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Soil & Water Conservation Society of Metro Halifax (SWCSMH)
Forms of Phosphorus in water
(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."
"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.
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
"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."
"......, 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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