Hydrobiologia 334: 157-161, 1996. 157K. A. Coates, Trefor B. Reynoldson & Thomas B. Reynoldson (eds), Aquatic Oligochaete Biology VI.( 1996 Kluwer Academic Publishers. Printed in Belgium.

Reversal of eutrophication in four Swiss lakes:evidence from oligochaete communities

Claude Lang & Olivier ReymondConservation de lafaune, Marquisat 1, CH 1025 St-Sulpice, Switzerland

Key words: biomonitoring, eutrophication, indicator species, lake, oligochaetes, recovery, zoobenthos

Abstract

Following the limitation of phosphorus inputs, total phosphorus concentrations decreased substantially between1980 and 1990 in four lakes of western Switzerland. Tubificid and lumbriculid communities of Lakes Geneva andNeuchfitel responded clearly to this decrease. Indeed, mean relative abundance of species typical of oligotrophicconditions (mostly Stylodrilus heringianus and Spirosperma velutinus) doubled in oligochaete communities ofboth these deep lakes (> 40 m). These changes indicated that both lakes were meso-eutrophic around 1980, butmesotrophic since 1990. In contrast, oligochaete communities of Lakes Morat and Joux, which consisted mostlyof tolerant species (Tubifex tubifex, Potamothrix hammoniensis, and Limnodrilus hoffmeisteri), did not indicatean improvement of environmental conditions between 1980 and 1990. In Lake Joux the ratio of chironomid tooligochaete biomass was a more simple indicator of change than the species present in oligochaete communities.

Introduction

Until now the biological responses of lakes to theincrease of phosphorus concentrations in the water arebetter documented than those following its decrease(Levine & Schindler, 1989). But, as more and morelakes of the northern hemisphere begin to recover fromman-made eutrophication, this aspect attracts moreattention. However, studies are in general concentrat-ed on the responses of phytoplankton (Saas, 1989),whereas the zoobenthos is often neglected.

The recovery of a lake is not complete as long asits sediments, especially those of the profundal, arenot recolonized by the species (or at least by theirecological equivalents) which prevailed therein, beforethe onset of eutrophication. Indeed, the sediment islocated on the receiving end of all processes going onwithin a lake (Levine & Schindler, 1989). Therefore,the restoration of its pristine state, as far as this goalis realistic, is the landmark of a successful recovery ofthe whole lake.

Following the removal of phosphorus by sewagetreatment plants and the ban of phosphorus in deter-gents, total phosphorus concentrations decreasedbetween 1980 and 1990 in the water of Lakes Geneva,

Neuchatel, Morat, and Joux (Table 1). In the presentstudy, we compare the responses of zoobenthic com-munities (especially those of oligochaetes) to the abate-ment of eutrophication in these four lakes of westernSwitzerland (Lang & Reymond, 1992, 1993a, b, c,1995). Our goal is to answer the following question:how to monitor the recovery of the studied lakes fromeutrophication with zoobenthos?

Stations and methods

Studied lakes can be divided into two groups accord-ing to their morphometry (Table 1): two large and deeplakes (Lakes Geneva and Neuchatel), two small andshallow lakes (Lakes Morat and Joux). In each lake,sampling sites for zoobenthos were located (Fig. 1) asfar as possible from the main external inputs of organ-ic matter (large rivers and sewage treatment plants).In that way, they were mostly exposed to organic sed-imentation derived from phytoplankton whose inten-sity varies according to the trophic state (Baines &Pace, 1994). Sampling sites, located in the profundal,have been visited before and after the abatement ofeutrophication (Tables 2, 3). Detailed results of these

158

Table 1. Characteristics of the studied lakes. Sources: Blanc etal., 1991; Liechti, 1989; Pokorni, 1991; Vioget,1991.

LakeCharacteristic Geneva Neuchdtel Morat Jouxl)

Depth mean (m) 153 64 25 21max. (m) 310 153 45 33

Lake surface (km 2) 582 215 23 9Volume (km3) 89.0 13.8 0.55 0.16Altitude of lake surface (m) 372 429 429 1004Total phosphorus (mg m- 3) in 1980 82 (37.5)2) 67 (37.1)2) 100 (71.5)2) 45 (34.0)2)

in 1990 55 (25.2) 29 (16.0) 50 (35.6) 25 (18.9)

More than 4 mg 02 1-1

up to a depth of (m) 260No. of total circulation per year 0.13)Theoretical residence time (year) 11.9

Area of drainage basin (km2) 7975

Average altitude of the basin (m) 1670

Number of inhabitants (thousand) 760

153

2

8.2

2672

780

245

10-15

2

1.2

693

66

10-15

2

0.85

211

7

1) Covered by ice two or three months every winter.2) Phosphorus concentration divided by the mean depth (log transformed).3) Total circulation only after a cold winter (i.e. every ten years); total circulation down to 150 m every winter.

