Winter and spring mixing depths affect the trophic status and composition of phytoplankton in the northern meromictic basin of Lake Lugano

The trophic state of Lake Lugano is still too high to be acceptable, despite extensive recovery measures undertaken in recent decades which have resulted in a reduction of the external phosphorus load to the deepest of the lake's basins (northern basin; Zmax=286 m) to fairly acceptable values. Since meromixis was established in the middle of last century, the deep hypolimnion of the northern basin (the layer between ca 100 m and the bottom) has contained high quantities of nutrients (especially phosphorus) which are a major potential source of internal load. When there are particularly strong winter mixing events, a portion of this phosphorus reserve is redistributed along the upper water column (0-100 m). The impact of meteo-climatic conditions on the plankton biocenosis were analysed using data collected in the northern basin (Gandria station) during the three-year period 1998-2000. The phytoplankton composition, which is typical of eutrophicated waters, shows marked interannual variations, also depending on the degree of mixing of the waters at the start of the vegetative period. Though there is no steady pattern of typical dominant species / master species in the lake, there is a seasonal succession characterised by a marked development of diatoms in spring, and a predominance of chlorophyceans and cyanobacteria in summer and autumn. Under present conditions, the mechanisms of internal replenishment of nutrients towards the euphotic layer, due to the phenomena of late winter and spring mixing, constitute a significant source of nutrients for the spring and summer growth of phytoplankton. On the other hand, pronounced mixing phenomena, like those occurring in the two-year period 1999-2000, can reduce the hypolimnetic nutrient reserves and cause a decrease in the trophic potential of the basin, contrasting with an increase in algal biomass in the euphotic zone.