AnnlsLimnol. 31 (3) 1995 : 185-199
Aquatic Coleóptera (Hydraenidae and Elmidae) as indicators of the chemical characteristics of water in the Orbigo River basin (N-W Spain)
F. Garcia Criado1 M. Fernandez Alaez
Keywords : Hydraenidae, Elmidae, indicator species, León, Spain.
The responses of 32 Hydraenidae and Elmidae species to six chemical parameters - alkalinity, CI", S04" ", COD, N03\ and to• tal P have been analysed in the Orbigo river basin. The study of the distribution in relation to these factors by means of elabora• ting their ecological profiles pointed out the indicator species for each one of these parameters, according to their reciprocal spe• cies-factor information.
Hydraenidae et Elmidae (Coleóptera) indicateurs des caractéristiques chimiques de l'eau dans le bassin de la rivière Orbigo (N-O Espagne)
Mots clés : Hydraenidae, Elmidae, espèces indicatrices, León, Espagne.
Les réponses des 32 espèces d'Hydraenidae et d'Almidae à 6 paramètres chimiques - alcalinité, Cl", S04" ", DCO, N03" et P total - ont été analysées dans le bassin de la rivière Orbigo. L'étude de leur répartition en relation avec ces facteurs et la recherche de leurs profils écologiques ont mis en évidence les espèces indicatrices pour chacun de ces paramètres.
1. Introduction In the province of León (N-W Spain) there has been no previous research of this kind. We have presented Few studies have been reported on the families Hy• some preliminary data from our study carried out in draenidae and Elmidae in Spain. Furthermore, these the Orbigo basin (García Criado et al. 1994, García studies have been limited to general considerations Criado et al., in press), and pointed out the tolerance li• about habitat preferences. Only recently have authors mits of species for 10 physical and chemical parame• such as Puig (1983), Sáinz-Cantero (1985), Sáinz- ters. In this work we extend these studies through mo• Cantero & Alba-Tercedor (1991), Díaz Pazos (1991) re detailed investigation on the autoecology of species and Rico (1992) taken interest in determining the in• of both families. We aim at exposing the preferences fluence of physicochemical parameters on the distribu• of these species within the ranges of values found for tion of species of these families. several chemical factors. 2. Material and methods The Órbigo basin lies mostly on siliceous soil, which alternates with calcareous soil in the northern areas (ri• vers Luna and Torrestio). The depth of the water• 1. Departamento de Biología Animal. Facultad de Biología. courses prospected was seldom more than 50 cm. Universidad de León. 24071 León. Spain. Samples were taken seasonally over an annual cycle 2. Departamento de Ecologia, Genética y Microbiología. Facultad de Biología. Universidad de León. 24071 León. Spain. (from April 1991 to February 1992) and were collected
Article available at http://www.limnology-journal.org or http://dx.doi.org/10.1051/limn/1995017 186 F. GARCIA CRIADO, M. FERNANDEZ ALAEZ (2) from 37 sites along the rivers of the basin (Fig. 1), al- were collected by five minutes kick-sampling. Only ways from lotie environmentsl(middlê~of~the river) presence/absence data of adults have been considered with pebble, cobble and boulder substratum. Beetles in this study.
