Content of Total Organic Carbon in Finnish Lakes and Its Relationship to Catchment Characteristics

During autumn 1987, 987 randomly selected lakes throughout Finland were surveyed. Drainage lakes are most common (70%), followed by headwater lakes (17%). The lakes in the survey span most of the variation in humic matter concentration found in aquatic ecosystems (total organic carbon (TOC) = 0.5–47 (mg∙L−1, colour = 0–600 mg Pt∙L−1. The median TOC, colour, and chemical oxygen demand (CODMn) are 12 mg∙L−1, 100 mg Pt∙L−1, and 15 mg O2∙L−1, respectively. TOC is significantly related to other water quality parameters including the following: CODMn (r2 = 0.92), colour (r2 = 0.86), total iron (r2 = 0.69), total nitrogen (r2 = 0.53), and total phosphorus (r2 = 0.41). TOC and nutrient concentrations are low in northern Finland compared with southern and central regions. Multiple linear regression shows that the proportion of the catchment area covered by peatlands is the most important catchment variable determining TOC concentrations. In northern Finland, where catchment areas are less disturbed compared with s...