Freshwater Contamination (Proceedings of Rabat Symposium S4, April-May 1997). , /-n 1AHS Publ. no. 243, 1997 167
Nutrient concentrations and planktonic biomass (chlorophyll a) behaviour in the basin of the River Avon, Warwickshire, UK
IAN FOSTER, SERWAN BABAN, SUE CHARLESWORTH, ROZ JACKSON, STEVEN WADE Centre for Environmental Research and Consultancy, NES (Geography), Coventry University, Priory Street, Coventry CV1 5FB, UK PETER BUCKLAND, KEITH WAGSTAFF & SARAH HARRISON Environmental Quality, Environment Agency, Riversmeet House, Newtown Industrial Estate, Northway Lane, Tewkesbury, Gloucestershire GL20 8JG, UK
Abstract The 2210 km2 basin of the Warwickshire River Avon has experienced increased nutrient concentrations and loadings over the last two decades from both point sources (especially sewage treatment works) and diffuse agricultural runoff. In addition, attached and planktonic biomass appears to have increased as has the incidence of nuisance blue-green algae. This paper presents preliminary data obtained from a major water quality sampling programme initiated in 1994. High chlorophyll a levels are found in three monitored reservoirs and in the lower reaches of the Rivers Alne and Dene, two major tributaries of the Avon which receive no major sewage effluent inputs. Whilst there appears to be an increase in chlorophyll a concentration with distance downstream in the main Avon system, nutrient concentrations do not appear to be the most critical control on the spatial distribution of planktonic biomass. It is suggested that river Si02 concentrations limit spring diatom productivity and that river sinuosity may impose a morphological control on residence time and reseeding.
INTRODUCTION
Over the past several decades, the River Avon basin has seen diverse and intensive urban growth together with agricultural development and intensification. In consequence, both water use and nutrient (N and P) rich waste discharges have increased substantially. Symptoms of accelerated eutrophication, including the growth of attached and planktonic algae, which lead to diurnal variations in dissolved oxygen concentrations, have characterized the main river and some of its tributaries. During the extreme low flows of 1990, nuisance blue-green algal blooms of Oscillatoria agardhii occurred in the lower reaches of the River Avon. Such characteristics are undesirable from both a water quality management and recreation/resource perspective. The river Avon meets all of the criteria set by the UK Department of the Environment (DOE) which defines eutrophic (running) waters. In December 1992, the Avon and a major tributary, the River Arrow, were put forward as candidates for designation as "Sensitive Areas" under the EU Urban Waste Water Treatment Directive (UWWT) (EC, 1991). For designation as a sensitive area under the UWWT Directive, a watercourse must receive sewage effluent from "a qualifying Sewage Treatment Works (STW) discharge" serving a population of greater than 168 Ian Foster et al.
(b)
SOWE |-X— WYKEN SLOUGH BROOK Stanford X -x WITHY BROOK Res'r a -X- COOMBE -X SMITE ^ — AVON SHERBOURNE —X- -X- v POOL BROOK Clifton â/x^X INCHFORD BROOK X „ \ X « FINHAM -x—£—x- CLAYCOTON BROOK rj\ BROOK Finham CLIFTON BROOK Stoneleigh (X) a SWIFT Draycote Eathorpe Res'r a D -X -X- _£Lv_ TACH -X-X LEAM Longbridge f| BROOK W Marton RADFORD • BROOK f D Barford (X) a D i RAINS ITCH EN BROOK .THELSFORD -X- ' BROOK
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a x Stratford 1 Milcote
WYNYATES BROOK Lower Bittel Aloester x x Res'r Bordesley ® $ T~ ~ STOUR X) â Broom Clifford f f a â -X-
Fig. 1 The Warwickshire River Avon. Location (a) and a schematic map showing the distribution of major tributaries, STWs and sampling stations (b). Nutrient concentrations andplanktonic biomass in the Avon basin, Warwickshire, UK \^g
