Phosphorus compounds in the dissolved and particulate phases in urban rivers and a downstream eutrophic lake as analyzed using 31P NMR

Highlights: • Transformation of phosphorus (P) compounds was investigated in a river-lake system. • The organic P compound classes such as monoester-P and DNA-P increased in Lake Barato. • DNA-P was only detected in the dissolved fraction in Lake Barato. • Pyro-P was only present in the particulate fraction both in the rivers and in the lake. Phosphorus (P) discharges from human activities result in eutrophication of lakes. We investigated whether the forms of phosphorus (P) in rivers with high effluent loads flowing through urban areas of Sapporo, Japan, were transformed when transported downstream into a eutrophic lake, namely Lake Barato. We hypothesized that the inorganic P supplied from the rivers might be transformed to organic forms in the lake. The results showed that soluble reactive phosphorus (SRP) and particulate inorganic phosphorus (PIP) dominated in the river discharge to the lake. Suspended solids in the rivers were rich in iron (Fe) so PIP was associated with Fe. A comparison of the concentrations at the river mouth and 4.5 km downstream showed that the concentrations of SRP and PIP were lower at 4.5 km downstream than at the river mouth, whereas the concentrations of organic P (i.e., dissolved organic phosphorus and particulate organic phosphorus) were similar. The results from solution ³¹P nuclear magnetic resonance spectroscopy of lake water showed that pyrophosphate was only present in the particulate fraction, while orthophosphate diesters (DNA-P) were only present in the dissolved fraction. Riverine samples contained orthophosphate (ortho-P) only, while lake samples contained ortho-P, orthophosphate monoesters, and DNA-P. The results suggest that the P forms, particularly those of dissolved P, shifted from inorganic to organic forms as the water was discharged from the river to the lake.