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Seasonal variation of densityand biomass of hydracarina(acari) in a north‐patagonianreservoir (neuquén, argentina)Beatriz R. de Ferradás a b , Francisco J. Kaisin a c &Andrea S. Bosnia a da Avda. Angel Gallardo 470, Buenos Aires, 1405,Argentinab Miembro de la Carrera del Investigador del CONICET,c Becario de Iniciación del CONICET,d Investigadora del Museo Argentino de CienciasNaturales “Bernardino Rivadavia”,Published online: 19 Nov 2008.

To cite this article: Beatriz R. de Ferrads , Francisco J. Kaisin & Andrea S. Bosnia(1987) Seasonal variation of density and biomass of hydracarina (acari) in anorth‐patagonian reservoir (neuquén, argentina), Studies on Neotropical Fauna andEnvironment, 22:3, 113-127, DOI: 10.1080/01650528709360725

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Studies on Neotropical Fauna and Environment 0165-0521/87/2203-0113S3.OÒVol. 22 (1987), No. 3, pp 113-127. © Swets & Zeitlinger

Seasonal Variation of Density and Biomass ofHydracarina (Acari) in a North-Patagonian Reservoir

(Neuquén, Argentina)

Beatriz R. de FERRADÁS1, Francisco J. KAISIN2

and Andrea S. BOSNIA3

B.R. de FERRADÁS, F.J. KAISIN & A.S. BOSNIA (1987): SeasonalVariation of Density and Biomass of Hydracarina (Acari) in a North-Patagonian Reservoir (Neuquén, Argentina).Studies on Neotropical Fauna and Environment 22 (1987), pp. 113-127.

The variations in density and biomass of Hydracarina have been analyzedthroughout a year in Ezequiel Ramos Mexia reservoir (Neuquén, Ar-gentina) as a part of an integrated study on zoobenthos. Four quantitativesamples were collected monthly with a Ponar grab (400 cm2) in 1 littoraland 4 sublittoral zones from June 1983 to June 1984. Some limnochemicalcharacteristics were measured at each site. Hydracarina imagines andnymphs were identified and quantified at species level. Mean annual densityand wet biomass per zone ranged from 81 to 1936 ind/m2 and from26.0 to 201.4 mg/m2 respectively. Both parameters decreased in Novemberremaining minimum during the summer until February. The highest valueswere recorded in late February and late May reaching 6006 ind/m2 and596.3 mg/m2 respectively.Species composition and relative dominance were considerably differentamong zones, with Oxus patagonicus, Neocalonys longipalpis, Hygrobatesampliatus and Limnesia patagónica predominating.Parasitism of water mite larvae on Chironomidae imagines was recorded.The infestation incidence in collected specimens was between 2.9 and8.9%, and its most frequent intensity was between 1 and 2 larvae perhost.

B.R. de Ferradás1, F.J. Kaisin2, A.S. Bosnia3, Avda. Angel Gallardo 470.(1405) Buenos Aires, Argentina.

Introduction

The Hydracarina have been used as biological indicators due to their sensibilityto water quality (Biesiadka, 1979; Biesiadka & Kowalik, 1980; Kolkwitz &Marsson, 1909; Kowalik, 1978; Kowalik & Biesiadka, 1981; Schwoerbel, 1959,1964; Sladecek, 1973; Walter, 1922). Likewise, they have been investigated

1 Miembro de la Carrera del Investigador del CONICET2 Becario de Iniciación del CONICET3 Investigadora del Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"

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114 B.R. FERRADAS ET AL.

from the point of view of parasitism, since larvae of most water mites areectoparasites of imaginai aquatic insects such as Chironomidae (Díptera),Zygoptera (Odonata) and Trichoptera (Böttger, 1965; Hevers, 1980; Smith& Oliver, 1976). Water mites may exert a significant influence on insectpopulations when they act as both predators and parasites on the same insectpopulation (Davids et al., 1978; Ellis-Adam & Davids, 1970; Mullen, 1975;Wiles, 1982).

This study is a part of a project on the ecology and dynamics of zoobenthosin Embalse Ezequiel Ramos Mexia, a young North-Patagonian reservoir (Kaisinand Bosnia, in preparation); in this community the water mites are a wellrepresented group.

