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Appendix 5.
Response to reduced phosphorus concentration in the Priors Farm
catchment.
1. Introduction
This project originally aimed to determine the chemical and environmental impact of a
large cattle yard at Priors Farm on the chemistry and ecology of the agriculturally
impacted stream in the Priors Farm catchment (Avon river basin). The cattle shed was
was due to be converted into a car park but unfortunately, this planned land use change
did not occur during the time period of this project, and so could not be used. As an
alternative, the impact of a phosphorus (P) mitigation option was simulated by
investigating the effect that a change in stream P status would have on the ecology at
the site. To achieve this, the bioavailable P concentration in the river was artificially
reduced by a P stripping methodology developed in a previous study (Bowes et al.,
2007), using iron (II) sulphate dosing. The addition of FeSO4 to river water results in a
reaction with the bioavailable soluble reactive phosphorus (SRP) in the water column,
converting it to a non-bioavailable precipitate. This reaction is routinely used by the water
industry as a means of stripping phosphorus from sewage effluent. The installation of
three flumes within the river channel allowed three SRP concentrations to be
simultaneously produced using this P-stripping methodology, and the effect that these
SRP reductions had on periphytic algal growth rate was quantified. Periphyton are
presumed to be a key driver in the eutrophication process in rivers, as they can rapidly
utilise increases in nutrient concentration, and play a major role in the shading out of
macrophytes due to their rapid colonisation of newly emerging leaf surfaces (Hilton et al.,
2006).
2. Methods
An array of three plastic flumes (width 0.3 m, length 4.5 m) were floated in the River Sem
at Priors Farm, allowing river water to flow through them at a set depth (approx 3 to 4
cm). The flumes were sited immediately downstream of the Priors Farm road bridge, as
this was a relatively straight part of the river channel, and had minimal riparian
vegetation (thereby providing full sunlight to the flumes). The flumes were anchored into
position by metal rods set into the river bed, which pass through sleeves at the corners
of the flume array, allowing the flumes to move up or down with changing river depth
(Figure 1). Prior to the start of the experiment, the sides and base of each channel were
scrubbed to remove algae and sediment. Stone blocks were placed in the entrances to
the 3 flumes (Figure 2), to reduce the flow velocity and increase the residence time of
the water flowing through the flumes to ca. 2 minutes. The blocks were carefully
positioned to give the same flow velocity in each flume (0.035 +/- 0.003 m s-1).
Three treatments were randomly assigned to the flumes;
 Flume 1. FeSO4 addition, aiming for a reduction in river SRP concentration to ca.
50 µg l-1.
 Flume 2. FeSO4 addition, aiming for a reduction in river SRP concentration to ca.
150 µg l-1.
 Flume 3. No FeSO4 addition (control)
1
Figure 1. Design of flume experiment.
2
Periphytometers
Slate substrates
Sump for collecting iron
precipitate / debris
Floats
Iron additions
Stone blocks to reduce
and control flow
velocity
Velocity in channels
Figure 2. Photo of flume experiment at Priors Farm, looking downstream.
A concentrated solution of FeSO4 was added to the upstream end of Flumes 1 and 2,
using a peristaltic pump. After 30 minutes (to allow the system to equilibrate), samples
of water were taken from the downstream portion of each flume, filtered immediately
through a 0.45 µm filter (to stop the phosphorus stripping reaction), and analysed for
SRP concentration (Murphy and Riley, 1962). The FeSO4 drip rates were adjusted until
the desired SRP concentrations were achieved.
The experiment began on 21st August 2008, and ran for 9 days. Two cleaned slates
were placed into the downstream portion of each flume, to provide a growth substrate for
epilithic algal growth. Nutrient-diffusing periphytometers (Matlock et al., 1998) (Error!
Reference source not found.) were placed at the downstream extent of the channel.
