Eutrophication of Lakes L&man and Neuchltel (Switzerland) indicated by oligochaete communities

Claude Lang Conservation de la Faune, Ch. du Marquisat 1, CH-102.5 St-Sulpice, Switzerland

Keywords: aquatic Oligochaeta, benthos, eutrophication, indicator communities, lake, tubificid

Abstract

In 197880, oligochaete communities of meso-eutrophic Lake LCman(Lake of Geneva) were compared to those of mesotrophic Lake Neuchatel. Worm species were classified into three groups corresponding to their increasing tolerance to eutrophication: (1) oligotrophic species, mostly Peloscolex velutinus, Stylodrilus heringianus; (2) mesotrophic species, mostly Potamothrix vejdovskyi, P. bedoti; (3) eutrophic species, mostly Potamothrix hammoniensis, P. heuscheri, Tubifex tubifex. In both lakes, eutrophic species constituted the bulk of the communities in terms of absolute abundance. However, relative abundance of mesotrophic and eutrophic species was higher in Lake LCman; oligotrophic species were more important in Lake Neuchatel. These data confirmed the trophic classification of lakes based on chemical parameters. The number of zero values, which perturbated statistical analysis, was reduced by using species groupings instead of isolated species. Thus, making the lakes more comparable even if different species were present in each one. Relative density values based on all samples were distributed among4 density classes for the 3 species groupings. The 12 resulting frequencies described the community structure expressed in terms of eutrophication. Further- more, these frequencies may be used for comparison of eutrophication levels in several lakes.

Introduction

In the present study, oligochaete communities of meso-eutrophic Lake L&man (Lake of Geneva) were compared to those of mesotrophic Lake Neu- chatel. In both lakes, oligochaete communities changed strongly according to depth and to area (Lang& Lang-Dobler, 1980a; Lang&Cuvit, 1981). Therefore, a whole set of oligochaete communities, reflecting a large range of environmental condi- tions, was available for comparison. This compari- son resulted in: (1) definition of the specific data necessary to compare the lakes; (2) the evaluation of oligochaetes as an indicator of eutrophication.

Different approaches were possible to show the relationship between benthic communities and the trophic level of lakes. Community structure may be

Hydrobiologia 115, 131-138 (1984). 0 Dr W. Junk Publishers, Dordrecht. Printed in the Netherlands.

defined either directly by component species or in- directly by regrouping species with similar sensitivi- ty to eutrophication. The first approach - using directly component species - was feasible only if the species were restricted to narrow trophic ranges; as were the species constituting the 15 characteristic chironomid communities described by Saether (1979).

The second approach - regrouping species - ap- peared more practical with species being present in a broad range of trophic conditions and whose relative density increased in some point of this range. This kind of method has been used for dia- toms in rivers, for polychaetes in the sea, for oligo- chaetes in lakes(Lange-Berthalot & Lorbach, 1979; Bellan, 1980; Lang & Lang-Dobler, 1980b), and it has been applied in the present study.

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Stations and methods

Characteristics of the lakes (Table 1)

Phosphorus concentrations were increasing with time in both lakes. Oxygen concentrations were decreasing in the deepest area of Lake L&man where- as they remained high in the profundal of Lake Neuchstel (Fahrni, 1982). The persistence of high oxygen concentrations in Lake Neuchitel was at- tributed to its orientation in the axis of the two prevailing winds, favoring a strong mixing.

Sampling stations and methods

In Lake Lkman, 294 stations were distributed regularly in the whole lake (1 station per 1.96 km2). In Lake Neuchbtel, 77 stations were located on 20 equidistant transects perpendicular to the coast, distributed in the whole lake. On every transect, stations were situated at a depth of 40,60, 90 and 120 m. The mean depth of the sampling stations corresponded to the mean depth of each lake (Table 1). Stations were visited during the spring 1978 in Lake L&man, some during the summer 1979 and most (68.8%) during the summer 1980 in Lake Neu- chbtel.

