REGULATED RIVERS: RESEARCH & MANAGEMENT, VOL. 5, 77-87 (1990)

TIME SCALES FOR THE RECOVERY POTENTIAL OF RIVER COMMUNITIES AFTER RESTORATION: LESSONS TO BE

LEARNED FROM SMALLER STREAMS

ULRIKE FUCHS AND BERNHARD STATZNER Zoologisches Institut I , Universitat Karlsruhe, Postfach 6380, 7500 Karlsruhe, FRG

ABSTRACT

German politicians have promised that the River Rhine will be sufficiently restored within twelve years to permit salmon to live there again. Obviously the large rivers in Central Europe are more isolated from each other than smaller streams, and communities donating potential colonizers (if they exist at all) are further apart for possibly restored large rivers than for smaller streams. Thus, recovery can be expected to be faster in small streams than in big rivers after restoration (or reduction of detrimental human influence). Therefore, two restoration projects in German lowland streams, which differ in their degree of isolation, can serve as an indicator to the time periods which could at least be expected for the recovery of Central European rivers.

Under optimal conditions (almost completely intact communities upstream and downstream of a 400 m restored reach) in North Germany, sufficient recovery of benthic macroinvertebrate fauna could be achieved in relatively short periods. However, in a rather isolated stream reach in the Upper Rhine valley (closest intact lotic ecosystems of a comparable type were found 20-25 km away) a sufficient recovery of benthic macroinvertebrate fauna was not achieved within five years after restoration, although there was high diversity of physical habitats and the water quality was acceptable (except for two oil accidents in the fourth and the fifth year).

Hence, we conclude that recovery of a large Central European river ecosystem like the Rhine, which has lost a large number of its former species and is more isolated than small streams, will require more than twelve years to reach a state significantly different from the present one.

KEY WORDS Lotic ecosystems Restoration Isolation Recovery Inoculum Benthic macroinvertebrates Central Europe

INTRODUCTION

At the beginning of 1988 the German media announced: ‘Politicians have promised an effective restoration of the River Rhine, so that an autochthonous salmon population will be able to live in the Rhine catchment in the year 2000’. We do not know the scientific background which specified that this 12-year period would be sufficient. In our view, reduction of pollution and improvement of the physical habitat are hardly manageable within a few years, and re-establishing salmon populations is difficult for various reasons (see Quinn et al., 1983; Quinn and Groot, 1984 for a review on the biological background of migration and Egglishaw et al., 1986 for an example of how land use in the whole catchment affects salmon).

In general, running water ecology supports the view that lotic ecosystems are highly resilient and should thus have a good chance of rapid recovery after restoration (Fisher, 1983; Minshall et al., 1983). However, the speed of recovery depends on the existence of inocula (= communities donating potential colonizers) within the restored catchment. In the Rhine a considerable number of the species, which originally comprised these inocula, are lacking today (Caspers, 1980a, 1980b; Kinzelbach, 1978; Schmitz, 1986), so that species immigration from other catchments is necessary for ecological recovery. How rapidly such immigrants will be able to reach the Rhine remains an open question.

The degree of isolation of large rivers in Central Europe is much greater than that of small streams. Therefore, we expect the recovery of lotic communities after ecologically improved management to be

08869375/90/010077-11$05.50 0 1990 by John Wiley & Sons, Ltd.

78 U. FUCHS AND B . STATZNER

faster in smaller streams than in rivers. If, then, the recovery of relatively isolated small streams requires a period of many years, that of large rivers will certainly need much longer.

Here we report the results of two restoration experiments on benthic macroinvertebrates in small lowland streams. The speed of recovery is described for a stream reach with a rich inoculum in North Germany (Schierenseebach) and a rather isolated stream in the Upper Rhine Valley (GieBbach).

METHODS AND STUDY AREAS

Methods and the study area are described in detail in Fuchs (1988), Hering (1979), and Statzner (1979). Quantitative benthic samples were obtained by various methods (box samplers, modified Surber

samples, Eckmann grabs). Sample size, sample frequency as well as the mesh size used for processing the samples differed (in the mollusc-dominated Schierenseebach 0.4 and 0.5 mm respectively and in the insect-dominated GieBbach 0.2 mm). In addition, qualitative samples were also studied to record new species.

