Spectral Characterisation of the Osterseen Lake District - EARSeL ...

3.2. In situ data
Simultaneously to the overflights, water samples were taken and Secchi depth was measured in each lake of the
Osterseen Lake District at 1m and 3 m water depth, and in cases of high Secchi depth also at 5 m. The samples
were analysed in the laboratory for the optically relevant water constituents chlorophyll, suspended matter, and
Gelbstoff. Chlorophyll analysis was conducted by photometric measurement of the transmission of the solution at
665 nm and 750 nm after the samples were filtered through a 1 µm filter, extracted [9], and centrifugated.
Suspended matter was measured as total, organic, and inorganic concentration. The water samples were filtered
through a dried and weighed 1 µm glass fiber filter and dried for two hours at 105°C and then for four hours at
550°C with weighing after each step. The total suspended matter is the dry weight after the first step subtracted by
the filter weight, the inorganic matter is the weight after the second step, and the organic matter is the loss between
these two weighings. For the determination of the Gelbstoff absorption, the water samples were sucked through a
0.2 µm filter. The transmission was measured with a Perkin-Elmer Lambda-2 dual beam spectrophotometer in 10
cm and 5 cm cuvettes. From these two transmission spectra the Gelbstoff absorption was determined including the
pure water correction using the absorption spectrum from [10]. The in situ data are given in Tab. 1 including also
the spectral slope to additionally characterize Gelbstoff spectrally.
Table 1. In situ measurements of water constituents on June 5 th , 2001
Lake Secchi Depth Chlorophyll-a Total suspended Gelbstoff [m
[m]
[µg/l] Matter [mg/l]
-1 ] Gelbstoff
Spectral Slope S
Waschsee [1 m] 1.9 6.2 0.9 0.62 0.0150
[3 m] 41.7 4.6 0.77 0.0138
Schiffhüttensee [1 m] 1.5 16.7 3.4 0.65 0.0133
[3 m] 22.4 4.9 0.53 0.0147
Sengsee [1 m] 3.8 2.0 1.5 - -
[3 m] 2.0 2.9 0.69 0.0142
Fohnsee [1 m] 6.9 0.5 0.6 0.64 0.0134
[3 m] 2.0 0.9 0.15 0.0114
Westl. Eishaussee [1 m] 5.0 0.9 0.7 0.52 0.0148
[3 m] 1.7 1.7 0.41 0.0132
Östl. Eishaussee [1 m] 4.9 1.2 0.5 0.43 0.0137
[3 m] 2.3 1.4 0.58 0.0148
Gr. Ostersee - S [1 m] 1.9 1.8 2.3 - -
[3 m] 1.8 3.2 0.32 0.0152
Gr. Ostersee - N [1 m] 1.5 2.2 2.2 0.41 0.0117
[3 m] 1.8 2.9 0.38 0.0125
Westl. Breitenauersee [1 m] 3.5 1.2 1.9 0.36 0.0126
[3 m] 1.2 1.9 0.56 0.0154
Lustsee [1 m] 7.5 - 0.4 0.67 0.0143
[3 m] 0.9 - 0.49 0.0139
[5 m] 0.2 1.0 - -
Stechsee [1 m] 4.5 0.7 4.1 0.57 0.0145
[3 m] 0.6 3.7 1.08 0.0116
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