Sediment oxygen demand and bottom sediment environment in the northern part of the North Basin of Lake Biwa, Japan

Abstract

Sediment oxygen demand ( SOD ) and chemical composition of the profundal zone ( ≥ 80 m water depth ) of the northern part of the North Basin of Lake Biwa, Japan were analysed to identify the mechanism of formation of large-scale oxygen-deficient waters. Sediment core samples for SOD measurements were taken from five stations ( depth of 40–95 m ) . In addition, surface sediment samples for investigation of elemental composition ( carbon, nitrogen, and phosphorus ) and origin ( autochthonous or allochthonous ) of organic matter were collected from the entire area ( 65 stations ) . The horizontal distributions of organic matter content ( g m -2 ) was almost uniform at a depth of ≥ 40 m, except for a limited zone. The percentages of organic matter ( % ) that predominantly consisted of autochthonous organic matter was higher in the profundal zone. The horizontal distribution of SOD at each measuring time was nearly uniform for all stations at a depth of ≥ 40 m, though SOD varied seasonally from 192 to 271 mgO 2 m -2 d -1 . The plausible reason for the uniform distribution of SOD at a depth of ≥ 40 m was the uniformity in organic matter composition and hypolimnion water temperature in this region. These findings suggest that oxygen dynamics in the water column above the bottom sediment greatly affects the mechanism of formation of large-scale oxygen-deficient waters.