Variations of the Nutrients Loads to the Mainland U.K. Estuaries: Correlation with Catchment Areas, Urbanization and Coastal Eutrophication

Estuarine, Coastal and Shelf Science (2002) 54, 951–970 doi:10.1006/ecss.2001.0867, available online at http://www.idealibrary.com on

Variations of the Nutrients Loads to the Mainland U.K. Estuaries: Correlation with Catchment Areas, Urbanization and Coastal Eutrophication D. B. Nedwell, L. F. Dong, A. Sage and G. J. C. Underwood Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, U.K. Received 9 March 2001 and accepted in revised form 1 July 2001 The annual loads of nutrients (TOxN equal to nitrate+nitrite; ammonium, phosphate, silicate) to all the estuaries on the mainland of the United Kingdom were estimated from data on water flow through gauging stations at the tidal limits of estuaries, and from concentration measurements under the Harmonised Monitoring Scheme of nutrient concentrations in water samples from these stations. The annual loads of nutrients showed distinct regional variations, with estuaries along the west coast of Wales and northern Scotland having much smaller loads than those along the east coast of England. The largest nitrogen loads were of TOxN, and ammonium loads were usually small in comparison. The Severn, Mersey, Humber and Thames had the highest loads, although these were small in relation to the larger continental European estuaries. Loads of TOxN per unit of catchment area were surprisingly constant (about 105 moles N km
2 y
1). The nutrient loads showed that most U.K. catchments were influenced by human activity, the majority being in the ‘ moderately influenced ’ category. Nutrient loads were also normalized for the area of each estuary, as a measure of the relative influence of nutrients on the receiving estuaries. The ratios of N:P, N:Si and P:Si in the annual loads suggested that most estuaries were likely to be, if anything, P limited rather than N or Si limited. However, crude annual loads may conceal significant seasonal variations. The spring maximum chlorophyll a concentrations in coastal waters adjacent to each estuary were significantly correlated with the log total annual loads of TOxN, ammonium and phosphate (but not silicate) for each estuary, providing a direct link between a measure of the degree of biological response in coastal waters and the nutrient load through the estuaries. There were no significant correlations between spring maximum chlorophyll a concentrations and either catchment-normalized or estuary-normalized nutrient loads. There was significant correlation between catchment area-normalized loads of phosphate and an urbanization index for the catchments, but not with the catchment area-normalized loads of the other nutrients.  2002 Elsevier Science Ltd. All rights reserved. Keywords: nutrient loads; U.K. estuaries; Harmonised Monitoring Scheme; catchment areas

Introduction The development of towns and cities, and the expansion of populations and associated industry, has led to the widespread discharge of both municipal and industrial effluents into river and estuarine systems. This, along with changes in agricultural practices, has lead to increased concentrations of nutrients in estuaries, particularly nitrogen and phosphorus, correlated to the number of human inhabitants of a riverine catchment system (Wollast, 1983; Peierls et al., 1991). The over-abundance of benthic algae and phytoplankton in estuarine and coastal waters is often blamed on these excessive inputs of nutrients (Peierls et al., 1991). When adequate light is available, phosphorus and nitrogen are the nutrients that limit phytoplankton growth in aquatic systems, and Si for diatoms. Primary production in estuaries and coastal waters is 0272–7714/02/060951+20 $35.00/0

generally held to be limited by nitrogen availability, while freshwater phytoplankton tend to be limited by phosphorus availability, although the extent and severity of N limitation in the marine environment remains open to question (Hecky & Kilham, 1988; Boynton et al., 1982). Consequently, nutrient standards for estuarine and coastal waters have usually been designated in terms of acceptable nitrate concentrations, nitrate being the most abundant form of inorganic nitrogen. The U.K. is required through various international agreements and commitments (The OSPAR Convention; the North Sea Conference Declarations; European Community Directives) to reduce the quantities of nutrients from entering the sea from land-based sources. Nutrients such as nitrogen and phosphorus are included in these commitments because of their potential to cause eutrophication in estuaries and the sea, although at present no legislation exists governing maximum  2002 Elsevier Science Ltd. All rights reserved.

952 D. B. Nedwell et al.

acceptable concentrations of nutrients within estuaries. Furthermore, in turbid estuarine and coastal waters even high nutrient concentrations may not result in significant stimulation of primary production because of light limitation of algal growth. Although there are many studies relating to nutrient concentrations within both oligotrophic and eutrophic estuaries (Fichez et al., 1992; Balls, 1994; Balls et al., 1995; Nedwell & Trimmer, 1996; Ogilvie et al., 1997; Sanders et al., 1997a, b), there is no current classification of British estuaries in relation to their nutrient status. We present data on the loads of nitrogen, phosphorus and silicate, both on a gross load and on a per unit area basis for each estuary in the mainland U.K to permit direct comparisons to be made.

45 46 42

49 51 52 48 50

47

53 55 54

41

57

56

58 59

60 61 62 63

40 39

64 66 65 67

36 34 38

37

Methods Currently 155 British (including Northern Ireland) estuaries are identified (Davidson et al., 1991; Figure 1). However, several of these are estuaries for more than one river e.g. the Wash has several smaller estuaries combining to produce one large system, as has the Thames and the Humber. By grouping these smaller estuaries, 93 major estuarine systems can be identified on mainland Britain (excluding northern Ireland and offshore islands such as the Shetlands). Table 1 lists the 93 U.K. mainland estuaries, and the rivers that enter them. Where secondary rivers enter an estuary, they are shown in Table 1 as subsidiaries of the primary river. Also in Table 1 is shown the catchment area of each estuary (see below) and the estuary area (Davidson et al., 1991). The annual nutrient loads to the 93 U.K. mainland estuaries were derived by using the water flow and concentration data for the gauging station on each river nearest to the tidal limit (see Table 1). The annual average water flow and annual average concentration of each nutrient were obtained from the Harmonised Monitoring Scheme (HMS; Simpson, 1980) data set held at the U.K. Environment Agency’s Environmental Data Centre, Twerton, England. Gauging station numbers and positions were obtained from Natural Environment Research Council (1998). In a statistical appraisal of the data Hurley et al. (1994) concluded that the HMS data contained good quality and ample data for ammoniacal nitrogen, nitrate and orthophosphate. The frequency of samples varied from region to region, and with the size of the river. The various approaches for estimating discharges of water-born loads over a long period of time may result in large differences depending upon the method of calculation used (Walling & Webb, 1985; de Vries &

44

43

35 33 68

32 31 26 27

15

25 24 23 22 21 20 19 17 18

69

30 29 28

70 71

73

16 11

12 9 8 7 6 14 13 10 3

2 1

74

5

75 76 77

4

84

83

78

80

82

85 87 86 89 88 92 91 90

72

81

79

93

F 1. Positions of the 93 mainland U.K. estuaries.

Klavers, 1994). Differences between calculated loads may be particularly important with particle-associated determinands which tend to be strongly related to water flow rate, but less so with dissolved determinands such as nutrients (Walling & Webb, 1985). Our initial procedure was equivalent to method 1 of de Vries and Klavers (1994) where the annual nutrient load was derived by multiplying the annual average water flow (m3 s
1) by the annual average concentration of each nutrient (mg l
1): Method 1

where L is annual load estimate, K is a conversion factor to take account of sampling frequency and units for discharge and concentration, n is number of samples, Ci is concentration on sampling date, Qa is

U.K. estuary nutrient loads 953 T 1. The 93 U.K. mainland estuaries covered by the Harmonised Monitoring Scheme data (see Fig. 1 for location). Where more than one river enters an estuary they are shown as secondary rivers. The tidal limit gauging stations used for load calculations, the catchment area for the gauging station, the corrected catchment area for the whole estuary, and the estuary area are also shown

Gauging station No. 1 2 3

Primary river

No.