Each channel contained a control periphytometer (containing deionised water) and one
containing a concentrated solution of orthophosphate, which allowed the effect that resupplying P to the system would have on algal growth (Fig. 3). These periphytometers
would enable changes in algal growth rate within the flumes to be attributable to either P
limitation, or a physical affect that may be associated with iron sulphate addition, such as
3
toxicity or turbidity. Water velocities in each flume were checked twice per day, and
adjusted were necessary by adjusting the positions of the stone blocks. The iron
sulphate drip rates were also checked regularly, and fresh iron sulphate solution was
prepared each day. The slate substrates were regularly inspected for the presence of
invertebrates, and any found were removed with tweezers, to avoid loss of algae.
Invertebrate grazing at this site was not a major factor, with only a few snails per day
being present. Grazing of the algae by fish was not observed at the site.
Figure 3. Details of (a) Periphytometer design and (b) positioning of periphytometers at
the downstream end of the flumes.
2.1. Chemical analysis
Water samples (20 ml) were taken from each flume (ca. 4 per day), from directly above
the slate substrates, immediately filtered, and transported back to the laboratory. All
flume samples were analysed for SRP concentration within 4 hours of being taken, to
avoid errors associated with sample instability. In addition, bulk water samples (500 ml)
were taken manually, directly from the River Sem, 10 m upstream of the site of the flume
experiment. These samples were analysed for total phosphorus (TP), total dissolved P
(TDP), ammonium, nitrate, nitrite, total dissolved nitrogen, dissolved silicon, and major
anions and cations.
2.2. Chlorophyll-a analysis
After nine days, the experiment was terminated. The stone blocks were repositioned to
stop all flow through the flumes, to minimise potential loss of algae during sampling. The
slates were carefully removed, and the colonising algae and deposited sediment
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transferred into a glass jar using a scrubbing brush. These jars were immediately
transported back to the laboratory, and stored in the dark at 4 oC.
The algal and sediment slurries were analysed for chlorophyll-a analysis, as a proxy
measure of the algal biomass that had developed on each slate. The samples were
diluted to a total volume of 400 ml. They were then thoroughly homogenised by
constant stirring. Three 20 ml subsample was taken from each sample by pipette, using
a wide bore pipette tip so that course material (such as filamentous algae) was not
excluded from the sample. These 20 ml subsamples were filtered through a Whatman
GF/C filter paper, to remove the algae. Each filter was placed into a centrifuge tube
containing 20 ml of 90% acetone, and stored at 4 oC in the dark, overnight. Next
morning, the filter paper was removed, the samples centrifuged, and then the overlying
solution was analysed spectrophotometrically to determine the chlorophyll-a
concentration (Bowes et al., 2007; Marker, 1972).
The periphytometers were also removed from the flumes at the same time as the slate
sampling. They were carefully dismantled, and the filter papers (that provide the algal
growth substrates were analysed for chlorophyll-a concentration using the method
described above.
3. Results and discussion
3.1. Water Chemistry of the Priors Farm stream during flume experiment
The results of the water analysis of the bulk water samples from the Priors Farm stream
are shown in Table 1. The river had a high nutrient loading during the experimental
period, with phosphorus concentrations greatly exceeding the Environment Agency’s
target of 100 µg L-1 for lowland clay rivers. Most of the phosphorus load (78%) was
transported in dissolved form, despite the high loading of fine suspended sediments from
the catchment. The TDP concentrations closely matched the SRP concentrations
measured in the control flume, indicating that almost all of this dissolved P was in a
readily bioavailable form.
The mean TDN:TDP nutrient ratio was 15:1, which is close to the Redfield ratio of 16:1.
implying that the supply of these two major plant nutrients are ‘in balance’, and either
could become limiting with the minor fluctuations in nutrient concentrations that naturally
occur in rivers. It also implies that any small reduction in SRP concentration would
increase the N:P ratio beyond the Redfield ratio, thereby making phosphorus limiting to
algal growth. However, the use of the Redfield ratio under these high nutrient
concentrations may not be of use, as both N and P are likely to be in excess, and algal
growth rate is more likely to be limited by light or flow velocity. Using the methodology
developed by Mainstone et al., which takes this nutrient loading into account by
comparing the mean SRP concentration to the N:P ratio, the stream should be classified
on the boundary between ‘occasionally P limited’ and ‘occasionally N limited’ during the
growing season (Mainstone and Parr, 2002). Therefore, based on these two traditional
approaches, it would be expected that any reduction in SRP concentration would result
in a step reduction in periphyton biomass. The third major essential element for diatom
growth is silicon. The proposed Redfield nutrient ratio for diatoms is C:Si:N:P =
106:15:16:1. The dissolved silicon concentration in the River Sem is clearly in excess,
with a Si:N:P ratio of 25:15:1 (Table 1).
