An Italian environmental group warned on Monday that mass tourism is slowly eroding the Venice lagoon, which it said is also threatened by major real estate development and an inadequate transport network. Architect Cristiano Gasparetto said a 1988 study indicated that while the acceptable maximum number of tourists for Venice is 33,000 daily, today the average figure is 59,000. Alessandra Mottola Molfino, head of the Italia Nostra non-governmental organisation, said the figure \"is too high for such a fragile city\". The group\'s experts say that as a result of this human tide, aquatic transport has soared in the city, causing the gradual destruction of the lagoon ecosystem, with its mix of sea and fresh water and its relatively shallow bed which is home to plants which can oxygenate water. Propellers of ships cruising at ever greater speed push up the underwater silt which is sucked in by tides, with Venice increasingly becoming less of a lagoon and more and more a bay. The Venice lagoon is an inlet of the Adriatic Sea, with access to sea waters largely restricted by a series of sand bars at the lagoon\'s entrance. Substantial freshwater inputs used to flow through the lagoon as well, but over the past several centuries most of the freshwater has been diverted directly to the Adriatic. The lagoon\'s water now possesses a salinity level nearly as high as that of the Adriatic. Luigi D\'Alpaos, a professor of hydrodynamics at Padoa University, said that in the space of 70 years, the laguna bed had dropped on average by a meter (about three feet). Italia Nostra proposed a sharp reduction of the number of tour groups, even though it realises that such a step \"would momentarily lead to a drop in trade flows\" and apparently to a \"decline of the local economy\". It suggested developing other activities such as university research or ecotourism to \"create a richer economy than one solely based on (mass) tourism\". Italia Nostra also slammed excessive real estate speculation, including plans by Venice authorities to build a nearby city, Tessera-City, complete with offices, a casino, warehouses and a high-speed rail link. The rail link would require construction of a nine-kilometer-long tunnel under the laguna and would further lower its bed, according to Gasparetto. \"Venice is really under threat. We must find a balance between immediate needs and the future to ensure sustainable development,\" said Mottola Molfino.