10 000 pe. Under this Directive, 1 pe is defined as the organic biodegradable load having a 5-day biochemical oxygen demand (BOD) of 60 g of oxygen per day. The River Avon upstream of Evesham (Fig. 1) drains an area of 2210 km2. It is located in a lowland (<150 m AOD), predominantly agricultural area and has a mean annual discharge at Evesham of c. 15.2 m3 s"1 (1310 Ml day"1) (NRA, 1992). Annual rainfall for the basin ranges from 720 mm in the northeast to 599 mm in the south, and average effective rainfall ranges from c. 250 mm to less than 150 mm year"1. The Avon comprises nine major sub-basins and receives effluent discharge from five qualifying discharges under the UWWT Directive in addition to over 60 sewage effluent discharges which do not qualify under the UWWT Directive (Fig. 1). In addition to controls imposed by nutrient availability, temperature and day length, recent investigations into the maintenance and survival of river plankton in the UK have been discussed in detail by Reynolds (1988; 1995) and by Reynolds et al. (1991) who have suggested a number of hydraulic controls constraining the survival and re-population of rivers with phytoplankton following population decline and/or removal from river reaches by high velocities. Reynolds (1992) has also suggested that there is a propensity for phytoplankton populations to be higher in the lower reaches of rivers than in headwater streams but acknowledges that proximity to zones of standing water along the channel in bays, pools, underbanks and blind side- arms may contribute to between-bloom survival of fluvial populations. In an attempt to establish the potential effects of these controls on phytoplanktonic biomass in the River Avon, measurements of both channel gradient and sinuosity have been included in the data collection programme as described below.
SAMPLING PROGRAMME
In 1994, 79 sites were selected for water quality sampling. These included 58 river sites, the five major qualifying STWs, 13 minor STWs and three reservoirs/lakes (Fig. 1). The qualifying STWs, main River Avon sites and major tributaries were sampled at weekly intervals; minor STWs and smaller tributaries were sampled fortnightly and the remaining sites were sampled at monthly intervals. On-site determinations were made of pH, redox potential, dissolved oxygen (mg l"1 and % saturation), temperature and river discharge. Sub-samples were filtered on site through 0.45 /xm membrane filters, and 11 water quality determinands were measured in the laboratory within two days of sample collection. These measurements included suspended solids, BOD, COD, Kjeldahl-N (unfiltered), Total P (unfiltered), Total P (filtered), ammonia (filtered) TON (filtered) orthophosphate (filtered), orthophosphate (unfiltered) and chlorophyll a (the latter was measured additionally at 19 stations, including 3 reservoirs, with sampling from April to September in the first monitoring year only; see Fig. 1). At the beginning of the second full year of measurement in April 1996, Silica (Si02) concentrations were also determined on field filtered sub-samples. For the 1994-1995 period, Si02 data were available for a limited number of sites from the Environment Agency Quality Information System data archive. All laboratory analyses were undertaken at the Environment Agency laboratory in Nottingham, UK. 170 Ian Foster et al.
For the 16 river sites at which chlorophyll a was monitored, information on river gradient and river sinuosity, which is defined as the ratio of stream length to valley length, was derived from Ordnance Survey 1:25 000 scale topographic maps. Measurement of stream length included all bifurcating reaches and side arms of the river as depicted on the blue line network. An increase in sinuosity therefore provides an index of potentially longer residence time and incorporates potential seed areas from pools of standing water.