Catchment area (km2)

River

Estuary

Estuary area (km2)

9037 9027 9028 9030 9035 8426 8326 3227 10033 10034 10035 10037 10013 10009 10012 10011 10010 10024 10042 10025

49002 49001 50002 50001 50012 52007 52005 54057 56001 56005 56002 55023 57008 58001 57005 57009 58002 59001

48 209 826 663 54 75 202 9895 912 98 216 4010 179 158 455 145 191 228

50 216 826 921 54 75 202 9996·3 1124·6 106·9 222·4 4061 223 273 510 163 260 236

50 216 1801

1·15 8·39 15·93

277

65·29

15512

556·84

223 273 673

1·2 1·87 1·6

260 236

59002

46

162

162 1318

11·29 7·85 no data 95·24 82·95

Towy (Tywi) Taf

10026 10027

60010 60003

1090 217

1101 217 424

54·48

Eastern Cleddau Western Cleddau

10028 10039 10030 10041 10042 10032 10017 10015 10019 10021 10020 10022 10016 10023 10007 10008 10003 1002 1005 1006 1008 1016 1015

61001 61002 62001 63001 60006 63002 64001 64002 64005 65007 65001

198 183 894 170 130 182 471 75 111 52 69

894 173 130 182 476 75 281 52 69

65004 66011 66001 66006 67020 69007 68001 69032 71001 71014 70002

48 345 404 194 1817 660 622 90 1145 128 198

48 345 404 194 2118 2030 1370 90 1360 128 198

48 345 598

3·02 no data no data 0·18 19·54 1·17 9·99 0·85 no data 3·04 3·43 14·94 1·2

2118 3400

161·01 89·14

90 1686

14·13 119·24

2776

454·62

1023 1011 1014 1012 1013 1022

72002 72004 73010 73005 73008 75002

275 983 247 209 131 663

275 983 247 209 131 663

663

no date

Secondary river

Red (Hayle) Camel Torridge Taw Yeo

4

Parrett Tone

5

Severn Usk Afon Lwyd Ebbw Fawr Wye

6 7 8

Rhymney Ogmore Taff Ely

9 10 11 12 13 14

Neath Tawe Nant Y Fendrod Loughor Carmarthen Bay

Teifi Ystwyth Gwili Rheidol (Aberystwyth) Dovey (Dyfi) Dysynni Mawddach Dwyfawr (Pwllheli Harbour) Glaslyn Ogwen (Traeth Lafan) Gwyrfai (Foryd Bay) Conwy Clwyd

28 29

Dee Mersey

30 31

Alt Ribble

Elwy Weaver Darwen Douglas

207·6 216·8 894 173 130 182 476 75 281 52 69

Morecambe Bay Wyre Lune Leven Kent Bela

33

HMS code

Milford Haven

15 16 17 18 19 20 21 22 23 24 25 26 27

32

Corrected total catchment area (km2)

Derwent

954 D. B. Nedwell et al. T 1. Continued

Gauging station No. 34

Primary river

Secondary river

Esk Lyne

36 37 38 39

Dee Cree Water of Luce Garnock Irvine Annick Lugton

40

Clyde Kelvin White Cart Black Cart Leven

41 42 43 44 45 46 47

Lochy Carron Thurso Wick Shin (Dornoch Firth) Conon (Cromarty Firth) Inner Moray Firth Beauly Ness Nairn

48 49 50 51 52 53 54 55 56 57 58

Findhorn Lossie Spey Deveron (Banff Bay) Ugie Ythan Don Dee North Esk (St Cyrus) South Esk (Montrose Basin) Tay Earn Dighty water

59 60

No.

Catchment area (km2)

River

1019 16002 16003 16004 1020 1021 16005 16006 16007 17011 17009 17010 17012 17001 17002 17003 17004 17005 11008 11009 11010 11001 11002 11003

76007 78003 79006 80001 77001 77005 80002 81002 81003 83009 83005 83008 83007 84013 84001 84012 84017 85001 91002 93001 97002

2286 925 471 199 842 191 809 368 171 184 381 91 55 1903 335 235 103 784 1252 138 413

2397 957 799 199 842 191 809 368 171 184 381 91 55 1903 335 235 103 784 1252 138 413

3005 4001

575 962

575 962

11004 11005 11006 11006 12001 12002 12003 12004 12005 12006 12007 13006 13005 13003 13002 13004 13001 14005 14001 14002 14003 14006 14007 14008 14009 14010 14011 15001 15002 2009 2012

5001 6007 7004 7002 7003 8006 9001 10002 10003 11001 12001 13007 13008 15006 16004 14002 14001

849 1839 313 216 216 2861 442 325 523 1273 1370 732 488 4587 782 127 307

1074 1966 313 216 216 2987 955 325 523 1336 2116 732 488 4760 782 127 307

17002 18002 18005 17001 17005 19001 19006 19007 20001 21009 21022 22001 22007

424 181 210 122 195 369 107 330 307 4390 503 570 287

424 181 210 122 195 369 107 330 307 4390 503 636 331

HMS code

Inner Solway Firth Eden Annan Nith Urr Water

35

Corrected total catchment area (km2)

Eden Forth Leven Devon Allan Carron Avon Almond Water of Leith Esk

61 62

Tyne (Tyningham Bay) Tweed

63 64

Coquet Wansbeck

Whiteadder

Estuary

Estuary area (km2)

4352

420·56

1035

11·34

809 368 171 711

11·44 47·28 12·28 2·04

3854

54·85

1252 138 413 575 962 3353

no data no data no data no data no data 116·63 92·32

216 216 2987 955 325 523 1336 2116 732 488 5669

no data 0·56 0·49 1·02 no data 2·82 0·23 0·97 0·35 8·42 121·28

307 1938

10·41 84·01

307 4893

5·07 2·36

636 331

no data 0·15

U.K. estuary nutrient loads 955 T 1. Continued

Gauging station No. 65 66 67 68 69

70

71

Primary river

No.

Catchment area (km2)

River

Estuary

Estuary area (km2)

2923 2044 2061 4012

23001 24009 25009 27050

2176 1008 1264 308

2935 1174 1930 360

2935 1174 1930 360 19427

7·92 2 13·47 0·3 303·57

Hull Ouse Aire Don Wharfe Derwent Trent Idle

4001 4003 4004 4007 4013 4014 3006 3009

26002 27009 27003 27021 27002 27041 28009 28015

378 3315 1932 1256 759 1586 7486 529

852 3520 2057 1739 913 1586 8231 529 6521

666·54

Welland Nene Ely Ouse Mid Lev Main Drain

5502 5510 5651 5683

31004 32001 33035

717 1634 3436

717 2368 3436 884

15·34

Wensum Bure

5714 5722 5810 5820 5830 5840 6010 6105 6106 7001

571 313 578 255 993 190 9948 1036 303 1386 345 519 135 401 379 154

25·31 23·35 51·84

11287

47·45

1386 345 519 135 401 533

64·41 8·63 3·76 0·47 1·24 1·71

Rother

571 165 578 238 247 190 9948 1036 303 1256 345 206 135 396 139 154

578 255 1183

7004 7005 7006 7008 7007

34004 34003 36006 37005 37010 37008 39001 38001 37001 40003 40011 40004 41003 41004 41019 41011