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Table 1. River Sem water chemistry data.
DATE
20/08/2007
21/08/2007
22/08/2007
22/08/2007
23/08/2007
23/08/2007
24/08/2007
24/08/2007
25/08/2007
26/08/2007
27/08/2007
28/08/2007
29/08/2007
29/08/2007
30/08/2007
30/08/2007
TIME
TDP
TP
NH4
NO2
NO3
TDN
Si
µg L-1 P µg L P mg L-1 NH4 mg L-1 mg L-1 N mg L-1-N mg L-1
SO4
mg L-1
Cl
mg L-1
Na
mg L-1
K
Ca
Mg
mg L-1 mg L-1 mg L-1
12:30
14:00
10:00
12:30
08:30
13:30
10:30
16:30
11:00
11:00
12:00
13:00
08:30
13:00
11:00
13:30
220
186
176
196
406
330
268
230
208
196
200
192
182
182
176
172
334
282
262
262
530
434
354
262
228
248
254
224
210
206
210
214
0.100
0.104
0.068
0.080
0.068
0.068
0.052
0.068
0.048
0.060
0.056
0.048
0.068
0.064
0.056
0.044
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
2.0
2.1
1.8
1.9
4.9
4.1
2.8
2.3
1.5
1.1
1.1
1.0
1.4
1.2
1.6
1.6
3.8
3.6
3.0
3.4
6.7
5.7
4.2
3.8
2.7
2.2
2.0
2.0
2.3
2.2
2.5
2.4
4.6
4.9
5.1
5.3
5.4
5.2
5.3
5.2
5.4
5.6
5.7
5.8
6.0
5.8
6.0
5.9
45.5
54
62
63
66.5
69.5
71.5
74.5
80.5
84.5
89
91.5
92
92
92
92.5
24
25.5
26.5
27
43
40.5
38.5
37
34
34
34.5
34.5
35.5
35.5
36
36
16.0
18.2
18.9
19.4
24.6
24.2
23.9
24.4
23.6
24.8
25.3
26.1
27.4
26.9
28.2
28.1
8.1
8.4
8.3
8.7
15.6
15.2
13.1
12.7
11.1
10.7
10.1
9.7
9.9
9.7
9.6
9.6
71.0
74.9
79.9
81.9
95.1
92.8
91.3
91.8
93.2
97.9
99.5
101.0
103.5
103.5
105.4
105.5
6.0
6.5
6.9
7.1
8.8
8.6
8.3
8.3
8.4
8.7
8.8
9.0
9.1
9.0
9.0
9.0
Average
220
282
0.07
<0.1
2.0
3.3
5.5
76.3
34
23.7
10.7
93.0
8.2
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3.2. Flume experiment
3.2.1. SRP concentrations
The results of the SRP analysis are shown in Fig. 4. The SRP concentration of the
River Sem (as measured in the control flume, which is unmodified river water) varies
widely between 140 and 370 µg L-1, with a mean concentration of 219 µg L-1. In fact,
other spot SRP samples taken directly from the river during this experimental period
observed a number of sporadic SRP concentrations of up to 800 µg L-1 (23rd and 26th
August) and, in one instance, as high as 1500 µg SRP L-1 (27th August).
The addition of FeSO4 to two of the flumes successfully reduced the SRP concentration
of the incoming river water. The two iron sulphate addition rates to Flumes 1 and 2
resulted in step reductions in mean SRP concentration to 68 and 124 µg SRP L-1
respectively (equivalent to 31 and 57 % of the current SRP loading in the River Sem).
Because the FeSO4 dosing method removes a certain quantity of the SRP load, the SRP
concentrations observed in the two treated flumes followed a similar pattern to the SRP
concentrations observed in the control channel, with similar concentration ranges.
Figure 4. SRP concentrations in the three flume channels. High P stripping = Flume 1.