Study Area

The E. Ramos Mexia reservoir was filled in 1972 by closing the Limay River. It is situatedat 39°25'S and 68°51'W between the provinces of Neuquén and Rio Negro, at 381 m abovesea level.The reservoir belongs to the Rio Negro basin (Fig. la). The Limay River has its origin inNahuel Huapi Lake and its mean flow is 751 mVsec. The Collón-Curá River and some temporarycourses such as the Picún Leufú Creek are its main tributaries. The hydric regime of the LimayRiver and the power plant (3350 million KW/year) are responsible for water level variationin the lake which may reach 7 m amplitude between summer (maximum) and winter (minimum).The reservoir is located in an area with a windy dry climate where a bushy steppe developson a clay and sandstone soil. Its surface is about 816 km2 with a shore length of 316 km,maximum length and width are 59.5 km and 18.8 km respectively. Its volume is 20.2 km'and the depth reaches 70 m, and 47% of the surface has a depth less than 20 m (Kaisin, 1985).

Taking into account the heterogeneity of the littoral and sublittoral environments, five samplingsites were chosen (Fig. lb). Station 1 near the dam, is located in a dendritic shore with asteep slope with a dense vegetation, mainly composed by Potamogetón berteruanus and Nitellaopaca (Table 2), predominates during summer. Station 2 and 3 are situated in the NE portionof the reservoir. Station 2 is thickly carpeted, throughout the year, by Nitella hyalina accompaniedby N. clavata. Chara globularis, P. berteruanus and Ranunculus aquatilis; whereas at Station3 a marshy pteridophyte, Pilularia mandonii, predominates among other species such asMyriophylum elatinoides and Crassula aquática. Station 3 is covered by water only betweenNovember and March. Station 4 is close to the mouth of Limay River and has no vegetation.Station 5 located at the end of a long bay, has macrophytes such as N. clavata and P. berteruanuswhich are distributed in patches.

Methods

Four monthly samples were taken with a Ponar grab (400 cm2) from June 1983 to June 1984.These were sieved with a 0.25 mm mesh in the field and put into a sugar solution (1.12 density)in the laboratory (Anderson, 1959). The organisms were picked with the aid of a manualmagnifying glass (6X) and fixed in 4% formalin, blotted and weighed at 0.1 mg sensitivityand preserved with Koenike solution. The nymphs and imagines were clarified by Cloral hydrateand mounted. No larvae were found due to the mesh size and the sorting method.

From each station and along the shore, 1519 Chironomidae imagines were collected duringthe sampling period and preserved in 70% ethanol in order to register the infested specimens.

The wetweight of macrophytes was estimated every two months taking 15 samples with thePonar grab. The plants were centrifuged and weighed at 0.1 g sensitivity.

Temperature, dissolved oxygen, alcalinity, hardness, conductivity, pH, chloride, calcium andsuspended solids were measured monthly from the water above the sediment with a HACH

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HYDRACARINA IN A NORTH-PATAGONIAN RESERVOIR 115

SWn

Fig. 1. Map showing location of E. Ramos Mexia reservoir (a) and position of the samplingsites (b).

apparatus. The water and sediment samples were taken by means of a Kajak-Brinkhurst corermodified by the authors. Granulometry by hydrometry, organic matter content by combustionwith potassium dicromate and total nitrogen content by Kjeldahl method, were determined from thefive first centimeters of sediment.

The Secchi disc struck bottom often, hence the times that the bottom was exposed to lightwere registered; this is taking into account the fact that the light reaches three times Secchidisc value.

Diversity was calculated by means of the H' index (Shannon and Weaver, 1963).Mean annual density and biomass were estimated taking into account the difference in lapses

between two successive sampling dates according to Krueger and Martin (1980) formula;

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116 B.R. FERRADAS ET AL.

Table 1. Physical and chemical data for stations. Values above the dashed line refer to thewater (5 cm above the bottom) and values below it refer to the sediment (the uppermost5 cm). The data stated are average numbers for the sampling period except for thefirst four parameters. The numbers in brackets are standard errors. For transparencysee text.

Station

depth (n)

transparency (%)

temperature (°C)

dissolved oxygen (mg/1)

conductivity ^mho/cm

alcalinity (rag/1)

hardness (mg/1)

chloride (mg/1)

calcium (mg/1)

pH

suspended solids (mg/1)

clay (%)

silt (%)

sand (%)

organic matter (%)

nitrogen (%)

1

0.35-4.75

100

6-20

3-12

82.2(1.85)

28.5(0.87)

32.1(1.80)

7.4(0.69)

20.6(1.00)

7.4(0.06)

19.3(6.37)

9.5(3.6)

62.6(0.8)

27.8(4.6)

2.21(0.29)

0.17(0.02)