1716

39·75

Test Itchen Blackwater

7012 7013 7011

42004 42023 42014

1040 415 105

1196 415 105 2779

2·39

Avon Stour

8100 8201

43005 43007

324 1073

1706 1073 648

38·05

Piddle Frome

8300 8400 9001 9002 9003 9008 9011 9013

44002 44001 45004 45005 45001 46002 46003 46008

183 414 289 203 601 381 248 102

184 464 426 283 1194 381 475 102

426 283 1194 381 475 102 1338

0·43 0·36 18·74 3·7 8·63 2·14 39·62

9014 9015 9017 9023 9024 9025

47011 47015 47001 47004 48011 48003

79 197 917 136 169 87

79 197 956 136 171 110

171 110

3·05 24·82

Secondary river

Tyne Wear Tees Esk Humber

Breydon Water Stour Colne Blackwater

75

Thames

Chelmer Lee Roding 76 77 78 79 80 81

Medway Pegwell Bay Rother Cuckmere Ouse Arun

82

Southampton water

84 85 86 87 88 89 90 91

Christchurch Harbour Poole Harbour Axe Otter Exe Teign Dart Avon Plymouth Sound Plym Tavy Tamar Lynher

92 93

HMS code

Wash

72 73 74

83

Corrected total catchment area (km2)

Fowey Fal

956 D. B. Nedwell et al.

mean annual water flow based on averaged daily flow measurements. The method is simple and permits at least broad comparisons between the annual nutrient loads between U.K. estuaries. We calculated the loads for 1995 and 1996, the last 2 years for which complete data were available, and then used the average annual load values for these 2 years. Such long-term averaging may underestimate peak loads during periods of very high flow. Better precision is achieved by determining the nutrient fluxes over shorter periods of time, and then summing the fluxes to determine the annual load. We also calculated the monthly nutrient loads from 1995–1998 using the monthly averaged water flow rates and monthly averaged nutrient concentrations for the Thames, Mersey, Severn, Humber, Clyde, Colne and Conwy, which included the largest estuary loads from U.K. estuaries, and some of the smaller ones. The annual load was derived by summing all the monthly loads for each estuary (method 2). We compared the estimates of annual nutrient loads by both methods for these seven estuaries.

catchment. This is probably reasonable in relation to the magnitude of the differences in nutrient loads between geographical areas that we report (see below). The total annual nutrient loads for each estuary were used to obtain two other indices. First, the nutrient load per unit of catchment area was derived by dividing the annual estuarine nutrient load by the total area of catchment (km2) for any particular estuary. Secondly, a measure of the relative influence of the nutrient load in each estuary was obtained by dividing the annual estuarine nutrient load by the area of each estuary given in Davidson et al. (1991). The dissolved nutrients reported to the HMS data base (Simpson, 1980) are total oxidized nitrogen (TOxN, which is nitrate+nitrite); ammoniacal nitrogen; orthophosphate; silicate. The standard analytical methods used to obtain these data are described in Simpson (1980). Particulate nutrients are not included within these data, but only soluble nutrients. Silicate was not measured in estuaries in some regions of the U.K. during 1995 and 1996, and the absence of silicate loads for some estuaries (see Figures) reflects lack of data, not absence of silicate.

Method 2 L is annual load estimate, K is a conversion factor to take account of sampling frequency and units for discharge and concentration, Cm is the mean monthly nutrient concentration, Qm is the mean monthly water flow. In several estuaries the gauging station nearest to the tidal limit did not cover all branches of a river system entering an estuary, or was not at the tidal limit (see Natural Environment Research Council, 1998), and in these cases the calculated load had to be corrected to allow for the proportion of the estuary’s catchment which was not included in the gauging station flow. To achieve this the interactive digital terrain model (Morris & Flavin, 1990) at the Centre for Ecology & Hydrology, Wallingford, was used to obtain an estimate of the total catchment for an estuary to its tidal limit, and this area was compared with the catchment area drained by the particular gauging station within the estuary catchment (given in Natural Environment Research Council, 1998). This gave a ratio of gauging station catchment area: total estuarine catchment area, and the nutrient load calculated for any gauging station was then corrected by this ratio to give the nutrient load from the total catchment, and hence the corrected total nutrient load to an estuary. An underlying assumption in this approach is that the nutrient load per unit area of catchment is constant within any particular estuary

Estimates of outputs of sewage treatment works to estuaries In order to establish the relative importance of nutrient loads discharged directly into estuaries, rather than entering via rivers, STWs on several of the major British estuaries were located and the local Water Authority approached for discharge data. STW discharge data is not available for many of the smaller estuaries with small STWs, and also many estuaries have no STWs. Results Figure 2 shows the annual fluvial loads of TOxN, ammonium, orthophosphate and silicate to the U.K. estuaries, in Mmol (moles106) of the particular nutrient y
1 as a mean of the loads for 1995 and 1996. Because of very large variations between nutrient loads to some estuaries compared to others, linear plots obscure the data for those estuaries with smaller loads. The nutrient loads are therefore shown as semilogarithmic plots to allow the loads to be seen for all estuaries. Firstly, in general the TOxN loads are at least an order of magnitude greater than loads of ammonium and phosphate. The annual loads of silicate are similar in magnitude to those for TOxN, at least in those estuaries for which data is available. It is evident that

10 000 TOxN load (Mmol N y–1)

TOxN

Mersey

Severn

Humber Thames

Clyde

1000 100 10 1 0.1

Ammonium load (Mmol N y–1)

0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 NH4+

1000

100

10

1

0.1 0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Phosphate load (Mmol P y–1)

1000

PO43–

100 10 1 0.1 0.01 0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Silicate load (Mmol Si y–1)

10 000

SiO22–

1000

100

10 No data

No data

1 0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Estuary number

F 2. Annual fluvial loads of nutrients to U.K. estuaries (mean of 1995 and 1996 loads). Units are in Mmoles y
1 of N, P or Si. Bars indicate range of 1995 and 1996 loads.

958 D. B. Nedwell et al.

Sum of monthly average (Mmol N y–1)

4000

3000

2000

1000 y = 1.3963X – 104.9 r2 = 0.91

0

1000

2000

3000

4000

Yearly average (Mmol N y–1)

F 3. Comparison of the annual TOxN loads to the Thames, Mersey, Severn, Humber, Clyde, Colne and Conwy estuaries during 1995–1998 calculated by either annual average load (method 1) or summed monthly loads (method 2). Broken line shows equivalence; solid line shows regression of y upon x.

there were large variations in the nutrient loads to estuaries around the U.K. Several regions have estuaries with significantly higher annual loads of TOxN, including the Severn estuary (estuary number 5 in Table 1), the Mersey (estuary 29), the Clyde (estuary 40), the Humber (69), the Thames (75), and those around the Solent (estuaries 83–84). These estuaries are draining catchments with generally high nitrate soils, often in ‘ Nitrate Vulnerable Zones ’ (Hornung, 1999, and see http://www.environment-agency.gov. uk//s-enviro/viewpoints/3compliance). In contrast, the west Wales estuaries (16–26) and the north Scottish estuaries (41–46) had particularly low TOxN loads. These drain relatively infertile catchments, with low population densities. Figure 3 compares the estimates for seven U.K. estuaries of annual TOxN loads for each year 1995–98 derived by annual averages of water flow and concentration with those estimated from summed monthly loads. It can be seen that there is good correlation of the load data to a straight line (r2 0·91, P<0·05) and the monthly summation method gave rather higher estimates of loads, as reported also by Walling and Webb (1985). This is presumably because the short periods of peak nutrient loads, at times of peak water flows, are better represented by the monthly loads than by a yearly average. The regression coefficient for the relationship suggests that annual load estimates derived from annual averages (method 1) need to be

