Decomposition Activity and Nutrient Regeneration Rates in the Hypolimnion of the North Basin of Lake Biwa

Abstract

In the course of a recent survey on nutrient dynamics in the north basin of Lake Biwa, an abundant accumulation of nitrate was observed in the hypolimnion during the stagnation period, and the regeneration rate of nitrogen in the hypolimnion was estimated to be 40.5 mmol⋅m^-2⋅mon^-1. However, it was calculated on the basis of oxygen consumption that the C : N ratio of organic matter mineralized within the hypolimnion should'have been 19-21, which suggested a low regeneration efficiency of nitrogen relative to carbon. Dissolved phosphate was found to be accumulated in small concentrations near the bottom, but the regeneration rate of phosphate was very low (0.22 mmol⋅m^-2⋅mon^-1), and the activity of hypolimnetic biota seemed to contribute hardly at all to phosphorus recycling in this basin. Possible factors determining the regeneration efficiency of nutrients are discussed in relation to the physiological properties of the hypolimnetic microbial metabolism.