Fig. 1. Geographical situation of the Órbigo river basin and sampling sites. Fig. 1. Situation géographique du bassin de la rivière Órbigo et stations de récoltes. (3) AQUATIC COLEÓPTERA AND CHEMICAL CHARACTERISTICS IN A RIVER BASIN 187
All the 148 collected samples were taken into ac• Next the profile of absolute frequencies was arran• count together, regardless of their time of collection. ged, this is the number of samples in every class of Samples without Hydraenidae and Elmidae specimens describer in which each species is present. Although were excluded from statistical processing, so that 101 the pattern of this profile gives a preliminary idea of was the final number of analyzed samples. These how the taxon responds to environmental variables, it samples have been excluded as the lack of specimens may be misleading as the number of samples included is often due to causes other than chemical factors (high in every class (that is to say, the overall profile) is not water levels and difficulty of sampling, siltation...). uniform. Besides, it is necessary to take into account Besides their inclusion would have produced gaps in the frequency of each species in the sample lot to cor• the profiles making it difficult to interpret the results. rect the differences between the features of rare and Some factors likely to influence the distribution of frequent species. For these reasons, a profile of correc• Coleóptera or those positively correlated with the de• ted frequencies C(k) was calculated in order to produ• gree of organic pollution of water were considered in ce a more reliable image of the pattern of species dis• the analysis. Such selected factors are : alkalinity, CI", tribution in relation to the describer. For this purpose we used the formula of Daget & Godron (1982) : S04", chemical oxygen demand (COD), N03" and to• tal P. On the one hand, parameters such as pH and oxy•
gen concentration were not included since their values C(k)- W • M> W throughout the basin were homogeneous and gave ve• R(k) ' NR ry little information on beetles distribution. Because conductivity is closely correlated with alkalinity, its in• where, besides the previous notation : clusion was considered redundant. C(k) = median frequencies of the species E in every class of describer. The data were analyzed as follows. Firstly, the ob• U(k) = number of sampling sites for every class of served range of values for each habitat parameter (des- describer in which species E is present. criber) was divided into a series of intervals (classes of U(E) = total number of sampling stations in which describer). The samples were assigned to intervals ac• species E is present. cording to their value for the describer. In this way, we established the number of samples occurring in each In order to select those taxa which give the most in• class of describer. Within each describer, care has been formation with regard to every parameter, and avoid taken for the amplitude of intervals to be as homoge• analysing all of them, the reciprocal species-describer neous as possible. However, it was not possible to es• information has been estimated. For this, we used the tablish perfectly even intervals since it is convenient formula put forward by Godron (1968) : that the number of sites included in each class is as uni• form as possible (Nakache 1973). Nk U(k) U(k) NR Nk V(k) V(k) NR
Firstly, it is important to determine the sampling I(L,E) = I log2 . +1 log2 quality. We calculated the entropy linked to the descri• l NR R(k) U(E) l NR R(k) V(E) ber resulting from the sampling achieved by means of the following formula (Daget et al., 1972) : where, besides the previous notations : V(k) = number of sampling stations for each interval Nk R(k) NR in which species E is absent.