Sch ierenseebach The Lower Schierenseebach is situated in Northern Germany near Kiel and connects two lakes (stream

width: approximately 6 m, mean annual dischage: about 200 1 SKI). A 400 m long section (= experimental reach) flowed through land used for cattle grazing. No riparian forest was present along most of its length, while upstream and downstream the banks were stocked with woods (alder and beech). In the reaches with riparian forest the richest invertebrate fauna known in Northern Germany occurred (Bottger and Statzner, 1983). The water quality in the experimental reach was 11-111 according to the ‘MELUF’ scale (1987, I is the best, V the worst water quality).

Local farmers weeded the experimental reach a little by hand in the late autumn of every year prior to 1974. In 1974 the water authority became responsible for channel maintenance and began mechanical weeding of the water plants in the experimental reach, using machines in November of that year (Statzner and Stechmann, 1977). The reach was dredged the following May (Bottger and Statzner, 1983). Owing to increased growth of weed after dredging, the reach was subsequently weeded once or twice a year using machines (Bottger, 1977) and routine dredging was planned by the water authority every three years. In 1977 the maintenance of the reach became the responsibility of the Zoological Institute in Kiel, a little weeding was carried out again in autumn and trees, mainly alder, were planted in single to triple rows on the banks (Bottger, 1978).

Giebbach The GieBbach is situated in the Upper Rhine valley on the outskirts of Karlsruhe. Its original mean

stream width was approximately 2.3 m and its mean discharge 10-20 1 s-’. Its banks used to be stocked with fruit and poplar trees, which were almost completely removed between 1972 and 1979. Land in the catchment was used for agriculture. Whether or not growth of the reed population started after the riparian trees were felled is unknown, but in 1982 the whole channel and large parts of the banks were densely covered by Phragmites australis (channel: up to 640g dry weight m-2; bank up to 2000g dry weight m-’). The channel was dredged once a year prior to the experimental period. This was the fate of almost all streams in the Upper Rhine Valley. Streams where this did not occur were concentrated in an area about 20-25 km away from the GieBbach, and these contained a relatively rich invertebrate fauna (Braukmann, 1987).

In the autumn of 1982 a 1-1 km reach of the Gieljbach was restored with the cooperation of the State of Baden-Wiirttemberg, the City of Karlsruhe, and the Zoological Institute Karlsruhe, which also became responsible for the future maintenance of the channel. Large parts of the mud layer at the bottom were removed (comparable to the procedure carried out in previous years) and five moderately excavated pool sections (some of which were wider than 5.5 m in parts) were created by dredging.

In addition, several riffle sections (stone size 10-40cm in diameter) were created and several large single stones were put into the stream. Such large and coarse bed material is rather unnatural for a

RECOVERY OF RIVER COMMUNITIES 79

lowland stream, but the natural coarse material-woody debris, donated by the restored riparian forest--could not be expected in the stream for decades.

In the autumn of 1982 and 1983 three rows of woody plants were planted on each bank, the majority of which were alder. An increased water supply was obtained at a weir further upstream, so that the discharge of the whole GieBbach (not only of the restored section) was increased to about 501 s-'. Sections of the GieBbach outside the restored area were managed as before and dredged once per year. These sections served as a control.

The water quality of the GieBbach was usually 11-111 according to the scale of MELUF (1987), but in April 1986 and January 1987 the GieBbach was polluted with oil (Fuchs, 1988).

RESULTS

Schierenseebach Of the various groups of macroinvertebrates living in the total Schierenseebach the majority of species

also occurred in the experimental reach before the first weeding by the water authority (Table I). Although the drift increased considerably during that weeding and the lentic species especially drifted out of the section in large numbers (Statzner and Stechmann, 1977), no significant reduction in the species number was observed in the next spring (October 74 to April 75). The dredging, however, reduced the species number considerably, and the big molluscs (Unionidae and Viviparus) did not reappear until the autumn of 1975. Owing to the dense weed stands in autumn 1975 several caddisfly species appeared which had not previously been found in the section. By 1978 the species numbers in most fauna groups were similar to those before the dredging (Table I).

The density of Potamopyrgus jenkinsi, the predominant species in the experimental system, fluctuated too much to allow recognition of any pattern related to the treatment of the experimental section (Table

Seasonal density fluctuations in groups of benthic macroinvertebrate taxa other than P. jenkinsi mean that only the results of similar seasons are comparable (Table 11). Dredging reduced invertebrate density immediately. The amphipod Gammarus pufex came into its main reproduction period just after dredging and its abundance increased considerably, while most other groups occurred in reduced numbers (Table 11, samples from the beginning and end of May 75). A distinct shift towards an increased abundance of species living in weeds occurred by autumn, with several dragonflies, two species of the waterbug Sigara, and two species of the caddisfly Agraylea. By 1978 the density came closer to the predredging values, although weed-prefering species were still more predominant than before April 75 (Table 11).

