The Long-Term Thermal Impact of Reservoir Operation and Some Ecological Implications

Man's Influence on Freshwater Ecosystems and Water Use (Proceedings of a Boulder Symposium, July 1995). IAHS Publ. no. 230, 1995. 245

The long-term thermal impact of reservoir operation and some ecological implications

B. W. WEBB & D. E. WALLING Department of Geography, University of Exeter, Amory Building, Rennes Drive, Exeter EX4 4RJ, Devon, UK

Abstract A long-term study of the thermal impact of a regulating reser voir in southwest England is reported. Detailed records collected for the regulated River Haddeo and the neighbouring unregulated River Pulham in a 13-year period following the attainment of top water level in the reservoir (Wimbleball Lake) reveal that the main thermal effects of impoundment and regulation have been to raise mean water temperature, eliminate freezing conditions, depress summer maximum values, delay the annual cycle and reduce diurnal fluctuation. Long-term records also reveal pronounced year to year contrasts in the impact of reservoir construction, which can be largely explained by fluctuations in the volume of runoff released from the reservoir in the summer period, or passing the spillway of the dam in the winter period. Combination of data on daily mean water temperatures with published biological models derived from laboratory studies suggests that the thermal modification associated with reservoir construction has had a greater impact on the life cycle and growth of brown trout than on the development of selected mayfly and stonefly species. Considerable inter-annual variability in the extent of the predicted biological impacts indicates the need for long- term, as well as detailed, records in order to define rigorously the physical and ecological consequences of impoundment.

INTRODUCTION

There have been very many studies of the impact of reservoir construction and asso ciated river regulation on downstream thermal regime and the consequences for the ecology of water courses below impoundments. However, although these investigations have encompassed schemes which differ greatly in environmental setting, in purpose and in position and number of dams within the river system (e.g. Neel, 1963; Williams, 1968; Nishizawa & Yambe, 1970; Collings, 1973; Ward & Stanford, 1979; Edwards & Crisp, 1982; Petts, 1984; Rader & Ward, 1988; Brittain & Saltveit, 1989; Marchant, 1989; Tuch & Gasith, 1989; Voeltz & Ward, 1989; O'Keeffe et al, 1990; Saltveit, 1990; Liu & Yu, 1992; Gippel &Finlayson, 1993; Tvede, 1994), few of the conclusions concerning water temperature behaviour and its biological implications have been based on long-term studies. For example, the major published investigations of water tempera ture in regulated rivers of northern England and Wales (Lavis & Smith, 1972; Cowx et al, 1987; Crisp, 1987) have relied on between one and five years of data. Although these and many other studies have provided valuable insights into the effects of 246 B. W. Webb & D. E. Walling

impoundment, a short-term perspective may be limiting. In the absence of long-term records, it is impossible to assess how the physical and biological impacts of regulation vary in response to factors such as the maturation of water quality within reservoirs, significant inter-annual variability in hydrometeorological conditions and changes in the operation schedule of regulation schemes. The present study reports an investigation of a reservoir in southwest England, which has spanned more than 17 years and has provided the opportunity not only to quantify the long-term impact of impoundment and river regulation, but also to assess and explain inter-annual variability in the effects on water temperature and on selected aspects of the freshwater biota.

