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The distribution, abundance and life-histories of stoneflies

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by cooling-water from a power station

Article  in  Hydrobiologia · July 1971

DOI: 10.1007/BF00036844

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Hydrobiologia, vol. 38, 2, p. 339-377, 1971 .

The Distribution, Abundance and Life-Histories ofStoneflies (Plecoptera) and Mayflies (Ephemeroptera)

in a British River, Warmed by Cooling-Waterfrom a Power Station

by

T. E . LANGFORDCentral Electricity Research Laboratories,

Kelvin Avenue, Leatherhead Surrey

INTRODUCTION

Of the invertebrate groups found in stony streams and river, thePlecoptera and Ephemeroptera are generally regarded as the mostsensitive to ecological disturbance (HYNES, 1960) . It might be ex-pected therefore, that these animals would be among the first to beaffected by a heated discharge .

The River Severn at Ironbridge (Salop) is warmed by cooling-water discharged_frarrl Ironbridge `A' electricity generating station .For at least 2 km downstream of the outfalls the temperature of theriver water may be raised by values of 0.5°C to 7.2°C, dependingupon the rate of discharge of the river (dilution factor) and thepower output, by the power station . (LANGFORD - 1970) .

Collections of Plecoptera and Ephemeroptera were taken atmonthly intervals in 1965 and at weekly intervals in 1966-67 fromreaches upstream and downstream of the power station . The life-histories of the eight most abundant species were determined asclosely as possible for both heated and unheated reaches . Of thePlecoptera Taeniopteryx nebulosa (LINN) was the only common spe-cies, and at the time of writing (November 1968), no account hasbeen published of its life-history in a British river . The life-historiesof four of the seven commonest Ephemeroptera have not previouslybeen described in detail, viz. Heptagenia sulphurea (MULL), Baetis

Received Sept 22, 1970 .

339

bioculatus, (LINN), Boetis vernus (CURT) and Baetis buceratus (ETN) .The other common species were Ephemerella ignita (PoDA), Baetisrhodani (PICTET) and Baetis pumilus (BURM) . Their life-histories in theSevern are compared with those found in other rivers and regions .(MACAN, 1961 et seq .)The study forms part of a long-term project dealing with the

ecological effects of heated effluents on British rivers .

DESCRIPTION OF THE RIVER AND SAMPLING STATIONS

Physical CharacteristicsIronbridge `A' power station is situated on the west bank of the

River Severn, in the upper-middle reaches, approximately 100 kmupstream of the tidal limit . For some 40 km upstream of the powerstation, the river meanders in wide `oxbow' loops through a broad

TABLE I

Description and Location of Sampling Stations .

340

Abbey

SJ. 647045Bridge

0.5 km upstream(Buildwas)

of warm wateroutfalls fromIronbridge `A'

C,'-Garage Bend

SJ. 663036(Coalbrookdale)

0.6 km downstreamof the warm wateroutfalls

EGasholder

SJ. 679034(Ironbridge)

2 km downstream ofthe warm wateroutfalls

Average Maximum and CurrentStation Location width of Minimum Velocity

River Depth of Water (low water)

50-60 m Low summerLevels0.3-1 .0 mMaximumapprox. (flood)4-5 m

Main . Stream0.5-0.75 m/secSome slackwater at margins

40-50 m Low0.5-1.5 mHigh(in flood)4.5-5 .5 m

Main Stream0.4-0.6 m/secSome slackwater at margins

70-80 mLow0.3-1 .0 mHigh3-4 m(in flood)

Main Stream0.6-1 .0 m/secLittle marginalslack

valley . Short, shallow stony runs, alternate with slower, deeperreaches until just upstream of the power station ('A' on Fig. 1),where the river is approaching the Ironbridge gorge, when thechannel becomes straighter and the current becomes generallyfaster. At normal flow rates, the river at `A', `C' and `E' (see TableI), the main sampling stations, is shallow and turbulent with a sub-stratum of limestone rocks up to 20 cm in diameter, laid on a matrixof coarse sand (Table I) . All three sampling stations were basicallysimilar with very similar floras .

In the upper-middle reaches, the Severn is often subject to suddenspates and at such times depths of up to 5 m of water may be re-corded at the sampling stations .

Between, the stony runs at `A', `C' and `E' the river is deeper(1 .5 to 2.5 m) at normal flows and generally slower (0 .3 to 0 .5m/sec) with steep banks and a substratum of hard clay, overlaid bycoarse sand . In these reaches, small patches of Nuphar lutea, Poto-

River Severn Near Ironbridge .

Substratum

Basically coarse sand,overlaid by limestonechunks from 3 cm - 25cm in diameter .Clay margin, sand banksoccasional backwaters .

Basically coarse sand,overlaid by limestonechunks from 3 cm - 25cm in diameter .Some short sandy runs .

Clay at margins .Occasional pools at edge .

Basically coarse sandwith limestone chunksplus bricks and somedebris .

Clay banks

Common Plants

Remarks

Ranunculus spp . Fast, flowing tur-very abundant from bulent riffle reachMay-October . Some stretches for aboutalgae . Little patches

50-60 m .of Myriophyllum sp .Grasses at margins .

Ranunculus spp .

Fairly fast but avery abundant . Some

little deeper thanalgae and

Abbey Bridge . NotM_yriophyllum .

such a turbulent reach50--60 m long .

Grasses overhangingmargins .

Ranunculus spp .very abundant, somealgae. Mosses com-mon in fastest parts .

Grasses overhangingmargins .

Wide and fast . Moreturbulent and slightlyshallower than AbbeyBridge. Reachstretches for about100-150 m .

34 1

mageton natans and Typha spp occur at the margins and in small back-waters .

Chemistry of the River WaterThe river water is fairly hard with very little indication of organic

pollution (Table II) . B.O .D. values varied from 0 .9 to 5 .0 ppm, andthe higher levels were probably due to run-off from the rich agri-cultural land bordering the river, plus the effects of a well-treatedsewage effluent from Shrewsbury sewage disposal works some 15 kmupstream of the power station . There are no other power stations andvery little industrial development upstream of Ironbridge .

The river water is taken into the power station at Tl (Fig. 1) andpassed once through the cooling system before being returned to theriver. There is negligible loss through evaporation and no chlorineor other chemicals are added during the process .

INTAKE

RAILNABRIDGE

I

342

COOLING TONERS/~

I'll' STATION)

Fig . 1 . Map of river Severn around Ironbridge "A" and"B" power stations sam-pling stations and location of temperature recorder . (see text for explanation) .

Between `A' and `E', very small amounts of sewage and industrialeffluent are discharged to the river but, owing to the high dilutionfactor, these do not have any significant chemical effects (Table II) .

