Temporal and Vertical Variability in the Relationship Among Organic Matter Indices in a Deep Reservoir Ecosystem

Abstract

ABSTRACT Temporal and vertical variability in the relationship between biochemical oxygen demand (BOD), chemical oxygen demand (CODMn), and total organic carbon (TOC) were investigated in a deep reservoir (Lake Soyang, Korea) and compared with other major reservoirs in Korea. The average oxidation efficiencies of BOD and COD were low (16% and 36%, respectively), compared with TOC analysis. Weakly positive correlations could be observed between TOC vs. BOD (r = 0.64) and TOC vs. CODMn (r = 0.66) in the upper layer, whereas significant correlations were not observed in the middle and deep layers of the water column. In the upper layer, CODMn was more strongly correlated with particulate organic carbon (POC) than with dissolved organic carbon (DOC), even though DOC is ten times more abundant than POC. This might imply that DOC in the lake is more resistant to permanganate oxidation than POC. The oxidation efficiency of BOD, an indicator of biodegradability, varied among water layers (upper > deep > middle layer). The lower efficiency in the middle layer may be due to an input of recalcitrant allochthonous organic matter during the summer monsoon, when water is known to flow into this layer of the lake. The oxidation efficiency of CODMn also varied significantly by water layer (upper ≅ middle > deep layer). Because of poor and variable oxidation efficiencies, BOD and CODMn measured at any particular time or location should be used with caution as indices of organic matter content in lakes.