Gieobach In 1982, before restoration, several groups of lotic macroinvertebrates, such as Tricladida,

Ephemeroptera, Plecoptera, and Simuliidae were absent from the GieBbach while others, such as Gastropoda, Heteroptera, Coleoptera, Megaloptera, and Trichoptera were represented by only one species (Table 111). By 1987 about twice as many species appeared in the restored section compared with the control section. Three orders with a total of eleven species newly colonized the restored section, as well as six species of Trichoptera, three of Coleoptera, and four of Odonata. Furthermore, we found two species of Elmidae, two Simuliidae, and 23 Chironomidae species for the first time (Table 111). But, when the ecological requirements of the newly recorded species were considered, it was evident that most of them were ubiquitous and showed no special preference for small lowland streams (Table IV).

Sixteen frequent species of different functional feeding groups were selected for further quantitative research. Using the U-Test we compared their abundances in the control section with the abundances in the different parts of the restored section (Figures 1, 2, and 3).

Species which exhibited high abundance in the GieBbach before restoration, such as Radix peregra, Gammarus roesli, and Sialis lutaria fluctated in their densities and were sometimes more common in the control section. All other species became more abundant in the restored section or showed no significant

11).

Tab

le I

. Sp

ecie

s nu

mbe

r of

var

ious

gro

ups

of b

enth

ic i

nver

tebr

ates

in

the

Schi

eren

seeb

ach

foun

d in

the

tota

l sy

stem

and

in t

he

expe

rim

enta

l rea

ch, w

hich

was

wee

ded

betw

een

Oct

ober

197

4 and

Apr

il 19

75 an

d dr

edge

d be

twee

n A

pril

1975

and

May

197

5 (fir

st

colu

mn

2-3

days

aft

er d

redg

ing,

sec

ond

colu

mn

4 w

eeks

aft

er d

redg

ing)

. In

autu

mn

1977

the

stre

am w

as re

stor

ed a

nd m

aint

enan

ce

redu

ced

to a

litt

le w

eedi

ng (

afte

r B

ottg

er a

nd S

tatz

ner,

198

3; H

erin

g,19

79;

and

Stat

zner

, 19

79).

Num

bers

in

brac

kets

ind

icat

e sp

ecie

s w

hich

wer

e no

t fo

und

in t

he e

xper

imen

tal

sect

ion

befo

re t

he d

redg

ing

Tot

al S

chie

rens

eeba

ch

Exp

erim

enta

l Se

ctio

n of

the

Unt

ere

Schi

eren

seeb

ach

(197

4-19

75)

Oct

. 74

Apr

. 75

M

ay 7

5 M

ay 7

5 O

ct. 7

5 Ja

n.-M

arch

78

Apr

.-Ju

ne78

Tric

ladi

da

Gas

trop

oda

Biv

alvi

a H

irud

inea

C

rust

acea

E

phem

erop

te

Plec

opte

ra

Odo

nata

H

eter

opte

ra

Col

eopt

era

Meg

alop

tera

T

rich

opte

ra

3 19

10 9 2

ra

9 2 13 5 7 1 40

1 11 9 8 2 5 0 6 3 3 1 18

0 11 9 5 2 5 1

6 3 3 1 20

0 6 4 0 2 0 1 0 1

2 1 a

0 0

7 7

4 4

2 6

2 2

4 3

0 0

1 3

1 2

1 1

0 4(

1)

12

22(6

)

c ?