CHLOROPHYLL a
The Department of the Environment (DOE, 1993) define the occurrence of planktonic algal blooms in running waters in terms either of the mean annual concentration (average >25 mg m"3) or the maximum observed concentration (> 100 mg m"3) of chlorophyll a. Since chlorophyll a data are not available for a full year, the former cannot be calculated. However, the boxplots and time series plots of Fig. 2 and Fig. 3 indicate the 25 and 100 mg m"3 levels as a guide. Summary statistics for the chlorophyll a sampling stations are given in Fig. 2 as boxplots. The boxes contain 50% of the values between the 75th and 25th percentiles of the frequency distribution whilst the whiskers extend to the maximum and minimum values excluding outliers and extremes. An outlier (o) is defined as a value greater than 1.5 box lengths away from the box, and an extreme value (*) as more than three box lengths away from the box. The line across the box indicates the median value of the data set. A number of observations arise from these data. First, 11 of the 19 monitoring sites (including the three lakes/reservoirs) have maximum chlorophyll a values exceeding 100 mg m"3. Several, but not all, of the monitoring points where maximum concentrations fall below the 100 mg m3 threshold are located in headwater tributaries of the Avon (upstream of Rugby Newbold STW) and in its sub-basins with
Stanford Reservoir AVON CLIFTON Swift Brownsover AVON LAWFORD AVON STARE BRIDGE Sowe Stoneleigh Draycote Reservoir Itchen Marton Learn Eathorpe Learn PD AVON BARFORD Dene Wellesbourne • -rmu oo AVON STRATFORD : HT Stour C.C. gjj-L Lower Bittel Res. hwr—u Arrow Bordesley Alne Alcester Arrow Broom o 100 200 300 AVON EVESHAM -3 Chlorophyll â (mg m ) Fig. 2 Boxplots for data obtained from the 19 chlorophyll a monitoring stations (main Avon stations in capital letters plotted downstream from headwaters to Evesham). Nutrient concentrations and planktonic biomass in the Avon basin, Warwickshire, UK 171
no major qualifying STW discharges. Secondly, high median concentrations are recorded in the Lower Bittel Reservoir, and in the Rivers Arrow and Dene. Thirdly, highest extreme values are recorded in Stanford Reservoir and in the Rivers Alne, Arrow and Avon (Evesham). The latter site is only some 8 km downstream of the Avon confluence with the Arrow-Alhe system. The Dene enters the Avon some 9 river kilometres upstream of Stratford at which point maximum chlorophyll a concentrations also exceed 100 mg m"3.
300 300 - Alne (Alcester) - - Arrow (Broom) - - Arrow (Bordesley) 200 200
100 100
Apr May Jun Jul Aug Sep Oct Nov Apr May Jun Jul Aug Sep Oct Nov 200 Stour (C.C.) 80- Avon (Stare Bridge) - - - Avon (Lawford) » - - - Dene (Wellesbourne) 60- Avon (Clifton)
40- 100 20- /M
0- —•—i— —i 1 1 1 1 Apr May Jun Jul Aug Sep Oct Nov
200 Leam (Princes Drive) Apr May Jun Jul Aug Sep Oct Nov - - - Swift (Brownsover) 150 Leam (Eathorpe) Avon (Stratford) 200- Avon (Evesham) 100 Avon (Barford)
100
Apr May Jun Jul Aug Sep Oct Nov
— Sowe (Stoneleigh) Apr May Jun Jul Aug Sep Oct Nov • - Itchen (Marton) 100 \-- 19 9 5 - 100 - 25
DOE (1993) maximum and average thresholds
Apr May Jun Jul Aug Sep Oct Nov defining the presence of planktonic blooms. 19 9 5 Fig. 3 Time series plots of chlorophyll a concentration at monitoring stations in the River Avon and its sub-basins. 172 Ian Foster et al
Chlorophyll a time series plots are presented in Fig. 3. At all river monitoring sites, chlorophyll a levels increase dramatically in early May 1995 and decline rapidly in late May or early June. Whilst the minor tributary streams evidence no significant secondary peak in late summer, secondary August/September peaks are recorded in the Rivers Swift and Dene and in the River Avon at Stare Bridge, Barford and Evesham. Neither the River Arrow nor the River Alne experience significant increases in chlorophyll a concentration in late summer. The River Stour, draining calcareous rocks of the Cotswold edge, experiences a slow increase in chlorophyll a from April to late June whilst the River Dene has 4 peaks exceeding 100 mg m3 in May, July, August and September. Seasonal trends in lakes and reservoirs are complex. Lower Bittel reservoir has sustained high concentrations throughout the sampling period whilst Draycote Water evidences a May/June peak in concentration. Peak chlorophyll a levels in Stanford reservoir exceed 900 mg m"3 in June and July but remain below 25 mg m"3 for the remainder of the sampling period. Chlorophyll a concentrations in the River Sowe at Stoneleigh, less than 1 km downstream of the largest STW in the Avon catchment (Coventry, Finham), rarely exceed 25 mg m3, although minor increases in concentration are recorded in April and August.