THE WIMBLEBALL STUDY

Wimbleball Lake, which is located in the River Haddeo on the eastern margins of the Exmoor upland in southwest England (Fig. 1A), was constructed as a dual-purpose reservoir to regulate the mainstream of the River Exe and to provide a direct supply for part of Somerset (Battersby etal., 1979). Impounding began in mid-December 1977 and the firstoverflo w was recorded in November 1980 (Fig. IB). The catchment feeding the reservoir has an area of 29 km2 and mean annual rainfall and runoff of 1330 and 910 mm, respectively. Details of the Wimbleball Scheme and its operation are given in Webb & Walling (1988a). A compensation release of 9.1 Ml day"1, which is required at all times, is made up by seepage from springs lying just downstream of the dam, as well as by water released directly from the reservoir. The latter is mainly taken from the top 25 m of the water body. In order to assess the effects of Wimbleball Lake, the water temperature monitoring network established in the Exe Basin by the University of Exeter (Fig. 1A) includes a site on the River Haddeo situated 0.4 km below the dam (Upper Haddeo) and a station on the River Pulham (Pulham), which is a neighbouring tributary similar to the River Haddeo in all respects except that it is unregulated. Continuous measurements have been made at these sites with commercially-available mercury-in-steel thermographs during a period extending from Spring 1976 until the present day. Measuring bulbs are anchored to the stream bed in order to prevent exposure to the air and thermographs are checked against weekly readings taken with a standard laboratory-calibrated thermo meter. The latter procedure rarely reveals discrepancies exceeding 0.2°C. For the purposes of data analysis, water temperature information has been abstracted as hourly records from the thermograph charts. Work reported previously (Webb & Walling, 1988a; 1988b) has demonstrated that closure of Wimbleball Dam had an immediate effect on the thermal regime of the River Haddeo, which continued during the three-year filling phase of the reservoir. The present study is based on 13 years of detailed data following the attainment of top water level and covers the period from 1 January 1981 to31 December 1993. The effect of the Wimbleball Scheme on water temperature is assessed by comparing hourly records for the regulated River Haddeo and unregulated River Pulham, while the ecological implica tions of regulation are ascertained by combining water temperature data with published biological models derived from laboratory studies (cf. Webb & Walling, 1993). Long-term thermal impact of reservoir operation 247

Altitude in metres ES

200 300 400 500 600 Weeks since 1st January 1978 Fig. 1 The study reservoir and location of the study sites (A) and weekly variation in drawdown of Wimbleball Lake (B).

LONG-TERM THERMAL IMPACTS

It is clear from 13 years of detailed records that the main effect of the construction of 248 B. W. Webb & D. E. Walling

Wimbleball Lake on the thermal regime of the River Haddeo has been to increase the mean water temperature value, eliminate freezing conditions, depress summer maxima, delay the annual cycle and reduce diurnal fluctuation(Fig . 2). Average temperature at Upper Haddeo was 0.6°C higher than at Pulham and temperatures in the regulated river

Water Température {°C} A 1 Maximum ,_ Pulham 2 Mean 3 Minimum Upper~^"v 4 Maximum diurnal range Haddeo ^**>>«^_ 5 Mean diurnal range 6 Minimum diurnal range peratur e (°C ) E 11- —— UH 19.6 10.3 1.6 6.3 1.2 0.1 x-—, ~~ v, £ 1- PU 21.3 9.7 -0.9 10.1 2.6 0.1 1 1 1 1 1 1 i 11 l|!

Percentage of time

U O 16 - c o 14 IÏ Pulham / / \ \ S 3 '3 S 12 / / \\ H io- Kl O \, S g* // Upper Haddeo Pulham - C 10 5 <" a

-1 1 1 1— I I I i I i I I 1 1 J F M A M J J A M J J SOND Month Month D U U 16' o Upper Haddeo l¥l4- Upper Haddeo c a g 3 04 | g.H B 10 3 § I S Pulham ID H C Pulham

£ 6- MAM J J A SOND M J J A S O N D Month Month Fig. 2 Water temperature statistics and duration curves (A) and the average annual cycle of mean (B), mean maximum (C), mean minimum (D) and mean diurnal range (E) values based on the period 1 January 1981 to 31 December 1993 for the regulated (Upper Haddeo) and unregulated (Pulham) river stations. Long-term thermal impact of reservoir operation 249