increased by a factor of 1·4 to correct to the monthly averaged method. This factor is not large when compared to the order of magnitude of regional variations in loads observed (Figure 2). The annual loads of ammonium (Figure 2) were generally an order of magnitude smaller than those of TOxN, and showed much less consistent regional trends, although they were particularly high for the Severn (5), around the Mersey (29) and Clyde (40), and the estuaries on the east coast of England. Ammonium inputs are probably derived largely from STWs, and such variability of ammonium loads reflects differences in the population densities, in the amounts of STW effluent discharged, and the nitrification efficiency of the STWs above each gauging station. Phosphate loads were an order of magnitude less than those for TOxN. but the geographical pattern of variation resembled that of TOxN. Silicate loads were of similar magnitude to TOxN, but (at least for those estuaries where data was available) exhibited comparatively little geographical variation. Silicate loads tend to reflect catchment mineralogy, and are relatively independent of anthropogenic influences (Hessen, 1999). While it is not unexpected that the estuaries of large rivers with large catchments would have greater annual nutrient loads than small rivers with small catchments, the catchment area-normalized loads permit comparisons removing such differences in catchment size. The loads of nutrients derived km
2 catchment gives information about the relative export loads of nutrients per unit of catchment area, including leaching of nutrients from soil to surface waters, and sources such as STW inputs or animal wastes above the gauging station. Figure 4 shows that the catchment-normalized TOxN load to most U.K. estuaries was relatively constant, averaging 1·1 105 moles N km
2 y
1. Some of the west Wales (16, 17, 24) and north Scottish (41–46) estuaries do have catchment area-normalized loads lower than the average; while others (29, 30) seem to have particularly high catchment area-normalized loads; but most are constant around 105 moles N km
2 y
1. Catchment area-normalized P loads were very much lower (average 4·85103 mole P km
2 y
1) than N, and showed much greater geographical variation. There were increases of the catchment areanormalized P loads around the Mersey (29–31), and all around the south east coast of England from the Tyne to the Solent (60–76). It is difficult, with an incomplete data set, to be confident about catchment area-normalized Si loads, although it appears [Figure 4(d)] that there are very large geographical variations

1e+7 TOxN load (mol N km–2 y–1)

TOxN 1e+6

1e+5

1e+4

1e+3

Thames Medway

Mersey

Ammonium load (mol N km–2 y–1)

1e+5

NH4+

Tyne Tweed

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Clyde

5

Alt

0 1e+6

1e+4

1e+3

1e+2 0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Phosphate load (mol P km–2 y–1)

1e+5

PO43–

1e+4

1e+3

1e+2

1e+1 0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Silicate load (mol Si km–2 y–1)

1e+7

SiO22–

1e+6 1e+5 1e+4 1e+3 1e+2

No data

No data

1e+1 0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Estuary number

F 4. Catchment area-normalized annual nutrient loads to mainland U.K. estuaries. Bars indicate range of 1995 and 1996 loads.

960 D. B. Nedwell et al. 1000 N:P = 16:1

TIN:P

100

10

1 0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

10 P:Si = 1:16

P:Si

1

0.1

0.01 No data

No data 0.001 0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 N:Si = 16:16

TIN:Si

100

10

1 No data No data 0.1 0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Estuary number

F 5. Atom ratios of annual nutrient loads (mean of 1995 and 1996 loads) to U.K. mainland estuaries. The horizontal lines on each graph indicates the Redfield atom ratios for the elements.

which may reflect differences in silicate mineralogy in the various catchments. Figure 5 shows the atom ratios between annual loads of nutrients entering the estuaries compared to the ratios required by algae for balanced growth, as indicated by the Redfield ratios of 106 C:16 N: 1 P:16 Si (Harris, 1986). They are expressed as the ratio between total inorganic nitrogen (TIN which is TOxN+ammonium) and phosphate; as TIN:Si and as

P:Si. The horizontal lines indicate the Redfield values for each of the ratios. The relative ecological influence of the nutrient load on a receiving estuary may be indicated by the nutrient load per unit area of estuary. Clearly, other things being equal a large area of estuary may be better able to cope with a given nutrient load than a smaller estuary. The estuary-normalized loads are shown in Figure 6. It can be seen immediately that there are

TOxN load (Mmol N km–2 y–1)

1000

TOxN

100 10 1 0.1 0.01

Ammonium load (Mmol N km–2 y–1)

0

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 NH4+

10 1 0.1 0.01 0.001 0

Phosphate load (Mmol P km–2 y–1)

5

100

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

100

PO43–

10 1 0.1 0.01 0.001 0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Silicate load (Mmol Si km–2 y–1)

10 000

SiO22–

1000 100 10 1 No data

No data

0.1 0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Estuary number

F 6. Estuary area-normalized annual nutrient loads (mean of 1995 and 1996) to U.K. mainland estuaries. Bars indicate range of 1995 and 1996 loads.

962 D. B. Nedwell et al. T 2. Loads of nutrients in Sewage Treatment Works effluents input directly to estuaries below the tidal limit (average of loads for 1995 and 1996). Loads are in units of Mmol nutrient y
1, and the STW input load is shown as a percentage of the total (STW+fluvial) load

Estuary number

STW TOxN

Fluvial TOxN

%

STW NH4+

Fluvial NH4+

%

STW PO3
4

Fluvial PO3
4

%

5 Severn 29 Mersey 32 Morecambe Bay 39 Garnock 65 Tyne 66 Wear 67 Tees 69 Humber 70 Wash 72 Stour 73 Colne 74 Blackwater 75 Thames 76 Medway 77 Pegwell Bay 78 Rother 81 Arun 82 Southampton Water 83 Christchurch Harbour 91 Plymouth Sound

30·5 8·8 1·4 6·5 8·5 1·0 5·8 0·9 19·6 0·5 4·8 1·0 741·0 19·5 0·1 0·7 0·7 17·9 13·5 6·5

1996 2667 210 66 308·7 95·4 225·8 2970·8 426·3 45·6 43·0 177·2 2305·6 225·3 36·4 22·7 156·1 210·0 468·7 150·3

1·5 0·3 0·7 9·0 2·7 1·0 2·5 0·0 4·4 1·1 10·0 0·6 24·3 8·0 0·3 3·0 0·4 7·8 2·8 0·04

256·4 83·2 44·2 76·5 166·3 10·5 64·2 88·2 56·9 0·6 19·5 0·1 477·0 77·1 0·1 0·1 0·1 54·9 0·6 115·1

285 1295 75 5·3 235·5 23·3 99·6 251·5 69·4 0·3 21·1 1·8 483·0 85·7 0·35 0·4 1·7 6·1 4·9 2·9

47·4 6·0 37·1 93·5 41·4 45·1 39·2 26·0 45·1 66·7 48·0 5·3 49·7 47·4 22·2 20·0 5·6 90·0 10·9 97·5

26·7 9·8 6·4 6·4 15·2 1·6 5·7 9·7 6·5 0·1 6·2 0·3 156·3 24·8 0·0 0·3 0·3 13·1 5·1 20·7

93·3 146·8 12 1·9 22·5 9·8 14·4 226·9 20·6 2·0 7·6 9·7 257·3 33·3 2·3 0·9 15·3 4·3 19·9 1·5

22·1 6·3 3·5 76·2 41·1 14·0 28·4 4·1 24·0 4·8 44·9 3·0 37·8 42·7 0·0 25·0 1·9 75·3 20·4 93·2

large differences in the nutrient impact on U.K. estuaries, with two orders of magnitude variation in the estuary-normalized loads of all four nutrients. Two areas of particularly high estuary-normalized TOxN load were estuaries 50–55 on the coast of Scotland around Aberdeen, estuaries 62–69 from the Tweed to the Humber, and some of the estuaries around the Solent on the south coast of England. These estuaries also tend to have high estuary areanormalized loads of P and Si, although ammonium loads again are very much more variable than those of the other nutrients. Estuaries in other geographical areas, including most of the Welsh estuaries and those in northern Scotland, have very much lower estuary area-normalized nutrient loads. Table 2 shows for 20 British estuaries the data for TOxN, ammoniacal nitrogen and phosphate inputs from STWs to the estuary below the tidal limit gauging station, together with the fluvial nutrient load data for comparison. Figure 7 shows for 1995–1998 the monthly fluvial loads of nutrients into the Thames, together with the estimated inputs of nutrients from STW effluents discharged into the estuary below the tidal limit gauging stations. The seasonal changes of fluvial nutrient loads with peak loads during winter, and the particularly dry winter of 1996–1997, can be seen clearly. The inputs of nutrients from STW effluents show no

seasonality, but are relatively constant throughout the year. Figure 8 shows for the Thames estuary the ratios of monthly loads of N,P and Si from the rivers alone and from the rivers combined with the STW inputs. Discussion Our analysis of the nutrient loads to estuaries as indicated by the HMS data provides a comprehensive picture of geographical variation of nutrient loads to U.K. coastal waters. There has been considerable concern over the historical trends of increasing concentrations of nutrients (and presumably of nutrient loads) in river waters (e.g. DOE/CDEP 1986; Turner & Rabelais, 1991), and the consequent impact of these nutrients on biological processes. Similar trends of increase of nutrients in coastal waters have also been reported (Hickel et al., 1993; Allen et al., 1998). Eutrophication in response to nutrification of receiving waters is regarded as of major importance in some European estuaries, and a considerable amount of legislation has been directed towards controlling any adverse effects. While there is considerable geographical variation, it is apparent that annual nutrient loads to U.K. estuaries are small compared to those reported from the much larger European and North American