Stations were sampled with a Shipek grab in Lake Ltman, with a corer (type Kajak-Brinkhurst) in Lake Neuchbtel. Furthermore in 1979, 15 sta- tions (10 to 35 m deep) were sampled by a diver using hand-pushed cores in Lake Neuchitel. Sedi-

Table 1. Characteristics of Lake L&man and of Lake Neuchltel

ment samples were constituted by 16 cm2 cores, 10 to 20 cm long, in both lakes. One 16 cm2 core was collected from the sediment, inside the Shipek grab, at every station of Lake LCman. In Lake Neuchstel, 10 cores were taken by the corer per station in 1979, 4 per station in 1980.

Worms were present in 197 of the 294 samples taken in Lake L&man. Cores without oligochaetes, attributed to low sampling efficiency of the Shipek grab (Brinkhurst, 1974), were not included in sub- sequent analysis. Numbers of worms collected by the diver and by the corer were comparable. The core, taken from inside the Shipek grab, tended to underestimate the number of worms, but less than if the whole Shipek was sampled.

Identification of oligochaetes

For both lakes, tubificid and lumbriculid worms, stained in Bengal Rosa, were separated from the sediment with a sieve (0.2 mm mesh size aperture). All worms which were present in the cores from Lake L&man were mounted in Hydramount of Gurr. A random subsample of 50 worms was used if more than 50 individuals were present. Worms were counted in every core from Lake Neuchatel and their mean density per core was calculated. A ran- dom subsample of 30 to 60 worms was taken from all the cores pooled together and mounted in Water Mounting Medium of Gurr. The relative density of every species in the subsample was related to the total mean worm density per core to calculate their

Variable Unit

Lake surface km2 Mean depth m Maximum depth m Lake volume km3 Theoretical water residence time year Area of drainage basin km2 Average altitude of the drainage basin m Altitude of lake surface m Number of inhabitants in the drainage basin 103 Average concentration of total phosphorus at 1979 spring overturn mg m-3 Lowest concentration recorded during oxygen 1979 in the profundal mg 1-1 More than 4 02 1-l to a depth of mg up m Lowest average concentration recorded in the whole lake mg 1-I Trophic level

Sources: Fahrni (1982), Lachavanne (1980), Sollberger (1974), B. Pokorni (pers. commun.).

L&man Neuchltel

581.5 214.6 153 64 310 153

89.0 13.8 11.9 8.2

7 975 2 672 1 670 780

372 429 760 245

90 56 1.7 8.0

260 153 7.4 9.0

meso-eutrophic mesotrophic

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absolute density per m2. Immature worms were identified according to the shape of setae and other characters.

ing expressed as a percentage of the total density per sample. The relative density represented the community structure, i.e. the relationship between species groupings.

Previous studies Median density

The studies of Monard (1919) in Lake Neuchatel and of Juget (1967) in Lake Ltman were used as references. Monard’s study consisted of 71 stations sampled in 1917 and 19 18 with a modified Stein- mann dredge. Stations were located in the northern part of the lake; their mean depth was 67 m. Juget’s study consisted of 58 samples taken with an Ekman grab in 10 stations visited between 1958 and 1967; the mean depth of the stations was 95 m.

Data analysis methods

Species groupings

Some worm species, frequent and abundant in one lake, were absent in the other. Also, most spe- cies were absent in too many samples (too many zeros) to perform statistical analysis of their density (Green, 1979). To overcome these problems, oligo- chaete species, with similar tolerance to eutrophica- tion, were regrouped. Species 1 to 3 of Table 2 - pooled together - represented the oligotrophic spe- cies, i.e. the species typical of oligotrophic lakes. Species 4 to 11 represented the mesotrophic species; species 12 to 18, the eutrophic species. The used classification of species in terms of eutrophication was discussed elsewhere (see ‘Oligochaetes as indi- cators’ in the discussion). Using species groupings instead of the isolated species reduced the number of zero values. Furthermore, the lakes were more comparable even if different species were present in each one.

Types of data

Two types of data were analysed in this study, the frequency of occurrence and the density. The fre- quency of occurrence represented the number of samples where a given worm species or a species grouping were present, irrespective of their abun- dance(Table 2). The absolute density was based on the number of individuals of a given species group- ing per sample. The relative density was based on the number of individuals of a given species group-

The value of absolute (or relative) density of a species grouping may be summarized by the arith- metic mean or the median. In this study, the median was used because this statistic was more robust than the mean, especially if extreme values were present (Reckhow, 1980). For example, the mean value of the absolute density was higher in Lake L&man than in Lake Neuchatel; the reverse was true for the median. Furthermore, the median corresponded to the assumption of the Mann-Whitney test which was used to compare the density of species group- ings in both lakes (Conover, 1971).

