Lake Garda

Ecological characterization

- Figure 2. Lake Garda

Lake Garda s the largest Italian lake and its basin is astride three different regions: the largest area is in Trentino-Alto Adige region (1,269 km2), followed by Lombardy region (615,7 km2) and Veneto region (340 km2).

As the largest Italian lake, Lake Garda contains 30% of the whole natural and artificial lake water resource in Italy and as one of the most important Italian freshwater resource, it has been regularly investigated (monthly samplings) since the mid 1990s and included in the Italian Network for the Long Term Ecological Research since 2006 (LTER, www.lteritalia.it).

Lake Garda (mean altitude = 65 m a.s.l.) occupies a deep fluvial valley originated during the superior Miocene (5-6 millions year BP) modified during the successive Quaternary glaciations. The combined erosion-glacial origin explains the narrow and elongated shape of the lake, the NS orientation of its main basin, the steep shores and slopes (Figure 2) and the position of lake bottom 285 m below sea level (cryptodepression).

The catchment of Lake Garda is relatively small in relation to the lake area (6:1) and mainly composed by sedimentary rocks (60%), while both crystalline rocks and secondarily deposited glacial and fluvial sediments account for the remnant 20% (Sauro, 2001).

Due to its large water mass, the climate is temperate and characterized by mild winters which favor the growth of a typical Mediterranean vegetation composed by oaks, cypresses, lemon and olive trees, and many other species (Figure 3). 67% of the drainage basin of the lake is covered by woods and semi-natural meadows (this class reaches almost 90% in Trentino), agricultural areas account for 12,5%(concentrated in the southern portion of the catchment), and freshwaters for 17% (Figure 4). The remaining 3.5% is covered by artificial surfaces, such as settlements, roads etc (Figure 5).

- Figure 4 Land use in Lake Garda basin - Figure 5

The artificial areas are generally located in the neighbourhood of the lake, especially in the southern part of the basin with a high exploitation of the shores, and alongside the main valleys in the Trentino area. Artificial area are subdivided in urban fabric (80%), industrial, commercial units and infrastructures (12%), and artificial, non-agricultural vegetated areas for a 5%. Agricultural areas are divided in 45% heterogeneous agricultural area, 24% arable lands, 19% permanent crops, 12% pastures.

Protected areas also cover a large part of the basin (about 40% = 878 km2, of which 50% is located in Lombardy, 38% in Trentino and 29% in Veneto) (Figure 6).There are 3 parks (2 regional parks in Lombardy and 1 provincial in Trentino), 34 sites of community importance, and 10 special protection areas. In Trentino there are also 7 provincial biotopes. These areas cover in total of surface.

- Figure 6. Map of the protected natural areas within the lake basin

The hydrological balance of Lake Garda is controlled by the discharge of the main lake inlet, the River Sarca, whose catchment represents ca. 50% of the whole lake catchment. Discharge of River Sarca (mean annual discharge = 30.5 m3 sec-1), despite being regulated because of the numerous power plants located along its course, still shows the typical pattern of glacial rivers (i.e. maximum discharge in late spring - early summer). On the other hand, the discharge of the other numerous smaller inlets of Lake Garda is mainly controlled by meteorological events. Since 1960, Lake Garda can receive up to 500 m3 sec-1 of water deviated from the River Adige through the canalization Adige-Garda, built to avoid floods during extreme weather events. For the same reason, also the natural lake outlet (River Mincio) was regulated in the 1960s and at present can reach water discharge up to 200 m3 sec-1 . The mean annual outflow is however around 58 m3 sec-1 (Salmaso et al., 2009).

Due to the relatively small water input the theoretical retention time of Lake Garda is of 26.6 years and the lake is classified as warm mono/meromictic and oligomictic, as water mixing occurs once in late winter-early spring and usually involves only the upper 150-200 m of the water column while complete water circulation occurs at irregular intervals in relation with exceptionally cold winters. However, in accordance with the warming recorded in other deep lakes at the southern and northern border of the Alps, an increase in water temperature has also been measured in Lake Garda in the last years.

Thermal dynamics are of crucial importance for the ecology of lakes since they affect both water oxygenation and nutrient redistribution within the water column. Higher oxygen saturations and nutrient concentrations in the water column of Lake Garda were recorded after complete water mixing.

At present Lake Garda is classified as oligo-mesotrophic (spring mean total phosphorous concentration around 20 µg L-1), but during the last 35 years the lake has experienced a significant increase in phosphorus content (Figure 7). The highest phosphorus concentrations were recorded in 2005 and represented the final stage of the gradual process of nutrient increase started in the early 1970s (Salmaso, 2010) related to uncontrolled discharge of urban sewage waters into the lake.

- Figure 7. Concentrations of total phosphorus along the entire water column, measured in samples taken in March 1997 to 2011 (Source: APPA)

About 95% of the population is served by the sewage system, over 90% of which is served by the treatment system. Wastewater coming from municipalities in the mid and lower lake areas is collected by two separated systems. A system collects wastewater from Brescian municipalities south of Salò and conveys it to Peschiera treatment plant. The other system collects wastewater produced between Salò and Gargano and at Toscolano conveys it to the Veneto side by means of an underwater pipe. From there, a special collector collect wastewater of that side and conveys it to Peschiera treatment plant.

A few surveys have highlighted the inadequacy of the ring trunk sewer in controlling nutrient loads. These problems have apparently contributed to the increase of phosphorus concentrations in the lake until 2005 (Mosello & Giussani, 1997). Today, the total phosphorus input is 1921.4 t P year-1, 64% of which originating from agriculture, 26% from cattle-breeding, and only 10% from urban sewages. Industrial and other productive activities are indeed relatively scarce and their contribution to nutrient input can be therefore considered as negligible.

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