In September 1967 a 24-hour survey of dissolved oxygen wascarried out at two places upstream and two downstream of thecooling water outfalls . No consistent differences were observed be-tween any of the four sampling stations. At most times during theyear the river water is well oxygenated, usually over 70% .

TABLE H

Chem

ical

Cha

ract

eris

tics

of

the

Seve

rn,

Upst

ream

and

Dow

nstr

eam

of I

ronb

ridg

e `A

' Po

wer

Stat

ion

.

Loca

tion

(Sam

plin

gPoint)

Diss

olve

d Di

ssol

ved

5 day

B.O

.D.

(P.p.m

.)

4 hour

Perman-

gana

teValue

(p.p.m

.)

Ammonia

NH

3(p

.p.m

.)

Nitr

ate

NO,

(p.p.m

.)

Cloh

ride

(p.p.m

.)

pH

Total

Calc

ium

Susp

ende

d

asSolids

40% Ca

.(p

.p.m

.)

Hardness

02(p

.p.m

.)02

(% S

atn)

Buil

dwas

High

14.8

155

4.4

11.0

0.94

2.6

478.

4131

76(u

pstr

eam)

ALow

9.2

710.

91 .

2ni

l0.

313

7.7

1.0

28

Coal

port

High

14.8

160

5.0

11.4

0.72

2.4

448 .

2No Records

(dow

nstr

eam)

ELow

9.5

721.

20.

6ni

l0.

313

7.6

The Temperature of the River WaterThe natural temperature of the Severn in this region is much

more stable than that of small streams (LANGFORD - 1970) withdaily fluctuations rarely exceeding 2 .5 °C. The most obvious effectof the effluent was to exaggerate the normal diurnal fluctuations byraising the daytime temperature . Daily increases varied from 0 .5 to

33

- 360020

3200

17 .5

- 2800

IS

2400

12.52000

101600

344

800 53

400

(a)

1

13

II30

I I0 2'6

+

1

120

/

1

`

!

I\o

0-0

. . 1,~ 1

090 /

0 04r ,

1965

III

21

25

29

33

WEEK NUMBER

39I

1

III

41

45

1

O

L

49

I

Fig. 2. Mean weekly river temperatures . Upstream and downstream of Iron-bridge power station .

7 .2 °C above ambient, while mean weekly temperatures were raisedby values of 0.2°C to 4.0°C (Fig . 2) . Upstream of the power station,natural water temperatures reached absolute maxima of 20 .0 °C in1965 and 20.6°C in 1966. Downstream, daily maxima were 24 .8°Cand 23.5°C respectively. These maxima were usually only observedfor short periods, rarely more than 3-4 hours in any day . Highestmean weekly temperatures were 18 .0°C (1965) and 18.1'C (1966)upstream, while downstream weekly means reached 21 .5 °C (1965)and 19.6°C (1966) .

Long term differences on a time/temperature basis have beendemonstrated . During the winter the downstream reaches may be3 5 weeks in advance of the upstream reach in accumulating de-gree-hours (above 0 °C) . In summer the advance may be up to threeweeks .

Regular transect measurements at `A', `C' and `E' showed thatthere was no temperature stratification and the warm water wasthoroughly mixed in passing over the shallow, stony riffles . Also, thechange in temperature from `C' to `E' was rarely more than 0 .2 °C .

METHODS

The problems of sampling stony streams and rivers have beendiscussed by many authors, but are well summarised by MACAN(1958) . In the Severn several techniques were tried, including a`shovel' sampler (MACAN, 1958), a Surber sampler (SURBER, 1937)and a specially constructed dredge. None of these could be usedconsistently owing to the large variations in current velocity anddepth at different times of year .

The most effective and adaptable technique was by using a hand .held net in a standardised manner . The nets were held on triangularframes and two mesh sizes, 24 per cm and 8 per cm were used in thesampling procedure . Each collection consisted of 3 to 5 `digs' witheach type of net, 6--10 digs in all per station . For each `dig' the sub-stratum was disturbed with the foot and animals dislodged, caughtin the net held just downstream (HYNES, 1961) . Measurements inthe field showed that approximately 0 .1 to 0.125 square metres weredisturbed for each dig .

A disadvantage of this collecting method is that organisms at-tached to stones are not easily dislodged . To compensate for this,five stones, each 10 15 cm in diameter, were washed separately inthe fine net each month . Trial collections in January 1966 showedthat many animals were found in the marginal grasses overhanging

345

TABLE III

Total Numbers of Each Species of Plecoptera and Ephemeroptera Collected From 3 Sam-pling Stations (Excluding April 1967)

346

Species(*indicates "casual" species)

Abbey

GarageBridge

Bend

Gasholder(Correctedfrom 30)

PLECOPTERATaeniopteryx nebulosa 89(61%) 57(73%) 87(73%)Amphinemura sulcicollis 7

6 8Leuctra fusca 21

7 11Leuctra moselyi/L . inermis* 12

1 1Isoperla grammatica 11

2 1Capnia bifrons* 2

1 4Brachyptera risi* 1Brachyptera putata/Rhabdiopteryx anglica* 1Nemoura erratica 2

1 6Nemoura cinerea 1Nemoura avicularis* 2

TOTAL PLECOPTERA 145

78 119

EPHEMEROPTERAEphemerella ignita 1221(21%) 1940(32%) 1549(27%)Ephemerella notata*Heptagenia sulphurea 844(14%) 657(11%) 812(14%)Rithrogena semicolorata* 1

4 2Ecdyonurus dispar* 2

1 4Ecdyonurus insignis* 3 -Ephemera danica 2

- 1Brachycercus harrisella* 1Baetis rhodani 345

241 283Baetis buceratus 578

341 404Baetis pumilus 134

77 101Baetis vernus (tenax) 115(50%) 85(31%) 103(37%)Baeits bioculatus (scambus) 633

806 1013Baetis spp . (tiny nymphs) 1041

213 199Habrophlebia fusca 15

11 5Paraleptophlebia submarginata* 1

Centroptilum luteolum 30

23 17Centroptilum pennulatum* 1Cloeon dipterum* -- 1

CAENIS spp. (moesta)(macrura) 768(13%)1414(25%) 1186(21%)(rivulorum)

TOTAL EPHEMEROPTERA 5730

5717 5681

Total Collections 40

40 40

Total % of 7 most abundantEphemeroptera species 98%

99% 99%

the river and `margin' samples were subsequently taken separatelyeach time. Formalin (20%) was added in the field and sorting wascarried out in small aliquots under a low power binocular micro-scope .