H(L) = I log2 i NR R(k) V(E) = total number of sampling stations in which species E is absent. where : This value was taken as a measure of the indicator R (k) = number of sampling stations in every class of value of the species in relation to the describer. Since describer. we are working with 32 species and 6 parameters it is NR = total number of sampling stations (101). not useful to analyze such a high number of features. It Nk '= number of classes of describer taken into ac• is much easier to handle just those features of species count. which provide useful information. These are the ones By comparing this value with the one for maximum with highest I(L ; E), that is, the highest indicator value for the describer. For every parameter we have selec• entropy H(L)max = log2 (Nk) we obtained the sampling ted the ten species whose values of reciprocal informa• quality Q(L) = H(L)/H(L)max, which indicates the loss of information in the sampling. Higher is the value, lo• tion species-describer were highest ; their corrected wer is the loss of information. frequencies are represented by histograms (Figs. 2-7). 188 F. GARCIA CRIADO, M. FERNANDEZ ALAEZ (4)
These are the species which are more highly linked to 3.2. Choride (Fig. 3) the factor under consideration. A sampling quality of 0.88 was obtained. Because of Finally, we calculated the barycenter which allows the small number of samples with a CI concentration us to summarize the information given by each one of exceeding 10 mg/1, all samples above this level were the ecological profiles by means of a single value. It included in a single interval, even though values ex• allows arrangement of the species along the ecological tended up to 23.2 mg/1. Only two species, Elmis mau- gradient and can be taken as a measure of its ecologi• getii and Esolus pygmaeus, occur in the site with maxi• cal optimum. For this calculation, the expression used mum chloride level. The most representative species was the following (Daget & Godron 1982) : for this parameter can be classified into : — Taxa from areas with a low concentration : Hy• 6 v m. draena inapicipalpis, Hydraena emarginata, Hydraena In 1 ibérica and Elmis aenea. With a wider range, Hydrae• where : na brachymera may also be included in this group. irij = index of frequency (value of corrected frequen• — Taxa from areas with a high concentration : Eso• cy for every class of describer in which the species is lus pygmaeus is closely linked to sites with high levels present). of chloride, as shown by the values of corrected fre• quencies. Oulimnius rivularis shows a wider range. dj = number of the corresponding class. — Taxa with a wide range of distribution : Elmis 3. Results maugetii, Limnius opacus and Oulimnius troglodytes. 3.3. Sulphate (Fig. 4) During the study 1,202 Hydraenidae and 2,771 El• midae belonging to a total of 32 taxa were collected The sampling quality value is 0.84. The maximum value of S0 " " concentration was 34.7 mg/1 ; again on• (Table I). Regarding the subspecies Limnius perrisi 4 subcarinatus (Sharp 1872), we have followed Rico ly Elmis maugetii and Esolus pygmaeus were present (1992), in synonymizing it with Limnius perrisi cari- at sites with such concentration. Among the most si• natus (Perez-Areas 1865). gnificant species three groups can be considered : Table I presents, for each species, the indicator value — Taxa from areas with a low concentration : Hy• (reciprocal infon iation species-describer) for each of draena brachymera, Hydraena inapicipalpis and Hy• the 6 variables studied, the barycenter, and the average draena emarginata. Although with a wider distribu• indicator value (arithmetic mean of the indicator va• tion, Hydraena exasperata has a tendency to occur in lues for the six parameters). The latter value accounts areas with reduced level of sulphates. for the potential of the species for being a global indi• — Taxa from areas with high concentrations : Esolus cator for these parameters. This table shows as well the pygmaeus and, on a lesser scale, Oulimnius troglodytes. relative abundance, which is the percentage of samples — Widely distributed taxa : Elmis maugetii, Limnius in which the species was present. opacus and Oulimnius rivularis. Though with impor• The response of the species more closely linked tant gaps in its profile, Esolus parallelepipedus also (those giving more information) to each of the sur• has a wide range. veyed parameters is discussed below. 