Spectral Characteristics of Phragmites australis and Its Response to Riverine Nitrogen and Phosphorus Contents in River Reaches Restored by Reclaimed Water

Reclaimed water is an important water source replenishing rivers and lakes for urban landscape. Higher contents of nitrogen and phosphorus in reclaimed water will cause eutrophication, disrupting the balance of hydro-ecology. Hyperspectral technology was applied to analyze the spectral characteristics of the emergent plant Phragmites australis, and the spectral characteristics response of P. australis leaf to nitrogen and phosphorus contents were explored in the Chaobai River restored by reclaimed water. Results showed that concentrations of total nitrogen (TN), total phosphorus (TP), chlorophyll a (Chl a), and dissolved oxygen (DO) were 1.85-18.16 mg·L ^-1, 0.01-0.36 mg·L^-1, 0.60-47.45 μg·L ^-1, and 4.24-11.4 mg·L^-1, respectively. Although the river water eutrophication was serious, it was still in an oxygen-rich environment. The results showed that there were significant differences in the concentrations of TN, TP, and Chl a among sampling sites (P<0.05) in multiple analysis of variance. With the increasing of riverine TN concentrations, the reflectance of leaf spectrum in the visible band lowered and the position of red edge also moved towards higher wavelength (i.e., redshift). The riverine TN and TP contents had significant correlations with the absorbance value log(1/R) in the visible band in correlation analysis, and the correlation coefficients between TN and log(1/R) were higher than that of TP. The difference of TN concentrations could be inferred by the spectrum of P. australis leaf to a certain extent, while the effect of TP on spectral characteristics was weaker than TN. TN was selected to establish fitting models with different spectral indices. Based on the photochemical reflectance index (PRI), the modified chlorophyll absorption ratio index (MCARI) and the derivative chlorophyll index (DCI), the exponential equations explained 62.4%-70.9% of TN (P<0.05), which could be useful for quantitatively monitoring of nitrogen contents in reclaimed water. This research proved practicability of plant spectrum technology in water eutrophication monitoring, providing a scientific basis for ensuring water quality safety and ecological security in rivers and lakes restored by reclaimed water.