Effects of Hydrological and Biological Factors on Fine Benthic Organic Matter at a Forested River in Yahagi Basin, Japan

The role of fine particle organic matters (FPOM) is important in the energy source of benthic organisms and microbes and organic transportation from terrestrial water ecosystem to costal water ecosystem. Accumulated FPOM at a river bed is called Fine benthic organic matter (FBOM). It is considered that FBOM is the source of FPOM at the flood period. But the mechanism of the accumulation and the flush of FBOM are not concerned in research. In this research, we examined effects of hydrological and biological factors on FBOM. The purpose of this research is 1) the routine sampling of FBOM for the collecting basic FBOM data, 2) the examination of the relationship between of the water level or the current velocity and the mass of FBOM, 3) the determination of the FBOM mass of microbial decomposition by a double exponential model. This study was conducted in Tomoe river which was substream of Yahagi river whose length was 117km, located at central of Honshu Island, Japan. The above sea level of the sampling site is 320m and 75km upstream from an estuary. The drainage basin is covered over 90 % by a forest. The stream bed is covered by the coarse fragment and sand. The barrage of a hydroelectric power plant is located at 100m downstream from the sampling site. The sampling area was 30 cm diameter divided by the polyethylene barrel. These samples were divided into two fractions: Coarse-Sediment, over 1mm and Fine-Sediment, 2mm - 63μm. FBOM and CBOM were determined from the amount of carbon containing in Fine-Sediment and Coarse-Sediment. Total C and Total N were measured by CN coder. The water level of river was measured by a level rod and the surface current velocity was measured by a floater at a sampling day. Sampling of FBOM was performed every 2 weeks from the end of August 2008 to the middle of April 2009. The heavy precipitation(98.0 mm/day) was measured at the end of August. This precipitation caused highest water level and fastest current velocity of river during this investigation. The water level of river decreased gradually after the end of August. There were small rain events during September to December, but these events did not let the water level increased. The mass of FBOM collected 0.2(g/m2) at the end of August. The mass of FBOM was increased and decreased between September and October. The mass of FBOM was 4.5(g/m2) at the end of October, subsequently decreased 0.5(g/m2) gradually at April 2009. These investigations showed that FBOM is decreased severely by the large flood, but the other factors are influenced to the FBOM fluctuation. As the effect of hydrology on FBOM, the mass of FBOM was decreased with increasing the water level during the water level increasing period. But the mass of FBOM had the negative relationship with the current velocity during the water level deceasing period. CN ratio of CBOM was larger than that of FBOM. This means that FBOM is biodegraded by organisms from CBOM or CPOM. Then the calculated result of the ratio of decomposition of FBOM was showed that the biodegraded mass of FBOM from CBOM might be less affected than hydrological factors during 2 weeks at investigation site.