Absorption properties of in-water constituents and their variation among various lake types in the boreal region

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 aCDOM(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 SCDOM(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 aCDOM and SCDOM calculated over various wavelength ranges. Despite the significant relationship between DOC concentration and aCDOM, almost five-fold variation was noted in DOC-specific CDOM absorption at 442nm. This variation was clearly associated with concurrent changes in SCDOM. 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 SNAP(380–700) varied from 7.6 to 12.8μm−1. Despite of the apparent covariation between weight of suspended particulate matter (SPM) and aNAP, 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.