2

0.30-5.75

100

6-20

0-12

204.8(51.79)

31.5(2.43)

43.1(6.24)

15.0(4.04)

23.9(1.62)

7.4(0.07)

29.7(8.02)

13.2(2.6)

49.1(2.5)

37.7(1.2)

1.78(0.52)

0.17(0.04)

3

0.00-2.80

100

15-21

8-13

96.5(15.84)

25.8(2.01)

31.2(6.00)

8.8(2.83)

25.0(4.47)

7.9(0.14)

18.7(8.96)

6.9(5.2)

25.2(5.1)

67.9(10.3)

1.62(0.23)

0.12(0.02)

4

2.50-6.50

40

4-21

8-12.5

138.5(28.98)

32.9(2.17)

60.4(14.72)

7.4(0.62)

25.6(1.94)

7.6(0.08)

63.0(14.41)

18.4(0.2)

48.8(2.5)

32.8(0.4)

1.42(0.34)

0.10(0.01)

5

0.50-5.80

80

3.5-19.5

8-13

118.5(8.35)

30.8(2.03)

45.5(3.46)

8.9(0.64)

26.7(2.20)

7.6(0.04)

25.8(7.18)

9.0(4.4)

9.2(2.9)

81.8(7.3)

1.04(0.26)

0.10

k-1

(Di+i-Di) (Nj+l+Nj)/ Dk-1

i=l 2

where D i is the number of days between the first sampling day and the i th date: N i isdensity or biomass on the i th date and D k is the total sampling period in days. By thismethod mean values were between 4.1 and 15.4% lower than the results obtained by the

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HYDRACARINA IN A NORTH-PATAGONIAN RESERVOIR 117

conventional method. The representativity index was employed as R = n ij x 100 / N i wheren i is the i th species density in j th station and N i is the accumulated density of the i thspecies.

Results and Discussion

Abiotic factorsTemperature, pH, alcalinity, calcium and chloride showed neither significative temporal variationsnor differences between stations (Table 1). Dissolved oxygen, hardness, conductivity, suspendedsolids, transparency, and granulometry, organic matter and total nitrogen of sediment contributedto a differential limnochemical characterization of the sites.

Station 1 had a dark sediment with a substantial percentage of silt, organic matter and nitrogen.Dissolved oxygen was low in December and January (6 and 3 mg/1). Conductivity, as in station3, was similar to open waters (V. Conzonno, pers. com.) and hardness was a little higher than30 mg/1. The sediment of station 2 had the proportions of silt, organic matter and nitrogenlower than at station 1. In March and April, minimum values of dissolved oxygen were recorded(0 and 4 mg/1) but these values would not be constant during this time due to water mixtureby frequent and strong wind. Conductivity was, in general, twice that in the other zones andin June 1983 the maximum record for the reservoir (750iimho/cm) was reached. Electrolyteconcentration close to the bottom water was greater than that at the surface as a result ofthe chemical exchange processes between the sediments and the water body, as pointed outby Reiss (1977) for Central Amazon lakes.Hardness values comparable to those of station 5 (about 45 mg/1). Stations 3 and 5 had asandy sediment whereas station 4 had a greater percentage of silt and clay. Dissolved oxygen wasnear saturation in the three sites.

As to the transparency of water, only at station 4 and 5 the light did not reach the bottomon each measurement (40 and 80% of the times respectively) and particularly at station 4 wherethe suspended solids were high (63 mg/1).

Composition and DiversitySeventeen taxa were present but only four were found in all stations: Neocalonyxlongipalpis, Oxus patagonicus, Limnesia patagónica and Arrenurus neuquenensis(Table 3).The dominant species were O. patagonicus, L. patagónica and Hygrobatesampliatus. According to the zone, this dominance was represented by oneor another: in stations 1 and 2 O. patagonicus represented 95.6 and 74.2%of the total water mites respectively; whereas H. ampliatus reached 99.2%at station 4 and L. patagónica no more than 52% at station 5. Where thesespecies were not dominant, they however remained common. At station 2most of the taxa were present and a great number of them showed high valuesof representativity index (R. index). Station 1 was the richest in Unionicolasp. and Neumania sp., station 3 in O. patagonicus, Piona errática andLimnohalacaridae, station 4 in H. ampliatus and station 5 in Mideopsischoconensis. Distribution of some species is related to the kind and abundanceof submerged vegetation as Davids et al. (1981) pointed out in a Dutch lakewhere Neumania sp.and Piona spp. prefer vegetated zones (in this case stations1, 2 and 3, Tables 2 and 3). Hygrobates occupied bare bottom (station 4).From the point of view of representativity, stations 2, 3 and 4 were significative.Stations 1 and 5 did not contribute substantially to the Hydracarina com-position of the reservoir.