did not rise above 20°C, nor fall below 1.5 °C, during the 13-year study period. Tempe rature duration curves for the regulated and unregulated rivers (Fig. 2A) reveal that the Wimbleball Scheme has had a greater effect in increasing temperatures in the low to intermediate range than in reducing high values. It is also clear that impoundment and river regulation have reduced the extent of diurnal water temperature fluctuationi n the River Haddeo. The average diurnal range at Upper Haddeo is less than half that at Pulham, while the maximum daily fluctuation recorded between 1981 and 1993 ex ceeded 10°C in the unregulated river but was ca 4°C less in the regulated water course. The delay in the annual cycle of water temperature fluctuation at Upper Haddeo compared with Pulham is evident from study-period mean values for individual months (Fig. 2B). The peak in the annual march occurs in July for the unregulated river, but in August for the regulated water course. Lowest mean water temperature is recorded in February for both catchments, but the spring rise and autumn fall are delayed at Upper Haddeo, with particularly strong differences in temperature occurring during the period from September to December. Similar contrasts between Upper Haddeo and Pulham are evident for the annual cycle of mean maximum water temperature, although values are more markedly lower in the spring months (Fig. 2C). Mean minimum values of water temperature are higher for Upper Haddeo than for Pulham in all months, but the contrast is small in the period between May and July (Fig. 2D). Plotting of study-period mean values for individual months also shows that intra-annual variation in daily water tempe rature range follows a similar pattern in the regulated and unregulated rivers, although it is clear that daily fluctuations are suppressed in all seasons below the reservoir (Fig. 2E).

INTER-ANNUAL VARIABILITY IN THERMAL MODIFICATION

Long-term records not only indicate the overall impact of the construction of Wimbleball Lake on downstream water temperatures, but also highlight pronounced year to year contrasts in the effects of the scheme. For example, although mean water temperature has generally increased in the River Haddeo as a consequence of regulation, annual statistics (Table 1) show that this effect was absent in 1982 but reached + 1.2°C in 1989, 1991 and 1992. Similarly, no depression of the maximum temperature recorded at Upper Haddeo compared with that at Pulham was evident in 1991 and 1992, but large reduc tions (>3.5°C) were observed in 1982 and 1984. Water temperature minima were higher in the regulated catchment for every year (Table 1), but the magnitude of the difference varied from +1.1 °C in 1982 to 5.9°C in 1992. Annual statistics also reveal that differences in mean and maximum daily ranges in water temperature between Upper Haddeo and Pulham varied considerably from year to year (Table 2). The impact of the Wimbleball Scheme on minimum daily water temperature range was less marked and less variable. This is because in regulated and unregulated catchments alike, water temperature fluctuation is restricted, at least for a few winter days, by overcast or freezing conditions. Inter-annual variability in the effects of impoundment and regulation on the seasonal cycle of water temperature is highlighted by plotting the range in difference between Upper Haddeo and Pulham recorded during the study period for monthly mean values of maximum, mean and minimum temperatures, as well as diurnal fluctuation (Fig. 3). 250 B. W. Webb & D. E. Walling

Table 1 Annual water temperature statistics for Upper Haddeo (regulated) and Pulham (unregulated).

Upper Haddeo Pulham Difference (UH -- PU)

Year Max Mean Min Max Mean Min Max Mean Min

1981 17.0 9.9 3.6 17.6 9.5 0.6 -0.6 + 0.4 + 3.0 1982 16.5 9.8 1.6 20.2 10.1 0.5 -3.7 -0.3 + 1.1 1983 18.0 10.2 3.4 21.3 9.8 0.2 -3.3 +0.4 + 3.2 1984 16.0 10.1 4.1 19.7 9.9 2.1 -3.7 +0.2 +2.0 1985 16.0 10.2 3.0 18.5 9.1 -0.3 -2.5 + 1.1 + 3.3 1986 15.9 9.2 2.5 19.4 8.8 -0.5 -3.5 +0.4 + 3.0 1987 17.4 9.8 2.8 19.1 9.3 -0.9 -1.7 + 0.5 + 3.7 1988 17.5 10.1 4.9 17.6 9.5 1.0 -0.1 + 0.6 + 3.9 1989 19.6 11.5 5.7 19.9 10.3 1.8 -0.3 + 1.2 + 3.9 1990 18.3 11.4 6.2 19.9 10.4 2.0 -1.6 + 1.0 +4.2 1991 17.7 10.8 4.1 17.6 9.6 -0.2 + 0.1 + 1.2 +4.3 1992 17.7 11.0 6.8 17.7 9.8 0.9 0.0 + 1.2 + 5.9 1993 16.7 10.3 5.0 17.4 9.4 1.1 -0.7 + 0.9 + 3.9

Temperature values given in °C; Max, Mean, Min maximum, mean and minimum values, respectively.

