Investigation of Water Quality in a River with Significant Algal Growth

Abstract

Compared with those in lakes and sea areas, the increase in the amount of organic matter (internal production) caused by algal growth in rivers is poorly understood. Therefore, to clarify the effect of internal production in rivers on water quality, we investigated the stagnant section of Hanamune River, where significant algal growth is observed, focusing on flow conditions and the elemental composition of particulate matter from which algae are fractionated. As a result, we were able to classify algae observed in rivers into the proliferative and inflow/mixed types depending on pH and dissolved oxygen saturation level. In addition, elemental composition ratios suggested that algae accounted for much of the particulate matter in the stagnant section of Hanamune River. From this, the term ‘biochemical oxygen demand (BOD) derived from algae (BOD_-Algae) ’ was defined as the BOD originating from particulate matter (P-BOD) in the stagnant section of the Hanamune River. By using chlorophyll-a as an indicator of algal abundance, the formula［BOD_-Algae (mg L^-1) ］ = 0.24 × ［Chl-a (μg L^-1) ］^0.66) was derived. From the obtained results, it was possible to quantitatively understand the effect of river internal production on BOD.