Total phosphorus analyses: is the convenience of automated inline-UV digestion robust enough for freshwater nutrient monitoring?

Figures & data

Table 1. Study site classifications, locations, and average TP estimations with associated error for both digestion methods.

Name	Type	Latitude	Longitude	Manual digestion		Inline-UV digestion
				Mean TP (µg/L)	Standard deviation (µg/L)	Mean TP (µg/L)	Standard deviation (µg/L)
Buffalo	Lake	48.06231	−118.89899	33	2.1	22	2.9
Roosevelt	Reservoir	47.94769	−118.98659	33	2.5	15	1.2
Banks	Reservoir	47.93501	−119.03041	32	1.5	6	0.6
Rebecca	Lake	48.05531	−118.93730	141	1.5	164	8.1
North Twin	Lake	48.28915	−118.36769	72	6.7	70	2.1
South Twin	Lake	48.27314	−118.38106	35	7.6	30	1.2
Owhi	Lake	48.22753	−118.90479	104	9.0	86	7.0
Round	Lake	48.29527	−118.32225	71	2.9	64	1.0
Sanpoil	River	48.22838	−118.69953	73	1.7	92	1.0
Snake	River	46.16237	−116.92293	90	2.0	29	1.7
Rock	Lake	47.13940	−117.72504	49	7.2	33	2.9
McGinnis	Lake	48.03570	−118.89880	38	1.5	30	9.2
Sunnyside	Pond	46.72924	−117.19215	328	14.2	333	3.5
Newman	Lake	47.77293	−117.08523	22	5.3	13	0.6
Fernan	Lake	47.67021	−116.74794	42	1.0	42	0.6
Hauser	Lake	47.77044	−117.01806	65	0.6	51	1.7
Spokane	River	47.68353	−117.11181	33	2.5	44	4.4
Hayden	Lake	47.75415	−116.75491	34	0.6	45	1.2
Liberty	Lake	47.64830	−117.09170	54	13.3	42	11.5
Coeur d'Alene	Lake	47.67307	−116.78757	30	1.5	40	1.0
North Fork Clearwater	River	46.50569	−116.31662	30	2.9	34	1.5
Main Stem Clearwater	River	46.50041	−116.31809	45	3.6	44	0.6
Garfield	Pond	46.99791	−117.19166	228	2.0	271	1.5
Horddeman's	Pond	46.73871	−116.97430	361	7.0	270	26.2
Winchester	Lake	46.23436	−116.62347	177	17.3	171	3.8
Moose Creek	Reservoir	46.87442	−116.42051	85	4.9	83	1.5
Potlach	River	46.62387	−116.63339	83	6.1	95	0.6
Paradise Creek	River	46.73129	−117.08627	207	6.2	233	5.9
Palouse	River	46.89080	−117.37478	335	2.5	341	0.6
Spring Valley	Reservoir	46.78553	−116.75516	38	2.1	27	2.6
Wawawai	Reservoir	46.63473	−117.37800	100	2.3	97	4.2
Lower Granite	Reservoir	46.62903	−117.38083	87	15.3	95	1.2

Table 2. Comparison of TP recovery from third-party QC reference material.

Method	n	Mean TP (µg/L)	Standard deviation (µg/L)	Certified value (µg/L)	Acceptable limits (µg/L)
Autoclave	7	205	1.7	207 ± 3.64	179 to 234
UV	5	205	11.4

Figure 1. Comparison of log-log transformations of inline-UV and autoclave TP digestion methods. Points represent mean of triplicate samples. Solid black line represents best fit linear model (p < 0.001, R² = 0.8110) and gray polygon represents 95% confidence intervals. Dotted line represents a 1:1 relationship between methods.

Figure 2. Histogram of percent difference between TP digestion methods for all samples (n = 96). Grey bar represents samples that displayed <10% difference between methods (n = 34), and white bars outlined in black represent samples that displayed >10% difference (n = 62).

Figure 3. Graphical representation of (a) homogenized sample within a sample cup, (b) possible separation of positively and negatively buoyant particles within sample cup leading to underestimation of TP, and (c) possible congregation of neutrally buoyant particles within sample cup leading to overestimation of TP concentration.