3.4. COD (Fig. 5) 3.1. Alkalinity (Fig. 2) The sampling quality obtained was 0.95. There was The value of sampling quality observed for this va• little polarization in the distribution of the studied taxa riable was 0.87, which is relatively high. This descri• to this factor. Elmis aenea and Oulimnius tuberculatus ber is, therefore, sufficiently sampled. From the spe• were only present in areas with low COD, the rest of cies with high indicator value for this factor, Hydraena the species were in a relatively wide gradient of values testacea, Hydraena inapicipalpis, and, to a lesser ex• or showed slightly defined tendencies. Among these tent, Hydraena brachymera, are typical of scarcely al• species, Hydraena brachymera occurred more fre• kaline waters ; the rest of the species were generally in• quently in areas with low levels and Esolus parallele• different to this factor. Only Hydraena emarginata, pipedus and Oulimnius troglodytes occurred in sites though widely distributed, shows a slight tendency to• with relatively high values. However, no specimens wards alkaline areas. This parameter, within the values were collected from localities with COD levels existing in the basin, barely influences the distribution above 20 mg 02/l, even though sites with values of the species, most of which exhibit wide tolerance. up to 53 mg 02/l were detected. AQUATIC COLEÓPTERA AND CHEMICAL CHARACTERISTICS IN A RIVER BASIN
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XI) a ft. O c g Ü fi. o tu es a c s S II v es S 5 60 t O Cj C ft. a Q C3 cy O CJ •S .S .2 t3 Vi C C K K K K C -5 .5 s 3 2 o tü tu tu , i, s. I I 5 5 • • CS > ci ci tf oo on o O"esiCiTfviV0t~-000v d —• Vi NQ [ es es ci ci CtCiTfvivoh-ooov es es cs Cl Cl x> es es cs es es t2 F. GARCIA CRIADO, M. FERNANDEZ ALAEZ
a) H. testa cea b) H. brachymera
O 0.25 0.5 1 1.5 2 25 >25 0.25 0.5 1 1.5 2 25 >Z5 Alkalinity (meq CO_Ca/I) Alkalinity (meq CQ,Ca/I)
c> H. inapicipalpis H. emarginata rj6
"D 4 a> „ O ° £ 2 fcl
O0 125 0.5 1 1.5 2 25 >25 0.25 0.5 1 1.5 2 25 >25 Alkalinity (meq CQCa/1) Alkalinity (meq CQ,Ca/l)
e) H. exasperate D. brev/s cr 6 CD 5 4
o 31 CD 2 1
0 0.25 0.5 1 1.5 2 25 >25 0.25 0.5 1 1.5 2 25 >2.5 Alkatinrty (meq CQjCa/i) Alkalinity (meq CCfeCa/l)
g; E. maugetii h) E. parallefepipedus
0.25 0.5 1 1.5 2 25 >25 o 0.25 0.5 1 1.5 2 25 >25 Alkalinity (meq CQjCa/l) Alkalinity (meq CQ,Ca/l)
I) L. opacus /> O. troglodytes cr 6 cr 6
Aikaïnity (meq COyCa/l) Alka&nity (meq CG,Ca/D
Fig. 2. Corrected frequencies of alkalinity for the ten taxa with highest indicator values for this parameter. Fig. 2. Fréquences corrigées de l'alcalinité pour les 10 taxa avec les valeurs indicatrices les plus élevées pour ce paramètre. AQUATIC COLEÓPTERA AND CHEMICAL CHARACTERISTICS IN A RIVER BASIN 191
a) H. brachymera b) H. inapicipalpis
2 4 6 8 10 >10 4 6 8 10 >10 Chloride (mg/l) Chloride (mg/l)
c) H. emarginata d) H. ibérica
6 8 10 >10 4 6 8 10 >10 Chloride (mg/l) Chloride (mg/l)
E. aenea /) E. maugetii cr 4 CO 3 T5 a> 2 O
g) E. pygmaeus h) l_ opacus 25 cr
/; O. rivularis \) O. troglodytes
Chloride (mg/l) Chloride (mg/l)
Fig. 3. Corrected frequencies of the chloride concentrations for the ten taxa with the highest indicator values for this parameter. 3. Fréquences corrigées des concentrations de chlorure pour les 10 taxa avec les valeurs indicatrices les plus élevées pour ce paramètre. 192 F. GARCIA CRIADO, M. FERNANDEZ ALAEZ (8)
a) H. brachymera b) H. Inapicipalpis
Sulphate (mg/1) Sulphate (mg/1)
H. emarginata d) H. exaspérala
4 6 8 10 12 16 20 >20 Sulphate (mg/l) Sulphate (mg/l)
e; E. maugetii f) E. parallelepipedus
Sulphate (mg/l) Sulphate (mg/l)
g) E. pygmaeus h) L. opacus
6 8 10 12 16 20 >20 6 8 1 0 1 2 16 20 >20 Sulphate (mg/l) Sulphate (mg/l)
i) O. rivularis J) O. troglodytes
Sulphate (mg/l) Sulphate (mg/l)