Tab

le 1

1. D

ensi

ty o

f va

riou

s gr

oups

of

bent

hic

mac

roin

vert

ebra

tes

(mea

ns f

or t

he to

tal

Unt

ere

Schi

eren

seeb

ach

base

d on

num

erou

s sa

mpl

es fr

om th

e bo

ttom

sur

face

and

from

dee

per

laye

rs o

f th

e su

bstr

ates

, cal

cula

ted

for t

he 2

0 m

ost d

omin

ant

taxa

onl

y; m

eans

for t

he

expe

rim

enta

l sec

tion

base

d on

at l

east

ten

sam

ples

from

the

botto

m s

urfa

ce);

see

Tab

le I

for f

urth

er d

etai

ls (

afte

r Bot

tger

and

Sta

tzne

r,

1983

; Her

ing,

197

9; a

nd S

tatz

ner,

197

9). N

umbe

rs i

n br

acke

ts i

ndic

ate

dens

ity o

f sp

ecie

s w

hich

wer

e no

t fo

und

in t

he e

xper

imen

tal

sect

ion

befo

re t

he d

redg

ing

Unt

ere

Schi

eren

seeb

ach

Exp

erim

enta

l se

ctio

n of

the

Unt

ere

Schi

eren

seeb

ach

(197

4-19

75)

Oct

. 74

Apr

. 75

M

ay 7

5 M

ay 7

5 O

ct. 7

5 Ja

n.-M

arch

78

A

pr.-J

une

78

(Ind

m-*

year

) (I

nd m

-')

Tnc

ladi

da

Gas

trop

ods*

P.

jenkinsi

Biv

alvi

a H

irud

inea

C

rust

acea

E

phem

erop

- te

ra

Plec

opte

ra

Odo

nata

H

eter

opte

ra

Col

eopt

era

Meg

alop

tera

T

rich

opte

ra

t 269

1136

24

1077

13

5 67

4 13

5

t t t 135

1359

7 t

9 29

7 22

551

106

224

1018

64 0 36

173 77

69

988

0 32

8 11

7817

13

9 44

358 89

124 26

190 14

19

660

0 65

1421

2 26 0 40 0 13 0 42 1 7

133

0 21

9 23

207

129 13

1119

1 0 0 4 1 0 65

0 2(

2)

727

4308

11 9

3(4)

38

2 27

305

92

335

102

52

118

45

0 32

5

278

(13)

34

62

3 43

51

32

843(

280)

30

86

27

(0.7

)

301(

2)

2(2)

16

1(6)

19

34

316

105

376 26( 0

.2)

13

40(0

-6)

49

75(0

.6)

32

335(

19)

E m < w

*=Ex

clud

ing

Pota

mop

yrgu

s jenkinsi.

'=

not

with

in th

e 20

mos

t do

min

ant t

axa.

82 U. FUCHS AND B. STATZNER

Table 111. Species number of various groups of benthic invertebrates in the GieBbach, found before the restoration (1982) in the subsequently restored and control sections, and after restoration in the restored section (1983-1987) and in the control section (1983-1987). which was dredged and weeded as previously (after Fuchs, 1988)

Total Gieljbach Control section Restored section 1982 1983-1987 1983-1987

Tricladida 0 1 1 Gastropoda 1 2 4 Bivalvia 3 3 3 Hirudinea 4 4 5 Crustacea 2 3 3 Ephemeroptera 0 1 6 Odonata 0 0 4 Heteroptera 1 1 6 Coleoptera 1 4 7 Megaloptera 1 I 1 Trichoptera 1 1 7 Simuliidae 0 0 2 Chironomidae* t 45 68

*= Species number of Chironomidae in the restored and the control section based on data from 1983 to 1985 (Huber, 1986). '= no data.

differences between the two sections (Figures 1 , 2 and 3) . Insect larvae, such as Baetis s p p . , Hydropsyche ungustipennis, and Odagmia ornatu very soon became more abundant in the riffles than in the control section, but they became less abundant or disappeared in 1987 after the oil accident (Figure 1). Non-insects, such as Polycelis nigra, Glossiphonia complanata, and Erpobdella octoculata increased in their abundance in the riffle and also in the pool only after a longer period (Figures 1 and 3)

DISCUSSION

The two examples of restoration projects presented here emphasize a well-known aspect of ecology: the more a system is isolated, the smaller the number of species is which colonize it over a certain period of time (MacArthur and Wilson, 1967).

The experimental reach of the Upper Schierenseebach was restored under optimal conditions, i.e. upstream and downstream of the experimental reach the most intact benthic macroinvertebrate fauna known in the whole North German plain occurred. However, even under such optimal conditions a sufficient recovery of populations may take longer than one year.

