Determination of the contribution of light attenuating substances in lakes and comparison of different lakes with respect to optical properties
The contribution of particulate and dissolved matter to light attenuation in six Swiss lakes was determined. The lakes were compared with respect to their optical properties and composition.
Light attenuating substances were broadly classified as larger particulate matter (LPM), suspended particulate matter (SPM) and dissolved organic matter (DOM) on the basis of the range of particle size. The light absorbance (~attenuance) of these particles was measured. The beam and vertical attenuation coefficients-c and Kd - as functions of wavelength on the
PAR region respectively were calculated. They were also calculated for specific wavelengths. Thus determined optical properties were, then used for making comparisons among the lakes.
Total suspended solids (TSS), particulate organic carbon (POC), dissolved organic carbon (DOC) and chl-a concentrations were measured to compare the lakes regarding their particle content and productivity.
The lakes revealed differing substances as the most important contributory component to light attenuation. Etang de la Gruère (EDG), for instance, is almost totally dominated by DOM, whereas SPM is the most important light attenuating factor in Lago Bianco. In fact, SPM has the highest importance in all the lakes except EDG. LPM, on the other hand, though it has some contribution at different depths, seems to have no effect (except for Horw basin) on the light regime when averaged over depth.
The highest light attenuation was observed in EDG followed by L.Bianco. Lake Brienz took the third place. Lake Alpnach and Horw basin showed closely similar absorption at the longer wavelengths, but the former was higher at the shorter ones owing to higher DOM. Lake Thun was the last.
The optical properties of the lakes depend on season. Measurements conducted on earlier dates showed L.Brienz and L.Thun to attenuate light much higher than L.Alpnach and Horw basin.
Vertical attenuaiton coefficient Kd (λ) in the PAR region proved to be the highest in EDG and the lowest in L.Thun and Horw basin. L.Bianco was the second. Lake Brienz took the 4th position following L.Alpnach. The seasonal dependence of optical properties caused by the change in composition of water was also noted from the changed beam attenuation coefficient (c ) and Kd (λ) values (eg, Lakes Brienz, Thun and Horw basin).
L.bianco with the highest concentration of SPM showed the highest, and EDG with abundance of DOM, the lowest reflectance respectively. Transmission of light showed the expected negative correlation with the measured values of TSS. But optical back scattering (OBS) didn’t show a good correlation with TSS and chl-a concentrations.
A linear model was formulated on the basis of KdPAR from the in situ measured broad band PAR and measured light absorbing substances ( chl-a, DOC and TSS) . ctot was also computed from the calculated c values of the different components of water and the water itself. Moreover, an exponential model was found to fit well with the attenuance of DOM in EDG. Likewise, c of SPM at 430nm was found to fit a linear model whe
regressed on TSS and chl-a.
The productivity of Lake Brienz is the lowest and that of Lake Alpnach, the highest. Others are placed in between. The HPLC results were well correlated with the fluorometrically determined chl-a distribution.