As to diversity, stations 2 and 5 had similar results with relatively high

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118 B.R. FERRADAS ET AL.

Table 2. Wet biomass of aquatic macrophytes at stations 1 to 5 in Ezequiel Ramos Mexia reservoir.

station species wet biomass(g/m^)

Potamogetón berteruanus*1 Nitella opaca* 7-804

Potamogetón striatus

Nitella hyalina*Nitella clavata

2 Potamogetón berteruanus 400-1270Ranunculus aquatilisChara globularis

Pilularia mandonii*3 Myriophylum elatinoides 346-563

Crassula aquática

4 without vegetation

Nitella clavata*Potamogetón berteruanus 28-255

•: dominant species

values of H' index in opposition to station 4 where it was minimum. Therichness of station 2 is related to the constant presence of an abundant (1270g/m2) and varied vegetation (Table 2) which favours water mite development.In such environments the species number and their density are, in general,much greater than in bare bottom communities (Biesiadka and Kowalik, 1980).This may be a consequence of a high variety of microhabitats, substratumand refuges against predators. Stations 2, 3 and 5 (in which plants were presentduring all the sampling period) showed a higher diversity than stations 1 and4 (Table 3).Station 1 presented abundant vegetation but only in summer, while station4 had no plants. The value obtained for station 5 probably results from therelative importance of the non dominant species which have a pronouncedimportance in the value of the index (Poole, 1974).

Density and biomassIn broad outline, all species decreased in density and biomass from Novemberto February reaching minimum values during the summer (Fig. 2). An absenceof water mites was registered only in December at station 1 and in Novemberat station 2. Mean values are presented in Table 4. The maximum value (1936ind/m2 and 201.4 mg/m2) occurred at station 4 where most biomass belongedto H. ampliatus due to its size and dominance. In late February and in lateMay density and biomass exceeded 6000 ind/m2 and 590 mg/m2.

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HYDRACARINA IN A NORTH-PATAGONIAN RESERVOIR 119

Table 3. Mean annual density of water mites, maximum representativity (in brackets) for eachspecies and Shannon and Weaver diversity index.

Station

0.4

Hygrobates arapliatus Viets 0.4

Oxus patagonicus Lundblad 419.7

Neocalonyx longipalpis Lundblad 5.3

Limnesia patagónica Lundblad 4.4

Arrenurusneuquenensis R.de Ferradas 3.5

Piona errática Marshall

Arrenurus valdiviensis Viets K.O.

Krendowskia convexa (Ribaga)

Arrenurus oxyurus Ribaga

Mideopsis choconensis Cook

Koenikea sp.

Flabellifrontipoda sp.

Linnohalacaridae

Hydryphantes jujuyensis Nordenskiold

Neumania sp.

Piona setipes Cook

Unionicola sp.

2.6

174.9 822.5(56.9)

769.0 147.5(82.5)

138.9(58.4)

131.2(81.6)

50.3

20.0

6.4 30.0(78.3)

1920.5(99.8)

7.7

1.9

1.9

1.9

1.9

4.4

0.4

0.4(100)

25.7(98.5)

19.3(97.5)

6.4(100)

1 .3

3.9(100)

2.6(86.6)

1.3(100)

1 .3(100)

1.8

2.5(100)

20.3

7.9

42.2

4.1

0.5

5.9(44.0)

Diversity (H')

0.4(100)

0.36 1.89 1.24 0.08 1.86

The second most important zone was station 2 with an annual mean densityof 1286 ind/m2 and an annual mean biomass of 176.7 mg/m2. Maximumvalues were recorded in June (4100 ind/m2) and May (550 mg/m2). These

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120 B.R. FERRADAS ET AL.

f l 2 7 S

J J A S O N D J F ' M ' A ' M '

J J A S O N D J F M A M J ' J ' A ' S ' O ' N ' D ' J ' F ' M ' A ' M

6 1000-

J ' J ' A ' S ' O ' N ' D ' J ' F ' M ' A ' M '

Fig. 2. Seasonal variation in density (solid line) and wet biomass (dotted line) of water mitesat stations 1-5 in E. Ramos Mexia reservoir.

results cannot be attributed to any particular species because of the highheterogeneity; nevertheless, since N.longipalpis is big, it contributed substan-tially to the water mite biomass.