In March 1966, conical, floating emergence traps were con-structed to catch emerging adults . These were only partially success-ful as summer spates brought down masses of Ranunculus and debris,which caused the traps to sink. Weekly collections of sub-imaginesand imagines were taken 1966, from vegetation near the river mar-gin to confirm identifications and fix approximate emergence dates .

RESULTS

The Species RecordedThirteen species of Plecoptera and twenty-one species of Ephe-

meroptera were recorded over two and a half years (Table III) . Ofthose not common to all reaches, all six were only recorded rarelyand with no consistent pattern of occurrence . Taeniopteryx nebulosawas the most common and abundant species of the Plecoptera .

Seven species of Ephemeroptera, excluding the Caenidae werereasonably abundant. These seven plus the three Caenis spp . madeup over 95% of the total numbers of Ephemeroptera collected .Fifteen species were found at some time in all of the reaches .

Seven species of Plecoptera and eight species of Ephemeropterawere recorded so rarely that it was concluded that they were non-residents (casuals) which had drifted down from upstream . Speci-mens of Amphinemura sulcicollis, Leutra fusca, Haprophlebia fusca andCentroptilum luteolum were collected regularly, though always in smallnumbers. It seemed likely that these species were in fact resident,though at the limits of their respective ecological ranges .Nymps of Baetis vernus and Baetis tenax are taxonomically indis-

tinguishable though MACAN (1961) considers them to be ecologicallydistinct. This applies also to nymphs of Baetis scambus and Baetisbioculatus . Although it is difficult to be absolutely certain in all cases,evidence from collections of imagines and subimagines, togetherwith ecological factors suggest that the species in this part of theSevern are B. rernus and B. bioculatus .

Distribution and AbundanceIn Table III, all of nymphs collected are standardised on a 40-

collection base, for comparison .There are only slight differences in the relative abundance of each

species at `A', `C' and `E', with no really consistent pattern . At both

347

`C' and `E' Taeniopteryx was relatively more abundant than at `A',owing to the other Plecoptera being generally less so, but except forthis the Plecoptera faunas were very similar . Also, at both `C' and`E' Caenidae were more abundant than at `A' . It is obvious, how-ever, that there were no major changes in the overall plecopteranand ephemeropteran faunas between `A' and `E' .

During winter and spring, nymphs of some species were moreabundant in the marginal grasses overhanging the river, than in themain-stream. These included tiny Baetis spp . and to some extentTaeniopteryx (Table IV) . In summer Ephemerella ignita and B. bio-culatus were also common in the margins .

Plecoptera were found in both margin and mainstream collec-tions but the rarer species were most often in the margins . Of theEphemeroptera, Heptagenia sulphurea, which is flattened and livesunder stones, was rarely collected from the margins, while Centrop-tilum luteolum, a slow water species (MACAN, 1961) was never foundin the mainstream .

It is perhaps significant that the species found most commonly inmargin collections i .e . Baetis spp, and E. ignita are those which other

TABLE IV

Distribution of some Plecoptera and Ephemeroptera at Three

348

Station

A

Species25.3 .66 29.4 .66 20.5.66 23 .6.66 27 .7 .66 10 .1 .67

M M/S M M/S M M/S M M/S M M/S M M/S

Taeniopteryx nebulosa 2 - - 4 8 2Amphinemura sulcicollis 3 - 1Leuctra fusca -- 1 12Leuctra moselyi - 3 - 1Isoperla grammatica - 3 - 2 1 3 -

Ephemera danica - 1Ephemerella ignita 31 11 54 27 93 420 30 92Heptagenia suiphurea - 55 1 77 26 19

1 17 8 134Baetis spp . (tiny) 18 22 - 3 7 24 5 426 70Baetis buceratus 29 31 63 11 15 31

1 38 14 120 2Baetis rhodani 15 29 30 29 3 25 - 9 1 7 116 19Baetis pumilus 5 10 9 9 4 11 - 2

1 13 38 -Baetis vernus 5 10 7 20 - 2 6

1 4 6 2Baetis bioculatus 3 8 5 75 13 126Habrophlebia fusca 3 2 1 1 - 4 2Centroptilum luteolum 2 - 3 2 - 4 6CAENIS spp . 11 52 18 49 11 9 16 97 23 25 22

M =MarginM/S = Mainstream

which other workers have found to be most active and frequent inthe 'drift-fauna' (ELLIOT, 1968) . No doubt, nymphs carried by themainstream current find a fairly easy foothold in the slower, gentlercurrents of the margin .

Comparisons With Other Rivers and RegionsTable V summarises the Severn data and compares the mayfly

and stonefly fauna with those of other streams and rivers . FordWood Beck (MACAN, 1957 et seq), the Afon Hirnant (HYNES, 1961et seq) and the Dartmoor stream (ELLIOT, 1967 et seq) are small,moorland hill streams, mainly draining less calcareous areas . TheLincoln streams (LANGFORD, 1969) drain chalk and limestone areasand consist of small freely-running streams and slower canalisedreaches. `x' is a small, rapid stream draining the clay area just up-stream of Ironbridge and `y' is a larger Shropshire stream drainingthe rich land to the west of the Severn . The Wharfe is a large,northern upland river and the Great Ouse a slow flowing, siltedlowland river. Only species common to the Severn are shown and notall the species which have been recorded from each river or stream .

Stations on the River Severn 1.966--67 (Margin/Main Stream) .

34-9

CI

E

25.3.66 29 .4.66 20 .5 .66 23.6 .66 27 .7 .66 25 .3.66 29 .4 .66 20.5 .66 23 .6 .66 27 .7 .66 10 .1 .67

MM/SMM/SMM/SMM/SMM/S MM/SMM/SMM/SMM/SMM/SMM,/S

6 1 3 6

2 1 1 1 1

1 - 1 2 - 3

1 1 -4 92 71 25 319 35 93 4 33 88 117 461 50 17417 28 12 - 43 13 19 15 27 1 35 1 4

2 8 9 1 3 11 4 -- 5 1

13 8 1 1 17

7 56 152 25 59 1 25 12 20 9 25 13 47 2 12 23 3 29 16 25 11 6

24 95 7 10 25 - 2 12 28 43 8 8 29 11 -- 32 79 7 2 5 20 7 10 13 1 3 6 7 10 7 22 5 33 1