Table 2 Annual statistics for diurnal water temperature fluctuation at Upper Haddeo (regulated) and Pulham (unregulated).

Upper Haddeo Pulham Difference (UH -- PU)

Year Max Mean Min Max Mean Min Max Mean Min

1981 3.9 1.4 0.3 8.0 2.3 0.3 -4.1 -0.9 0.0 1982 3.9 1.5 0.3 8.6 2.7 0.4 -4.7 -1.2 -0.1 1983 6.3 1.4 0.2 6.9 2.5 0.3 -0.6 -1.1 -0.1 1984 2.6 1.1 0.1 8.6 3.0 0.4 -6.0 -1.9 -0.3 1985 2.8 0.9 0.1 9.4 2.6 0.4 -6.6 -1.7 -0.3 1986 4.4 1.1 0.1 7.4 2.4 0.2 -3.0 -1.3 -0.1 1987 5.5 1.2 0.1 9.9 2.7 0.2 -4.4 -1.5 -0.1 1988 3.8 1.0 0.1 7.7 2.3 0.2 -3.9 -1.3 -0.1 1989 3.4 1.2 0.1 8.3 2.9 0.2 -4.9 -1.7 -0.1 1990 4.0 1.2 0.2 10.1 3.0 0.3 -6.1 -1.8 -0.1 1991 3.5 1.1 0.1 7.5 2.4 0.1 -4.0 -1.3 0.0 1992 4.4 1.1 0.2 7.6 2.5 0.5 -3.2 -1.4 -0.3 1993 3.8 1.0 0.2 7.0 2.4 0.3 -3.2 -1.4 -0.1

Temperature values given in °C; Max, Mean, Min = maximum, mean and minimum diurnal fluctuation, respectively.

In terms of mean and extreme temperatures (Fig. 3A-C), year-to-year variations in the impact of the reservoir are least for October and greatest for January. For example, the difference in monthly mean value between the regulated and unregulated water course ranged from +1.8 to +2.7°C for October but from -1.8 to +4.0°C for January (Fig. 3A). Inter-annual variability in the effects of regulation is relatively pronounced for the mid-summer months (May-July) in the case of mean maximum temperatures (Fig. 3B) but this pattern is not so well marked for mean minimum values (Fig. 3C). Although mean daily water temperature fluctuation showed a consistent reduction in the regulated Long-term thermal impact of reservoir operation 251 river for all months of the study period, year-to-year variations in this effect are greatest in the period from April to July and least for the mid-winter months (Fig. 3D). Inter-annual variability in the impact of the Wimbleball Scheme can be related to fluctuations in the volume of runoff originating from the reservoir. Plotting of the diffe rence in mean temperatures between the regulated and unregulated water courses against mean discharge measured close to the tail bay of the dam reveals systematic trends for most months (Fig. 4) which are also consistent with the causes of thermal modification identified for the Wimbleball Scheme. It has been demonstrated in earlier studies (Webb & Walling, 1988a; 1988b) that an increase in the spring flow feeding the River Haddeo below the dam is the most likely cause of the modified temperature regime at Upper Haddeo, since runoff from the reservoir cannot be shown to provide a source of rela tively cool water in summer nor of relatively warm water in winter. Water temperature at Upper Haddeo therefore reflects the mixing together of two contrasting sources of runoff (spring flow and flow from the reservoir) and variations in the volume and tempe rature of these components will influence differences in temperature between the regu lated and unregulated river sites. In the winter months from January to March, spring flow is generally warmer than runoff from the reservoir and in the unregulated river. Thus, in those years when flows from Wimbleball Lake are low during this period and spring flow dominates in the River Haddeo, temperatures at Upper Haddeo may be appreciably higher than those at Pulham (Fig. 4). However, in years when the months of January to March are wetter and higher volumes of relatively cold runoff pass the spillway of the dam, the warming influence of the spring flow is masked and the diffe-

Month Month Mean Diurnal Range D Mean Minimum 3 ' '

