Assessing internal phosphorus load – Problems to be solved

Figures & data

Table 1 Indicators of internal load in stratified (A) and polymictic lakes (B)

A. Stratified, mono- or dimictic (deep) lakes
Severe hypolimnetic anoxia
Profiles: increasing TP and DRP with depth
Seasonal: increasing hypolimnetic TP and DRP throughout summer
Concomitant iron, manganese or reduced gas development
Fall turnover: blooms, increased turbidity
Mass balance:
• More TP leaving the lake than entering (negative retention)
• Less TP retained than predicted (from qs)
• Higher TP concentration than predicted
B. Polymictic (shallow) lakes
Seasonal: increasing TP and DRP throughout summer, even in upper water layers
Turnover events during summer: blooms, increased turbidity
Thin oxic sediment layer; occasional anoxia in weed beds and open water during quiescent conditions (early morning)
Occasional iron, manganese or reduced gas development during quiescent conditions
Mass balance:
• More TP leaving the lake than entering (negative retention)
• Less TP retained than predicted (from qs)
• Higher TP concentration than predicted

Figure 1 (A) P and DRP profiles at the dam of eutrophic Brownlee Reservoir, Snake River, CO, 11 August 1999; (B) P, dissolved oxygen (DO) and iron profiles (TFE = total; SFe = soluble; FE2 = ferrous) in oligotrophic Chub Lake, Ontario, Sep 13, 1982 (note lake characteristics in Table 2).

Figure 2 Comparison of DRP with TP from individual measurements in the anoxic hypolimnion of eight stratified Canadian lakes (with data from Nürnberg and Peters 1984b).

Table 2 Characteristics of lakes and reservoirs used as examples and case studies (Nürnberg, unpublished data; avg = average, max = maximum).

Name, Location	Area (km^2)	Mean Depth (m)	Mixing Regime*	Trophic State ^**	Summer Avg Epilimnetic TP (μ g/L)	Max Hypolimnetic TP (μ g/L)
Brome Lake, Southern Quebec	14.6	5.7	s/m	m	15	150
Brownlee Reservoir, Snake River, ID	47.5	32.3	s/m	e-h	80-130	500
Cherry Creek Reservior, Denver, CO	4	3.2	m	e	75	200
Chub Lake, Muskoka, Ontario	0.34	8.9	s	o	9.3	86
Lake Mitchell (reservoir), SD	3.1	3.7	s/m	h	320	320
Pyhäjärvi (Lake), Finland	155	5.5	s/m	m	18	100

Figure 3 L_{int_1} estimate from in situ TP increases in polymictic Lake Mitchell Reservoir, SD, for summer 2001.

Figure 4 TP and chlorophyll concentration for three growing seasons in polymictic Brome Lake, Quebec.

Table 3 Internal load estimates (L_{int_1}, equation 1) for 11 summers (summer, su = May to “End-Date”) from in situ TP concentration changes in polymictic reservoir Lake Mitchell, SD.

			TP (mg/L)			Internal TP Load
	End-Date	Days	TP at Date	Change	Lake Volume 10^6 m^3	(kg)	(mg/m^2/su)
1991	19-Aug	60	0.208	0.058	11.43	662	226
1992	26-Aug	42	0.282	0.117	12.43	1455	476
1993	18-Aug	67	0.620	0.421	13.70	5768	1775
1994	13-Aug	63	0.372	0.212	12.83	2719	875
1995	11-Aug	59	0.149	−0.121	14.20	0	0
1996	21-Sep	76	0.320	0.055	13.70	1507*	464
1997	14-Aug	70	0.340	0.165	13.49	2227	696
1998	20-Aug	126	0.488	0.140	13.49	2675	591
1999	25-Sep	121	0.320	0.060	13.20	792	249
2000	27-Aug	97	0.770	0.560	12.43	6963	2279
2001	17-Sep	96	0.540	0.327	12.43	4066	1331
Average		80	0.401	0.181	13.03	2621	815
w/o 1995		82	0.426	0.211	12.91	2883	896
Note: The days of release are days between sampling efforts; when the available efforts were not spread to the beginning (1996), adjustments of internal load (kg) were made based on the daily rates (*). In some years, TP concentration dropped in September; therefore estimates recorded only until August were not adjusted and may underestimate internal load of the whole summer.

Figure 5 Comparison of measured net P retention (R_meas from mass balance) and predicted gross retention (R_sed) for Cherry Creek Reservoir, Denver, CO (Table 2). Differences are due to internal load.

Table 4 Comparison of long-term median net and gross internal load estimates (equation 5) according to Method 2 in several water bodies. Years indicates the sample size as the number of years with mass balance estimates.

		Gross Net
Lake or Reservoir	Mixing State	(mg/m^2/yr)		Years (#)
Lake Wilcox	Dimictic	280	62	9*
Cherry Creek Reservoir	Polymictic	255	59	15
Lake Pyhäjärvi	Partially polymictic	66	13	26

Figure 6 Comparison of different internal load estimates for polymictic Cherry Creek Reservoir, Denver, CO. L_{int_3} estimates are based on a constant RR and moderately variable active areas.

Figure 7 Comparison of different internal load estimates in partially polymictic Lake Pyhäjärvi, Finland. L_{int_3} estimates include annual changes in summer temperature. Note that there was a variable biomanipulation effort between years with fish catch exporting as much TP as the outflow. An increasing trend with time is detectable for both estimates (R² = 0.21 for L_{int_2}; R² = 0.43 for L_{int_3}).

Figure 8 Comparison of observed summer TP averages with those predicted from the TP model (equation 18 with R_sed = R_pred of equation 14 and L_int) for 33 non-alpine lakes with data from Nürnberg (1998; R² = 0.84, p < 0.0001, slope is not significantly different from one and t-test on differences is not significant, p = 0.383). The line of perfect prediction (solid) and the 95% confidence band around the regression line (broken) are shown.

Figure 9 Comparison of observed Jul-Sep TP averages with those predicted from the TP model (equation 18 with R_sed = R_τ of equation 17 and k = 2.7 and L_{int_1}) for 15 years of polymictic Cherry Creek Reservoir, Denver, CO (p < 0.01, R² = 0.38, n = 15). The line of perfect prediction is shown.

Figure 10 Comparison of observed summer TP averages with those predicted from the Vollenweider model (with R_v of equation 13 for v = 10, and no provision for internal load) for 44 stratified, non-alpine lakes (including 33 of Fig. 8, computed with data from Nürnberg, 1998. R² = 0.72, p = 0.0001, n = 44, slope = 0.697, SE = 0.068 significantly different from one, and t-test on differences significant, p < 0.05). The 95% confidence bands are shown (solid). Bands of the TP model of Fig. 8, are shown for comparison (broken).

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