Diffusive phosphorus fluxes in relation to the sediment phosphorus profile in Big Traverse Bay, Lake of the Woods

Figures & data

Figure 1. Sediment sampling station location in Big Traverse Bay, Lake of the Woods, Minnesota, USA. Star represents the sampling station location in the present study while triangles denote station sampling in Big Traverse Bay in 2011. The black dot represents the location of the vertical thermistor array in 2015 (Edlund M and Heathcote A, St. Croix Watershed Research Station, Science Museum of Minnesota, unpubl. data).

Figure 2. Variations in mean (±1 standard error, n = 6) diffusive phosphorus (P) flux under (a) anaerobic and (b) aerobic conditions versus collection date and temperature.

Figure 3. Variations in (a) anaerobic (black circles) and (b) aerobic diffusive phosphorus fluxes (gray circles) versus temperature (n = 6 for each temperature). Dotted lines denote the 95% confidence intervals while the solid line represents the regression line (SAS 1994). Black and gray triangles denote diffusive phosphorus fluxes determined from Big Traverse Bay east and west stations in 2011.

Figure 4. Variations in (a) mean monthly bottom temperature ∼0.5 m above the sediment surface and predicted monthly (b) anaerobic and (c) aerobic diffusive P flux estimated from the regression equations presented in Figure 3. Striped columns denote indirectly estimated bottom temperature based on historic vertical profiles.

Figure 5. Vertical variations in sediment phosphorus (P) fractions from a sediment core collected in Big Traverse Bay in February 2015. Redox-sensitive P (i.e., redox-P) is the sum of loosely bound and iron-bound P fractions and biologically labile P (i.e., bio-labile P) is the sum of redox-P and labile organic P. Vertical dashed lines denote the mean buried P concentration below the 6 cm sediment depth.

Table 1. Estimation of annual long-term internal phosphorus (P) loading from a sediment core collected in Big Traverse Bay in February, 2015, modified from Rydin et al. (2011) as the difference between surface and buried P multiplied by an average sedimentation rate of 660 g/m²/yr.

Variable	Total P	Bio-labile P	Redox-P
P concentration in upper 1 cm (mg/g)	0.964	0.509	0.391
P concentration below 6 cm (mg/g)	0.524	0.124	0.098
Gross deposition (mg/m^2 y)	637	336	258
Burial (mg/m^2 y)	346	82	65
Annual internal P loading (mg/m^2 y)	291	254	193
Burial efficiency (%)	54	24	25

Table 2. A comparison of ranges (minimum, mean, maximum) in annual internal phosphorus (P) loading from large, shallow, polymictic systems in Northern USA and Canada versus Big Traverse Bay, Lake of the Woods. “Partial net” internal P load refers to mass P balance approaches that include some deposition of internal P loads back to the sediment while “gross” internal P load includes only P flux from the sediment into the water column (Nürnberg 2009).

		Annual internal P load (mg/m^2/yr)
System		Minimum	Mean	Maximum	Reference
Big Traverse Bay, Canada, USA	Grossa	38		1171	This study
	Grossb	193		291	This study
Lake Diefenbaker, Saskatchewan, Canada	Partial net	398			North et al. (2015)
Lake Erie, Western Basin, Canada & USA	Gross	157		332	Matisoff et al. (2016)
Lake Simcoe, Ontario, Canada	Gross	86			Nürnberg et al. (2013)
Lake Winnipeg, Alberta, Canada	Partial net to gross	259		468	Nürnberg and LaZerte (2016)

^a Laboratory-derived diffusive P flux under aerobic (left column) and anaerobic (right column) conditions

^b Derived from the sediment redox-P (left column) and total P (right column) vertical profile

Table A1. Replicate diffusive phosphorus (P) flux regression analysis for various dates. r² = coefficient of determination, CI = confidence interval, SE = standard error of the mean.

	Aerobic diffusive P flux (mg/m^2/d)				Anaerobic diffusive P flux (mg/m^2/d)
rep	r^2	Rate	Lower 95% CI	Upper 95% CI	r^2	Rate	Lower 95% CI	Upper 95% CI)
	July 2014 @ 18 C
1	0.84	0.22	0.15	0.29	0.99	9.77	8.04	11.51
2	0.96	0.64	0.45	0.74	0.97	5.49	4.14	6.85
3	0.84	0.12	0.08	0.16	0.98	6.83	5.47	8.19
4	0.78	0.18	0.11	0.25	0.99	6.99	5.80	8.18
5	0.85	0.33	0.22	0.43	0.97	5.92	4.44	7.41
6	0.83	0.2	0.13	0.27	0.96	11.71	8.57	14.85
Mean		0.28	0.19	0.36		7.79	6.08	9.50
SE		0.08	0.06	0.08		0.99	0.75	1.26
	August 2014 @ 25 C
1	0.88	0.22	0.12	0.31	0.99	16.04	14.27	17.81
2	0.96	0.13	0.10	0.17	0.99	18.71	16.83	20.60
3	0.99	0.53	0.49	0.58	0.99	20.31	17.96	22.66
4	0.97	0.45	0.35	0.55	0.99	12.48	11.33	13.62
5	0.98	0.39	0.32	0.46	0.99	9.22	8.24	10.20
6	0.91	0.42	0.27	0.57	0.99	23.97	20.85	27.09
Mean		0.36	0.28	0.44		16.79	14.91	18.66
SE		0.06	0.06	0.07		2.19	1.88	2.51
	February 2015 @ 5 C
1	0.76	0.09	0.02	0.15	0.98	1.47	1.23	1.71
2	0.58	0.05	0.01	0.11	0.99	0.46	0.43	0.49
3	0.77	0.09	0.02	0.15	0.99	0.76	0.70	0.82
4	0.49	0.02	−0.01	0.05	0.99	1.15	1.06	1.25
5	0.76	0.03	0.01	0.05	0.72	0.15	0.07	0.24
6	0.73	0.04	0.01	0.08	0.99	0.65	0.51	0.78
Mean		0.05	0.01	0.10		0.77	0.67	0.88
SE		0.01	0.00	0.02		0.19	0.17	0.22
	May 2015 @ 11 C
1	0.99	0.13	0.11	0.15	0.96	6.52	5.46	7.58
2	0.86	0.03	0.01	0.05	0.95	2.79	2.26	3.32
3	0.96	0.06	0.05	0.08	0.94	2.81	2.26	3.37
4	0.93	0.06	0.04	0.08	0.90	1.86	1.36	2.36
5	0.87	0.16	0.08	0.24	0.88	1.38	0.96	1.79
6	0.87	0.11	0.06	0.15	0.98	1.69	1.47	1.90
Mean		0.09	0.06	0.13		2.84	2.30	3.39
SE		0.02	0.01	0.03		0.77	0.67	0.88

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