M J J A Month Month

Fig. 3 The range in temperature difference between the regulated (Upper Haddeo) and unregulated (Pulham) river stations recorded for individual months in the period 1 January 1981 to 31 December 1993 for mean (A), mean maximum (B), mean minimum (C) and mean diurnal range (D) values. 252 B. W. Webb & D. E. Walling

5- 3-, January February March 4- 3- 2- 3- . . 2- . 2- .. . 1 - r. ¥ 1- 1- ' 0- • . • ^ 0- \ > • D * • 0- -1- • , a. .1. • * , K „ , \. . ! _2- O 6 o!s 1 L5 2 2.5 0 0.4 0.8 1.2 1.6 0 0.5 1 1.5 2 2.5 3 t. 0) > 2 i] . April 0.5-1 May 1.5-, June T3 o • • . ,f3 0.5- "3 o- „ -0.5- . • • p60 • • • C *\ -0.5- D . * • T3 -1- • • -1.5- C - * -1.5- •. T3 * ,, S 5-, c October November December 'Z 2.8- 4- . 3- Hc) M 2.4- 3- 2- <4-< * • 2- S i# 2- 1- '". ' _ 1- ••' 1.6- -i 0- — 0- 0 0.1 0.2 0.3 0.4 0 0.2 0.4 0.6 0 Flow at Reservoir Tailbay (m3 s"1) Fig. 4 Difference in mean water temperature between the regulated (Upper Haddeo) and unregulated (Pulham) rivers in relation to mean flow from Wimbleball Lake for individual months in the period 1 January 1981 to 31 December 1993. rence in temperature between Upper Haddeo and Pulham is reduced. In some winters, runoff from Wimbleball Lake may be colder than flows in the unregulated River Pulham, because of the greater exposure associated with the large surface area of the reservoir. Given sufficiently high flows passing the spillway in these circumstances, monthly mean temperatures at Upper Haddeo may be lower than those at Pulham. In the summer months from May to August, the relationship between temperature differences and runoff from the reservoir is reversed (Fig. 4). At this time of year, the spring flow is generally cooler than the runoff in the unregulated river or originating from the reservoir. Therefore, in years when relatively little water is being released Long-term thermal impact of reservoir operation 253

from the reservoir during the summer period, the influence of spring flow dominates in the regulated river and monthly mean temperatures at Upper Haddeo are appreciably lower than those at Pulham (Fig. 4). This effect is diminished or even reversed for those years when more water is released from Wimbleball Lake. The temperature of runoff from the reservoir in the mid-summer period may exceed that of the unregulated River Pulham and in these circumstances relatively large releases from Wimbleball Lake will result in higher temperatures at Upper Haddeo compared with Pulham. As groundwater heats up during the summer period, the capacity of spring flow to reduce water tempera tures in the River Haddeo diminishes. By September, temperatures at Upper Haddeo exceed those of Pulham, even in years when flows from the reservoir are very low (Fig. 4). Relationships between temperature differences and runoff from the reservoir are more scattered for the months of April, October and November, which are transitional periods between the winter and summer situations. In April, contrasts between the temperature of the spring flow and runoff from the reservoir are generally not marked. In October and November, variability in flow from the reservoir is relatively low and the warming effect of spring flow dominates in the regulated river. A clear inverse relationship between temperature difference and runoff from Wimbleball Lake is evident for December, but even in years when relatively high flows passing the spillway of the dam, the warming effect of the spring flow in the regulated river is not obliterated (Fig. 4). Similar relationships linking temperature differences between the regulated and unregulated water courses to flow from Wimbleball Lake could be plotted for monthly mean maximum and minimum temperature values. However, inter-annual variability in the differences between monthly mean diurnal temperature ranges at Upper Haddeo and Pulham is relatively small (Fig. 3D) and is not generally related to the volume of flow originating from the reservoir. Contrasts between Upper Haddeo and Pulham tend to reflect conditions promoting diurnal fluctuations in the unregulated rather than in the regulated water course. Flow regulation and the effects of spring flow moderate diurnal temperature ranges recorded at Upper Haddeo in all years, while diurnal fluctuations at Pulham reach maximum values in years, such as 1984 and 1990, that are characterized by a favourable combination of low discharges and strong solar heating during the spring and mid-summer periods.