NUTRIENT CONCENTRATIONS
Time series of N and P concentrations for four river monitoring stations together with the annual river discharge and Si02 concentrations at Evesham are shown in Fig. 4. In rivers unaffected by major STW discharges (e.g. Avon at Clifton, Alne at Alcester, Stour at Clifford Chambers, Leam at Eathorpe and Dene at Wellesbourne), TP concentrations are generally below 1.4 mg l"1 and TN concentrations below 15 mg l"1. Nevertheless, even these relatively small tributaries receive effluent discharge from minor STWs and, as shown in the previous section, frequently experience significant planktonic blooms. In contrast, the Sowe at Stoneleigh has TP concentrations above 7 mg l"1 and TN concentrations exceeding 25 mg l"1. The seasonal pattern in nutrient concentration broadly reflects the seasonal pattern in river discharge as shown in Fig. 4 for Evesham. Since STW discharges remain relatively constant throughout the year, the temporal trends in P reflect the diminishing dilution effect of natural flow during the late spring and summer months. With the exception of the Sowe, this dilution effect is less evident for N, the source of which is predominantly diffuse. Neither average nor seasonal trends in nutrient concentration appear to have a significant impact on the distribution of planktonic algae in the Avon system. In order to analyse this problem further, an attempt has been made to examine the relationship between water quality and planktonic biomass in a multivariate analysis presented below. From the available Si02 data at Evesham (Fig. 4), it would appear that a significant proportion of the chlorophyll a in spring is associated with the presence of diatoms. Dissolved Si02 may be a limiting factor in diatom productivity at this time of year since it is depleted to trace concentrations at Evesham, and is below the limit of detection in the Lower Bittel reservoir. Whilst Si02 levels decline in association with the August bloom at Evesham, Si02 does not appear to be limiting. N 25 Avon (Cifton) 20
15 E 10 z 5 .A 0 rrrwC'â^ 30
25
~ 20
15 mg l z— 10
5 t 0 -r-^-l-^'^Y^-l^.^'-' -,,/\
20 Arrow (Broom) Arrow (Broom)
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0 .£$£^ . |_^ j^_ .'-i i/'Y '-1 •- r v'i -r 01 Dec, 11 Mar" 19Jun ' 27 Sep ' 01 Dec ' 11 Mar ' 19Jun ' 27 SeFp g4 20 Jan g5 30 Apr gs 08 Aug gg 16Nov 94 20 Jan 95 30 Apr g5 08 Aug g5 16Nov 95 95 95
NH3 (filtered) 16 P04 (filtered)
12 TON Particulate P E, 8 OJ O TN (Kjeldahl-N+TON) 4 Total P
0 01 Dec ' 11 Mar ' 19Jun ' 27Sep N (Kjeldahl) 94 20 Jan g5 30 Apr 95 08 Aug g5 16 Nov
Fig. 4 Selected time series plots for N and P concentrations and river discharge and SiO, concentrations at Evesham. 174 Ian Foster et al.
