The synergistic effect of elevated CO₂ and phosphorus on reservoir eutrophication

Figures & data

Table 1. Select reservoir characteristics for Beaver Lake and Lakes Brittany, Norwood, Rayburn, and Fayetteville. Data from Ben Thompson's master's thesis and Bryant Baker's master's thesis, and from the Arkansas Department of Environmental Quality.

Reservoirs	Surface area km^2	TN (ug/L)	TP (ug/L)	Chl-a (ug/L)	pH
Beaver Lake	113	800	5	5	8.53
Lake Brittany	<0.2	400	15	6	7.88
Lake Norwood	<0.2	650	20	15	8.14
Lake Rayburn	<0.2	700	32	19	8.37
Lake Fayetteville	<0.2	690	92	38	8.24

Figure 1. Reconstructed carbon dioxide partial pressures (PCO₂) for (a) Beaver Lake (1966–2010) and Lakes (b) Brittany, (c) Norwood, and (d) Rayburn (2011 and 2012). Note the different scale on (a). The dotted line represents the current global PCO₂ level.

Figure 2. Reconstruction of carbon dioxide partial pressures (PCO₂) during June, July, and August in (a–c) 2011 and (d–f) 2012 at Beaver Lake, northwest Arkansas. The dotted line represents the current global PCO₂ level.

Table 2. Summary of CO₂ partial pressure (PCO₂) for 22 June, 22 July, and 22 August 2011 and 2012.

	Carbon dioxide partial pressure (PCO2 µatm)
Month	Maximum	Minimum	Median	Mean
22 June 2011	209	40	89	99 ± 3
22 July 2011	86	18	34	44 ± 1
22 August 2011	66	14	37	35 ± 1
22 June 2012	339	8	55	76 ± 4
22 July 2012	195	32	58	74 ± 2
22 August 2012	361	131	203	228 ± 4

Figure 3. Changes in δ¹³C and measured PCO₂ over (a) a 24-hour period at Lake Fayetteville, northwest Arkansas, and (b) relationship between modeled and measured PCO₂ for Lake Fayetteville, northwest Arkansas. The dotted line represents the current atmospheric PCO₂ level.

Figure 4. The relationship between chlorophyll a (Chl-a) and particulate carbon (PC) at (a) 250 µatm, (b) 400 µatm, and (c) 550 µatm.

Figure 5. Algal biomass response reported as (a, b, and c) particulate carbon (PC) and (d, e, and f) chlorophyll-a to different levels of PCO₂ along a phosphorus concentration gradient. The r² values represent the linear fit with P concentration as the x-variable.

Table 3. Pairwise comparison of phosphorus treatment and corresponding p-values for particulate carbon (PC), chlorophyll a (Chl-a) and carbon to phosphorus (C:P), carbon to nitrogen (C:N), and nitrogen to phosphorus (N:P) ratios. Results were from a 2-way ANOVA, but there were no significant differences in PCO₂, so only P treatment values are shown. Values <0.05 represent significant differences.

P treatment comparison	PC	Chl-a	C:P	C:N	N:P
(ug/L)	p-value
100 vs. 5	<0.001	<0.001	<0.001	NS	<0.001
100 vs. 10	<0.001	<0.001	<0.001	NS	<0.001
100 vs. 25	<0.001	0.002	<0.001	0.005	<0.001
100 vs. 50	NS	NS	0.021	0.018	NS
75 vs. 5	<0.001	0.002	<0.001	NS	<0.001
75 vs. 10	0.001	0.012	<0.001	NS	<0.001
75 vs. 25	<0.001	NS	<0.001	0.03	0.002
50 vs. 5	NS	NS	<0.001	NS	<0.001
50 vs. 10	0.004	NS	<0.001	NS	<0.001
50 vs. 25	0.002	NS	<0.001	NS	NS
25 vs. 5	<0.001	NS	<0.001	NS	<0.001
25 vs. 10	<0.001	NS	<0.001	0.033	NS

Figure 6. Algal carbon to phosphorus (C:P), carbon to nitrogen (C:N), and nitrogen to phosphorus (N:P) molar ratio across a phosphorus gradient at carbon dioxide partial pressures (PCO₂) of 250 µatm, 400 µatm and 550 µatm.