1 1- 7 20 7 1 3 3

1 3 114 - 23 16 42 5 2

2 16 4 31 194 2295 1 3 1 3 1

2 9

---14 20 2 20 - 65 10 14 3 31

5 10 - 7 58 118 17 -- 13

TABL

EV

Taeniopteryx nebulosa

Amphinemura sulcicollis

Leuc

tra

fusc

aLe

uctr

a mo

sely

iLe

uctr

a in

ermi

sIs

oper

la g

ramm

atic

aCapnia bifrons

Brac

hypt

era

risi

Rhadiopteryx anglica

Nemo

ura

erra

tica

Nemoura cinerea

Nemoura avicularis

++ + + 0 0 0 0 0 0 0 0 0

+ + 0++ 0

+++++ 0

+ +

+++

++

++ 0 + 0

++ 0

++ 0

++ + +

++

+++++

+ +

++ 0

+++

++ 0

Ephemerella ignita

+ +

~+

++ +

+ +

+0

+ + +

+ +

0+

+Ep

heme

rell

a no

tata

0+

Hept

agen

ia s

ulph

urea

+++

+++

Rith

roge

na s

emic

olor

ata

0+++

+ + +

+++

+ +

+ +

0Ec

dyon

urus

dis

par

0Ephemera danica

0+

++

+Br

achy

cerc

us h

arri

sell

a0

0Baetis rhodani

++

+++

++

++

++

++

++

0+++

Baet

is b

ucer

atus

++

Plecoptera and Ephemeropte

ra C

ommo

n to

the

Sev

ern

and

some

other British Rivers and Streams

.

Stream or River

Fordwood Afon

Lincoln

Local Small Streams

River

Grea

t*Dartmoor

Species

Severn

Beck

Hirn

ant

Stre

ams

x

yWharfe

Ouse

Stre

ams

0 -

occa

sion

al o

nly

+ -

pres

ent

+ +

-common

+ + +

-co

mmon

/abu

ndan

t at

rig

ht s

easo

ns

Baetis

pumilus

++++

+++

0++

0Baetis

vern

us++

++

0Baetis

bioc

ulat

us+++

++

++

0Haprophlebia

fusca

00

+Paraleptophlebia

submarginata

00

+0

Centroptilum

luteolum

+++

++

Centroptilum

penn

ulat

um0

0+

Chloeandipterum

+++

+++

Caenis

moesta

+++

++

++

Caenis

rivulorum

+++

+++

++

Caenis

macrura

+++

+++

Common Species

Middle

Reac

hes

Plec

.12

55

65

32

Only0

5Ephem

.20

63

155

59

9

5

From the table, it appears that the upper-middle reaches of theSevern support species characteristic of both upland and lowlandrivers. Compared with the hill streams, the Plecoptera fauna isrelatively sparse but the species which do occur most commonly arefound over a wide range of altitudes and calcium concentrations(LANGFORD, 1969) . Both the Afon Hirnant and the Lincolnshirestreams have more species common to the Severn than any otherstream, but there is only one species, i .e . Isoperla grammatica, whichis common to all the stony rivers . T. nebulosa, the commonest speciesin the Severn is obviously tolerant of a very wide range of bothaltitude and calcium (LANGFORD, 1969) .

Of the Ephemeroptera, both E. ignita and Baetis rhodani are com-mon to every river and stream, while Rithrogena semicolorata is onlyabsent from Lincolnshire and the Great Ouse . The rivers with mostspecies common to the Severn are the Wharfe, the Great Ouse andthe Lincolnshire group, all either relatively large or draining cal-careous lowlands . H. sulphurea, Ecdyonarus dispar, Ephemerella notata,Brachycercus harrisella and Baetis buceratus are not very common andare probably restricted to larger, stony rivers or lakes (MACAN,1961) .

The Plecoptera and Ephemeroptera fauna suggest that this partof the Severn is a `transitional' reach providing niches for both hillstream and lowland river species .

LIFE HISTORIES

Comparison of Fine and Coarse Net CatchesJONASSON (1955) concluded that the life-histories of some insects

have been falsely interpreted owing to the use of too large mesh sizesin sampling nets . Other authors (MACAN, 1958 ; HYNES, 1960) havealso shown significant differences in length/frequency distributionsof insect nymphs caught by fine and coarse meshed nets .

In the Severn, the finet net usually caught more nymphs in thesmaller size ranges (2 mm and under) but there were very few differ-ences in the total numbers of large nymphs caught by either net(Fig. 3) . Only in Beaus spp. is there a very marked bias towardsmaller nymphs in the fine net catches . These small, cylindricalnymphs are more likely to escape through the meshes than the rela-tively broad or squat nymphs like Heptagenia or Ephemerella .

Sampling with the finer net allowed life-histories of some speciesto be determined more precisely . For example, tiny nymphs ofEphemerella ignita were first collected by the finer net on April 29 in1966 . The coarse net did not catch nymphs of this species until the

352

FINENETS

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Baetis spp

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mm SIZE GROUPS

Fig . 3 . Size/frequency histograms to compare coarse and fine net catches ofEphemeroptera nymphs for all sampling stations at Ironbridge, river Severn .

next collection on May 20, by which time some were half-grown .Similarly in April 1967, nymphs only occurred in fine-net samples .

The length/frequency histograms in Figs . 4 to 11 were compiledfrom both fine and coarse net catches, corrected to allow for similarcatches of medium sized nymphs (3-6 mm) in both nets .

PLECOPTERA

Family Taeniopterygidae

Taeniopteryx nebulosa (LINN)

HYNES (1958) gives the flight period for adults as February toApril, mainly February and March . LANGFORD (1969) recorded tinynymphs (2-3 mm long )from Lincolnshire streams in July andfully-grown nymphs from mid-December to mid-February, withoccasional stragglers as late as March . SVENSSON. (1966) recordedsmall nymphs in his streams during July but adults did not emergeuntil late March when the ice cover melted .

In the Severn at A, small nymphs first appeared in May and Junein 1966 but did not reappear until September (Fig. 4) . AfterSeptember nymphs grew at a rate of about 1 mm per week untilmaturing in December or early January . In the heated reaches, atC and E the growth and hatching patterns were essentially similar,

353

SEP .

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NOV .

1967

J

- E06 -

ME

_ a

TI

iII

I

I

1

1

I

II

I

mmIIIIIIIIIII11II12

4

6 8

10 12

2 4

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8

10

2

4

6

8

10 12

SCALE : Z= 5 nymphsFig. 4. Length/frequency of Taeniopteryx nebulosa nymphs collected from riverSevern from upstream and downstream of Ironbridge power station (vert .hatched line = approx. period of emergence) .

JAN .

JUN .

354

r1

APR .

T

except that nymphs were not found during June, July or August .In April 1967, small nymphs were collected from C and E but notfrom A (upstream) .

EPHEMEROPTERA

Family Ephemerellidae

Ephemerella ignita (PoDA)

This is generally regarded as a summer-growing species in Britain(MACAN, 1961) but nymphs have been recorded in most months ofthe year (LANGFORD, 1969) . Most authors agree that nymphs beginto hatch from the overwintering eggs during spring or early summer .MACAN (1957) recorded nymphs in late May from Lake Districtstreams, while HYNES (1961) collected nymphs from the Hirnantduring March . In both streams nymphs were absent after Septem-ber. Recently, however, LADLE (pers comm) has recorded smallnymphs during January in Dorset streams .