BIOLOGICAL IMPACTS

By making a number of simplifying assumptions (cf. Weatherley and Ormerod, 1990), information on daily mean water temperatures can be combined with published bio logical models derived from laboratory studies in order to assess the potential impact of the modified thermal regime of the River Haddeo on selected invertebrate and fish species.

Invertebrate development

The mayfly Baetis rhodani and four species of the stonefly in the genus Leuctra were 254 B. W. Webb & D. E. Walling investigated because they occur in the study rivers, include spring, summer and autumn species and are known to be variously sensitive to water temperature effects. Results of simulations of egg and larval development averaged for the period 1981-1993 suggest that regulation has had a relatively modest impact on the life cycle and growth of these invertebrates. Predicted incubation periods were slightly longer at Upper Haddeo than at Pulham for eggs of L. nigra and B. Rhodani fertilized in spring, while hatching was predicted to occur a few days earlier in the regulated compared with the unregulated river for B. rhodani and L. geniculata fertilized in late summer and autumn (Table 3). These effects can be attributed to the occurrence of lower temperatures during the spring and higher temperatures during the autumn in the River Haddeo below Wimbleball Dam (Fig. 3A). The stoneflies L. hippopus and L. moselyi, which are known to have a rela tively insensitive relationship between hatching time and water temperature, were asso ciated with small differences, on average, in simulated egg incubation periods for Upper Haddeo and Pulham. Predictions of egg development for individual years reveal greater variability in the difference between the regulated and unregulated river (Table 3). For example, the incubation period of B. rhodani fertilized on 1 March was predicted to be six days shorter than at Pulham in 1985 but a week longer in 1982, while that of L. geniculata fertilized on 1 September was eight days shorter in 1990 but only two days shorter in 1982. Inter-annual differences were least pronounced for the thermally insensitive species L. Moselyi. Predictions of the time between hatching and emergence of nymphs show that, on average, larval development of B. rhodani is retarded by a few days in the regulated river for eggs fertilized on 1 March because of the slower spring rise of water tempe rature, but this effect is absent for eggs fertilized on 1 May (Table 4). Simulations for individual years, however, suggest that differences in larval development of the mayfly

Table 3 Simulated egg development for selected mayfly and stonefly species at the study sites. Incubation period is expressed as number of days to 50 % hatch and has been calculated from water temperature data using relationships for B. rhodani given in Elliott (1972) and for Leuctra spp. given in Elliott (1987a).

Species Date of Average value for 1981-1993 Range of difference fertilization (UH - PU) in individual years Upper Haddeo Pulham B. rhodani 1 March 41.0 40.3 -6 (1985) to +7 (1982) 1 September 15.3 18.8 -6 (1986) to -1(1982) L. nigra 1 April 43.4 41.0 -1(1981) to +8(1987) 1 May 35.7 32.5 -1(1981) to +7(1987) L. geniculata 1 August 22.9 24.0 -4 (1985) to +1(1988) 1 September 23.7 27.7 -8 (1990) to -2(1982) L. hippopus 1 March 39.2 39.4 -5 (1985) to +4 (1982) 1 May 32.1 30.4 -1(1981) to +4(1985) L. moselyi 1 July 25.4 24.5 -1 (1989) to +3(1983) 1 August 23.9 24.8 -3 (1985) to 0 (1982) Long-term thermal impact of reservoir operation 255

Table 4 Simulated period in days between hatching and emergence of invertebrate larvae at the study sites calculated from water temperature data using relationships for B. rhodani given in Elliott et al. (1988) and for L. nigra given in Elliott (1987b). Larval length assumed to be 0.6 mm on hatching for both species but 9 and 7.5 mm on emergence for B. rhodani and for L. nigra, respectively.