The Giefibach probably represents the other extreme, but the degree of its isolation from quasi-natural lotic ecosystems is nowadays not the exception, but the rule. Thus, under such conditions, most newly colonizing species were rather ubiquitous. Species typical of quasinatural lowland streams in the Rhine Valley (Braukmann, 1987) or elsewhere in Central Europe (Statzner, 1979; Tolkamp, 1980) failed to colonize the experimental reach. From 49 species (Gastropoda and Insecta), which Bottger (1986) named as rheotypical lowland species, only three (Vetia caprai, Limnius volckmari, and Hydropsyche siltalai) colonized the Giefibach. Thus, recolonization of the Giefibach was a slow process, despite the acceptable water quality and the considerable length of the restored reach, which was not too short to maintain isolated populations of many zoobenthos species in other lowland streams (Higler and Repko, 1981 ; Statzner, 1979).

It was mainly the change of the physical habitat in the restored reach, which favoured the colonization of immigrants. Mobile, flying species, which prefer running water for optimal oxygen conditions, food supply, or stony habitat for oviposition, such as Hydropsyche angustipennis, Baetis spp., and Odagmia

RECOVERY OF RIVER COMMUNITIES 83

Table IV. List of the species, which occurred exclusively in the restored section of the GieBbach (without Chironomidae) and information on their preferred biotopes (after Illies, 1978)

Species

Bithynia tentaculata L. Lymnea stagnalis L. Theromyzon tessolatum (0. F. Miiller) Baetis buceratus Etn. Baetis rhodani Pict. Caenis horaria L. Caenis luctuosa Burm. Platycnemis pennipes (Pallas) Coenagrion puella (L.) Aeshna cyanea (Miiller) Aeshna mixta Latreille

Corixa punctata (Illig.) Nepa cinerea L. Notonecta glauca L. Limnoporus rufoscutellatus (Latr .) Veliu caprai Tam. Scarodytes halensis halensis (Fabr.) Hydraenu nigrita Germ. Elmis maugetii Latreille Limnius volckmari Panzer Hydropsyche siltalai Dohler Limnephilus lunatus Curtis Limnephilus rhombicus L. Anubolia nervosa Curtis Potamophylax latipennis Curtis Athripsodes aterrimus Steph. Eusimulium latigonium Rz. Odagmia ornata (Mg.)

Preferred biotopes

Brooks-streams, lakes, brackish water Freshwater in general Ectoparasitic on birds Brooks-streams Brooks-streams Rivers-streams, lakes Riverestreams Rivers-streams, lakes Lakes, peat-bogs Brooks-streams, lakes, pools, ponds, peat-bogs Brooks-streams, lakes, pools, ponds, brackish water, inland salt water Freshwater in general, brackish water Brooks-rivers, lakes, pools, ponds Freshwater in general Freshwater in general No information given Brooks-rivers Underground water, springs, brooks-rivers Brooks-streams, hygropetric zone Brooks-streams No information given Lakes, brackish water Freshwater in general, brackish water Lentic water in general Brooks-streams, lakes Lentic water in general, brackish water Brooks-rivers, lake outlets, weirs, etc. Ubiquitous

ornata colonized the riffles rapidly. Less mobile species, which prefer coarse material for oviposition, such as Polycelis nigra, Glossiphonia complunata, and Erpobdellu octoculuta increased their densities only after three to four years.

However, even in this case study of a small stream restoration project funded by State and City pollution accidents which endangered the recovery could not be prevented. Two oil spills in 1986 and 1987 affected the project in the period covered by this paper and in spring 1988, after a massive sewage inflow due to malfunction of canalization, enormous numbers of dead macroinvertebrates were found stranded on the banks.

Thus in our cultural landscape, a sufficient recovery seems impossible even in a small stream like the Gieljbach within five years, This result shows a considerable variance from the high recovery potential observed in many other lotic ecosytems (Table V) to which the Schierenseebach was closer than the GieSbach. Consequently, it is very unlikely that the recovery of large Central European rivers, which are known to have successions in parts of their ecosystems lasting over long periods (Bravard et al., 1986) can reach a significant improvement compared with the prerestoration level within periods shorter than decades. For many species the point of no-return may already be passed, and disturbance through accidents, such as that at Sandoz, can hardly be excluded for a period of decades. Therefore, the view that restoration of our large river systems is possible within relatively short periods is too optimistic in our opinion.

