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Phosphate (PO43-)

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

Modified: January 31, 2016                                          Phosphorus species

Contents:

Img-pin.gif  Overview

Img-pin.gif  Forms of Phosphorus in water


Overview

(Excerptstracts from Hudson, J.J., Taylor, W.D., and Schindler, D.W. 2000. Phosphate concentrations in lakes. Letters to nature. NATURE. 406:54-56. [incidentally, Prof. Dr. David Schindler is recipient of the first-ever Stockholm Water Prize in 1991; this prize is intended as an aquatic science equivalent to the Nobel Prize!])


"Phosphate is an important nutrient that restricts microbial production in many freshwater and marine environments. The actual concentration of phosphate in phsophorus-limited waters is largely unknown because commonly used chemical and radiochemical techniques overestimate the concentration.

Here, using a new steady-state radiobioassay to survey a diverse set of lakes, we report phosphate concentrations in lakes that are orders of magnitude lower than estimates made spectrophotometrically or with the frequently used Rigler bioassay.

Our results, combined with those from the literature, indicate that microbes can achieve rapid turnover rates at picomolar nutrient concentrations. This occurs even though these concentrations are about two orders of magnitude below the level where phosphate uptake is estimated to be half the saturation level for the pico-plankton community.

Also, while phosphate concentration increased with the concentration of total phosphorus (TP) and soluble reactive phosphorus (SRP) in the lakes we sampled, the proportion of phosphate in the total phosphorus pool decreased from oligotrophic to eutrophic lakes. Such information, as revealed by the phosphate assay that we use here, should allow us to address hypotheses concerning the concentration of phosphate available to planktonic microorganisms in aquatic systems."

Conclusions:

"Although our steady-state bioassay is not suitable for routine monitoring, it is appropriate for testing hypothesis that address the phosphate concentration in phosphorus-limited pelagic systems. Our approach may also be adopted to examine the concentration of other nutrients.

Most important, our results indicate that phosphate is at picomolar concentrations in lakes across a broad range in total phosphorus, and orders of magnitude lower than measured with the best widely used methods."

Methodology:

"We have employed a new method for estimating the regeneration of dissolved phosphorus (defined as that phosphorus which passes through a filter of 0.2-µm pore size). Most of the phosphorus regenerated by the plankton is phosphate, and the rest is likely to be substrates for phosphatases. Although phosphate uptake cannot be directly measured, the uptake constant (k) for phosphate can be easily and accurately measured. Therefore, with the regeneration rate and the uptake concentration measured, we can solve for the concentration of phosphate (that is, phosphate uptake = k × [PO43-] = regeneration rate). We refer to this measure of phosphate as a steady-state estimate."

"Phosphate concentrations were determined for 56 lakes from three major physiographic regions of North America (Rocky Mountains, Interior Plains and Canadian Shield), spanning a nutrient gradient of 0.058-4.5 µM of total phosphorus (TP, which is the concentration of all phosphorus in the water, including dissolved phosphorus and phosphorus in plankton). In addition, we obtained concentrations of soluble reactive phosphorus (SRP) for 14 of these lakes from the literature. The colorimetric determination of SRP is still widely used as an estimate of phosphate concentration (for example, see APHA) and provides a contrast with our steady-state estimates. We also compare the steady-state phosphate with Rigler bioassay determinations of phosphate in two Canadian Shield lakes."

"Uptake constants (k) for phosphorus were rapid, 0.02-1.1 min-1, and indicate that phosphorus was limiting in all lakes. Dissolved phosphorus regeneration had a range of 210-26,000 pMh-1. The steady-state estimates of phosphate concentrations were between 27 and 16,800 pM. This range narrowed to 27-885 pM when three lakes were removed from the data set. These lakes were identified as outliers in the regression analysis of phosphate on TP."

Discussion:

"......, the magnitude of the differences and the consistency of our phosphate estimates indicate that there is a large discrepancy between phosphate and SRP. In no instance, even in eutrophic Halfmoon Lake (with a TP value of 2.6 µM), did SRP concentrations approximate the steady-state concentrations."

"There are two well-documented explanations for this discrepancy. First, the SRP approach requires a water-filtration step to isolate the soluble phosphorus from the particulate phosphorus (phosphorus bound in plankton). During this step an error is introduced because a portion of the particulate phosphorus is released into the soluble pool, probably as a result of cell damage. Second, the reagents that are used to determine SRP acidify the filtrate and lead to the release of bound phosphate. Both steps lead to overestimates of actual phosphate."

"Rigler bioassay estimates of phosphate in Mouse and Ranger Lakes were between 6 and 38 nM and represent concentrations that are approximately two orders of magnitude greater than the concurrent steady-state estimates. There is an obvious explanation for this discrepancy. The Rigler bioassay uses the Michaelis-Menten equation and uptake velocities for different additions of phosphate to determine the sum of the half-saturation constant for uptake (Ks) of the added phosphate and the unknown ambient phosphate concentration. Therefore, the assay can only provide a potential upper limit of phosphate, because the phosphate concentration cannot be separated mathematically from the apparent Ksfor the mixed community. Our results suggest that this apparent Ks is much greater than the phosphate concentration. Therefore, the Rigler bioassay is essentially an estimate of Ks and not of phosphate."

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