U.K. estuary nutrient loads 963

350

90 TOxN

80

300

75

250

70

200

65

150

60

100

55

50

50

0

0

J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D

10 8

80 NH4

+

70 60 50

6

70 4

40 30

2

20 0

J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D

STW ammonium (Mmol N)

Riverine ammonium (Mmol N)

85 STW TOXN (Mmol N)

Riverine TOxN (Mmol N)

400

10

18

Phosphate (Mmol P)

16

PO43–

14 12 10 8 6 4 2 0

J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D

150 Silicate load (Mmol Si)

2–

SiO2 100

50

0

J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D 1995 1996 1997 1998 Months

F 7. Monthly loads of nutrients in the fluvial flow to the Thames estuary between 1995 and 1998. Also shown is the STW nutrient loads discharging into the Thames estuary during 1995 and 1996. River, STW.

964 D. B. Nedwell et al. 12 P:Si = 1:16

10

P:Si

8 6 4 2 0

J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D

50 N:P = 16:1

TIN:P

40 30 20 10 0

J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D

80 N:Si = 16:16

TIN:Si

60

40

20

0

J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D 1995 1996 1997 1998 Months

F 8. Atom ratios of monthly nutrient loads into the Thames estuary in river flow alone during 1995 to 1998., and when the river and STW effluent flows are combined. River, River+STW. The horizontal lines on each figure indicate Redfield atom ratios.

estuaries. For example, the largest total inorganic nitrogen (TIN equivalent to TOxN+ammonium) loads to U.K. coastal waters were from the Severn (2280 Mmoles y
1), Mersey (3959 Mmoles y
1), Humber (3223 Mmoles y
1) and Thames (2788 Mmoles y
1). By comparison the Scheldt estuary exports 5190 Mmoles N y
1 (Billen et al., 1985), the Seine 6641 Mmoles N y
1 (Billen, unpublished data cited in Howarth et al., 1996), and the Rhine/Meuse 29 000 Mmoles N y
1 (OSPAR; de Vries et al., cited in Nienhuis, 1996). The large American rivers

export even larger loads-total N loads for the Mississippi 130 000 Mmoles N y
1 and the Amazon 223 000 Mmoles N y
1 (see Nixon et al., 1996). Howarth et al. (1996) give values for total fluvial N inputs into the North Sea of 1·22 Tg N y
1 (8·7104 Mmol N y
1) and total fluvial P of 0·099 Tg P y
1 (3·1103 Mmol P y
1). Our data indicates that total east coast, U.K., estuarine N loads are equivalent to only about 13% of the total fluvial N load to the North Sea, or about 2% of the total inorganic N input to the North Sea when inflows from

U.K. estuary nutrient loads 965

the North Atlantic are also included (Laane et al., 1993). The average N load from catchments around the North Sea is reported to be about 1450 kg N km
2 y
1 (Howarth et al., 1996), and the average TIN load to U.K. estuaries of 1·1105 moles N km
2 y
1 (1·4103 kg N km
2 y
1) is almost exactly equal to this value. Howarth et al. (1996) estimated the range of N fluxes from pristine rivers in the temperate regions around the North Atlantic basin as between 76 to 230 kg N km
2 y
1. Hessen (1999) suggested that for most pristine temperate water-sheds runoff will be in the range 80–200 kg N km
2 y
1; water sheds influenced moderately by human activity 500– 2000 kg N km
2 y
1; and heavily influenced catchments > 10 000 kg N km
2 y
1. The values obtained for the U.K. estuaries (average 1·12105 moles TIN km
2 y
1: 1·57103 kg N km
2 y
1) were, except for the west Wales and northern Scottish estuaries, much greater than for pristine catchments, indicating their nutrient-enriched status. Howarth et al. (1996) reported N inputs for a range of European estuaries from 650 kg N km
2 y
1 from the Viroin to as much as 4490 kg N km
2 y
1 from the Scheldt. These estimates of annual N loads do not include any contribution by dissolved organic nitrogen (DON) although Seitzinger and Sanders (1997) suggested that DON could contribute a significant part of fluvial total N load. However, in the large nutrified U.K. estuaries which contribute the majority of the inorganic N load it is unlikely that DON will greatly increase the total N load. For example, in the Great Ouse DON represented only 3–20% of the total N load (Rendell et al., 1997), and in the Thames <10% (D. Sivyer & M. Trimmer, unpublished data). The maximum catchment area-normalized N load for U.K. estuaries were those of the Mersey and the Alt (1·2106 and 1·1106 moles N km
2 y
1,: equal to 1·7104 and 1·5104 kg N km
2 y
1, respectively). Thus a few U.K. estuaries seem to be at the top end of the spectrum of catchment-normalized N loads, at least compared to other reported values, but the majority were within the ‘ moderately influenced ’ category of Hessen (1999). The data base for loads of other nutrients is much smaller. The average P load for catchments around the North Sea was reported as 117 kg P km
2 y
1 (Howarth et al., 1996), and the average for the U.K. estuaries was about 4·9103 moles km
2 y
1, equivalent to about 152 kg P km
2 y
1. This is above the European average, and the HMS data includes only dissolved P which may be only a small part of the total flux, which is often dominated by P

adsorped to suspended particulate material (e.g. Prastka et al., 1998). The ratios of loads of nutrients to estuaries also give valuable information for estuarine management (Figure 5). The ratios between nutrients may indicate those nutrients which either regulate the growth rates of algae in estuaries (Monod or Blackman type limitation) or have the potential to limit the production of algal biomass by such growth (Liebig type limitation). A particular nutrient will only become rate limiting to growth when the in situ concentration of that nutrient falls to a level at which uptake by algae is no longer at a nutrient-saturated maximum rate. Much of the concern about eutrophication and the management of nutrients in coastal waters has been with formation of ‘ nuisance ’ algal blooms (i.e. accumulation of high density of biomass) in response to nutrification. Nutrient ratios indicate which nutrient may become ultimately limiting to the production of algal biomass, but the imposition of that limitation will not occur until the actual concentration of the limiting nutrient has fallen to a value at which its starts to limit the growth rate. Redfield atom ratios of 106C:16N:1P: 16Si give ‘ typical ’ element ratios for algal biomass. Assuming that CO2 is never limiting in coastal waters, and that light is abundant, these ratios suggest that any N:P atom ratio greater than about 16:1 indicates potential limitation of algal biomass formation by phosphate, while ratios <16 indicate potential limitation of algal biomass production by N. Figure 5 shows that in the large majority of U.K. mainland estuaries TIN:P of annual loads was >16, suggesting that they are potentially limited by phosphate rather than N, with a few exceptions in West Wales (estuaries 11, 16, 17), Scotland (40, 44) and the English east coast (73, 75, 76, 79) which would appear to be N limited. Current belief is that while freshwaters tend to be P limited (see Hecky & Kilham, 1988), coastal waters tend to be N limited (Ryther & Dunstan, 1971; Howarth, 1988) but the body of evidence for consistent N limitation in coastal waters is not as firm as that for P limitation in fresh waters, and there have been other reports of P limitation in estuaries (Harrison et al., 1990; Rudeck et al., 1991). However, the load data would tend to support the idea of P limitation at the freshwater end of most estuaries. The east coast English estuaries have high inputs of treated sewage effluents which has relatively low N:P ratios (high phosphate content), which may tend to decrease any tendency to P limitation in receiving waters (Howarth, 1988). However, these nutrient ratios are based on annual nutrient loads and may conceal considerable seasonal and interannual variation of the ratios within these annual values (see later).

