Lake Como (Lario): Innatural Trophic Level Trend Due to Pollution
by
Anselmo Pizzala
Università degli Studi dell'Insubria, Como

The Lake Como's hydrographic basin covers an area of 4, 552 km2; it is mainly located in the north-western sector of Lombardy, and to a lesser extent in the Ticino and Grigioni districts (487 km2) of Switzerland. The lake stretches 90 km in the N-S direction, and 125 km in the W-E direction.

The pollutants are carried into the lake's waters through the hydrographic system, by means of run-off processes or meteoric precipitations.

37 streams carry their waters into the lake, as well as 2 main tributaries which flow into the lake's northernmost tip. These are the Adda River (carrying 50% of the total water volume that flows into the lake from tributaries) which is also the only emissary of the lake, near Lecco, and the Mera River, carrying 20% of the waters flowing into the lake from tributaries.

Located into a transversal cryptodepression of the Southalpine chain, Lake Como is the deepest of the Italian lakes, reaching -412 m in the offshore waters in front of the Argeno village.

Due to its characteristic "upside-down-Y" shape, the lake may be subdvided into three different basins: the upper lake (or nothern basin), the Como branch (southwestern basin) and the Lecco branch (southeastern basin). The southwestern basin is the deepest one, with a water exchange rate that is longer than the other two basins. The northern and southeastern basins are characterised by a more intense hydrologic activity. The theoretic water exchange time for the whole lake is around 4.5 years, one of the longest among the deep southalpine lakes.

Lake Como may be classified in terms of its glacial origin or on the basis of its thermal regime, a classification criterion that better defines the "ecologic vocation" of the water body.

In the case of the Lario, during mild and dry winters the vertical mixing of waters is only partial in its deepest portions; therefore, the lake is defined as oligomitic.

Phosphor, that represents the key factor in limiting the growth of the producers in the lake, comes from cultivated and uncultivated crops, as well as from domestic wastewaters produced by residents and tourists and from cattle-breeding and industrial wastewaters.

The natural condition of the lake's waters would be an oligotrophic one, with an average phosphor concentration of 7, 5 mg/m3; however, in the last decades, as a consequence of the increasing enrichment in nutrients, the lake has progressively become eutrophic.

It is well known that the excessive amount of nutrients, altering the balance of the lake's ecosystem, is a major cause of pollution.

Lake Como has been the focus of scientific interest since the dawning of the limnological researches in Italy; historically, two phases might be recognised: The first one, which began back at the times of Alessandro Volta (1745-1827), was characterised by interesting studies, although limited to the investigations of a few scientific issues; the second one, which began in 1986, was marked by more articulated studies and researches.

In 2003, researchers from the Faculty of Science of the University of Insubria (A. Pizzala, A. Cantaluppi), examined and selected a great deal of scientific data regarding phosphor and nitrogen in the lake's waters, collected during studies conducted on different occasions. These data have been compared and divided in 11 groups correspondent to 11 areas of the lake, in which the data might be grouped on the basis of detectable analogies. These zones were defined and named according to the sampling locations and the names of the nearby main geographic sites (Como, Cernobbio, Moltrasio/Torno, Argegno, Menaggio/Bellagio, S.Vito/Corenno Plinio, Colico, Bellano, Lierna/Grumo-Civenna, Abbadia Lariana, Lecco).

Therefore, it has been possible to elaborate a picture of the trophic evolution of the lake as a whole and as an assemblage of different zones. The historic series of phosphor and nitrogen have been presented by means of tables (with reference to their bibliographic sources) and graphs, useful for illustrating the trophic evolution of the lake. The Authors have chosen to work with average values of phosphor and nitrogen, focussing respectively on the hypolimnium, epilimnium and the whole water column.

For explanatory purposes, it is possible to take a look at the graphs relative to phosphor and nitrogen referred to the whole water column, respectively for the Como area (mostly subject to incoming wastewaters from textile factories) and the entire lake.

With regard to the whole lake, it is possible to highlight that the trophic evolution of its waters has shown an anomalous trend in the past few decades, markedly different from the normal increase of trophic levels to which any lake is inevitably subject.

It is possible to point out that, starting in the 80s, there has been an improvement of the trophic conditions of the lake's waters, caused by the decrease of the limiting factor represented by phosphor. The average amount of this element was 80 mg/m3 in 1977-78, a clearly eutrophic condition, but it sank to 35 mg/m3 in the 2nd half of the 90s, an almost mesotrophic condition.

It is not difficult to explain this difference from the natural trend in terms of the antrophic presence (industry and tourism) in the lake's basin; in the same way, it is possible to explain the observed decrease in phosphor in terms of the reduction of this element in the tensioactive products used by the textile industry. Such an improvement has been also possible thanks to the realization of water depuration systems, both at the domestic and the municipal level (i.e. the Comodepur depuration plant, where a large volume of wastewaters from dying factories are subject to treatment). A recent study, sponsored and supported by the Ministry for the Environment and the Como Provincial Administration, whose title is "Project aimed at the mitigation of eutrophic processes in the Lake como's waters" (Politecnico di Milano: Chiesa-coordinatore; Università degli Studi di Milano: Chiaudani-coordinatore, Negri, Borsani; Centro Volta: Bartesaghi, Pizzala, Taiana, CCR/CEE di Ispra: Premazzi, Rodari, Rossi; ATE s.r.l.; Lombardia Risorse s.p.a.) and whose final results were published in 1993, kept in consideration the incoming amounts of phosphor and elaborated water reclamation projects.

With an external phosphor input to the lake of around 379 tP/year, in 12 years its average concentration in the lake's waters would decrease from 40 mg/m3 to 25 mg/m3, an amount that would then tend to stabilize. However, reducing these external inputs by 50% (by means of adequate and sustainable methods), it would be possible to reach concentrations as low as 13 mg/m3 in 12 years, an amount that is larger than the natural one (around 7, 5 mg/m3), but that is compatible with a good quality of the lake's waters.

Date received: July 14, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-30.