4.2 Water Quality in Lake Trasimeno
Lake Trasimeno, a post-tectonic lake situated in central Italy, is the fourth largest
Italian lake (124 km
2 ). The lake is almost round with a diameter of about 11 km,
three small islands and an open bay colonised by aquatic vegetation in the southeastern part. Tourism, agriculture and livestock breeding are the most important
activities in the Trasimeno area: cultivated lands cover about 70 % of its catchment
area (intensive agriculture with irrigational needs is only present in 28 % of the
area). Lake Trasimeno is a closed lake, with shallow unstratified waters (average
bottom depth is 4.5 m; maximum depth is 6 m). The lake shows from mesotrophic
to eutrophic conditions, where principal critical parameters are phosphorous and
chl-a, despite biological evidence of eutrophic–hypertrophic conditions. Important
ecological constraints are recurring sediment resuspension phenomena and phytoplankton blooms, including those of cyanobacteria [79].
The concentrations of chl-a, SPM and CDOM were estimated using MIVIS data
acquired on 12 September 2009 in the eastern portion of Lake Trasimeno. The
atmospheric correction of MIVIS data was carried out using ATCOR-4. Then, the
conversion of water reflectance into concentrations was based on a spectral inversion procedure implemented in BOMBER. The bio-optical model implemented in
BOMBER was parameterised with data gathered from fieldwork activities
performed in 2008 and 2009.
Figure 3 shows the BOMBER-retrieved products in the optically deep waters
describing the concentrations of SPM, chl-a and a CDOM (440). The average values
of SPM, chl-a and CDOM were, respectively, 5.9 gm
À3 (standard deviation (SD) ¼
1.0 gm
À3 ), 1.1 mgm
À3 (SD ¼ 0.5 mgm
À3 ) and 0.35 m
À1 (SD ¼ 0.06 m
À1 ). These
values are reliable for the season, as in late spring lake waters have acceptable
quality that degrades from mid-summer until mid-autumn, when the lake is affected
by phytoplankton blooms [80]. Figure 4 shows the scatter plots of chl-a vs. SPM,
chl-a vs. CDOM and SPM vs. CDOM. The correlation is poor for each pair of water
quality parameters, confirming that Lake Trasimeno is typical Case-2 water.
4.3 Cyanobacteria Bloom in Lakes of Mantua
The lakes of Mantua are three small and shallow lakes surrounding the city of
Mantua; the Upper Lake is 3.7 km
2 and has a mean depth of 3.6 m, the Middle Lake
is 1.1 km
2 with a mean depth of 3.0 m and the Lower Lake is 1.5 km
2 with a mean
depth of 3.3 m. The system of lakes is fed by the Mincio River, the emissary of Lake
Garda that was dammed during the twelfth century. The average flow of the Mincio
River usually decreases in summer (up to À90 %) from the combination of
upstream water deviation for irrigation purposes and less frequency of rain, with
consequent water stagnation and increased residence time [81].
Imaging Spectrometry of Inland Water Quality in Italy Using MIVIS: An Overview
69
Lake Trasimeno, a post-tectonic lake situated in central Italy, is the fourth largest
Italian lake (124 km
2 ). The lake is almost round with a diameter of about 11 km,
three small islands and an open bay colonised by aquatic vegetation in the southeastern part. Tourism, agriculture and livestock breeding are the most important
activities in the Trasimeno area: cultivated lands cover about 70 % of its catchment
area (intensive agriculture with irrigational needs is only present in 28 % of the
area). Lake Trasimeno is a closed lake, with shallow unstratified waters (average
bottom depth is 4.5 m; maximum depth is 6 m). The lake shows from mesotrophic
to eutrophic conditions, where principal critical parameters are phosphorous and
chl-a, despite biological evidence of eutrophic–hypertrophic conditions. Important
ecological constraints are recurring sediment resuspension phenomena and phytoplankton blooms, including those of cyanobacteria [79].
The concentrations of chl-a, SPM and CDOM were estimated using MIVIS data
acquired on 12 September 2009 in the eastern portion of Lake Trasimeno. The
atmospheric correction of MIVIS data was carried out using ATCOR-4. Then, the
conversion of water reflectance into concentrations was based on a spectral inversion procedure implemented in BOMBER. The bio-optical model implemented in
BOMBER was parameterised with data gathered from fieldwork activities
performed in 2008 and 2009.
Figure 3 shows the BOMBER-retrieved products in the optically deep waters
describing the concentrations of SPM, chl-a and a CDOM (440). The average values
of SPM, chl-a and CDOM were, respectively, 5.9 gm
À3 (standard deviation (SD) ¼
1.0 gm
À3 ), 1.1 mgm
À3 (SD ¼ 0.5 mgm
À3 ) and 0.35 m
À1 (SD ¼ 0.06 m
À1 ). These
values are reliable for the season, as in late spring lake waters have acceptable
quality that degrades from mid-summer until mid-autumn, when the lake is affected
by phytoplankton blooms [80]. Figure 4 shows the scatter plots of chl-a vs. SPM,
chl-a vs. CDOM and SPM vs. CDOM. The correlation is poor for each pair of water
quality parameters, confirming that Lake Trasimeno is typical Case-2 water.
4.3 Cyanobacteria Bloom in Lakes of Mantua
The lakes of Mantua are three small and shallow lakes surrounding the city of
Mantua; the Upper Lake is 3.7 km
2 and has a mean depth of 3.6 m, the Middle Lake
is 1.1 km
2 with a mean depth of 3.0 m and the Lower Lake is 1.5 km
2 with a mean
depth of 3.3 m. The system of lakes is fed by the Mincio River, the emissary of Lake
Garda that was dammed during the twelfth century. The average flow of the Mincio
River usually decreases in summer (up to À90 %) from the combination of
upstream water deviation for irrigation purposes and less frequency of rain, with
consequent water stagnation and increased residence time [81].
Imaging Spectrometry of Inland Water Quality in Italy Using MIVIS: An Overview
69