Trends and relations among basic phytoplankton characteristics in the course of the long-term oligotrophication of Lake Maggiore (Italy)

Lake Maggiore underwent a process of eutrophication in the course of the 1960s and 1970s which caused the lake status to change from oligotrophy to meso-eutrophy, sewage phosphorus being the main responsible factor. Starting from the late 1970s the P load has been gradually reduced, so that total phosphorus in-lake concentration is now below 10 µg l-1 at winter mixing, compared to a maximum value of 30 µg l-1 in 1978. Nearly two decades after the start of this reversal in trophic conditions, long-term analyses become fruitful, not least because of a scarcity of published long-term data on lake re-oligotrophication, and consequently of agreed predictions on specific points. This paper presents such an analysis, regarding some phytoplankton characteristics (numbers, biovolume, cell size, chlorophyll a, species richness and composition) and their mutual relations in the time series 1981-1995, as well as recent primary production compared with early observations. Notwithstanding phosphorus reduction, the phytoplankton remained quite stable for many years. Only after 1987, when TP concentration fell to about 15 µg l-1 at mixing, definite changes in species diversity and composition appeared. At the same time, average cell size decreased abruptly and persistently, together with chlorophyll and total biovolume while cell numbers were unchanged or rising. Annual primary production was halved in comparison to a past reference period and the efficiency of the photosynthetic process was apparently enhanced.