966 D. B. Nedwell et al.

For those estuaries where Si data were available, the P:Si ratios were usually <1:16, indicating potential P limitation, except for estuaries around the Mersey and Morecombe Bay (29–32), around the Tyne (65), Tees (67), the Wash (70); and the Thames (75) where the ratio was >1:16. The N:Si ratios were usually >1:1, which indicated relative excess of N and potential for limitation by Si. Based on annual nutrient loads and assuming equally conservative behaviour for all nutrients, therefore, potential nutrient limitation in U.K. mainland estuaries would appear to be in the order P>Si>N, in decreasing order of limitation, and assuming equally conservative behaviour of all three nutrients within an estuary. While loads of N and P are anthropogenically elevated above pristine levels in many European estuaries, Si loads are held to be less influenced anthropogenically, and are regulated by the mineralogy of the catchment through erosion of the parent rock (Hessen, 1999). Seasonal variations of Si concentrations in estuaries have been suggested (Fichez et al., 1992; Balls et al., 1995) to result more from seasonal variations in biological removal than in seasonal changes of fluvial loads. While abundant Si may be present in coastal waters in early spring, it is often removed after the spring bloom (e.g. Justic et al., 1995; Del Amo, 1997a, b) so that diatoms become Si limited and the phytoplankton become dominated by microflagellates which do not require Si. Justic et al. (1995) have suggested that this change in the dominant type of phytoplankton from diatoms to microflagellates might have impact on the type of food web which is supported by the phytoplankton community. Diatoms tend to be grazed by copepods, which then support a consumer food web with fish as the top predators. In contrast, microflagellates tend to be grazed by flagellates, and their biomass enters the microbial food web. The contribution of nitrate loads from STWs directly into estuaries was usually trivial compared to the fluvial nitrate loads (Table 2): significant only in the Thames (24% of total), Garnock (9%), Medway (8%) and Southampton Water (8%). In contrast, loads of ammonium from estuarine STWs were usually a significant percentage of the total fluvial ammonium loads in the estuaries investigated. This is possibly not surprising as in most rivers nitrification would reduce to some extent the ammonium load from STWs discharge into the river above the tidal limit gauging station. Thus at least part of the fluvial ammonium load would appear as nitrate. Is there any correlation between the nutrient loads to estuaries and the degree of development of the various catchments? The Centre for Ecology and

Hydrology, Wallingford, have developed an ‘ urbanisation index ’ (URBEXT1990) from remote sensing data from the 1990 Land Cover Map of Great Britain (Centre for Ecology and Hydrology, Monks Wood). This expresses the area of built over urban area as a proportion of the total land area, while suburban area is similarly expressed but weighted 0·5 relative to urban area, to give an overall urbanization index. We obtained the urbanization index for every estuarine catchment and compared it with the catchmentnormalized nutrient load. Where complex estuaries had more than one river entering, we calculated the percentage of each river catchment which was urbanized, totalled them, and derived the urbanization index for the catchment for the whole estuary. There were two obvious outliers, the Mersey and Alt estuaries, which had a disproportionate effect on the analysis. Omitting these two outliers, there were no significant (P>0·05) correlations between the degree of urbanization of an estuary catchment and the TOxN, ammonium or silicate loads; but there was a statistically significant relationship (P<0·05) between the catchment urbanization and its P load. This might possibly not be surprising when P load is probably strongly influenced by the STW input from urban and suburban areas, and hence strongly correlated with human population density, while TOxN load is influenced both by STW effluents and from nitrogen leached from the soil. The relative impact of the river born nutrient loads on the estuaries that receive them is indicated by the estuarine area-normalized loads, which showed large geographical variations. One problem here is to define what actually is the area of an estuary. Should this include the area of the estuarine plume, maybe outside the physical seaward limits of an estuary but still an area where the impact of a river discharge may be seen (e.g. Trimmer et al., 1998)? We have taken a simplistic approach and used the area of each estuary within its seaward physical limits given by Davidson et al. (1991). [This is the definition of estuaries that has been used to assess in England and Wales the impact of nutrients under the Urban Wastewater Treatment Directive (91/271/EEC), and the Nitrates Directive (91/676/EEC), whereas Scotland includes the area of the estuarine plume for the same purpose.] On this basis the greatest impact of nutrients seems to be in many of those estuaries which also exhibit symptoms of eutrophication in terms of high chlorophyll a concentrations >10 mg chl a l
1 i.e. the estuaries around Aberdeen (50–55), along the English east coast, around the Solent and along the south coast (83–87) (see www.environment-agency.gov.uk/ s-enviro/viewpoints/4health/3 eutroph/4-3-3c.html).

U.K. estuary nutrient loads 967 20

20 r2 = 0.453 (P < 0.05)

15

Chlorophyll a (µg l–1)

Chlorophyll a (µg l–1)

r2 = 0.306 (P < 0.05)

10

5

0

1

10

100

15

10

5

0 0.01

1000 –1

10

100

1000

–1

NH4 (Mmol N y )

20

20 2

r2 = 0.196 (P < 0.05)

r = 0.403 (P < 0.05) 15

Chlorophyll a (µg l–1)

Chlorophyll a (µg l–1)

1 +

TON (Mmol N y )

10

5

0 0.01

0.1

0.1

1 PO4

3–

10

100

1000

–1

(Mmol P y )

15

10

5

0

1

10

100

1000

SiO22– (Mmol Si y–1)

F 9. Correlation between the average spring peak chlorophyll a concentration measured in the coastal sea water at the mouth of each English and Welsh estuary during the National Coastal Baseline Survey during 1993–1996, with the log of the annual load (mean of 1995 and 1996 loads) of nutrients to each estuary. Regression lines are shown.

One of the current concerns of environmental management is to develop indicators of eutrophication or response to nutrient loads. In view of the general nutrient limitation of primary production in aquatic systems it might be hypothesized that there might be some correlation between nutrient loads and indicators of biological activity. During 1993–1996 the U.K. Environment Agency carried out a National Baseline Survey around the coast of England which included sampling the water at a number of coastal stations during spring and measurement of chlorophyll a concentrations at this time. During the spring, before phytoplankton biomass became grazed down or dependent on nutrients recycled from mineralized organic matter, it might be argued that any correlation between nutrient load and phytoplankton biomass would be most apparent. From their coordinates we

determined those sampling stations which were at the mouths of estuaries, and for each estuary the average (1993–96) spring peak chlorophyll a concentrations (data obtained from the Environment Agency Data Centre, Twerton, U.K.) were regressed against the average annual (1995–96) estuarine nutrient loads. Average spring chlorophyll a concentrations increased asymptotically against the total loads to each estuary for all four nutrients, which indicated that the algae became limited by other factors at very high nutrient loads. There were significant (P<0·05) linear regressions between the average spring chlorophyll a concentration and the log of the loads of TOxN, ammonium and phosphate, but no significant correlation with silicate load (Figure 9). A higher proportion of the variation of the spring chlorophyll a concentrations was explained by the log of the load of