Fig. 4. Corrected frequencies of the sulphate concentrations for the ten taxa with the highest indicator values for this parameter. Fig. 4. Fréquences corrigées des concentrations de sulfates pour les 10 taxa avec les valeurs indicatrices les plus élevées pour ce paramètre. (9) AQUATIC COLEÓPTERA AND CHEMICAL CHARACTERISTICS IN A RIVER BASIN 193
a) H. brachymera b) H. inapicipalpis
CD
) í 6 8 10 12 14 19 COD (mg Cl/1) COD (mg Q/I)
c) D. brevfs d) E. aenea
4 6 8 10 12 COD (mg Q,/!) COD (mg q,/l)
e> £. rioloides f) E. angustatus
t 4 "S 3 CD o 21 CD O o 4 6 8 10 12 4 6 8 10 12 14 19 COD (mg QVI) COD (mg Cyi)
g) E. paratteiepipedus h) O. rivularis D" CD
«fc TJ CD o CD v_ i_ O 2 4 6 8 10 12 14 19 Ü 4 6 8 10 12 14 19 COD (mg q,/l) COD (mg Q,/1)
i) O. troglodytes /> O. tuberculatum cr5 £4 **— "S 3 CD o 2 CD
O 0 4 6 8 10 12 14 19 0 2 4 6 8 10 12 14 19 COD (mg Q,/l) COD (mg cyi)
Fig. 5. Corrected frequencies of the chemical oxygen demand (COD) for the ten taxa with the highest indicator values for this parameter. Fig. 5. Fréquences corrigées de la demande chimique en oxygène (DCO) pour les 10 taxa avec les valeurs indicatrices les plus élevées pour ce paramètre. 194 F. GARCIA CRIADO, M. FERNANDEZ ALAEZ (10)
3.5 Nitrate (Fig. 6) rant to pollution. Actually, extensive literature sup• A sampling quality of 0.96 was obtained. The ports the fact that these families are generally quite sensitive to the degradation of streams (Thomas distribution of N03" concentrations found in the ba• sin was skewed towards low values (< 1 mg/l), there• 1988, Rico 1992). However, the stated data here be• fore uneven intervals, of 0.2, 0.5, 1 and a final interval low are valuable in relative terms, as they allow us to including just over 2 units were used. Nitrate levels in compare various areas from the same basin. At the the basin were found not above 6.89 mg/l. Due to this same time, they underline the existence of some taxa scarce variation no noticeable differences can be ob• which are more demanding, in connection with those served in the response of the species, which on the parameters, than others. whole occupied a wide range of values. Only Hydrae• Of all 32 taxa collected, 18 proved to be indicators na inapicipalpis, Oulimnius tuberculatus and, to a les• for at least one of the describers used. That number ser extent, Dupophilus brevis were restricted to areas is reduced to 11 if we leave aside those taxa which with the lowest concentration. are only indicators of 1 or 2 describers. Those 11 3.6. Total phosphorus (Fig. 7) species are Hydraena brachymera, Hydraena inapi• cipalpis, Hydraena emarginata, Hydraena exaspera- The sampling quality for this parameter was 0.92. ta, Hydraena ibérica, Dupophilus brevis, Elmis mau• Phosphorus levels were relatively low, not exceeding getii, Esolus parallelepipedus, Limnius opacus, Ou• 4 [iM of P0 ". Hydraena'inapicipalpis and Limnius 4 limnius rivularis and Oulimnius troglodytes. These perrisi carinatus showed preference for low values. taxa are likewise the ones with the greatest average A similar tendency, though less strong, was exhibi• indicator value. Thus, these taxa give more global ted by Hydraena ibérica. The rest of the species are information about the variables under consideration. present along the greater part, or all, of the gradient They are also the most abundant species in the basin although Oulimnius rivularis and Oulimnius troglo• (Table I). dytes had a propensity for points with the highest concentrations and Hydraena brachymera showed With regard to pollution, shown by the levels of ni• an inclination for those points with the lowest trate, phosphorus, COD, chloride and sulphate, the concentrations. species Hydraena brachymera, Hydraena inapici• palpis, Elmis maugetii, Oulimnius rivularis, and Ou• Discussion limnius troglodytes are indicators for at least 4 of these 5 factors. Hydraena inapicipalpis especially These results are a preliminary approach to the au- shows a remarkable preference for low levels of pol• toecology of some species found in the basin. In or• lution, a feature already pointed out by Díaz Pazos der to reach final conclusions concerning the prefe• (1991). Hydraena