PLANKTONIC BIOMASS AND NUTRIENT CONCENTRATIONS
In order to analyse the relationship between nutrient concentration and biomass, average water quality values were extracted from the database for the period of chlorophyll a monitoring. These average data formed the basis of a Principal Components Analysis (PC A) of water quality/chlorophyll a interactions. Included within the analysis of river sites were data on sinuosity and river gradient derived from topographic maps. The PC A extracted five significant components with eigenvalues above 1, and explained almost 90% of the variance in the data. (Table 1). From this preliminary analysis it is evident that the major distinguishing water quality characteristics for the 16 rivers are N and P which load significantly on Component 1. Chlorophyll a, sinuosity and suspended solids concentration only load significantly on Component 5, whilst COD, DO, Kjeldahl-N, ammonia and pH load significantly on Component 2. Whilst chlorophyll a and sinuosity both load significantly on the same Principal Component, they are not significantly related to each other (Pearson's Product Moment Correlation Coefficient; p > 0.05). On the basis of the PC A solution, an attempt was made to classify the 16 rivers using the resultant component scores, a hierarchical cluster analysis (HCA) and discriminant analysis (DA). The results of the HCA are given in Fig. 5 which provides evidence for the existence of six major groups of rivers using this dataset. The allocation of sites to one of each of the six groups was confirmed by DA. On the basis of the classification, both the Rivers Sowe and the Avon at Evesham form individual reaches unrelated to other reaches in the Avon catchment. The Rivers Swift, Arrow, Stour and Alne form one major group, and the Learn, Itchen and the Avon at Starebridge and Lawford form a second major group. The middle reaches of the Avon at Barford and Stratford show similar characteristics to each other whilst the Dene is most similar to the Avon at Clifton.
DISCUSSION
The results and analysis presented above lead to a number of specific conclusions regarding the spatial and temporal distribution of chlorophyll a in the Avon and the chemical and physical characteristics of individual reaches of river which might influence the observed pattern. First, there is a tendency for chlorophyll a concentrations to increase downstream, generally confirming the conclusions presented by Reynolds (1995). However, exceptions to this pattern are especially apparent on the Dene at Wellesbourne which has a sinuosity of 1.275 and a gradient of 0.001 96. The gradient is considerably higher than that of the lower reaches of the Avon at Evesham (0.000 309) but with a slightly higher sinuosity than at Evesham (1.1172). Of greater significance to the potential for seeding the Dene at Wellesbourne is the existence of a small reservoir on the main stream some 3 km upstream of the monitoring station. However, similar potential seeding patterns from the Lower Bittel Reservoir are not observed on the River Arrow at Brownsover, which may be due to the high gradient (0.004 35) of this river reach. Secondly, there would appear to be no significant control on plankton production imposed by nutrient concentration other than the period of the spring diatom bloom when maximum Nutrient concentrations and planktonic biomass in the Avon basin, Warwickshire, UK 175
Rescaled Distance Cluster Combine
Site
STAREBRIDGE -i Leam (Eathorpe) —[ Itchen —' LAWFORD Leam (Princes Drive) BARFORD STRATFORD Swift (Brownsover) Arrow (Bordesley) Stour AI ne Arrow (Broom) CLIFTON Dene EVESHAM Sowe Fig. 5 Hierarchical cluster analysis identifying Isix major groups of river monitoring stations in the Avon basin. productivity could be limited by Si02 availability on the Avon at Evesham and in the lower Bittel reservoir. Thirdly, whilst there appears from the PC A that there is some association between sinuosity and chlorophyll a, the association is weak because the
