Differences in dissolved phosphate in shallow-lake waters as determined by spectrophotometry and ion chromatography
Phosphorus (P) plays important roles in aquatic ecosystems, but accurately determining phosphorus species (particularly bioavailable phosphate) is difficult. When phosphate concentrations are spectrophotometrically measured as soluble reactive P (SRP), the actual values are often overestimated. Ion chromatography is one method for accurately measuring orthophosphate concentrations. To clarify how the spectrophotometrical measurement overestimates actual phosphate concentrations, we compared estimates of phosphate concentrations in lake waters using both methods. SRP and orthophosphate concentrations in water samples collected from four shallow lakes around Lake Biwa, Japan, were determined monthly for 1 year by spectrophotometry and ion chromatography, respectively. SRP concentrations were frequently higher than those for orthophosphate in all lakes throughout the study period, suggesting that SRP and orthophosphate are not equivalent, although a significant relationship was found between them. Orthophosphate/SRP (ortho/SRP) ratios varied spatiotemporally in all lakes (range 0.11–1.04; mean 0.56), being well predicted by biological-mediated relevant parameters using a stepwise multiple logistic regression analysis (R2 = 0.76). The results implied that organic matter associated with biological activity contributes to the variability in ortho/SRP ratios. Such large variability in ortho/SRP ratios emphasizes the importance of accurate orthophosphate estimates for understanding P dynamics in aquatic ecosystems.
KeywordsBioavailable phosphorus Analytical methods Orthophosphate/SRP ratio Biological activity Freshwater lakes
We thank the members of our laboratories at The University of Shiga Prefecture for supporting the field and laboratory work. This study was supported through grants-in-aid for Specially Promoted Research in The University of Shiga Prefecture to MM, a grant-in-aid for Scientific Research (grant no. 18H03961) from the Japan Society for the Promotion of Science to SB, and a Chinese Scholarship Council Grant to RY. We thank Gareth Thomas, PhD, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.
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