulse injections of organic matter from lake shore after drought. We simulated the arid and flood periods exposing artificial substrates for 30 days near the lake shore. The substrates were recovered and analysed for organic carbon and total phosphorous, the limiting nutrient for this lake. The C and P release upon rewetting was measured after 3 and 30 days. The C:P ratio of the leaked material resulted two orders of magnitude lower than in freshly recovered organic matter from artificial substrates. We hypothesised that rain washout of lake shore in a low lake-level state may supply P enrichment, promoting the germination of A.lemmermannii akinetes, thus starting the bloom. Lingua abstractEnglish