Although station 3 was submerged only for 5 months, its mean values wererelatively high (1075 ind/m2 and 109.8 mg/m2) as a consequence of a fastcolonization. A similar pattern was observed in other benthic groups andmacrophytes (Kaisin, 1985; N. Gabellone, pers. com.). Maximum valuesoccurred in late January (2985 ind/m2 and 247 mg/m2) corresponding to themaximum density of the dominant species O. patagonicus (2538 ind/m2). Station

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HYDRACARINA IN A NORTH-PATAGONIAN RESERVOIR 121

Table 4. Mean annual density (ind/m2) and wet biomass (mg/m2) of Acari and some potentialhosts. In station 3 mean values were calculated for 5 months.

stations

Acariind/m2

mg/m2

Chironomidaeind/m2

mg/m2Chironominae (%)Orthocladinae (%)Tanypodinae (%)Diamesinae (%)

Odonataind/m2

mg/m2

Trichopteraind/m2

mg/m2

1

43931.6

844213634

81 .57.5

11.00

12830

1255

2

1286176,7

3127110644.339.116.50.1

592132

84601

3

1075109.8

117053870.422.86.80

0.52.5

1347

4

1936201.4

3889181415.90.283.90

00

11

5

8126.0

2387195558.65.535.90

0.35

1679

1 showed lower mean values than those mentioned above (439 ind/m2 and31.6 mg/m2). Despite its smaller size, 0. patagonicus may be considered asthe most influential species in biomass values due to its high density. Maximumvalues were observed on the same dates as they were in station 2; they didnot exceed 1700 ind/m2 and 130 mg/m2.

Finally, station 5 had the lowest mean values reaching only 81 ind/m2 and26.0 mg/m2. The most abundant species L. patagónica, and its big size suggeststhat it had a relevant contribution to biomass. The large hydracarina densitiesthat appear here are among the highest records obtained. Ezcurra de Drago(1966) and Di Persia et al. (1982) found a maximum of 133 ind/m2 in lenticand lotie environments of Paraná Medio; in Embalse Río Tercero the valuesdid not surpass 270 ind/m2 (Stahl and Kaisin, in preparation). On the otherhand, Reiss (1973, 1977) found mean values between 45 and 178 ind/m2 ina North-Brasilian pond and in a small lake from Amazonas whereas Gliwicsand Biesiadka (1975) recorded a maximum value of 2000 ind/m2 of Pionalimnetica in the pelagic zone of a Panamanian reservoir.

Relationships with insectsThe available bibliography indicates that most of the genera present in thisstudy are largely related, from the point of view of ectoparasitism, toChironomidae (Hevers, 1980; Kouwets & Davids, 1984; Oliver & Smith, 1980;Smith & Oliver, 1976). Some, such as Hygrobates and Piona can also affectTrichoptera (Smith & Oliver, 1976) but the density of Trichoptera was notimportant enough in station 4 (Table 4) to explain such a high number ofH. ampliatus. On the other hand, Arrenurus is a parasite of Odonata (Bouger,

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122 B.R. FERRADAS ET AL.

1965; Mitchell, 1959; Münchberg, 1960; Prassad and Cook, 1972; Smith 1978)and of Trichoptera (Smith & Oliver, 1976). These groups were particularlyabundant at station 2 (Table 4) and the highest representativity index ofArrenurus was found there (Table 3). The importance of Chironomidae canbe verified in Table 4, in terms of density and biomass.

Specific relationships between hosts and parasites were not established sincelarvae descriptions of Neotropical Hydracarina are unknown at present andmost of Chironomidae species from this region are new and unpublished (A.Paggi, pers. com). The genera and families found coincide with some of thosewhich Smith & Oliver (1976) pointed out for North America, except forHydryphantes, Neocalonyx and Flabellifrontipoda. Limnesia, Mideopsis andPionidae are associated mainly with members of subfamily Chironominae.Oxidae and Hygrobatidae are associated to the Orthocladinae while Kren-dowskia and Arrenurus (if it infests Chironomidae) are connected exclusivelywith Tanypodinae.

Variations in total and nymphal density of the principal species can beobserved in Fig. 3. O. patagonicus and H. ampliatus showed a majority ofimagines during most of the sampling period while the nymphs predominatedin the N. longipalpis population.Nevertheless, the three species decreased in density in November droppingnearly to zero values; in February the population started to increase and agreat number of nymphs were observed, particularly in the H. ampliatuspopulation.

