Algal blooms, circulators, waterfowl, and eutrophic Greenfield Lake, North Carolina

Figures & data

Figure 1. Greenfield Lake, Wilmington, NC, showing water sampling stations, major tributaries, the road circling the lake, and locations of Solarbee mixers.

Table 1. Nutrient input factors used for various waterfowl taxon or functional groups, provided as g/dry wt of TN or TP excreted per day in feces for the individual taxon or taxa group.

	TN (g dry wt)	TP (g dry wt)
Geese^1	1.57	0.49
Dabbling ducks^2	0.72	0.22
Diving Ducks^2	0.61	0.19
Muscovy Ducks^2	0.97	0.30
Cormorants^3	0.89	3.87
Gulls^3	0.44	0.24
Egrets and Herons^2	0.97	2.64
Coots^2	0.28	0.09
Grebes^4	0.20	0.89

¹Manny et al. (1994)

²Derived from waterfowl calculation.

³Marion et al. (1994)

⁴Derived from cormorant calculation.

Table 2. Selected water quality parameters in Greenfield Lake before (Jan 2000–Jan 2005) and after (Mar 2005–Dec 2013) lake rehabilitation measures in 2005, by individual station and all data combined. Presented as mean ± standard deviation/range.

Parameter and units		GL-P	GL-YD	GL-2340	Whole lake
Dissolved oxygen	pre	8.3 ± 4.1	7.8 ± 3.2	5.4 ± 2.0	7.1 ± 3.5
		(0.2–18.9)	(0.7–15.8)	(1.2–8.8)	(0.2–18.9)
(DO, mg/L)	post	9.1 ± 2.7	7.9 ± 2.6	5.6 ± 2.9	7.5 ± 3.1
		(3.8–17.0)	(2.5–14.8)	(1.0–14.7)	(1.0–17.0)
Turbidity	pre	7.3 ± 11.5	7.0 ± 11.5	4.1 ± 4.8	6.1 ± 9.8
		(1.0–53.0)	(1.0–54.0)	(1.0–28.0)	(1.0–54.0)
(NTU)	post	5.1 ± 6.0	4.3 ± 4.5	3.7 ± 4.3	4.4 ± 5.0
		(1.0–35.0)	(1.0–21.0)	(1.0–28.0)	(0–35.0)
TSS	pre	8.3 ± 15.9	8.6 ± 11.5	4.3 ± 3.2	7.0 ± 11.6
		(1.0–13.0)	(1.0–74.0)	(1.0–12.0)	(1.0–113.0)
(mg/L)	post	6.3 ± 7.3	6.8 ± 8.7	12.1 ± 25.3	8.4 ± 16.2
		(1.0–45.0)	(1.0–46.0)	(1.0–151.0)	(1.0–151.0)
Total ammonia	pre	118.6 ± 217.4^*^*	124.3 ± 281.2^*	75.4 ± 106.7	105.7 ± 212.3^*
		(5.0–1100.0)	(5.0–1680)	(5.0–647.0)	(5.0–1680.0)
(µg/L N)	post	46.7 ± 71.5	47.1 ± 71.1	64.8 ± 66.3	53.0 ± 69.8
		(5.0–390.0)	(5.0–410.0)	(5.0–310.0)	(5.0–410.0)
Nitrate + nitrite	pre	51.2 ± 75.5^*	54.7 ± 62.3	138.7 ± 154.8	82.1 ± 113.4
		(5.0–460.0)	(1.0–320.0)	(5.0–620.0)	(5.0–620.0)
(µg/L N)	post	34.4 ± 55.5	43.2 ± 58.9	155.3 ± 137.3	78.3 ± 107.4
		(5.0–260.0)	(5.0–240.0)	(5.0–470.0)	(5.0–470.0)
Total nitrogen	pre	1141 ± 1382	1170 ± 1452	838 ± 546	1047 ± 1195
		(70–8,710)	(120–9810)	(50–5100)	(70–9810)
(TN, µg/L)	post	911 ± 746	883 ± 753	967 ± 857	921 ± 784
		(50–3910)	(50–5000)	(50–5100)	(50–5100)
Orthophosphate	pre	31.7 ± 29.4	70.5 ± 290.5	65.4 ± 282.0	55.6 ± 232.1
		(5.0–120.0)	(4.0–2011)	(5.0–2013)	(5.0–2013)
(µg/L P)	post	33.8 ± 35.3	26.9 ± 23.4	20.6 ± 12.9	27.1 ± 26.0
		(5.0–180.0)	(5.0–100.0)	(5.0–60.0)	(5.0–180.0)
Total phosphorus	pre	100.9 ± 93.1	193.1 ± 558.4	144.7 ± 483.5	145.3 ± 425.4
		(10.0–560.0)	(20.0–3760.0)	(5.0–3440)	(10.0–3760.0)
(TP, µg/L)	post	100.9 ± 68.6	102.5 ± 60.8	85.3 ± 78.0	96.1 ± 69.7
		(10.0–390.0)	(40.0–420.0)	(10.0–420.0)	(10.0–420.0)
DIN/DIP	pre	28.1 ± 47.5^*	35.4 ± 64.1	27.2 ± 27.6	30.1 ± 48.2^*
		(0.4–223.6)	(0.1–385.3)	(0.1–146.1)	(0.1–385.3)
Molar ratio	post	8.2 ± 11.6	11.3 ± 16.8	30.9 ± 29.3	16.9 ± 22.9
		(0.5–75.3)	(0.8–93.0)	(1.1–119.6)	(0.5–119.6)
TN/TP	pre	45.9 ± 57.8	32.7 ± 24.2	50.9 ± 91.9	43.4 ± 55.7
		(0.7–321.4)	(0.4–98.5)	(0.8–425.1)	(0.4–425.1)
Molar ratio	post	25.1 ± 25.5	22.4 ± 23.3	35.0 ± 33.7	26.7 ± 28.3
		(0.9–173.2)	(0.8–93.0)	(0.7–186.0)	(0.7–186.0)
Chlorophyll a	pre	22.1 ± 35.9^*^*	20.3 ± 25.1^*^*	16.0 ± 27.8	19.4 ± 28.7^*
		(1.0–169.1)	(2.4–118.9)	(1.0–158.9)	(1.0–169.0)
(µg/L)	post	35.9 ± 44.5	37.6 ± 32.7	28.2 ± 39.7	33.8 ± 39.3
		(1.2–303.0)	(1.0–157.0)	(1.0–209.5)	(1.0–303.0)
BOD5	pre	3.9 ± 2.6	NA	2.4 ± 1.2	3.0 ± 2.1
		(1.3–9.0)		(1.0–4.4)	(1.0–9.0)
(mg/L)	post	3.6 ± 2.3	4.0 ± 2.0	3.2 ± 2.8	3.6 ± 2.4
		(1.0–16.0)	(1.1–9.2)	(1.0–14.0)	(1.0–16.0)
Fecal coliforms	pre	697 ± 1250^*	360 ± 737	1652 ± 8473	914 ± 5009
		(10–6000)	(1–3000)	(12–60,000)	(1–60,000)
(CFU/100 mL)	post	394 ± 939	468 ± 1697	2375 ± 10,539	1090 ± 6270
		(3–5300)	(3–12,000)	(5–60,000)	(3–60,000)