Table 2. Changes in tubificid and lumbriculid communities of Lakes Geneva and Neuchfitel before (1982-84) and after(1990-1992) abatement of eutrophication. Sources: Lang & Reymond (1992, 1993 b, 1995). FR = frequency (percentageof cores with species); RA = relative abundance (%); A = species absent, + = present; NS = not significant; * = significantP = 0.05 or less.

Geneva Neuchatel Geneva

40m 40m 150 m

Unit 1982 1991 1984 1992 1990Code Species

I Bichaeta sanguinea Bretscher

2 Bythonomus lemani Grube

3 Stylodrilus heringianus Claparede

4 Spirosperma velutinus (Grube)

Oligotrophic species 1-4

5 Potamothrix vejdovskyi (Hrabe)

6 Spirospermaferox (Eisen)

7 Psammoryctides barbatus (Grube)8 Potamothrix moldaviensis (Vejdovsky, Mrazek)

Mesotrophic species 5-8

9 Limnodrilus hoffmeisteri (Claparbde)

10 Limnodrilus profundicola (Verrill)11 Potamothrix heuscheri (Bretscher)

12 Potamothrix hammoniensis (Michaelsen)13 Tubifex tubiJex (MUller)

Eutrophic species 9-13

Oligotrophic species 1-4Mesotrophic species 5-8Eutrophic species 9-13

FR + +

10 24

39 44

46 65

66 78*

93 62

49 22

23 28

A A

95 78*

+ +

+ +

+ +

+ +

+ +

87 67*

RA 17 41*

51 29*32 30 NS

+ + +

A A 3

37 47 33

17 19 8

44 59* 45

0 23 33

2 13 0

0 0 0

0 9 A

2 33* 33

+ + +

+ + +

A A +

+ + +

+ + +

100 77* 84

16 33* 24

0 15* 1684 52* 60

No. of 16cm 2 cores 61 192 64 171 189No. of 16 cm 2 cores 61 192 64 171 189

159

Table 3. Mean relative abundance (%) of tubificid species and mean biomass (mg/16 cm2)of tubificids, Chaoborus and chironomid larvae in Lakes Morat and Joux before (1984, 85)and after (1991, 92) the abatement of eutrophication. Source: Lang & Reymond (1993 a, c).+ = present; NS = not significant. * = significant P = 0.05 or less.

Morat 40 m Joux 25 m

Zoobenthic taxa Unit 1984 1991 1985 1992

Tubifex tubifex Abundance 50.0 50.0 NS 23.8 63.6*

Potamothrix hammoniensis 50.0 50.0 0 0

Limnodrilus hoffineisteri 0 0 76.2 36.4

Tubificids Biomass 40.8 31.9 NS 43.2 42.7 NS

Chaoborus + + 7.1 18.4 NS

Chironomids + + 39.9 15.1*

All 40.8 31.9 NS 90.3 76.2*

No. of 16 cm2 cores 16 16 120 120

surveys are presented elsewhere (Lang & Reymond,1992, 1993a, b, c, 1995).

Zoobenthos samples (16 cm2 /sample) wereobtained with a corer, operated from the water surface,in Lakes Geneva and Neuchftel, Morat, and by a diverin Lake Joux. Sediments were sieved (mesh size aper-ture: 0.2 mm) and the retained material preserved in 5%formalin. Oligochaetes, chironomids, and Chaoboruswere picked, counted, and weighed after removingexcess water with blotting paper. Oligochaetes weremounted (Reymond, 1994) and identified to species,whereas chironomids and Chaoborus were not identi-fied.

Three categories of species were identified fol-lowing the guidelines of Lang (1991): species whosenumerical dominance in tubificid and lumbriculidcommunities indicates oligotrophic, mesotrophic andeutrophic conditions, are designated in Table 2. Rela-tive abundance of oligotrophic species was calculatedas a percentage of the total number of tubificid and lum-briculid worms present in each core. Juvenile worms(diameter less than 0.3 mm) were excluded from cal-culations to reduce the effects of seasonal variabilityon species abundance (Lang, 1991).

Relative abundance of oligotrophic worm specieswas used to monitor the recovery of Lakes Genevaand Neuchatel from eutrophication (Lang & Reymond,1992, 1993b, 1995). As these species were absent fromthe profundal of Lakes Morat and Joux, other benthicindicators were used: total composition of worm com-munities in Lake Morat (Lang & Reymond, 1993a);in addition to that indicator, biomass of chironomids,

tubificids, and Chaoborus was used in Lake Joux (Lang& Reymond, 1993c).