Fig. 4 exhibits the mean abundance of Chironomidae pupae recorded fromall stations. According to this picture, the emergence period was in springand summer even though some species emerged in winter but with smallerintensity (Kaisin, 1985). During this same time water mite eggs attached tomacrophytes (mainly Nitella hyalina and N. opaca) and larvae infestingChironomidae imagines were registered (Fig. 4). This indicates that larvaeaffected these insects only in spring and summer. The infestation incidencevaried between 2.9 and 8.9%. These records were lower than those obtainedin March and April 1983 (18.0 and 16.7%), which were similar to the valuesobtained by Kouwets & Davids (1984). The real infestation was sometimesunderestimated because of larvae detachment during preservation.

A single species may affect several hosts, for example Hydrachna portigerais a parasite of three species of Sigara (Corixidae) in an argentinian pond(R. de Ferradas, unpublished data).It is possible, also, that several species exploit the same host, but are locatedon different parts of the body or at different times throughout the year(Lanciana, 1970). In this study, the infestation intensity was between 1 and2 parasites stuck to the articular membrane between the head and thoraxor to the abdomen, but on rare occasions 13 larvae per host were found(Fig. 5). Pieczynski (1976) proposed that the availability of hosts during theparasitic phase is the limiting factor of water mite populations, and in thisreservoir their abundance is very important (Table 4). It was observed thatother factors such as habitat differentiation, particularly related to vegetation(Davids et al., 1981; Viets, 1924), available food or prédation by other aquaticinsects (Eriksson et al., 1980) can also influence the density of water mites.

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HYDRACARINA IN A NORTH-PATAGONIAN RESERVOIR 123

2600-

? 1950-

1300-

650-

Oxus patagonicus— Station 1- -S ta t i on 3

J ' J A S ' O N ' D J F M ' A M1981 1981

2400-

NeoCQlonyx longipalpisStat ion 2

J J A S O N D J F M A M

6000

J J A S O N D J F M A M

Fig. 3. Seasonal variations in density of main Hydracarina species at 4 stations in E. RamosMexia reservoir. Lines refer to total individuals, bars refer to nymphs.

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124 B.R. FERRADAS ET AL.

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Fig. 4. Mean density of pupae (dashed line) and infestation incidence (solid line) of Chironomidaeimagines in E. Ramos Mexia reservoir.

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Fig. 5. Infestation intensity of Chironomidae imagines in E. Ramos Mexia reservoir.

Mazzucchelli (in preparation) found Rollandia rolland's guts to be full ofHydracarina indicating that grebes can also be regulators of mite populations.

Another aspect under discussion is the damage to their hosts that parasitescan produce. Lower fertility, higher mortality and impediment to carry outactivities were only detected in Corixidae (Davids, 1973); but, the same maybe infered for other groups. This is easy to assume if one takes notice ofthe big size and numerosity that water mite larvae can reach (Lanciani, 1971;R. de Ferradas, unpublished data). Furthermore, some insects possess defencemechanisms against the mite infestation as Davids (1973) showed withwaterbugs or as Kouwets and Davids (1984) suggested with chironomids.

Final considerations

Specific composition, density, biomass and diversity of water mites differ fromone zone to the other. Those differences are related to the limnochemicalcharacteristics and the kind and abundance of vegetation at each site. Some

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environmental factors like sediment quality (which may have an importantinfluence on ionic concentrations and oxygen availability of the adjacentwater) influence some species distributions. However, the vegetation playsa principal role in structuring the water mite community studied. Thus, inlocalities with abundant, permanent and diverse macrophytes, the highest watermite diversity and representativity are found. While zones with temporaryvegetation or without vegetation snowed the lowest values. On the other hand,density and biomass are not affected by the presence of vegetation. In thoselittoral environments which are exposed during part of the year water mitesand other benthic groups rapidly colonize available habitats as they areproduced by rising water levels. Parasitism is an important factor in thedynamics and dispersion of water mites since density and biomass decreasecoincide with the emergence and infestation period of hosts (mainly Chiro-nomidae). Availability of hosts could not be demonstrated as a limiting factorin these populations; although it is probably important in the structure ofthe water mite community.

Acknowledgments

We are grateful to Dr. J.A. Schnack and Mrs. L. Stahl for their review of this manuscript;D. Arbet, E. González, J. Sanjurjo, S. Trubiano and A. Vega for their assistance in field andlaboratory work and Hidronor S.A. for its financial support. We give special thanks to Lie.A. Tablado for allowing us to use his personal computer and also for his valuable suggestions.

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