In the Severn (Fig . 5) during 1966, tiny nymphs were first col-lected on April 29 at all three sampling stations . The first emergingadults were collected on June 20, some 6-7 weeks later . From thenuntil early September adults were collected each week . Largestnumbers of nymphs were found in June or July at all stations . Smallnymphs were collected until mid June in 1965 and until July in 1966and there was obviously a "delayed hatching" process in operation(MACAN, 1957) allowing nymphs to hatch out over a prolongedperiod. Similarly the emergence period is protracted and eggs areprobably laid at different times during the summer . The fate of theseeggs is not clear . It seems likely that the earliest batches hatch duringthe same summer, while most of the later ones over-winter . On theother hand, however, the time of laying may also determine time ofhatching in the following summer, but as yet the exact mechanismis speculative .

This type of life-history, however, provides even a "summer" spe-cies with some flexibility to withstand climatic changes from year toyear .

Growth patterns and life-histories were essentially similar in bothheated and unheated reaches . However, in April 1967, tiny nymphsappeared in collections from `C' and `E' but not at `A' . In 1966,nymphs had appeared simultaneously at all three stations .

355

356

1965

MAY

JUN

JUL .

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1966

APR .

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UPSTREAMABBEY(A)

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1967 IAPR .

NO SAMPLE

NO SAMPLE III`I1II

2 4 6 8

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2 4 6 8SCALE Z = 10 nymphs

Fig. 5 . Length/frequency of Ephemerella ignita nymphs collected from river Severnupstream and downstream of Ironbridge power station .

DOWNSTREAM

GASHOLDER(E)

NO SAMPLE

NO SAMPLE

=I&

2 4

6

8 10 mm

I

2 4 6 8 10 mm

Family Ecdyonuridae

Heptagenia sulphurea (MULL)

This species is characteristic of large stony rivers and stony lakeshores in limestone areas (MACAN, 1961) . The adult emergenceperiod is described by KIMMINS (1954) as May to August, though in-formation on the growth of nymphs is scarce .

In the Severn nymphs of all sizes were recorded almost everymonth (Fig . 6) . Similarly, nymphs of all sizes overwinter, their sizeapparently depending upon the date of hatching. Tiny nymphswere abundant during July-April, but hatching began during June .It would seem from the histograms that recruitment slowed downduring October and that both hatching and growth of nymphs wasvery slow or had practically ceased during the winter . Comparedwith E . ignita, the histograms suggest that the earliest hatchingnymphs of H. sulphurea may not grow to maturity in the same sum-mer .

Nymphs hatched in June or July apparently grow steadily duringthe summer to emerge as adults in early May the following year .Later hatched nymphs, probably overwinter in their smaller stagesto grow and hatch later in the foliwing summer. Observations andcatches show that adults begin to emerge in May and small numberswere caught almost every week until early September .

The period of most rapid growth occurred in March, April andMay.

At all three sampling stations, recruitment, growth and emer-gence appeared to be very similar . Any subtle differences in growthand development were probably obscured by the protracted hatchingand emergence periods .

Family Baetidae

Baetis bioculatus (LINNE)

Nymphs were mostly collected during May to September fromthe Severn (Fig . 7) . In 1965, fully grown nymphs and `winged"`specimens were collected in late May and the latest recorded inOctober.

In 1966, no nymphs were taken at all until May 20 at `A' or `C'when some were already 5-6 mm long. A few small nymphs werecollected at `E' on 29 April and it appears that recruitment of nymphsbegan in earnest during early May . On 23 June subimagines and"winged" nymphs were collected and subsequently until mid-

357

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358

- L

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UPSTREAM

(A)

0

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Fig. 6. Length/frequency of Heptagenia sulphurea nymphs collected from riverSevern upstram and downstream of Ironbridge power station.

8

∎

10 12

2

4

6

8

SCALE J_= 5 nymphs

DOWNSTREAM(C)

(E)

NO SAMPLE

10 12 2

NO SAMPLE

L-

4 6 8

10

12

mm

2 4 6 2

4

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SCALE ,.& = 10 NYMPHS

Fig . 7 . Length-frequencies of Baetis bioculatus nymphs from river Severn upstreamand downstream of Ironbridge power station .

359

September. The earliest nymphs grew from + 2 mm to + 6 mm inabout 6 -1 weeks .

The life-history of this species in the Severn was similar to thatobserved by PLESKOT (1953) in Austrian streams, and also similarto that of E. ignita. Nymphs hatched and grew in a series of quicklysucceeding generations from May to September .

Baetis pumilus (BuRN1)

In 1965 final instar nymphs were collected in May, July andAugust, but numbers were generally low (Fig . 8) . In 1966-67,there were two reasonably distinct generations, an overwinteringone and a summer one. For the former, hatching and recruitmentof nymphs occurred in October and growth was slow duringNovember to January. Growth accelerated in March until the firstspring emergence in May.

Tiny nymphs were again collected in June and these apparentlygrew quickly during the summer to produce adults in July andAugust. During September the species was mostly in the egg stage .

Overwintering nymphs took some 30 weeks to grow from + 2mm to ± 7 mm, while the summer growers took some nine weeks togrow from ± 2 mm to ± 6 mm. Final instars of the summer genera-tion were approximately 1-2 mm shorter from head to tip of abdo-men than those of the overwintering group (MACAN, 1957) .

In Ford Wood Beck (MACAN, 1957), the life history of B. pumiluswas very similar, though nymphs were much more abundant in thesmaller stream . The life history and growth of this species wasessentially the same in both heated and unheated reaches, thoughnymphs occurred more sporadically at C and E than at A duringthe summer .

Baetis spp . (unidentified tiny nymphs)

Three other species, Baetis vernus, Baetis buceratus and Baetis rhodaniwere abundant, but the tiniest nymphs proved difficult to separateinto species. Some were easily recognizable even at 1-2 mm long,but many, particularly newly ecdysed nymphs, were not .

In Figs. 9 and 10, the identified nymphs are plotted in the shadedhistograms, whilst the numbers of tiny nymphs are shown in the un-shaded parts . The same numbers of tiny nymphs were used for bothfigures .

At `A' tiny Baetis nymphs were collected in every month of 1966with the largest numbers occurring during autumn and winter . Thispattern was essentially the same in the heated reaches .

360

1965MAY

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APR .

JUN .

mm

SCALE 1 = 5 nymphs

Fig. 8 . Length/frequencies of Baetis pumilus nymphs from river Severn upstreamand downstream of Ironbridge power station .