Species Date of Average value for 1981-1993 Range of difference fertilization (UH — PU) in individual years Upper Haddeo Pulham B. rhodani 1 March 137.4 133.7 -5 (1981) to +9 (1984) 1 May 123.2 125.2 -8 (1989) to +5(1982) L. nigra 1 April 570.5 583.3 -22 (1990)* to -3(1981)*

* refers to year of fertilization.

between the regulated and unregulated water course did not remain consistent throughout the study period. In contrast to B. rhodani, which is univoltine, L. nigra is semivoltine and has a much longer stage of larval development. Predictions for eggs fertilized on 1 April show that the time between hatching and emergence is reduced, on average, by ca 13 days at Upper Haddeo compared with Pulham, which can be attributed to more favourable conditions for overwintering stonefly nymphs in the regulated river. In some years, however, development of the stonefly larvae was up to 22 days faster at Upper Haddeo, while in others, it was as little as only 3 days faster (Table 4).

Trout development

Predictions suggest that thermal modification associated with the Wimbleball Scheme has had a larger impact on the life cycle and growth of brown trout (Salmo trutta L.) than on invertebrate development. On the basis of results averaged over the 13-year period, simulations indicate that in the regulated river, the incubation period for eggs fertilized on 15 November is shortened by 15 days, the rate of alevin development is accelerated by 24 days and the weight of under-yearling fish at the end of the calendar year following swim-up is increased by 5.2 g or ca 35% (Table 5). Computation of mean instantaneous growth rate per day indicates that under-yearling fish do not, on average, grow faster in the regulated than in the unregulated water course. The greater weight attained by 31 December must therefore be a function of earlier emergence and a longer period for growth in the regulated river. Prediction of the weight gained by a "standard trout" over the calendar year (Table 5), however, indicates that larger fish do grow faster at Upper Haddeo than at Pulham and this effect may be related to the effects of regulation in moderating high summer water temperatures (Edwards et al., 1979; Elliott, 1994). Faster egg and alevin development for trout at Upper Haddeo probably reflects higher winter water temperatures in the regulated water course. Predictions also reveal considerable year-to-year variation in the extent to which trout development is favoured in the regulated river. Incubation periods varied at Upper Haddeo from being more than 30 days shorter in 1985 to one week longer in 1981 than at Pulham, while the comparable range for days to emergence of the trout fry was 256 B. W. Webb & D. E. Walling

Table 5 Simulated development of brown trout {Saltno trutta L.) at the study sites. Relationships of incubation period and alevin development with water temperature used in the predictions are given in Crisp (1981, 1988). An alevin weight of 200 mg on emergence is assumed and weight of under-yearling fish on 31 December is calculated from the relationship given in Elliott (1975) and assumes feeding on maximum rations. Growth refers to mean instantaneous growth rate per day for under-yearling trout during the period between swim-up and 31 December and weight of standard trout on 31 December is calculated for a trout weight 18.6 g on 1 January (Crisp, 1987).

Development Average value for 1981-1993 Range of difference (UH — PU) in individual years Upper Haddeo Pulham

Incubation period (days)* 47.0 61.9 -32 (1985) to +7(1981)

Days to swim-up 91.9 115.7 -57 (1985) to +13(1982)

Weight of under-yearling fish 20.0 14.8 1.5 (1987)to 9.1 (1985) on 31 December (g)

Growth 0.014394 0.014548 -0.001044 (1986) to 0.001059 (1982)

Weight of standard trout on 138.6 120.5 0.1 (1982) to 44.8 (1985) 31 December (g)

* fertilization of eggs is assumed to occur on 15 November in the previous year.

between 57 days shorter in 1985 to 13 days longer in 1982 (Table 5). Under-yearling and larger fish put on more weight in the regulated stream in all years, but this advantage varied in individual years for the former from 1.5 to 9.1 g and for the latter from 0.1 to 44.8 g.

CONCLUSIONS

Although the impact of the Wimbleball Scheme on downstream thermal regime is rela tively modest compared with that of larger reservoirs in different climatic settings, it is consistent with findings reported for other impoundments in Britain that are relatively shallow and exhibit poorly developed stratification. The present study suggests that there is considerable inter-annual variability in the extent of thermal modification occurring below the reservoir which also affects the biological impacts, especially with respect to trout development, that are associated with the regulation scheme. This variability primarily reflects how the scheme is operated during the summer months and hydro- logical conditions during the winter period. It can be concluded from this study that both long-term and detailed records are required to define rigorously the physical and eco logical consequences of impoundment and river regulation.

REFERENCES

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