968 D. B. Nedwell et al.

ammonium (r2 0·45) than by phosphate (r2 0·41) or TOxN (r2 0·31). This might not be surprising as ammonium tends to be preferred to nitrate as a nitrogen source by algae (e.g. Dortch, 1990), even if the nitrate concentrations are very much higher than ammonium. Ammonium loads would, therefore, be the first nitrogen source to drive primary production in estuaries, whereas nitrate would tend to be utilized only after ammonium was depleted. These statistically significant relationships indicated, though, that the N and P loads to U.K. estuaries seem to regulate the coastal phytoplankton biomass during the spring, before it becomes dependent on regenerated nutrients (Dugdale & Goering, 1967; Eppley & Petersen, 1979). The lack of a correlation of chlorophyll a with silicate load indicated that diatom biomass was not, at least during the spring, limited by the availability of silicate. It is usually after the spring bloom that diatoms become limited by lack of silicate (e.g. Del Amo et al., 1997a, b; Underwood & Kromkamp, 1999) and it would be interesting to examine whether there is a significant relationship between the chlorophyll a concentration in estuarine and coastal waters and the estuarine silicate loads during summer. There were no statistically significant correlations between the average spring chl a concentrations and either the log of the catchment area-normalized nutrient loads or the log of the estuary area-normalized nutrient loads. This is possibly not surprising as the phytoplankton biomass in offshore coastal water (as indicated by the spring chlorophyll a maximum) will be influenced by the total load of nutrient emerging from an estuary. Other indicators, such as attached macroalgal biomass, or phytoplankton biomass within an estuary, may be more influenced by the relative impact of nutrients per unit area of estuary i.e. by the estuary area-normalized load. However, it should be appreciated that the impact of a nutrient load on an estuary will also be a function of its residence time within the estuary. This is indicated by the fresh-water flushing time (FWFT) (Balls, 1994; Monbet, 1992; Nedwell et al., 1999; Alber & Sheldon, 1999). Even a large nutrient load per unit area of estuary may not have a great impact within the estuary if the nutrients are flushed rapidly through the estuary. Monbet (1992) has shown that the average concentration of chlorophyll a within an estuary is a function of the tidal range. Thus, with fast flushing any impact by nutrification may be in the coastal seas rather than in the estuary itself. Again, despite high nutrient loads and a relatively long freshwater flushing time (100–200 days; Uncles & Radford, 1980) the Severn estuary is not subject to high chlorophyll a concentrations, but here it is probably high suspended

solids load and poor light penetration which limits primary production. Nonetheless, the correlations between the annual nutrient loads indicated by the HMS data with other indicators of possible eutrophication such as high chlorophyll a seem to suggest them as potentially useful management indicators of nutrient impact on estuaries and adjacent coastal seas. Examination of the monthly nutrient loads of the seven selected rivers over a 4 year period illustrated other important factors. We use the Thames as an example. Figure 7 illustrates both fluvial and STW load data for the Thames for this period, and it can be seen that while the fluvial loads varied both interannually and with season, the STW loads were essentially constant throughout the year, and a monthly average STW load was calculated. It was clear (see Figure 8) that in the Thames (and also in the Mersey, Severn, Clyde, Humber and Colne) the TIN:P atom ratios of the fluvial loads indicated P limitation during winter (TIN:P >16:1) but N limitation during summer (TIN:P <16:1). However, when the inputs of STWs to the estuary were also included it was apparent that the high P content of the STW effluents made the estuary, and the algae using the nutrients, potentially N limited throughout the year. This monthly data illustrate that the potentially limiting nutrient indicated by nutrient ratios during the active growing season in the spring and summer may be quite different to that indicated by the annual nutrient ratios, as the peak fluvial loads of TOxN during the winter occur at a time when algal growth is not nutrient limited but light limited. Thus the change from estuarine P to N limitation during winter probably has little ecological impact, but increased N limitation during summer may be significant. It may be more difficult to control the diffuse sources of N input in a catchment, compared to control of the dominant point sources of P from STWs. In contrast to the nutrified estuaries, in the low nutrient input Conwy estuary the nutrient ratios in the monthly loads indicated P limitation throughout the year. Acknowledgements This work was carried out under contract CWO694 from the Department of the Environment, Transport and the Regions, U.K. Government. We wish to thank Dr Richard Emmerson for help and encouragement during this work. Mr Martin Lees (Centre for Ecology & Hydrology, Wallingford) is thanked for help with the digital terrain model, and Mr Ron Thomas (Environment Agency Data Centre, Twerton) for making available the Harmonised Monitoring Scheme data. The opinions expressed in this report are

U.K. estuary nutrient loads 969

those of the authors, and do not represent Government policy, although they may contribute to the development of Government policy.

References Alber, M. & Sheldon, J. E. 1999 Use of a date-specific method to examine variability in the flushing times of Georgia estuaries. Estuarine Coastal and Shelf Science 49, 469–482. Allen, J. R., Slinn, D. J., Shammon, T. M., Hartnoll, R. G. & Hawkins, S. J. 1998 Evidence for eutrophication in the Irish Sea over four decades. Limnology and Oceanography 43, 1970–1974. Balls, P. W. 1994 Nutrient inputs to estuaries from nine Scottish east coast rivers; influence of estuarine processes on inputs to the North Sea. Estuarine Coastal and Shelf Science 39, 329–352. Balls, P. W., Macdonald, A., Pugh, K. & Edwards, A. C. 1995 Long-term nutrient enrichment of an estuarine system: Ythan Scotland (1958–1993). Environmental Pollution 90 (3), 311–321. Billen, G., Somville, M., DeBecker, E. & Sevais, P. 1985 A nitrogen budget of the Scheldt hydrographical basin. Netherlands Journal of Sea Research 19, 223–230. Boynton, W. R., Kemp, W. M. & Keefe, C. W. 1982 A comparative analysis of nutrients and other factors influencing estuarine phytoplankton production. In Estuarine comparisons (Kennedy, V. S., ed.). Academic Press, New York, pp. 69–90. Davidson, N. C., Laffoley, D. d’A., Doody, J. P., Way, L. S., Gordon, J., Key, R., Drake, C. M., Pienkowski, M. W., Mitchell, R. M. & Duff, K. L. 1991 Nature conservation and estuaries in Great Britain. Nature Conservancy Council, Peterborough, 422 pp. Del Amo, Y., Le Pape, O., Treguer, P., Quequiner, B., Menesquen, A. & Aminot, A. 1997a Impacts of high nitrate freshwater inputs on macrotidal ecosystems I Seasonal evolution of nutrient limitation for the diatom-dominated phytoplankton of the Bay of Brest. Marine Ecology Progress Series 161, 213–224. Del Amo, Y., Quequiner, B., Treguer, P., Breton, H. & Lampert, L. 1997b Impacts of high nitrate freshwater inputs on macrotidal ecosystems II Specific role of the silicic acid pump in the year-round dominance of diatoms in the Bay of Brest (France). Marine Ecology Progress Series 161, 225–237. De Vries, A. & Klavers, H. C. 1994 Riverine fluxes of pollutants: monitoring strategy first, calculation method second. European Water Pollution and Control 4, 12–17. DOE/CDEP 1986 Nitrate in water—a report by the nitrate coordination group. HMSO, London, 103 pp. Dortch, Q. 1990 The interaction between ammonium and nitrate uptake in phytoplankton. Marine Ecology Progress Series 61, 183– 201. Dugdale, R. C. & Goering, R. F. 1967 Uptake of new and regenerated forms of nitrogen in primary productivity. Limnology and Oceanography 12, 196–206. Eppley, R. W. & Petersen, B. J. 1979 Particulate organic matter flux and planktonic new production n the deep ocean. Nature 282, 677–679. Fichez, R., Jickells, T. D. & Edmunds, H. M. 1992 Algal blooms in high turbidity, a result of the conflicting consequences of turbulence on nutrient cycling in a shallow water estuary. Estuarine Coastal and Shelf Science 35, 577–592. Harris, P. G. 1986 Phytoplankton ecology. Chapman and Hall, London, 384 pp. Harrison, P. J., Hu, M. H., Yang, Y. P. & Lu, X. 1990 Phosphate limitation in estuarine and coastal waters of China. Journal of Experimental Marine Biology Ecology 140, 79–87. Hecky, R. E. & Kilham, P. 1988 Nutrient limitation of phytoplankton in freshwater and marine environments: A review of recent evidence on the effects of enrichment. Limnology and Oceanography 33 (4), 796–822.