I /I

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III2

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36 1

Baetis rhodani (PICTET)

MACAN (1957) showed that B. rhodani in Ford Wood Beck pro-duced two definite generations each year : a slow growing over-wintering one and a rapidly growing summer generation .

In the Severn the overwintering generation was obviously thedominant and the main flight period for adults was April to May(Fig . 9) . Although a few adults were taken throughout the summerand into October, numbers of nymphs were generally low duringJune to September.

The fate of summer nymphs is uncertain . From Fig . 9 it appearsthat the smaller nymphs left over after the April-May emergencemay have stopped growing or grown very slowly to emerge inSeptember or October (PLESKOT, 1953) . Alternatively, nymphs mayhave grown and developed rapidly in a series of small generations .All the evidence suggests that the former is most likely .

There are no readily apparent differences in the life history of thespecies at A, C or E .

Baetis vernus (CURT)

The difficulty in separating the smaller nymphs from those of B.buceratus makes it difficult to be precise about the life history of thespecies .

Using only the larger nymphs (Fig. 10), it appears that there maybe two emergence periods in the Severn, unlike in the Austrianstreams (PLESKOT, 1953) . Large nymphs were collected at A inApril 1966, but not in April 1967. In March 1966 nymphs up to 10mm long were recorded, mainly final instars. From May-Augustfew final instar nymphs were over 9 mm long.

Nymphs occurred slightly more sporadically at C and E than atA, but after July, nymphs were relatively scarce at all three stations .

Baetis buceratus (ETN)

Tiny nymphs were often, though not always, easier to identifythan those of B. vernus and the life history is fairly clear (Fig . 10) .

The main emergence was during April and May, though nymphsof all sizes occurred during both winter and summer .

The main recruitment occurred in October for the overwinteringgroup and growth was apparently rapid during March and April .The first "winged" nymphs were collected on April 29 (1966) andApril 11, 1967.

362

SCALE _t = 5 (B . rhodani) nymphs

0 = 10 (Baetis spp .) nymphs

Fig . 9 . Length/frequency of Baetis rhodani nymphs from river Severn upstream anddownstream of Ironbridge power station .

363

There was some recruitment during June and it is probable thatthese smaller nymphs, plus the smallest left over in May, produceda prolonged summer emergence . "Winged" nymphs were collectedregularly though not in every week during 1966 .

The life history of B. buceratus is similar to that of B. rhodani, thoughthe emergence period of the former ends somewhat earlier . NymphsofB. buceratus were practically absent from the river in September of1966 .There is no evidence of any changes in life histories between A,

C and E .

Notes on other SpeciesLarge numbers of Caenidae were collected in most months but as

nymphs of less than 3 mm were difficult to identify they were notincluded in the life history studies .

Haprophlebia fusca and Centroptilum luteolum were taken only inmargin samples . The largest nymphs of the former species werefound in April and May, while nymphs of Centroptilum occurred insummer, autumn and winter months .

Leuctra , fusca nymphs were collected mainly during June andAugust while Isoperla grammatica nymphs occurred all year round insmall numbers and various size groups .

DISCUSSION

These factors which influence the seasonal development andgrowth of insects have been discussed at length by DANILIEVSKII(1965) and the effects of temperature on the distribution andecology of aquatic organisms surnmarised by MACAN (1963) andNAYLOR (1965) .

The role of temperature in the life-history of any insect is obvious-ly not a simple one and it is not possible in a field study to separate,the effects of temperature from those caused by other factors suchas photoperiod, chemistry and food supply .

For this work on the Severn, however, it is reasonable to assumethat the only fundamental difference in the habitats at `A', `C' and`E' (upstream and downstream of the warm-water outfalls) was thevariable daily increase in temperature at `C' and `E' caused by thecooling-water discharge .

There are few published data available on the long-term effects ofheated effluents on the distribution and life-histories of stream in-sects . STARHMUHLER (1961) showed that some Plecoptera andEphemeroptera were absent where a hot spring raised the tempera-

364

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Fig . 10. Length/frequency distribution of Baetis aernus nymphs collected fromupstream and downstream of Ironbridge power station (river Severn 1965-67) .

8

10 mm2 4 6

365

(A) (C) (E)

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ture of an Austrian stream by 5°C . COUTANT (1962) also showedthat certain insects were absent or reduced in numbers where apower station effluent raised water temperature to more than 30°C .

To date, the actual long and short term effects of artificially in-duced sublethal temperatures on river insects in situ are largelyspeculative or based on very little evidence . HAWKES (1962) writesthat "apart from the more obvious lethal effect of high temperaturesin the summer, the increased winter temperatures may have im-portant ecological consequences" . Further, HAWKES states that"intermittent discharges of heated effluents would result in condi-tions unfavourable to the natural community" .LANGFORD (1970) has shown that the effluent from Ironbridge

"A" increases daytime temperatures in the Severn by up to 7 .2°C insummer and by up to 5°C in winter, for periods of 12 to 18 hours perday. The overall annual range, however, was increased only by 4°C .On most days, the diurnal fluctuation downstream of the stationwas still 100% greater than upstream .

Various authors (PLESKOT, 1953, MACAN, 1960a, 1963, 1964)have shown that the distribution and annual variations in emer-gence patterns of some Ephemeroptera appear to be related totemperature. On the other hand, Rithrogena semicolorata nymphs con-tinued to grow steadily throughout the year "regardless of tempear-ture" (MACAN, 1960b) .

The mode of action of temperature on the various phases ofinsect life-histories is difficult to determine and it is evident (DANI-LIEVSKII, 1965, MACAN, 1963) that there is no general rule for allspecies .

It is, however, possible to show how the annual cycles vary inrelation to annual temperature cycles, and to demonstrate how farthe ecological events may or may not be affected by the cooling-water discharge . It is hoped that at very least, the work will add tothe knowledge of temperature relationships of the species . Fromthis data, together with experimental data, it may be possible even-tually to forecast the ecological consequences of artificially raisedriver temperatures .

Temperature Relationships of the SpeciesUpstream of the Power Station (Natural)

There are three distinct types of life-history to be found amongthe species present .

(i) Nymphs that hatch in late summer or early autumn and growto maturity as temperatures fall i .e. winter growers e .g . Taeniopteryxnebulosa .

366

(ii) Nymphs that hatch out in spring or early summer and growto maturity during the warmest period of the year, (the winter ispassed in the egg stage), i .e. summer growers, e .g. Ephemerella ignita,Baetis bioculatus .

(iii) Nymphs that are present all the year round in various stagesof development, though growth is slow or almost stopped at certaintimes of the year . These species may have two definite generationsper year, e .g . Baetis pumilus, or prolonged emergence and recruit-ment periods from April/May to September or October, e .g . Hepta-genia sulphurea, Beatis rhodani, Baetis buceratus . In these species themain emergence occurred in May or early June .