00

P

Tab

le V

. T

ime

scal

es fo

r re

cove

ry o

f lo

tic m

acro

inve

rteb

rate

com

mun

ities

aft

er v

ario

us d

ata

Loca

lity

Tot

al r

ecov

ery

or

Aut

hor

equi

libriu

m d

ensi

ty

afte

r

Mea

n an

nual

or

Stre

am w

idth

T

reat

men

t ex

peri

men

tal

(m)

disc

harg

e (m

3 s-

I)

Mill

Cre

ek

1-2.

5 E

xper

imen

tal

shoc

k ac

idifi

catio

n Et

hyl

benz

ene-

cr

eoso

te-s

pill

Fly-

Ash

pon

d sp

ill

Aci

d sp

ill

Dra

inin

g an

d dr

edgi

ng

New

ly c

reat

ed c

hann

el

Cha

nnel

mod

ifica

tion

13-2

8 da

ys

Cai

rns

et a

l., 1

971

Roa

noke

Riv

er

Clin

ch R

iver

C

linch

Riv

er

Eas

t St

oke

mill

str

eam

M

oser

Cre

ek

1.2

6 m

onth

s

45

45

0.6

t

> 2

yea

rs

60 d

ays

1 ye

ar

109

days

< 1

yea

r

45-7

0 da

ys

8 3 C

risp

and

Gel

dhill

, 197

0 W

illia

ms

and

Hyn

es,

1977

W

hita

ker

et a

l.. 1

979

W

Coa

stal

pla

in s

trea

ms,

D

elaw

are

Ton

gue

Riv

er

t 3.

6

t 50

R

ecla

imed

fro

m c

oal

strip

-min

ing

and

new

ly c

reat

ed c

hann

el

Dro

ught

Fl

ood

due

to f

ailu

re

of a

res

ervo

ir da

m

and

subs

eque

nt d

roug

ht

Impr

oved

met

alm

ine

was

tew

ater

tre

atm

ent

Gor

e, 1

979

Littl

e A

ugla

ize

Riv

er

Ten

ton

Riv

er

t 30

10

t <

1 y

ear

1-3.

5 ye

ars

Gris

wol

d et

al.,

198

2 M

insh

all e

t al.,

198

3

Silv

er B

our

Cre

ek

1

4.7

10 y

ears

* C

hadw

ick

et a

l., 1

986

*= L

ack

of u

ndist

urbe

d he

adw

ater

s '=

no

data

RECOVERY OF RIVER COMMUNITIES 85

€D Ncontrol=N,,ffle(P,O) B Ncontroi Nriffle (pcQ1) @ Ncontrol "riffle (PCOJ)

nabolia nervosa imnephilus lunatus

ydropsyche angustipennis

pobdella octoculata oss iphonia cornplanata

82 83 84 85 86 87 YEAR

Figure 1. Comparison of the abundances of 16 selected species in the control section (= banks and channel covered with weeds and waterplants, low flow velocity, muddy bottom, weeded and dredged annually; 1982-1987) with that in the subsequently restored section (1982) and with that in the riffles of the restored section (= banks stocked with trees, higher flow velocity, stony bottom; 1983-1987). Indicated are the p-values of the U-Test (data from 12-16 samples in June and October of every year, after Fuchs,

1988). N = number of individuals

€3 €3 #

Ncont rol

Ncontroi Ncontrol

nabolia nervosa mnephilus lunatus

enis horaria is vernus arus roeseli

v I/ Erpobdella octoculata

lJu &'$ A7

82 83 84 85 8 6 87 YEAR

Figure 2. Comparison of the abundances of 14 selected species in the control section (1982-1987) with that in the subsequently restored section (1982) and with that in the runs of the restored section (= banks stocked with trees, low flow velocity, muddy bottom; 1983-1987). Baetts bucerutus and Baetrs rhodani were missing in these sections. For further details see legend of Figure 1

86 U. FUCHS AND B . STATZNER

rpobdella octoculata ssiphonia complanata

82 83 84 85 86 87

YEAR

Figure 3. Comparison of thc abundances of 13 selected species in the control section (1982-1987) with that in the subscqucntly restored section (1982) and with that in the pools of the restored section (= banks stocked with trees, negligible flow velocity. muddy bottom covcred with wafcrplants; 1983-1987). Baeits bucerafus, Raelis rhodani, and Cuenis horaria were rnisvng in thew

sections. For furthcr details see legend of Figure 1

ACKNOWLEDGEMENTS

The GieBbach-Project was funded by the City of Karlsruhe and the State Baden-Wurttemberg. We thank Mrs Liz Mole for linguistic advice.

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