^*indicates significant difference (P < 0.05) between periods

^**indicates highly significant difference (P < 0.01) between periods. BOD5 data for GL-YD before rehabilitation measures is not shown due to only 2 samples from that station during that period.

Figure 2. Water quality parameter concentrations in Greenfield Lake, 2006–2013, presented by sampling station as means ±1 standard error of the mean (n = 50 samples). Letters above the bars designate and rank significant (P < 0.05) differences among stations. Panel a = dissolved oxygen, panel b = nitrate, panel c = orthophosphate, panel d = DIN/DIP ratio, panel e = chlorophyll a, panel f = fecal coliform bacteria.

Table 3. Monthly average total waterfowl counts and estimated waterfowl nutrient deposition to the lake and shoreline.

Month and Yr	Average Bird Count per Day	Total N (kg dry wt)	Total P (kg dry wt)
Aug 2000	85.0	1.02	0.26
Sep 2000	95.6	1.12	0.37
Oct 2000	158.8	2.63	1.73
Nov 2000	349.8	4.77	3.46
Dec 2000	793.0	11.73	28.72
Jan 2001	688.3	11.12	20.28
Feb 2001	582.0	7.56	11.43
Mar 2001	563.2	6.87	11.45
Apr 2001	186.7	2.59	4.21
May 2001	92.2	1.57	2.38
Jun 2001	85.0	1.08	0.49
Jul 2001	102.5	1.72	0.68

Figure 3. Results of nutrient addition bioassay experiments from 1998 and 1999. Bars represent means of 3 replicates per treatment averaged over 3 days; ^*above bar represents significant (P < 0.05) difference from control. Panel a = September 1998, panel b = October 1998, panel c = April 1999, panel d = June 1999, panel e = August 1999.

Table 4. Number and percent of dates when water samples exceeded North Carolina water quality standards before (Jan 2000–Jan 2005) and after (Mar 2005–Dec 2013) lake aeration, grass carp additions, and herbicide treatments.

Parameter	NC standard	Pre-rehabilitation	Post-rehabilitation
Dissolved oxygen	5.0 mg/L	39/147 (27%)	33/168 (20%)
Turbidity	50 NTU	3/147 (2%)	0/168 (0%)
Chlorophyll a	40 µg/L	16/147 (11%)	52/168 (31%)
Fecal coliform bacteria	200 CFU/100 mL^*	60/147 (40%)	59/168 (35%)

^*Violation of the freshwater fecal coliform standard technically requires the geometric mean of 5 samples collected within 30 days to exceed 200 CFU/100 mL. The data from this study were collected monthly to bimonthly.

Manny BA, Johnson WC, Wetzel RG. 1994. Nutrient additions by waterfowl to lakes and reservoirs: predicting their effects on productivity and water quality. Hydrobiologia. 279/280:121–132.

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Marion L, Clergeau P, Brient L, Bertru G. 1994. The importance of avian contributed nitrogen (N) and phosphorus (P) to Lake Grand-Lieu, France. Hydrobiologia. 279/280:133–147.

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