Table 1 Principal components analysis of river water quality and morphological data (rotated component solution). Component: 12 3 4 5 BOD 0.88390 0.33430 0.27762 0.03007 0.08448 Chi a 0.41406 -0.42973 -0.31350 0.18021 0.59224 COD 0.45551 0.72225 0.17101 -0.03680 -0.04747 DO(%) -0.15868 -0.88761 -0.11013 -0.21998 -0.01400 DO (mg l"1) -0.14117 -0.91611 -0.16175 -0.17579 -0.15238 Flow 0.26900 0.13853 0.36856 0.83490 0.17275 Slope 0.32355 -0.23162 -0.17008 -0.64993 -0.03070 Kjel. N. 0.69732 0.60228 0.26569 0.06778 -0.13546 N:P -0.13884 0.09968 -0.88897 -0.05297 0.21617 NH3 0.68256 0.61862 0.17948 0.07123 -0.23454 pH -0.32187 -0.91371 -0.06441 0.11274 0.03333 P04 0.58988 0.40286 0.63336 0.22810 -0.05939 PP 0.36624 -0.12281 0.03372 0.83953 -0.12843 Redox 0.00792 0.20140 -0.09827 0.29887 0.77446 Sinuosity -0.12864 -0.06293 0.05527 -0.30087 0.76473 Susp. Sol. 0.91558 0.09618 0.00651 0.05929 -0.00555 Temp. 0.05396 0.41020 0.65038 0.29907 0.14588 TN 0.64851 0.46045 0.46773 0.21783 0.11190 TON 0.60386 0.40413 0.48412 0.27447 0.15988 TP 0.60760 0.39560 0.62634 0.23034 -0.06399 TP (Filt.) 0.60443 0.39569 0.63131 0.22381 -0.06823
1 51.0 11.3 9.5 9.3 5.7 2 51.0 62.3 71.8 81.1 86.8 1 explained variance (%), 2 cumulative explained variance (%), underlined values indicate significance at the 99% confidence level. 176 Ian Foster et al. fifth component only explains some 5.7% of the total variance in the dataset. Distinction between the 16 river sites, as evidenced by PCA and HCA is more strongly controlled by N and P in different forms and by DO, COD and pH than by morphometry or planktonic algal populations. In a heavily regulated lowland river system like the Avon, the spatial and temporal patterns of chlorophyll a distribution appear to be complicated by the large number of potentially controlling variables which are difficult to quantify in terms of simple indices. Whilst there is some evidence to support the existence of hydraulic controls, these are less clear in the Avon catchment than for those reaches of less heavily regulated river systems discussed by Reynolds (1988; 1995) and by Reynolds etal. (1991).
Acknowledgements This research project was funded by the UK Environment Agency (Formerly National Rivers Authority) of Severn Trent Region. River flow data were provided by the hydrometric section of the Environment Agency, Solihull and Severn Trent Water pic provided additional hydrometric data. The Environment Agency (National Laboratory Services), Nottingham, UK undertook the laboratory analysis. Suzanne Howarth and Mike Wooldridge are gratefully acknowledged for the sampling and field analysis. Thanks also go to Paul and Jason in the Cartographic Unit of Coventry University for drawing the diagrams.
REFERENCES
DOE (1993) Criteria and procedures for identifying sensitive areas and less sensitive areas (Urban Waste Water Treatment Directive) and Polluted Waters (Nitrates Directive) in England and Wales. HMSO, London. EC (1991) Council Directive Concerning Urban Waste Water Treatment 91/271/EEC (21 May 1991). Foster, I. D. L., Baban, S. M. J., Wade, S. B., Charlesworth, S. M., Buckland, P. J. & Wagstaff, K. (1996) Sediment- associated phosphorus transport in the Warwickshire River Avon, UK. In: Erosion and Sediment Yield: Global and Regional Perspectives (ed. by D. E. Walling & B. W. Webb) (Proc. Exeter Symp., July 1996), 303-312. IAHS Publ. no. 236. NRA (1992) Hydrometric Report and Catalogue 1991. NRA Severn-Trent Region, Solihull. Reynolds, C. S. (1988) Potamoplankton: paradigms, paradoxes and prognoses. In: Algae and the Aquatic Environment (ed. by F. E. Round), 285-311. Biopress, Bristol. Reynolds, C. S. (1992) Algae. In: The Rivers Handbook, vol. 1 (ed. by P. Calow & G. E. Petts), 195-215. Blackwell, Oxford. Reynolds, C. S. (1995) River plankton: the paradigm regained. In: The Ecological Basis for River Management (ed. by D. M. Harper & A. I. D. Ferguson), 161-175. J. Wiley, Chichester, UK. Reynolds, C. S., Carling, P. A. & Beven, K. J. (1991) Flow in river channels: new insights into hydraulic retention. Arch. Hydrobiol. 121, 171-179.