Medium P stripping = Flume 2. Control = Flume 3 (no FeSO4 addition)
3.2.2. Periphyton biomass
After running the experiment for nine days, epilithic algae had colonised the slates in all
three flumes. From visual inspection, all slates appeared to have similar quantities of
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algae (Figure ). The experiment was terminated at this point, as a major storm event
was forecast for the following days.
Figure 5. Photo of slates at end of experiment (30th August 2007).
The biomass of periphyton on each slate was quantified by chlorophyll-a analysis
(Figure ). All three treatments had similar quantities of algal biomass. These results
clearly show that the SRP concentration in the Priors Farm stream at the present time
has little control on algal biomass. The mean SRP concentration of 219 µg L-1 observed
during the flume experiment is not limiting algal growth, and there is little evidence that
the SRP concentration would become limiting at concentrations as low as 60 µg L-1.
Reducing the present SRP concentration of the stream to around 120 µg l-1 actually
resulted in the increased periphyton accrual rate, demonstrating that even such a major
step-reduction in phosphorus loading would be unlikely to result in an improvement in
the ecology of the river.
The highest mean chlorophyll-a concentration was observed in Flume 2, which was
being phosphorus stripped, resulting in a 100 µg L-1 reduction in mean SRP
concentration of the incoming water (Figure ). This indicates that periphyton in the
stream are not phosphorus-limited during the summer growing period. The control
channel (with the highest SRP concentration) actually had the lowest chlorophyll-a
concentration, which does not fit with the expected Von Liebig growth curve response.
Similar results have been observed in a previous study of the River Frome, Dorset, using
similar flume methodology, where increasing the ambient SRP concentration due to
phosphate addition (a > 200 µg L-1) also resulted in a small reduction in algal biomass,
compared with the biomass observed at 90 to 120 µg L-1 (Bowes et al., 2007). These
observations from the River Frome and Sem may be due to a change in algal community
structure with changing nutrient status. Excessive nutrient enrichment seems to cause
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an increase in the proportion of delicate filamentous algal species, which may be
preferentially lost due to sloughing. Therefore, under eutrophic conditions, periphyton
become flow limited, rather than nutrient limited. Another likely reason for the reduced
biomass in Flume 3 is that this flume probably received less direct sunlight than the
other 2 flumes, due to the easterly orientation of the study reach, and the steep bank on
one side of the channel (Figure 2).
Figure 6. Chlorophyll-a concentration on slates after 9 days. High P stripping = Flume
1. Medium P stripping = Flume 2. Control = Flume 3 (no FeSO4 addition)
There is no apparent difference between the quantities of chlorophyll on the slates in
Flume 2 and the front slate in Flume 1, indicating that phosphorus was not limiting algal
growth even when the mean SRP concentration was reduced to 68 µg L-1. However, the
rear slate in Flume 1 has 20% less algal biomass than the front slate. This may indicate
that P is becoming limiting at SRP concentrations <68 µg L-1. (The SRP concentration in
each flume is measured in between the two slates, but because of the way that the P
stripping works, the SRP concentration will actually decrease as the water passes along
the flume. Therefore, the rear slates will always experience slightly lower SRP
concentrations than the front slates). It is difficult to make any stronger conclusions,
based on these limited number of observations, and lack of replicates. Deployment of
further flumes was not possible at the Priors Farm site, due to the narrowness of the
stream (which was only wide enough to accommodate three flumes), and the lack of
river reaches that are not shaded by riparian vegetation.
In a similar series of studies on the River Frome, Dorset, the phosphorus limiting
concentration was determined to be ca. 90 µg SRP L-1 (i.e. decreases in SRP
concentrations below 90 µg L-1 resulted in a decrease in algal biomass (due to nutrient
9
limitation) whereas increases in SRP to >90 µg L-1 P resulted in no change in biomass
(due to P being in excess). In contrast, the results from this study indicate that the
phosphorus limiting concentration for the River Sem is below 70 µg L-1.