brachymera has a wider range, al• rences of each species, further studies extending the though with a tendency to appear in the less polluted sampling area to differently-characterized basins are areas ; the only exception is with regard to N03", for required. which it occupies a wide gradient. However Díaz Pa• When it comes to interpreting the results obtained, zos (op. cit.) indicates that in his study area it was not firstly we have to keep in mind that altitudinal and affected by organic pollution. Elmis maugetii, Oulim• organic pollution gradients largely overlap in this ba• nius rivularis and Oulimnius troglodytes occur over a sin (Fernández Aláez et al., 1988). For this reason it wide range of chemical conditions and were found in is difficult to distinguish whether the distribution of even the most degraded points along the basin. This a certain taxon depends on the chemical characteris• same thing has been demonstrated by Rico (1992) tics of water, or on factors linked to altitude or to the with Oulimnius troglodytes, which is found, as he distance from the river source. On the other hand, we states, in areas with the highest eutrophication. Elmis must not forget that this research solely deals with maugetii, although also present in the most polluted lotie environments. Therefore, specimens peculiar to reaches, occurs over a wide range of pollution and lenitic surroundings, such as Hydraena testacea, ha• shows preference for the intermediate part of it. Such a ve only been exceptionally collected. It is very like• relative tolerance to pollution has been observed by ly that lenitic species will display noticeably diffe• Puig (1983) at the Llobregat river, by Rico (op. cit.) in rent ecological preferences. Biscay, and by Grasser (1994) in Austria. Levels of nitrate, total phosphorus, chloride, sul• As for the rest of the taxa we can point out that Hy• phate and COD were not found to be very high. draena ibérica is present in the cleanest waters, which Conquently, we cannot strictly talk of species tole• agrees once again with what is stated by Díaz Pazos (1991) ; this fact is revealed by its response to the le- (11) AQUATIC COLEÓPTERA AND CHEMICAL CHARACTERISTICS IN A RIVER BASIN 195
a) H. brachymera b) H. inapicipalpis
4 >4 Nitrate (mg N-NO3/I) Nitrate (mg N-NCgi)
c) H. ibérica d) D. brevis
o 0.2 0.4 0.6 0.8 1 1.5 4 >4 o 0.2 0.4 0.6 0.8 1 1.5 2 4 >4 Nitrate (mg N-NCgT) Nitrate (mg N-NO^I)
e) E. maugetii f) E. paralleiepipedus
0 0.2 0.4 0.6 0.8 1 1.5 2 4 >4 o 0.2 0.4 0.6 0.8 1 1.5 2 4 >4 Nitrate (mg N-NCç/l) Nitrate (mg N-NO3/I)
g) L. opacus h) O. rivularis
0 0.2 0.4 0.6 0.8 1 1.5 2 4 >4 0 0.2 0.4 0.6 0.8 1 1.5 2 4 >4 Nitrate (mg N-NCyi) Nitrate (mg N-NQjTD
i) O. troglodytes J) O. tuberculatum
Nitrate (mg N-NO¡/I) Nitrate (mg N-NCyD
Fig. 6. Corrected frequencies of N03" concentrations for the ten taxa with the highest indicator values for this parameter. Fig. 6. Fréquences corrigées des concentrations de nitrates pour les 10 taxa avec les valeurs indicatrices les plus élevées pour ce paramètre. 196 F. GARCIA CRIADO, M. FERNANDEZ ALAEZ (12)
a) H. testacea b) H. brachymera
-o 4
© 3 tO , Ü 0 0.2 0.4 0.6 0.8 1 1.5 2 >2 o 0.2 0.4 0.6 0.8 1 1.5 2 >2 Total phosphorus (¿íM Total phosphorus (/i M PCf/I)
c) H. inapicipalpis d) H. emarginata cr 6] O" lC_D CD 5 »+— Tí 4 "O _CD 3 CD O Ü CD 2 CD k_ •t . b O • o Ü 0 Ü 0 0.2 0.4 0.6 0.8 1 1.5 2 >2 0 0.2 0.4 0.6 0.8 1 1.5 2 >2 Total phosphorus QjM PCf/Q Total phosphorus {^M PO^/l)
ej H. exasperata f) HJberica d-6 cr6 £ s •o 4 S 4 ® 3 .£ 3 8 2 CD 2 O 0 0.2 0.4 0.6 0.8 1 1.5 2 >2 Ü 0 0.2 0.4 0.6 0.8 1 1.5 2 >2 Total phosphorus (pM POjVI) Total phosphorus (jjM PC#I)
g) E. maugetii h) L perrisi carinatus à6
CD 5 T5 4 CD 3 Ü CD 2
o 0 0 0.2 0.4 0.6 0.8 1 1.5 2 >2 Total phosphorus (¿*M POJfl) Total phosphorus (jjM PC#T)
i) O. rivularis /; O. troglodytes
0 0.2 0.4 0.6 0.8 1 1.5 2 >2 o 0.2 0.4 0.6 0.8 1 1.5 2 >2 Total phosphorus faM POg/ï) Total phosphorus (pM PC#l)