How far temperature determines progress through the severallife-history phases is not clear, but it is obvious from Fig . 11, thateach species is tolerant of a fairly wide temperature range .

For example, nymphs of E. ignita appear only during spring andsummer, mainly over a natural range from 8 °C to 22°C in theSevern. On the other hand, nymphs ofH. sulphurea of all sizes survivethe whole temperature range down to 0°C . Newly hatched T. nebu-losa nymphs appeared at temperatures up to 17°C in the Severn andat up to 20°C in other waters (SVESSON, 1966), and then grew astemperatures and daylength decreased. Emergence took place dur-ing January and February .

The mechanisms which prevent too early emergence of insects arenot clear but it seems that fully grown nymphs may exhibit someresting phase or diapause until conditions are suitable .

CORBET (1954) has shown that the dragon fly, Anax imperator hasa larval diapuase, the onset of which is related to changes in day-length . MACAN (1957) also suggests that Rithrogena semicolorata avoidstoo early emergence by what may be a diapause stage in the largenymphs, and SVENSSON (1966) showed that although nymphs of T.nebulosa were fully grown by January, emergence did not take placeuntil March, when the ice-over lifted . ELLIOT (1967) has recordedemerging sub-imagines of B. rhodani as late as November on Dart-moor, but none were found in the Severn . MACAN (pers-comm) hassuggested that shortage of food in winter may be the reason for lackof growth and final development .

Hatching and recruitment of nymphs of most species also occurredover a fairly wide temperature range . Under natural conditions, at"A", nymphs of E. ignita began to hatch when mean weekly watertemperatures were above 6 .5°C, in April 1966 . B. bioculatus nymphsappeared later than those of Ephemerella and tiny specimens werepresent until August, just before temperatures began to fall .Tiny nymphs of T. nebulosa appeared in May 1966 when mean

water temperature was 12-13°C . Small nymphs were still present

367

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Fig. 11 . Length/frequency of Baetis buceratus nymphs collected from river Severnupstream and downstream of Ironbridge power station .

in June at temperatues of 16-17 °C. Maxima reached 18--19 °Cduring this month . In October 1966, tiny nymphs were still hatchingat temperatures of 6°C to 8°C . During July and August no nymphswere collected from A even though mean water temperatures werevery little higher than in June . KHOO (1964) describes T. nebulosa asshowing no diapause in the egg stage and BRINCK (1949) showedthat some Plecoptera in Sweden exhibit the same tendencies as Tae-niopteryx in the Severn, but he found that the tiny nymphulae werepresent all through the summer without growing .

The other species have more complex life-histories and growthand hatching periods tend to overlap and be more prolonged . Fig .12 summarizes schematically the temperature ranges over whichnymphs or adults of each species were collected . The approximateperiods of hatching, growth and emergence are indicated .

Most species show an obvious "delayed hatching" process(MACAN, 1957, KHOO, 1968), whereby eggs hatch over a widetemperature range . This is probably a survival mechanism, evolvedto enable species to survive sudden, unseasonal climatic temperaturechanges by keeping a reserve of unhatched eggs, in the event ofnymphal or adult mortalities .

A number of authors, for example EGGLISHAW & MACKAY (1967),MACAN (1957, 1960a) and BROWN (1961) have shown differences inrates of development of Ephemeroptera or Plecoptera during differ-ent years. All the authors concluded that temperature variationswere responsible for the differences . A similar phenomenon wasevident in the Severn . In May 1965, some nymphs of Ephemerella at"A" were considerably larger than those collected in May 1966 .The same phenomenon was observed with B . bioculatus . None of theother species showed this variability .LANGFORD (1970) showed, however, that the early winter of

1966 was in fact generally warmer than that of 1965, though rivertemperatures during week 16 (April) were higher in 1965 (8-12 °C)than in the same week of 1966 (4-8 °C) . Also, the mean water tem-perature stayed above 6 °C after week 13, in 1965, but not until afterweek 16 in 1966. This suggests that the onset of hatching may berelated to the date on which certain absolute or mean temperaturesare reached, rather than to a time-based temperature factor, i .e .degree hours . Temperature alone, however, is not a controlling fac-tor and development will also depend on favourable light and day-length conditions (DANILIEVSKII, 1965) .

The population densities of each species varied considerably dur-ing the year. The summer growers were most abundant in June orJuly, while H. sulphurea and other Baetis spp were most abundantfrom autumn to spring . Nymphs of these species were, however,

369

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Taeniopteryx nebulosa

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present in summer but only in very low numbers or as very smallinstars . It seems likely that the low population densities are due tobiotic factors, i .e. competition with summer species and food sup-ply, rather than specific temperature tolerances . The optimumperiod for growth in the overwintering species appeared to beFebruary to May .

Timing of Life Histories

It has been shown that in some streams, species which occupysimilar niches have their life-histories so arranged that there is asuccession of eggs, nymphs and adults throughout the year with verylittle overlap of similar stages in each species (HYNES, 1961) .

In the Severn this succession is not very distinct . B. bioculatusnymphs appear at a similar time to those of E . ignita though the peakabundance occurs about one month later . Of the other Baetidae, allhave a generation which reaches maturity in spring at about thesame time, and all have either a second summer generation or main-tain low level populations and emergence during the summer .

Downstream of the Power Station

In view of conclusions by other authors it seems that the biologicalimplications of temperature change may affect the occurrence anddistribution of species together with the time of hatching, the growthrate of nymphs and the emergence of adults .

Among the Ephemeroptera, there is no evidence of either reduc-tion in numbers, or absence of nymphs below the power station . Itis, therefore, safe to conclude that neither the absolute temperatureor the increased diurnal fluctuations have any significant effects onthe distribution and abundance of the species .

The apparently sporadic occurrence of some Baetidae at "C" and"E" during summer was probably a result of sampling error in asparse population and there was no consistent pattern . Tiny nymphsof T. nebulosa were not found in collections at `A"' in July andAugust 1966 . At "C" and "E", however, none were found in eitherJune, July or August . If these nymphs are restricted by summertemperatures it is probable that the higher June temperatures at"C" and "E" were responsible for their absence, though the evidenceis hardly significant . In September, however, nymphs appeared andgrew simultaneously at all three stations .

There is some slight evidence from Figs . 4 and 5 that both E.ignita and T. nebulosa nymphs began to hatch slightly earlier at "C"and "E" than at "A" in April 1967 .