This difference in P limitation could possibly be explained by the difference in river
typology. The River Frome is a predominantly groundwater fed chalk river, and the
flume studies on this river only took place during dry periods. Therefore the SRP
concentrations were relatively stable throughout the study periods. In contrast, the
Priors Farm catchment is a flashy, clay-dominated catchment, and (due to the
uncharacteristically wet summer of 2007) the study had to take place at a time that had a
large number of small rainfall events. Therefore, there were very large variations in SRP
concentrations in all flumes, and so the mean SRP concentrations used in this study
may not be a true indication of the nutrient status. For instance, Flume 1 had an
average SRP concentration of 68 µg L-1, but this concentration varied between 10 to 150
µg L-1 throughout the experiment. There were three periods where the SRP
concentration exceeded 140 µg L-1. It is known that algae can take up and store
phosphorus during such periods of high P availability (know as luxury consumption),
which sustains them during the periods where the P concentration is potentially limiting.
Therefore, the regular P spikes observed in the River Sem means that it has a lower
mean P limiting concentration than the River Frome.
3.2.3. Periphytometer results
The chlorophyll-a results from the periphytometers did not indicate the presence of any
P limitation in the flumes. The results from the paired samples (P addition and control)
in each flume showed that there was little difference in algal biomass as a result of
supplying SRP (Fig. 7). Only one of the flumes (Flume 2) showed a slight increase in
algal growth rate with increased P supply, but this can not be attributed to P limitation, as
the periphytometers in Flume 1(which had 50% lower SRP concentration than Flume 2)
showed a decrease in growth rate as a result of re-supplying P.
4. Conclusions
The algal growth rate data from the slates and periphytometers showed that the Priors
Farm stream is not phosphorus limited during the summer growing period. The iron
sulphate dosing methodology allowed the SRP concentration in the river to be
significantly reduced (to 31 and 57 % of the current SRP loading in the stream), and this
still had no effect on the periphyton growth rate. This suggests that reducing the mean
SRP concentration from its present 219 µg L-1 to 68 µg L-1 would have little or no effect
on the ecology of the River Sem. These two P-stripped flumes have N:P ratios of 27:1
and 49:1, which are both much higher than the Redfield ratio of 16:1. This strongly
suggests that nitrogen is also not limiting algal growth, and both nutrients are in excess.
The weekly water chemistry monitoring dataset (2004 to 2006) shows that the SRP
concentration at Priors Farm has only been less than 68 µg L-1 on two occasions, and
has a mean SRP concentration of ca. 200 µg L-1, indicating that phosphorus is never
limiting for algal growth at this site.
The original aim of this work was to see if we could identify an improvement in the river
ecology following the closure of a farmyard at Priors Farm. This flume study has clearly
shown that this land use change would have had no impact on the river biota. In order
to see an ecological improvement (in terms of algal growth), the nutrient loading in the
Priors Farm catchment would need to be reduced by at least 70 %.
10
Figure 7. Chlorophyll-a concentrations from periphytometers.
5. References
Bowes, M.J., Smith, J.T., Hilton, J., Sturt, M., M. and Armitage, P.D. (2007). Periphyton
biomass response to changing phosphorus concentrations in a nutrient-impacted
river: A new methodology for P target setting. Canadian Journal of Fisheries and
Aquatic Sciences, 64, 227-238.
Hilton, J., O'Hare, M., Bowes, M.J. and Jones, J.I. (2006). How green is my river? A new
paradigm of eutrophication in rivers. Science of the Total Environment, 365(1-3), 6683.
Mainstone, C.P. and Parr, W. (2002). Phosphorus in rivers - ecology and management.
Science of the Total Environment, 282, 25-47.
Marker, A.F.H. (1972). The use of acetone and methanol in the estimation of chlorophyll
in the presence of phaeophytin. Freshwater Biology, 2, 361-385.
Matlock, M.D., Matlock, M.E., Storm, D.E., Smolen, M.D. and Henley, W.J. (1998).
Limiting nutrient determination in lotic ecosystems using a quantitative nutrient
enrichment periphytometer. Journal of the American Water Resources Association,
34(5), 1141-1147.
Murphy, J. and Riley, J.P. (1962). A modified single solution method for the
determination of phosphorus in natural waters. Analytica chemica acta, 12, 31-36.
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