6.0
Greenfield Lake Water Quality
Three tributaries of Greenfield Lake were sampled for physical, chemical,
and biological parameters (Table 6.1, Fig.
6.1).
All three tributaries suffered from extreme hypoxia, with GL-JRB (Jumping
Run Branch), GL-LB (creek at Lake Branch Drive) and GL-LC (creek beside
Lakeshore Commons) all showing average concentrations below the state standard
(DO < 5.0 mg/L). Dissolved
oxygen levels periodically were 1.0 mg/L or less at all three tributaries during
the summer months (Table 6.1; Appendix B). Turbidity and suspended solids were generally low in the
tributary stations (Table 6.1). total
nitrogen and nitrate concentrations were highest at GL-LC, somewhat lower at
GL-LB, and lowest at GL-JRB (Table 6.1). Ammonium
concentrations were highest at GL-LB, and generally similar across the other two
tributary stations. Phosphorus
concentrations were similar at the three sites.
All three of these input streams maintained fecal coliform levels
indicative of poor water quality, with fecal coliform counts exceeding the state
standard for human contact waters (200 CFU/100 mL) three of seven times at
GL-LB, five of seven times at GL-LC, and five of seven times at GL-JRB.
There was one major algal bloom in June at GL-LC, with a chlorophyll a level of 95 mg/L.
Table 6.1.
Mean and (standard deviation) of water quality parameters in tributary
stations of Greenfield Lake, January - September 2004.
Fecal coliforms as geometric mean; N/P ratio as median; n = 7 samples for
all parameters.
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Parameter
GL-JRB
GL-LB
GL-LC
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DO (mg/L)
4.3 (3.3)
2.8 (2.1)
2.9 (1.8)
Turbidity (NTU)
3 (1)
2 (1)
3 (3)
TSS (mg/L)
3.0 (1.6)
2.9 (1.3)
3.6 (3.1)
Nitrate (mg/L)
0.093 (0.065)
0.244
(0.180)
0.457
(0.418)
Ammonium (mg/L)
0.086 (0.046) 0.189 (0.075)
0.100
(0.031)
TN (mg/L)
1.106 (0.563)
1.509 (0.929)
1.546
(0.545)
Orthophosphate (mg/L)
0.023 (0.017) 0.026 (0.014)
0.023
(0.013)
TP (mg/L)
0.070 (0.046) 0.097 (0.105)
0.074
(0.055)
N/P molar ratio
24.4
34.0
67.9
Fec. col. (/100 mL)
277
313
417
Chlor. a (mg/L)
4.1 (2.2)
1.8 (1.1)
15.6 (35.1)
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Three in-lake stations were sampled (Table 6.2).
Station GL-2340 represents an area receiving a considerable influx of
urban/suburban runoff, GL-YD is downstream and receives some outside impacts,
and GL-P is at Greenfield Lake Park, away from inflowing streams but in a
high-use waterfowl area (Fig. 6.1). Low
dissolved oxygen affected all three sites (Appendix B), with GL-P suffering from
DO < 1.0 mg/L from July through September when the surface was covered by a
dense bloom of duckweed (Lemna sp.) mixed with various algae.
Two major algal blooms occurred at GL-P and one at GL-2340, and one minor
bloom occurred at GL-YD. Turbidity
and suspended solids were low to moderate at these three sites, with high TSS in
September at GL-P. In contrast to
last year, fecal coliform concentrations were only problematic at GL-P (Appendix
B) with three of seven samples exceeding the State standard in
2004.
Nitrate
concentrations were similar among the three sites, while total nitrogen,
ammonium, and total phosphorus were highest at GL-P (Table
6.2).
This was a result of high
summer ammonium and organic N near the park resulting from decaying bloom
material. Inorganic N/P molar
ratios can be computed from ammonium, nitrate, and orthophosphate data and can
help determine what the potential limiting nutrient can be in a water body. Ratios well below 16 (the Redfield ratio) can indicate
potential nitrogen limitation, and ratios well above 16 can indicate potential
phosphorus limitation (Hecky and Kilham 1988).
Based on the median N/P ratios (Table 6.2), phytoplankton growth in
Greenfield Lake was close enough to the Redfield ratio that either nutrient
could be limiting at times. Our
previous bioassay work indicated that nitrogen was usually the limiting nutrient
in this lake (Mallin et al. 1999).
Phytoplankton
blooms are periodically problematic in Greenfield Lake, and usually consist of
green or blue-green algal species, or both together.
These blooms have occurred during all seasons, but are primarily a
problem in spring and summer. As
mentioned, three algal blooms exceeding the state standard of 40 mg/L
were recorded in our sampling during 2004, along with an intense surface scum of
duckweed for three months. Thus,
during 2004 Greenfield Lake was impaired by algal blooms, high fecal coliform
counts and low dissolved oxygen concentrations. The tributary stations were also impaired by high fecal
coliform counts and low dissolved oxygen. These
same problems have occurred in the lake for several years (Mallin et al. 1999;
2000; 2002; 2003; 2004).
Table 6.2.
Mean and (standard deviation) of water quality parameters in Greenfield
Lake sampling stations, January - September 2004.
Fecal coliforms given as geometric mean, N/P ratio as median; n = 7
samples collected.
_____________________________________________________________________
Parameter
GL-2340
GL-YD
GL-P
_____________________________________________________________________
DO (mg/L)
6.1 (1.7)
8.0 (3.9)
5.2 (5.0)
Turbidity (NTU)
2 (1)
2 (1)
6 (10)
TSS (mg/L)
4.6 (4.5)
6.3 (7.2)
22.4 (40.2)
Nitrate (mg/L)
0.109 (0.182)
0.060
(0.042)
0.101
(0.158)
Ammonium (mg/L)
0.024 (0.023) 0.044 (0.053)
0.134
(0.205)
TN (mg/L)
1.053 (0.474) 1.426 (0.900)
1.919
(1.409)
OrthopPhosphate (mg/L) 0.009
(0.007) 0.010 (0.009)
0.018
(0.021
TP (mg/L)
0.049 (0.046) 0.061 (0.036)
0.087
(0.079)
N/P molar ratio
22.1
22.1
22.1
Fec. col. (/100 mL)
61
28
153
Chlor. a (mg/L)
31.6 (56.6)
11.0 (7.8)
25.1 (25.6)
____________________________________________________________________