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Small nymphs of both species were collected downstream butnone were taken upstream. At that time downstream water tem-peratures were 11-12 °C i .e. 3-4 °C above ambient . In April 1966,maximum downstream temperatures were lower 7 .0 to 9 .5°C, i .e .0.5 to 1 .5 °C above ambient and at that time nymphs were collectedat all stations .

During March, 1966, daytime downstream temperatures exceeded10°C for more than a week but no nymphs of summer species werecollected and development in general was later than in 1965 at allstations .

From the field data it was not possible to show any differences inthe life-histories of nymphs, at the three stations . There were noobvious, or consistent differences between the length-frequencyhistograms of nymphs of any species . The differences between May1965 and May 1966 at "A"were in fact much more obvious .LANGFORD (1970) showed that in terms of degree-hours, the

downstream reaches may be 3-5 weeks in advance of the upstreamreaches in some seasons. It was suggested that this advance might bereflected in earlier hatching or emergence . It is evident, however,that no such advances occurred . If the changes at "C" and "E" canbe regarded as an advance for both Taeniopteryx and Ephemerella, itwas probably of the order of a week or less and not 3-5 weeks as thetemperature data might suggest . GLEDHILL (1960) also suggestedthat emergence of Plecoptera in Whelpside Ghyll was similarly un-related to time-temperature factors . It is evident that the cumulativeeffects of the increased temperature downstream of Ironbridge "A"have produced very little alteration in the life histories of any of thespecies .

All the data so far collected and presented, therefore, indicatethat the heated effluent from Ironbridge "A" power station hadvery little effect either on the distribution or on the ecology of theresident Plecoptera and Ephemeroptera during 1965-1967 . Wherethere are indications of advances in hatching these are not verysignificant, particularly when related to the natural variations fromyear to year . These natural variations and effects of the power sta-tion are being studied further in the hope that more detailed infor-mation on temperature tolerances and adaptations of life-historieswill help to clarify the possible effects of future temperature changesin river systems .

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SUMMARY

1 . The River Severn at Ironbridge is warmed by cooling-waterfrom Ironbridge "A" power station . Water temperatures over theyears 1965-1967 were raised by values ranging from 0.5 to 7 .2 °Cabove ambient .

2. Collections of Plecoptera and Ephemeroptera were taken fromthree reaches in the vicinity of the cooling water discharge . "A"was situated 400 m upstream, "C" 800 m downstream and "E", 2km downstream of the outfalls .

3. The river at all three stations was fast flowing and normallyfairly shallow with a stony substratum .

4. The water was alkaline with B .O.D. values of 0.9-5 .0 ppm .There were no significant chemical changes from "A" to "E" .

5. Natural water temperatures varied from 0 °C to 20 .6 °C. Down-stream of the warm water outfalls the range was 0°C to 24 .8°CMean weekly temperatures were raised by between 0.2 °C to 4°C .There was no temperature stratification at the three sampling sta-tions .

6. Thirteen species of Plecoptera and twenty-one species ofEphemeroptera were recorded. Ten species of Ephemeroptera (in-cluding three Caenis spp) constituted 95% of the total numbers col-lected .

7 . Distribution of nymphs at "A", "C" and "E" changed withriver level and season. Species found commonly in the marginalgrasses were those most active and common in invertebrate drift .

8 . When compared with the faunas of other rivers and streams,the Severn at Ironbridge is a transitional area, with species commonto hill streams and larger rivers .

9. The use of fine nets (24 meshes/cm) facilitated the more accu-rate determination of life-histories .

10 Life-histories of Taeniopteryx nebulosa, Ephemerella ignita, Hepta-genia sulphurea, Baetis pumilus, Baetis bioculatus, Baetis rhodani, Baetisvernus and Baetis buceratus are described for both heated and un-heated reaches .

11 . Under natural conditions at "A", three distinct types of life-history were defined among the eight species and the relationshipsbetween their life-histories and the natural temperature cycles differ .

(a) T. nebulosa is a winter grower, nymphs only develop to matu-rity after September, though some were found in May and June at"A" . Temperature range for nymphs was approximately 17 °C to0°C .

(b) E. ignita and B. bioculatus are both summer growers. Nymphsoccur over a range of about 6 .5°C to 22°C naturally .

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(c) The other species had more complex hatching, growth andemergence periods . H. sulphurea, B. rhodani, B. pumilus and B. buce-ratus nymphs grew quickly during February-May. Numbers werelow in the height of summer and the controlling factor is probablycompetition for food and space with the "summer" species .

12. All species show a range of temperature tolerance during eachlife-history phase . This tolerance is considered to be a mechanismevolved to give the species some flexibility to withstand naturalvariations in climate from year to year .

13. E. ignita nymphs and B. bioculatus nymphs were larger andbetter grown in May 1965 than in May 1966 . This difference wasprobably related to the date on which 6°C was first reached and notto the time/temperature factor during the whole winter .

14. There was no really obvious "succession" of species in theSevern as found in small streams .

15 . Effects of the power station effluent are considered from twoaspects. There is some evidence that small, early hatched nymphsof Taeniopteryx were absent from "C" and "E" in June, but not from"A" . Later nymphs grew and hatched simultaneously at all threestations. There is no evidence of any other effects on the distributionand abundance of species .

There may have been a small advance, of up to one week in thehatching times of Taeniopteryx and E. ignita in April 1967 but thedata are not highly significant . The advances of 3-5 weeks in tem-perature shown by LANGFORD (1970) are not reflected in the life-histories of Plecoptera or Ephemeroptera .

16 . All the evidence indicates that the heated effluent fromIronbridge "A" has no significant effects on the distribution andecology of any of the eight species . Nymphs of some of the Ephe-meroptera were collected over the whole range of river temperature,i.e. 0°C to 24.8°C, while the summer species were recorded over thewhole summer range, i .e . 6°C to 24.8°C . T. nebulosa was restrictedin its hatching and growth by temperatures over about 17°C bothupstream and downstream of the power station .

17 . It is suggested that the ability of each species to tolerate widetemperature range in different life-history stages is sufficient toenable the species to withstand and survive unnatural temperatureconditions, provided that these temperatures are not sustained, orlethal to all stages, i .e. egg, nymph or adult .

18. Further studies on the long-term variations in life-histories inrelation to river conditions are in progress .

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ACKNOWLEDGEMENTS

My thanks go to DR. T. T. MACAN for his help and advice withthe original manuscript and also to J . W. WHITEHOUSE and R. S. A .BEAUCHAMP for valuable suggestions and discussion . For assistancein the field and with figures, I have to thank DR. D. CRAGG-HINE,DR. R. J . ASTON and JOHN R. DAFFERN .

Finally, thanks are due to DR. J. S. FORREST for his permission topublish the data. The work was done from the Central ElectricityResearch Laboratories and is published with the permission of theCentral Electricity Generating Board .

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