Phosphorus drops, Garda healthy. The lake is returning to its origins

29 May 2023

The ideal trophic state is preserved, with values similar to a century ago Data presented at the Fisheries Forum at the Venetian Customs House: the water situation is improving due to climate change and less sewage spillage. 
There is an effect of climate change that is helping Lake Garda to preserve its ideal trophic state, which is now comparable to the situation of a century ago. This is the increasingly rare complete mixing of the waters caused by the increase in temperature in the first 50 metres of depth (almost one degree in the last thirty years, a high value): this favours the deposition of inorganic nutrients (phosphorous in primis) towards the bottom. If they were to rise to the surface, they would cause massive algal production, leading the lake towards eutrophication: pollution responsible for the proliferation of bacteria and the drop in oxygen that affects flora and fauna. 
Garda Forum Nico Sal-maso, head of the Hydrobiology Unit of the Edmund Mach Foundation in San Michele all'Adige, spoke about the effects of anthropic and climatic changes on the quality of Lake Garda during the 28th edition of the Garda Forum hosted at the Dogana Veneta in Lazise, an annual event organised by the three Garda Rotary Clubs (Rotary Club Riva del Garda, Rotary Club Peschiera e del Garda Veronese, Rotary Club Salò-Desenzano). 
This year's theme was fishing, the historical value, current situation and prospects of which were discussed in depth in a debate that, in addition to Salmaso, involved the Veneto Region ichthyologist Ivano Confortini and Pierlucio Ceresa, secretary general of the Garda Community, with speeches moderated by Filippo Gavazzoni, councillor of Peschiera and vice-president of the Garda Community. Also present at the opening of the conference was the vice-president of the Veneto Region, Elisa De Berti. Phosphorus concentrations increased from the 1970s until the mid-2000s, while from 2010 onwards the situation improved. 
Phosphorous is decreasing 'Something positive has happened,' Salmaso noted. 'We observe changes over the long term, so we hypothesise that what has decreased phosphorus is less spillage of effluent into the water,' and this would depend on the collector, laid between the 1970s and 1980s. "The trend is still downwards: the lake is approaching oligotrophy, it is now in a condition of oligomesotrophy," where oligotrophy means an aquatic environment characterised by a lack of dissolved nutrients, while mesotrophy is the previous stage. A sort of return to the origins, as was reconstructed by analysing and dating a sediment sample: "More than a hundred years ago, the phosphorus concentration was 5-10 micrograms per litre, which is what we are now aiming for," Salmaso pointed out. 
A balance that could be threatened by particularly cold and windy winters and the consequent mixing of water deeper down (the last one occurred in 2006) with the return of phosphorus to the surface, which in the summer months would increase the concentration of cyanobacteria biomass, a species that until now has been present in minimal quantities but which also produces toxic substances when phosphorus increases. 
Alien species Forty-two alien species have been catalogued in Lake Garda: among the most recently detected is Dreissena bugensis, a mollusc native to the catchment area of the Dnepr (Ukraine), like Dreissena polymorpha, which was discovered at the end of the 1960s and has since invaded the lake. Their arrival is not due to climate change, but to the more or less conscious introduction of man: in the case of the Dreissena, transport took place through the keels of the boats to which they were anchored, which is why one of the objectives of the Lake Contract currently being approved is to sanitise the hulls. Ichthyologist Confortini explained that about 110 professional fishermen operate on Lake Garda, 77 of whom along the Veronese shore (mostly from non-EU countries), plus a few hundred local sports fishermen and a few thousand from neighbouring provinces. The annual catch of professional fishermen is around 300-400 tonnes (mainly whitefish, lavaret, trout, pike, perch and tench). Bleak, which was abundant in the past, has drastically declined since the late 1990s. "The species have also changed because the lake conditions have changed, exploited for other activities that have sacrificed some environments," said Confortini. 
Hence the proposal for a protection plan to be implemented on two fronts: that of the local administrations, called upon to implement strategies to safeguard the lake's ecosystem and biodiversity, and the supra-municipal one to arrive at an "adequate fishing regulation capable of enhancing the fish resource in terms of both quantity and quality, making the catch sustainable", while also advancing the idea of establishing a quality mark for Lake Garda fish.

(L'Arena, 19 May 2023)