Classes of density

To analyse the data differently, density classes were defined and the frequency of samples was calculated for each class. One problem, however, was being objective in defining the limits for the density classes. Results reviewed by Wiederholm (1980) helped to solve this problem for the absolute density: they indicated that densities exceeding 10 000 worms m-2 were characteristic or organic pollution. This value was selected for the lower limit of the second density class, and this value was doubled for the lower limit of the third density class.

The density classes for the relative density were defined from results of previous factorial corres- pondence analysis (Lang & Lang-Dobler, 1980b). In this analysis, samples were described by the rela- tive density of the three species groupings. Rela- tionship between samples and the three species groupings were graphically displayed on a factorial plot. Samples close to a species grouping were characterized by high relative density of this species grouping; samples far from this species grouping were characterized by low relative density of this species grouping. Distribution of samples on the factorial plot according to their distance from spe- cies groupings permitted us to select limits for den- sity. This selection was more objective than for the absolute density.

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Typical communities

According to these data, the minimal value of the relative density necessary to consider one worm community as typical of a given species grouping was equal or superior to 50% for typical oligotro- phic and mesotrophic communities, and 100% for typical eutrophic communities. This difference was due to the resistance of eutrophic species to a broad range of conditions. In fact, the so-called eutrophic species make up a significant portion of the com- munity in oligotrophic lakes. For example, Pota- mothrix hammoniensis, a typical eutrophic species, was very frequent in the oligotrophic Lake NeuchC tel in 1918 (Table 2).

Exclusion of zero values

In Table 3, worm densities were analysed in two ways. First, the samples with zero values were in- cluded to describe the whole lake. Second, these

samples were excluded to describe exclusively the communities where a given species grouping was present (selected samples of Table 3). Densities of oligotrophic and of mesotrophic species groupings were higher in selected samples than in the whole lake because these groupings were absent in many samples.

Calculations and statistical tests were performed with the SPSS package (Nie, et al., 1975; Hull & Nie, 198 1).

Results

Frequency of occurrence (Table 2)

Some species, frequent in one lake, were absent in the other. Bythonomus lemani(in 1958-67), Po- tamothrix vejdovskyi and P. heuscheri were fre- quent in Lake L.&man, absent in Lake Neuchltel. P. moldaviensis and P. bedoti were frequent in Lake Neuchatel, but absent in Lake L&man (Table 2). At

T&&=2. Percentage of samples where a given worm species is present and mean relative density (%) of worm species in Lake L&man and in Lake Neuchltel.

No Worm species Percentage of presence in samples Significance Mean relative density (%) of Chi*

Neuchltel L&man Neuchltel L&man Ltrman Neuchatel 1918a 1958-67b 1979-80 1978 I978 1979-80

I Byrhonomus lemani Grube 0 63.8 0 1.5 NS 0.5 0 2 Stylodrilus heringianus Claparbde 33.8 62.1 50.6 9.6 * 2.9 9.8 3 Peloscolex velutinus Grube 17.5 62.1 31.2 5.1 * I.1 3.5 4 Potamothrix vejdovskyi Hrabh 0 46.6 0 36.5 * 16.5 0 5 Peloscolex ferox Eisen 14.0 27.6 15.6 10.7 NS 2.4 0.9 6 Psammoryctides barbatus Grube 11.2 13.8 7.8 1.5 * 0.2 0.6 7 Aulodrilus limnobius Bretscher 0 0 0 0.5 NS 0.1 0 8 Potamothrix moldaviensis

Vejdovsky & Mrazek 0 0 16.9 0 * 0 4.6 9 Potamothrix bedoti Piguet 0 (+I0 32.5 c+YJ * 0 6.7