Results

Macrofauna consisted mainly of tubificid and lumbri-culid worms in the samples collected in Lakes Gene-va and Neuchatel, and mostly of tubificid worms inLake Morat (Tables 2, 3). In contrast, chironomids andChaoborus larvae were abundant in samples collectedin Lake Joux.

Increase of oligotrophic worm species in LakesGeneva and Neuchatel after the abatement of eutroph-ication indicated an improvement of conditions withinthe sediments (Table 2). According to the recordedabundances, these lakes were deemed meso-eutrophicaround 1980, but mesotrophic since 1990 (Lang &Reymond, 1992, 1993b). In Lake Geneva however,the response of oligotrophic species was less clear at adepth of 150 m than at a depth of 40 m (Lang & Rey-mond, 1995). Indeed, the recovery of the deepest areastakes more time because distances to travel to recol-onize patches without oligotrophic species are greaterin the profundal than in the sublittoral.

In Lake Morat, relative abundance and biomassof the eutrophic worm species were the same afterand before the abatement of eutrophication (Table 3).In contrast, relative abundance of Tubifex tubifexincreased whereas biomass of chironomid larvae andtotal zoobenthos decreased in Lake Joux. These trends,which indicated deteriorating conditions in the profun-

160

Lal

m

Lake Geneva

10km

Fig. 1. Location (shaded areas) and depth of sampling sites in four studied lakes. Arrows indicate main organic inputs and outflows from thelakes.

dal, were attributed to the persistent dominance of thecyanobacteria Oscillatoria rubescens (Lang & Rey-mond, 1993c). This interpretation was confirmed bythe increase of chironomid larvae in 1994 followingthe disappearance of Oscillatoria in 1993 (Lang &Reymond, unpublished data).

Discussion

The above results suggest that the response of zooben-thos to the abatement of eutrophication depends,

among other factors, on the morphometry of lakes. Inthe two large and deep lakes, the oligochaete commu-nities responded clearly and positively whereas, in thetwo small and shallow lakes, zoobenthic communitieswere unchanged (Lake Morat) or even degraded (LakeJoux). However, as morphometry affects the extent ofeutrophication (Saas, 1989), this result reflects alsothe previous history of these lakes. For instance, LakeMorat was eutrophic in 1936 and T tubifex and P.hammoniensis were already the only species present inthe profundal (Rivier in Lang & Reymond, 1993a). InLakes Morat and Joux, the composition of zoobenthos

t

I

161

was affected by the shortage of oxygen which persistedin the profundal after the abatement of eutrophication(Table 1). But the increase of zoobenthic biomass withdepth indicated good environmental conditions for tol-erant species (Lang & Reymond, 1993a, c).

Depth of sampling sites, relative to the maximumdepth of each lake, could also affect the outcome ofcomparison between the four lakes. For instance, asampling depth of 40 m in Lake Morat corresponds,expressed as a percentage of the maximum depth(Table 1), to a sampling depth of 275 m in Lake Gene-va. And, as discussed previously (Lang & Reymond,1995), zoobenthos responded more slowly to the abate-ment of eutrophication at a depth of 150 m than a depthof 40 m. Therefore, to detect the first signs of recov-ery, it could be necessary to sample zoobenthos inthe littoral (5-10 m) of Lakes Morat and Joux. Forinstance, the mesotrophic species Spirospermna feroxwas observed in 1994 at a depth of 5 m in Lake Joux(Lang & Reymond, unpublished data). The gradualrecolonization of the profundal by this species can beused to track the recovery of Lake Joux.

The above results suggest some empirical rules tomonitor the recovery of the four studied lakes fromeutrophication with zoobenthos. Firstly, if oligotroph-ic worm species are present, use them: they are easy toidentify and they give clear indication on the trophicstate. For instance, the relative abundance of the olig-otrophic species was significantly different betweenthe three troughs present in the western basin of LakeGeneva (Petit Lac) whereas total biomass of zooben-thos was the same (Lang, 1986). In addition, their rel-ative abundance can be predicted from total phospho-rus concentration recorded in the water (Lang, 1990).Therefore, observed and predicted values can be com-pared and the analysis of discrepancies provided usefulinsights on the speed of recovery. Secondly, if olig-otrophic worm species are absent from the profundal,the percentage of Tubifex tubifex, Potamothrix ham-moniensis and P heuscheri in tubificid communitiescan be used as an indicator of environmental con-ditions. Since these species can withstand the mostextreme situations (Milbrink, 1980), the decrease oftheir relative abundance in tubificid communities indi-cates an improvement of conditions. However, in somecases as in Lake Joux (Table 3), total biomass ofzoobenthos, divided into biomass of oligochaetes, chi-ronomid and Chaoborus larvae, can be a more sim-ple indicator of change than the species present inoligochaete communities.

Acknowledgments

Comments by Reinmar Grimm, Trefor Reynoldson,and one anonymous referee have improved this text.

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