Hessen, D. O. 1999 Catchment properties and the transport of major elements to estuaries. Advances in Ecological Research 29, 1–41. Hickel, W., Mangeldorf, P. & Berg, J. 1993 The human impact in the German Bight: eutrophication during three decades. Helgolander Meeresunters 47, 243–263. Hornung, M. 1999 The role of nitrates in the eutrophication and acidification of surface waters. In Managing risks of nitrates to humans and the environment (Wilson, W. S., Ball, A. S. & Hinton, R. H., eds). Royal Society of Chemistry, Cambridge, pp. 155– 174. Howarth, R. W. 1988 Nutrient limitation of net primary production in marine ecosystems. Annual Review of Ecology 19, 89–110. Howarth, R. W., Billen, G., Swaney, D., Townsend, A., Jaworski, N., Lajtha, K., Downing, J. A., Elmgren, R., Caraco, N., Jordan, T., Berendse, F., Freney, J., Kudeyarov, V., Murdoch, P. & Zhu, Z. L. 1996 Regional nitrogen budgets and riverine N and P fluxes for the drainage to the North Atlantic Ocean: natural and human influences. Biogeochemistry 35, 75–139. Hurley, M. A., Hilton, J. & Butterwick, C. 1994 Harmonised monitoring data for England and Wakes—a statistical appraisal. Unpublished report, 71 pp. National Rivers Authority, Bristol. Justic, D., Rabelais, N. N., Turner, R. E. & Dortsch, Q. 1995 Changes in nutrient structure of river-dominated coastal waters: stoichiometric nutrient balance and its consequences. Estuarine Coastal and Shelf Science 40, 339–356. Laane, R. P. M. W., Groeneveld, G., De Vries, A., Van Bennekom, J. & Sydow, S. 1993 Nutrients (N, P, Si) in the Chanel and the Dover Strait: seasonal and year-to-year variation and fluxes to the North Sea. Oceanologica Acta 16, 607–616. Natural Environment Research Council 1998 Hydrological data United Kingdom. Hydrometric Register and Statistics 1991– 95 (Marsh, T. & Lees, M., eds). Institute of Hydrology, Wallingford, 207 pp. ISBN 0 948540 84 2. Monbet, Y. 1992 Control of phytoplankton biomass in estuaries: a comparative analysis of microtidal and macrotidal estuaries. Estuaries 15 (4), 563–571. Morris, D. G. & Flavin, R. W. 1990 A digital terrain model for hydrology. Proceedings of the 4th International Symposium on Spatial Data Handling 1: 250–262. Dept of Geography, University of Zurich, Zurich, Switzerland. Nedwell, D. B. & Trimmer, M. 1996 Nitrogen fluxes through the upper estuary of the Great Ouse, England: the role of the bottom sediments. Marine Ecology Progress Series 142, 273–286. Nedwell, D. B., Jickells, T. D., Trimmer, M. & Sanders, R. 1999 Nutrients in estuaries. Advances in Ecological Research 29, 43–92. Nienhuis, P. H. 1996 The North Sea coasts of Denmark, Germany and the Netherlands. In Marine benthic vegetation: recent changes and the effects of eutrophication (Schramm, W. & Nienhuis, P. H., eds). Springer-Verlag, Berlin, pp. 187–221. Nixon, S. W., Ammerma, J. W., Atkinson, L. P., Berounsky, V. M., Billen, G., Biocourt, W. C., Boynton, W. R., Church, T. M., DiToro, D. M., Elmgren, R., Garber, J. H., Giblin, A. E., Jahnke, R. A., Owens, N. J. P., Pilson, M. E. Q. & Seitzinger, S. P. 1996 The fate of nitrogen and phosphorus at the landsea margin of the North Atlantic Ocean. Biogeochemistry 35, 141–180. Ogilvie, B., Nedwell, D. B., Harrison, R. M., Robinson, A. & Sage, A. 1997 High nitrate, muddy estuaries as nitrogen sinks: the nitrogen budget of the River Colne estuary (United Kingdom). Marine Ecology Progress Series 150, 217–228. Peierls, B. L., Caraco, N. F., Pace, M. L. & Cole, J. J. 1991 Human influence on river nitrogen. Nature 350, 386–387. Prastka, K., Sanders, R. & Jickells, T. D. 1998 Has the role of estuaries as sources or sinks of dissolved inorganic phosphorus changed over time? Results of Kd study. Marine Pollution Bulletin 36, 718–728. Rendell, A. R., Horrobin, T. M., Jickells, T. D., Edmunds, H. M., Brown, J. & Malcolm, S. J. 1997 Nutrient cycling in the Great Ouse estuary and its impact on nutrient fluxes to the Wash, England. Estuarine Coastal and Shelf Science 45, 653–668.

970 D. B. Nedwell et al. Ryther, J. H. & Dunstan, W. M. 1971 Nitrogen, phosphorus, and eutrophication in the coastal marine environment. Science 171, 1008–1013. Rudek, J., Paerl, W., Mallin, A. & Bates, P. W. 1991 Seasonal and hydrological control of phytoplankton nutrient limitation in the lower Neuse River estuary, North Carolina. Marine Ecology Progress Series 75, 133–142. Sanders, R. J., Jickells, T., Malcolm, S., Brown, J., Kirkwood, D., Reeve, A., Horrobin, T., and Ashcroft, C. 1997a Nutrient fluxes through the Humber estuary. Journal of Sea Research 37, 3–23. Sanders, R., Klein, C. & Jickells, T. 1997b Biogeochemical cycling in the upper Great Ouse estuary, Norfolk, U. K. Estuarine Coastal and Shelf Science 44, 543–555. Seitzinger, S. P. & Sanders, R. W. 1997 Contribution of dissolved organic nitrogen from rivers to estuarine eutrophication. Marine Ecology Progress Series 159, 1–12. Simpson, E. A. 1980 The harmonization of the monitoring of the quality of rivers in the United Kingdom. Hydrological Sciences Bulletin 25, 13–23.

Trimmer, M. Nedwell, D. B., Sivyer, D. B. & Malcolm, S. J. 1998 Nitrogen fluxes through the lower estuary of the river Great Ouse, England: the role of the bottom sediments. Marine Ecology Progress Series 163, 109–124. Turner, R. E. & Rabelais, N. N. 1991 Changes in Mississippi River water quality this century: implications for coastal food webs. BioScience 41, 140–147. Uncles, R. J. & Radford, P. J. 1980 Seasonal and spring-neap tidal dependence of axial dispersion coefficients in the Severn—a wide, vertically mixed estuary. Journal of Fluid Mechanics 98, 703–726. Underwood, G. J. C. & Kromkamp, J. 1999 Primary production by phytoplankton and microphytobenthos in estuaries. Advances in Ecological Research 29, 93–153. Walling, D. E. & Webb, B. W. 1985 Estimating the discharge of contaminants to coastal waters by rivers: some cautionary comments. Marine Pollution Bulletin 16, 488–492. Wollast, R. 1983 Interactions in estuaries and coastal waters. In The major biogeochemical cycles and their interactions (Bolin, B. & Cook, R. B., eds). SCOPE Report 21: 385–407. Wiley, New York.