IO Ilyodrilus templetoni Southern 0 6.9 3.9 0.5 NS 0.3 0.1 I I Aulodrilus pluriseta Piguet I .4 34.5 0 4.6 NS 0.5 0 12 Limnodrilus profundicola Verrill 1.4 10.3 5.2 0.5 * 0. I 0.1 13 Limnodrilus claparedianus Ratzel 2.8 3.9 0 * 0 0.1 14 Limnodrilus hoffmeisteri Claparede 5.6 23.4 4.6 * 0.2 I.0 15 Limnodrilus species 12 to 14 74.1 74.0 35.0 * 6.9 8.4 16 Potamothrix heuscheri Bretscher 0 93.1 0 44.7 * 19.4 0 17 Potamothrix hammoniensis

Michaelsen 63.5 58.6 89.6 34.0 * 13.9 27.0 18 Tubtfex tubtjizx Mtiller 18.3 77.6 94.8 62.4 * 35.3 38.2 19 Oligotrophic species 1 to 3 90. I 74. I 59.7 13.7 * 4.6 13.3 20 Mesotrophic species 4 to I I 26.6 60.3 57.1 40.1 * 20.0 13.0 21 Eutrophic species 12 to 18 76.1 98.3 100.1 94.4 NS 75.4 73.4

Number of samples 71 58 77 197 197 77

The Chi* test is calculated from data L&man, 1978 and Neuchatel, 1979-80. NS not significant, * p < 0.05, - missing data; a Monard (1919), b Juget (1967), (+) species present in the lake, not in the above samples.

the beginning of this century, Bl’thonomus lemani was very frequent in Lake L&man, though absent in Lake NeuchPtel (Monard 1919). The other above species were more or less recent immigrants.

In 1978-80, Stylodrilus heringianus became more frequent than Peloscolex velutinus or Bytho- nomus lemani in both lakes. In 1918, Peloscolex velutinus was absent from the area directly influ- enced by the River Areuse (Monard, 1919). This species and B.ythonomus lemani seemed to be the ultra-oligotrophic species which were very frequent in most oligotrophic Swiss lakes at the beginning of the century (Piguet & Bretscher, 1913). Their re- placement by Stylodrilus heringianus indicated a subtle shift from very sensitive towards more toler- ant oligotrophic species.

The frequency of worm species in lakes was in some measure related to their mean relative density (Table 2). The most frequent species were, in gener- al, the most abundant in the community. Four spe- cies constituted the bulk of the communities in Lake L&man, three species in Lake Neuchatel. In both lakes, Tubij2.x tubifex was the most abundant

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species, followed by Potamothrix hammoniensis in Lake Neuchbtel, by P. heuscheri in Lake L&man.

In 1978-80, oligotrophic and mesotrophic spe- cies were more frequent in Lake Neuchltel than in LCman; the eutrophic species were present in al- most all samples. The frequency of oligotrophic species decreased strongly from Lake Neuchdtel 1918 to Lake Leman 1978 (Table 2). During the same period, frequency of mesotrophic species showed first an increase and then a decrease. Eu- trophic species constituted rapidly the bulk of the community. The frequency of these species group- ings was a robust criterion to compare eutrophica- tion of several lakes. However, many small samples were necessary to assess precisely species frequency in a whole lake.

Absolute density of species groupings (Table 3, No 1-6)

Overall, the total absolute density of three species groupings and the absolute density of oligotrophic and eutrophic species were higher in Lake NeuchC

Tuble3. Median absolute and relative density of 3 groupings of worm species in Lakes Ltman(L) and Neuchatel(N) and percentage of samples in four density classes.

No. Species groupings

Lake Median Significance Upper limits of density classes Units Significance Mann-Whitney - of Chi*