Fig. 7. Corrected frequencies of total P concentrations for the ten taxa with the highest indicator values for this parameter. Fig. 7. Fréquences corrigées des concentrations de phosphore total pour les 10 taxa avec les valeurs indicatrices les plus élevées pour ce paramètre. (13) AQUATIC COLEÓPTERA AND CHEMICAL CHARACTERISTICS IN A RIVER BASIN 197
vels of phosphorus and chloride. The same applies to 7). This fact could lead us to think that this taxon is re• Hydraena emarginata. On the other hand, Hydraena latively tolerant of organic pollution. Notwithstanding, exaspérala, the hydraenid species with the widest dis• the number of specimens collected was low and there tribution in the Orbigo basin, tolerates a wide range of is little available data to make final statements. the surveyed parameters. Within the Elmidae family, The analysis of the barycenter values (Figs 8, 9) al• Limnius opacus and Esolus parallelepipedus also lows us to notice clearly and briefly what we have dis• show a relatively broad gradient of these factors, with cussed so far. Esolus pygmaeus stands out, since it is no definite tendency towards either end. distinguished by its tolerance to relatively high levels The case of Esolus pygmaeus is striking. This spe• of nitrate, total phosphorus, COD, chloride and sulpha• cies, scarcely represented in the basin, only occurred in te. Next to it stands Oulimnius troglodytes. Elmis mau• three sites of its final reaches, precisely where pollu• getii, Limnius opacus and Oulimnius rivularis exhibit tion levels are highest. This can be observed in the cor• more intermediate positions, although they also show a rected frequencies for chloride and sulphate (Figs 6, certain degree of tolerance.
Fig. 8. Representation of the barycenter values for alkalinity, CI" and S04" of the 18 taxa that have proved to be indicators of any of the studied parameters. The numbers of species correspond to those in table I. Fig. 8. Représentation des valeurs de barycentre de l'alcalinité, des chlorures et des sulfates pour les 18 taxa qui peuvent être indicateurs de cha• cun des paramètres étudiés. Les numéros des espèces correspondent à ceux du tableau I. Fig. 9. Representation of the barycenter values for N03" and total P concentrations and for COD of the 18 taxa that have proved to be indica• tors of any of the studied parameters. The numbers of species correspond to those in table I. Fig. 9. Représentation des valeurs de barycentre des concentrations en nitrates et phosphore total et de la DCO pour les 18 taxa qui peuvent être indicateurs de chacun des paramètres étudiés. Les numéros des espèces correspondent à ceux du tableau I.
At the opposite end are Oulimnius tuberculatus and preference to the most degraded areas of the basin, but Hydraena inapicipalpis, both are present at sites with to intermediate ones. low values for the 6 analysed parameters. Similarly, al• Species of the family Elmidae are in general more though to a lesser extent, are Dupophilus brevis, Hy• tolerant of the slight degree of organic pollution in the draena brachymera and Hydraena ibérica. study area than those of the family Hydraenidae. This is supported mainly by their responses to factors such 5. Conclusions as chloride, sulphate and total phosphorus, there being a clear difference between both families. Among the 32 species of aquatic beetles studied, the However, further studies are required in order to dis• most demanding, with respect to water quality, seem to cern more distinctly the effect of altitude on this distri• be Hydraena inapicipalpis, Hydraena emarginata and bution. It would be particularly important to prospect Hydraena ibérica. On the other hand, Esolus pyg• more degraded rivers. Only that way, will it be pos• maeus, Oulimnius rivularis, Oulimnius troglodytes sible to define the autoecology of these species tho• and Elmis maugetii may be regarded as the most tole• roughly, and state how they respond to the pollution of rant species, although Elmis maugetii does not show watercourses. (15) AQUATIC COLEÓPTERA AND CHEMICAL CHARACTERISTICS IN A RIVER BASIN 199
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