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Abstract: We studied the absorption properties of in-water constituents among 15 boreal lakes belonging to various lake types. One of the lakes was also monitored frequently during its growth season to reveal the seasonal variation of absorption properties. Within the studied lakes, CDOM absorption a CDOM(442) varied from 0.4 to 15m−1. CDOM was clearly dominating the absorption signal at low wavelengths, but even at the red-end of the spectra its contribution to total absorption was notable, varying from 48 to 99% of total absorption (excl. water) at 442nm and from 5 to 86% at 665nm. Slope of the CDOM absorption spectra S CDOM(400–700) varied from 12.9 to 19.3μm−1. Higher slopes were obtained when the wavelength range was extended to the UV-range (350–700 and 300–700nm). Non-linear relationships were noted between a CDOM and S CDOM calculated over various wavelength ranges. Despite the significant relationship between DOC concentration and a CDOM, almost five-fold variation was noted in DOC-specific CDOM absorption at 442nm. This variation was clearly associated with concurrent changes in S CDOM. Non-algal particulate absorption varied from 0.037 to 1.8m−1 at 442nm, contributing <1 to 34% of total absorption at 442nm and <1 to 27% at 442nm. Slope of the NAP absorption spectra S NAP(380–700) varied from 7.6 to 12.8μm−1. Despite of the apparent covariation between weight of suspended particulate matter (SPM) and a NAP, six-fold variation was observed in SPM-specific NAP absorption at 442nm, which was mostly associated with organic content of the particle pool. Contribution of phytoplankton pigments ranged from 1 to 28% of total absorption at 442nm, and 13 to 86% at 665nm, respectively. Chlorophyll a (Chla) specific absorption of phytoplankton pigments varied from 0.012 to 0.038m2 (mg Chla)−1 at blue peak, and from 0.008 to 0.020m2 (mg Chla)−1 at red peak. Observed values cluster around the low-end of the previously reported range, indicating a high package effect of phytoplankton cells in lakes. Observed variation was modeled as function of Chla (ranging from 1.8 to 95mgm−3). Parameters and relationships presented in this study provide useful information for remote sensing of lakes, as well as for various ecological and geochemical applications. [Copyright &y& Elsevier]

Given the importance of colored dissolved organic matter (CDOM) for the structure and function of lake ecosystems, a method that could estimate the amount of CDOM in lake waters over large geographic areas would be highly desirable. Satellite remote sensing has the potential to resolve this problem. We carried out model simulations to evaluate the suitability of different satellite sensors (Landsat, IKONOS, and the Advanced land Imager [ALI]) to map the amount of CDOM in concentration ranges that occur in boreal lakes of the Nordic countries. The results showed that the 8-bit radiometric resolution of Landsat 7 is not adequate when absorption by CDOM at 420 nm is higher than 3 m−1. On the other hand, the 16-bit radiometric resolution of ALI, a prototype of the next generation of Landsat, is suitable for mapping CDOM in a wider range of concentrations. An ALI image of southern Finland was acquired on 14, July 2002 and in situ measurements were carried out in 15 lakes (18 stations). The results showed that there is a high correlation ( R2 = 0.84) between the 565 nm/660 nm ALI band ratio and the CDOM absorption coefficient in lakes. Analysis of 245 lakes in the acquired satellite image showed a normal distribution of CDOM concentration among the lakes. However, the size distribution of lakes was highly skewed toward small lakes, resulting in the CDOM concentration per unit lake area being skewed toward high values. We showed that remote sensing enables synoptic monitoring of the CDOM concentration in a large number of lakes and thus enables scaling up to the level of large ecosystems and biomes. [ABSTRACT FROM AUTHOR]

Abstract: Given the importance of coloured dissolved organic matter (CDOM) for the structure and function of lake ecosystems, a method to estimate the amount of CDOM in lake waters over large geographic areas would be highly desirable. Advanced Land Imager (ALI) images were acquired in southern Finland (in 2002) and southern Sweden (in 2003) together with in situ measurements of bio-optical properties of 34 lakes (39 measuring stations). Based on this dataset, a band-ratio type algorithm was developed using ALI band 2 and band 3 for estimating CDOM content (absorption of filtrated water at 420 nm) in lakes. Correlation between in situ measured CDOM and the remote sensing estimate of CDOM was high, r 2=0.73. The CDOM retrieval algorithm obtained on the basis of two images and in situ data was validated on a third ALI image (eastern Finland, 2002) that was available in the ALI image archive. In situ water-colour monitoring data from 22 lakes (27 measuring stations) in the third image were available in a database of the Finnish Environment Administration. The water-colour data were converted to CDOM absorption values, which were then compared to the results from a third ALI image. The correlation between remotely estimated and in situ CDOM values in the algorithm validation image was high, r 2=0.83. These results support the conclusion that CDOM content in lakes over a wide range of concentrations (a CDOM(420) between 0.68 and 11.13 m−1) can be mapped using Advanced Land Imager data. [Copyright &y& Elsevier]