0 9 999 19 999 500 000 nm* 0 9 49 100 %

1. Total L 5919.6 * 62.4 16.2 21.3 * (All samples) N 13750.0 40.3 33.8 26.0

2. Oligotrophic L 53.0 * 86.3 12.7 0.5 0.5 * (All samples) N 712.0 40.3 57.1 2.6 0

3. Oligotrophic L 933.8 NS

92.6 3.7 3.7 (If present) N 1206.5 95.7 4.3 0

NS

4. Mesotrophic L 233.3 NS

59.9 31.0 6.6 2.5 * (All samples) N 351.0 42.9 55.8 1.3 0

5. Mesotrophic L 3068.2 * 77.2 16.5 6.3 * (If present) N 2121.0 97.7 2.3 0

6. Eutrophic L 3631.4 * 5.1 68.5 11.7 14.7 * (All samples) N 10000.0 0 49.4 35.1 15.6

7. Oligotrophic L 0.004 * 86.3 4.1 5.6 4.1 * (All samples) N 4.649 40.3 28.6 20.8 10.1

8. Oligotrophic L 21.700 29.6 40.7 29.6 NS (If present) N 10.700

NS 47.8 34.8 17.4

9. Mesotrophic L 0.055 NS 59.9 3.6 14.7 21.8 * (All samples) N 4.167 42.9 14.3 35.1 7.8

10. Mesotrophic L 52.200 * 8.9 36.1 54.4 * (If present) N 15.500 25.0 61.4 13.6

I I. Eutrophic L 99.943 * 5.1 2.0 16.8 76.1 (All samples) N 83.333 0 0 22.1 77.9

NS

No, 1 to 6 absolute density, 7 to II relative density. * p < 0.05; NS not significant. not included.

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tel than in Lake LCman according to the values of the median, whereas mesotrophic species had the same density in both lakes. However, in selected samples, absolute densities of oligotrophic species was the same in both lakes, whereas mesotrophic species predominated in Lake Leman. These differ- ences were clearly due to the higher number of zero values recorded in Lake Leman (Table 2). In this case, estimation of the absolute density was modi- fied by different frequencies of occurrence.

Most absolute densities of oligotrophic and me- sotrophic species were included in the low density class, while, on the other hand, eutrophic species were more abundant in the high density classes. This difference demonstrated the pratical signifi- cance of these classes to indicate eutrophication.

Values of total densities were distributed differ- ently in both lakes: proportionately there were more low and high values present in Lake L&man than in Lake Neuchatel.

These data were difficult to interpret because eutrophication contributed to either the increase or the decrease of the absolute density of the worms. The absolute density of the worms increased with organic sedimentation only if the oxygen was not limited (Lang & Hutter, 1981); if this element was deficient, absolute density decreased as was shown in the deepest area of Lake LCman. Total densities were higher than 50 000 worms mm* (up to a maxi- mum of 430 000 m-*) in 7.6% of the samples from Lake L&man. These high density stations were ex- posed to the inputs of the RhBne River. Such high densities were not encountered in Lake Neuchatel.

Relative density of species groupings (Table 3, No 7-l 1)

Typical oligotrophic communities were scarce and their numbers were the same in both lakes, whereas typical mesotrophic and eutrophic com- munities were better represented in Lake Ltman than in Lake Neuchatel. Percentages of typical eu- trophic communities were, respectively, 53.3% and 14.3% (p < 0.001). In Lake LCman, typical com- munities (oligotrophic, mesotrophic or eutrophic) were present in 79.2% of the samples versus 32.2% in Lake Neuchltel. Intermediate communities were represented in the other samples.

The frequencies of samples in4 classes of relative (or absolute) density for the three species groupings

(i.e. 12 samples frequencies) described fairly well the structure of oligochaete communities expressed in terms of eutrophication (Table 3).

Factorial correspondence analysis, based on these 12 frequencies, may be used for comparison of several lakes. These data clearly demonstrated how a single value, such as the mean or the median, was not enough to define the eutrophication level of a lake by the oligochaete communites.

Discussion

Oligochaetes as indicators

Data presented in this paper were based on the fact that oligochaete species may be classified ac- cording to their tolerance to eutrophication. Classi- fication of species in terms of tolerance presented in Table 2 (No. 19-21) was very similar to that of Milbrink (1980) with two exceptions. Firstly, Pe- loscolexferox was considered in my study as meso- trophic rather than oligotrophic, and secondly, Limnodrilus profundicola became eutrophic in- stead of oligotrophic.

