Lake Garda’s Ecosystem at Risk: Non-Native Species and Environmental Signs
The scientific name is «Lagarosiphon major». It is an aquatic plant native to the lakes of South Africa, featuring a long stem that can reach up to one and a half meters in height, with small emerald green leaves attached in rosette. It is slowly colonizing the lakebed of Lake Garda and, since 1970, when it was first reported in some locations, it has now spread everywhere. «It is a particularly vigorous plant, it spreads quickly, displacing native plants and taking over their habitats.» Daniele Zanini, biologist, teacher, and collaborator at the Museo di Scienze Naturali di Verona, was able to study it up close during the international scuba diving gathering held in Assenza di Brenzone.
Two days attended by approximately 280 divers who took turns in cleaning operations of the lakebed. «To do something new, we had the idea to call a naturalist to explain the characteristics of the Garda flora and fauna,» explains Stefano Agarri Panigutti, manager of Athos Diving, where the event took place. The material collected by scuba divers—tires, bottles, cans, washing machine baskets, and even shoes—was carefully studied by Daniele Zanini.
The technique of environmental semiotics
«The technique is that of environmental semiotics, a new science used by botanists, naturalists, and environmentalists,» he explains. «Just as doctors identify diseases by signs, we do the same to define the quality of an environment. Our “signs” are the so-called bioindicators, animals or plants that live only under specific environmental conditions.» Among the bioindicators found in the sediments collected are «Pyrgula Annulata», a small gastropod living at a depth of about 5-10 meters, and «Autostropatamoblus pallites», or freshwater crayfish.
«Their presence indicates that the water is of good quality, lightly polluted, well oxygenated, and cold.» Regarding non-native species, several are present, and they are changing the lake’s habitat. Often, humans are responsible for their presence. For example, «Lagarosiphon» is commonly used to decorate aquariums; a hypothesis to explain its presence is that it was discarded into the water by people and found an ideal environment for proliferation here.»
But the greatest danger comes from the American crayfish, commonly farmed for food. It can be distinguished from the native freshwater crayfish, which is pale in color, by the presence of red spots on the abdomen. «This is a very robust species, not only competing for food with the native freshwater crayfish but also feeding on it, and it is particularly resistant to pollution. Another non-native species creating damage is «Dreisenna polymorpha», a bivalve mollusk, a filter-feeder like mussels. Native to the Caspian Sea, it was first found in the waters of Benaco in 1993. Since then, its population has grown exponentially.»
«It is found everywhere—on the lakebed, on boat hulls, clogging intakes and sewage outlets. Out of nine mollusk species caught, 99% were «Dreisenna». The problem is that it has no natural predators here, whereas in the Caspian Sea, it is a favorite prey of the «Moriglione», a duck that usually feeds on it, thereby controlling its development.» In addition to collecting human-discarded material, the divers also performed two core samplings, collecting sediments from two different areas of the lake opposite the Assenza locality.
«In one, the soil was clean, clear, and it was the site where we also found indicators of good water quality. The other was composed of dirty, black, foul-smelling sediments, which is not a good sign. We also found some «Chironomide», a type of small, red flies that survive in anaerobic environments, lacking oxygen, and rich in fecal substances. There was also an increase in filamentous algae, indicating higher levels of nitrates and phosphates. In other words, the nutrients in the lake have changed, favoring some plant and animal species at the expense of others.»
«I want to clarify,» Zanini concludes, «that this remains an informal investigation, and based on it, no judgments can be made. To properly assess water quality, environmental semiotics should be used alongside chemical and microbiological analyses, especially across the entire lake, not just in a small portion of it.»