CDOM concentrations of large Finnish lakes relative to their landscape properties

Abstract

Coloured dissolved organic matter (CDOM) modifies the light penetration into water bodies due to stronger absorbance of UV and short wavelengths of light. Therefore, in natural waters with high CDOM concentration, the spectrum of sunlight is shifted towards brown, also referred to as brownification. Here, the relation between the spectrophotometrically measured water colour (CDOM) and landscape properties is examined. These properties explained at best > 40% of the CDOM variability among the study lakes larger than 10 km2. The key “permanent” landscape variables were lake percentage (Lake%) in the uppermost catchment area, and the peat land coverage (Peat%) of the catchment, which indeed was strongly correlated with lake elevation above the sea level. High Lake % indicated low CDOM concentration, while high Peat% indicated the opposite. Relative to the Peat% of the catchment, the CDOM concentrations were, on average, slightly higher in medium-size lakes (area 10–100 km2) than in large lakes (area > 100 km2), while relative to Lake% the concentrations declined more in medium-size lakes.