Two important modifications of the previous classification of worm species in terms of eutrophi- cation (Lang & Lang-Dobler, 1980b) derived from a study made in two 35 m deep stations in Lake Leman (Lang & Hutter, 1981). In this study, the annual organic sedimentation was equal to 157 g C m-* in station 1, to 214 g in station 2. The absolute and the relative densities of oligotrophic species (Table 2, No 1-3) decreased strongly in station 2 compared to station 1. On the contrary, the abso- lute and the relative densities of Potamothrix hammoniensis and P. heuscheri increased in station 2. The relative density of Potamothrix vejdovskyi decreased in station 2 whereas its absolute density increased. The intermediate position of P. vejdovs- kyi between oligotrophic and eutrophic species suggested for this species a sensitivity corresponding to mesotrophic conditions. Potamothrix heuscheri displayed the same reaction as P. hammoniensis to increasing organic inputs. Therefore, this species was classified as eutrophic.

In a second study, in Lake Morat, a typical eu- trophic lake (Davaud, 1976), Limnodrilus profin- dicola constituted 16.2% of worm communities, the other two species being Potamothrix hammonien- sis and Tubifex tubzfex (Lang & Cuvit, 1981).

137

Tub/e 4. Compared eutrophication of Lakes L&man and Neuchltel according to oligochaete communities.

Type of data Species Observed trend groupings L&man Neuchltel

More eutrophic lake according to the trend

Trend increased by sampling design

Occurrence Oligotrophic + L&man - in L.&man frequency Mesotrophic + Leman

Eutrophic = =

Absolute All + Neuchltel - in L&man density Oligotrophic zz =

Mesotrophic + L&man Eutrophic + Neuchltel

Relative Oligotrophic = = + in L&man density Mesotrophic + L&man

Eutrophic + L&man

Frequency Oligotrophic = = + in L&man of typical Mesotrophic + L&man communities Eutrophic + L&man

Trend: + high; - low; = same in both lakes. Absolute and relative density was considered exclusively for samples where a given species grouping was present (Table 3).

Peloscolex ferox, classified as a typical oligotro- phic species in Swedish lakes (Milbrink, 1980) was dominant in Vierwaldstlttersee at the beginning of the century (Obermayer, 1922). Peloscolex veluti- nus, which was absent from this lake, was dominant in the other Swiss lakes. This observation suggests a kind of competitive interaction between these two species.

Therefore, classification of a given worm species according to eutrophication depended also on the other species present. For example, a species may be absent from a lake for mere zoogeographical reasons. This absence permitted another equivalent species to fill up its ecological niche and to take its role as an indicator of eutrophication. Consequent- ly, the attribution of a worm species to a trophic category must be reevaluated for each lake.

Differences in sampling design (Table 4)

According to several criteria, oligochaete com- munities indicated that Lake LCman is more eutro- phic than Lake Neuchatel. However, some of these results might have been biased by differences in sampling design. For example, Shipek grab tended to underestimate absolute density in Lake Ltman. In Lake Neuchatel on the other hand, results of several cores pooled together tended to: (1) increase the frequency of the presence of scarce species, (2) reduce the frequency of extremely high and low

values of the absolute density and (3) reduce the frequency of communities dominated by one spe- cies grouping.

Advantages of species groupings

Regrouping worm species according to their tol- erance to eutrophication presented several advan- tages: (1) the number of zero values was reduced, (2) the absolute and the relative density of worm cate- gories were quantifiable in both lakes, therefore amenable to statistical tests, (3) the use of species groupings instead of the isolated species bypassed the identification problem of immature individuals for some species, (4) the seasonal variations of spe- cies groupings were also less than those of the com- ponent species (Lang 8z Hutter, 1981) and (5) the use of species groupings overcame problems due to different zoogeographical distributions of worm species.

Acknowledgments

Mrs B. Lang identified oligochaetes of Lake Lit- man, and Mrs L. Faravel provided technical assist- ance. Mr J. M. Helbling assisted with statistical methods. Mrs S. Hurni corrected the English, and Miss J. Sonnay and V. Nicole typed the manus- cript. The comments of two anonymous reviewers

138

helped to improve the paper. Mr R. Ducret helped to sample Lake Neuchgtel. Samples were taken in Lake L&man by the‘Laboratoire de stdimentologie de l’universitt de Gen&ve’(Prof. J. P. Vernet). Stu- dies were supported in both lakes by the ‘Office f&dCral de la protection de l’environnement’.

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