Natural organic matter (NOM) in a limed lake and its tributaries

The chemistry of a limed lake and its main tributaries were studied for 3 years (1992–94) with an emphasis on natural organic matter (NOM). Increased transparency and decreased water colour indicated a general reduction of NOM in the lake. Increased A254 nm/A410 nm ratios in the epilimnion during summer and early autumn suggested degradation of higher molecular size organic matter into low molecular size NOM. Increase in ammonium and dissolved inorganic carbon concentrations in the lake was possibly due to the NOM degradation. Using budget calculations and the literature values, photodegradation and microbial activity were estimated to be the main mechanisms of the NOM removal. These mechanisms accounted for about 30–35% and 60–65% of the total loss of organic matter, respectively, in the summer and early autumn period. Low sedimentation rates indicate that co-precipitation of organic matter with calcium, aluminium and/or iron was of minor importance in these seasons.