Crustacean zooplankton species richness and ... - Springer Link

Hydrobiologia 325 : 163-172, 1996. © 1996 Kluwer Academic Publishers . Printed in Belgium.

163

Crustacean zooplankton species richness and associations in reservoirs and ponds of Aguascalientes State, Mexico Stanley I . Dodson' & Marcelo Silva-Briano2 'Department of Zoology - Birge Hall, University of Wisconsin, 430 Lincoln Drive, Madison, WI 53760, USA 2 Universidad Autonoma de Aguascalientes, Centro Basico, Av . Universidad SIN, Aguascalientes, AGS, Mexico Recieved 22 March 1995 ; in revised form 6 November 1995 ; accepted 29 November 1995

Key words : Aguascalientes, area, associations, crustacean zooplankton, Mexico, species richness, tropics

Abstract We report the species of crustacean zooplankton found in 19 resevoirs and ponds that were sampled at on at least two occasions in Aguascalientes State, Mexico, at a latitude of about 21°30' North Latitude . We collected a total of 33 cladoceran taxa, 15 calanoid and cyclopoid copepods, an Ergasilus copepod, 2 anostracans, one notostracan species and an amphipod . All these taxa had previously been reported in Mexico . As in more temperate areas, there was a significant correlation between the species richness and lake size . The species-area relationship for Aguascalientes was indistinguishable from that of more northerly lakes . The two most common associations of zooplankton species were (1) those small species that occurred with fish : Daphnia parvula, Diaphanosoma birgei, Leptodiaptomus siciloides, Mastigodiaptomus albuquerquensis, and Thermocyclops inversus, and (2) the large Daphnia schodleri that occurred with the predator Notonecta, in the absence of fish . With minor variations, these two associations also occur throughout temperate North America .

Introduction There is a long history of freshwater zooplankton studies in Mexico . Earlier studies highlighted either the aquaculture and other limnological practices of the indigenous cultures (Deevey, 1994, 1957 ; Turner, 1980) or the limnology of the larger lakes, such as Patzcuaro (U6 no, 1939 ; Ancona et al ., 1940 ; Rioja, 1940 ; Brehm, 1943 ; Deevey, 1957) . In addition, there have been several surveys of regional faunas in Mexico . Juday (1915) reported on Cladocera from littoral samples of water in or near Mexico City . Copepods were reported from Yucatan by Wilson (1936) and from the Mexico City area by Brehm (1955). Lindberg (1954) analyzed samples of zooplankton for cyclopoid copepods from collections taken by fisheries biologists in various parts of Mexico . Van de Velde et al . (1978) provided a list of 25 littoral and pelagic cladoceran species from southern-most Mexico . ArredondoFiqueroa et al . (1983) initiated a series of studies on the `maar' lakes of Puebla . Frey (1982) provided a species list and an annotated bibliography of the Cladocera of

Mexico, and remarked that there are serious problems with the taxonomy of the earlier studies . VillalobosHiriart et al . (1993) also listed the known branchiopods of Mexico and commented that poor information exists for copepods . Study of freshwater zooplankton of Mexico continues, with emphasis on species diversity in different regions (e .g., Martinez-Martinez, 1981 ; ArredondoFiqueroa et al ., 1983 ; Reid, 1988, 1994), speciation (Torrentera-Blanco & Dodson, 1995), and aquaculture (e .g ., Rico-Martinez & Dodson, 1992) . Recent limnological studies tend to focus on the stresses the human population is putting on lakes, reservoirs and ponds in Mexico (e .g ., VAsquez et al ., 1986 ; and in Lake Chapala : De Buen, 1943 ; Castellanos & Sanchez, 1974 ; DAvalos et al ., 1989). New species and subspecies of freshwater zooplankton continue to be described (e .g ., Kraus, 1986 ; Silva-Briano & Segers, 1992 ; Spicer, 1985) . The air traveler, taking an early morning flight across Mexico, sees the sun glinting off myriads of small artificial ponds ('bordos') of about 50-500 m 2

1 64 surface area . It is apparent that the vast majority of freshwater bodies in Mexico are bordos . Although abundant, small ponds lack the appeal of larger lakes and reservoirs and tend to be in general under-studied (e .g ., Sublette & Sublette, 1967 ; Loring et al ., 1988) . Perhaps the most extensive study of small, often temporary ponds of Mexico was done by Comita (1951) . The present study provides information about the zooplankton fauna of small permanent and temporary ponds of Aguascalinetes, a state in central Mexico . This study focuses on crustacean taxa at the species level in reservoirs and ponds in the state of Aguascalientes, Mexico . Occasionally, difficult or indistinct species are lumped into the category 'sp .' or 'spp .', meaning there is one or more species in the genus, but we aren't certain of the species-level identification . The purpose of the study was to create a species list for the state, to test a biodiversity hypothesis by comparing the species-area relationship in a low-latitude region to the curve for temperate North American lakes and ponds, and to document crustacean zooplankton species assocations characteristic of these reservoirs and ponds . The species list is important because there are few systematic surveys of zooplankton in Mexican lakes and ponds . Mexico sits between a well-known fauna in the upper part of the North American continent, and the less-well-known but quite distinct fauna of South America (Reid, 1994) . Surveys such as the present study can help resolve the question of influence of the two major faunas on the limnology of Mexico . Limnological lore suggests that tropical zooplankton communities are less diverse than those of temperate communities (Lehman, 1988) . Recent data does not altogether support this hypothesis . For example, Reid (1994) found no support for the hypothesis of greater diversity of north temperate benthic copepods compared to neo-tropical benthic copepods . Dumont (1994) refutes the concept of a less diverse tropical fauna of cladoceran species . Zooplankton species richness in temperate lakes depends on lake size, productivity and the nearness of adjacent lakes (Dodson, 1992) . Soto & Zuniga (1991) also report that zooplankton assemblages of Chilean temperate lakes are correlated with lake size and distance between lakes . When these factors are taken into account, do tropical lakes have lower diversity than temperate lakes? In this paper, we look at the effect of pond or reservoir surface area on species richness to compare planktonic crustacean communities of Aguascalientes State (latitude ca . 22 ° N) with similar com-

munities in the middle of the temperate zone in Europe and North America. Zooplankton in temperate zone lakes often show a pattern of species associations, in relation to the intensity of fish predation . Small species tend to occur either in lakes with intense fish predation and large species occur in lakes with little or no fish predation (Dodson, 1979) . We hypothesized that the same would be true in Aguascalientes .

Materials and methods Zooplankton were sampled over a period of several years in reservoirs and ponds in Aguascalientes State (Table 1) . Aguascalientes State is geographically tropical (it lies just below the Tropic of Cancer) but climatic data suggests it has a temperate climate (SEP, 1982) . The ponds never freeze, but they are significantly colder in the winter than in the summer. Average annual air temperatures are between 18 and 20 °C, with an annual range of variation of less than 10 °C . June is the warmest month ; January is the coolest . The rainy season is in late Spring . The ponds and reservoirs are at relatively high elevations (roughly 1700-2100 m) which are typically associated with a cooler climate than would be experienced at sea level, regardless of latitude . Nineteen reservoirs and ponds were sampled on at least two different dates (Table 1) . Logistical problems prevented a more extensive sampling program . We sampled a range of reservoir and pond types, with surface areas ranging between 12 m 2 and 1 .19 x 107 m2 (Table 2) . The reservoirs ('presas' : Tables 1 and 2) are permanent impoundments behind cement dams, with some control of water level . Smaller bodies of water were either temporary or permanent . These ponds include 'bordos' (artificial ponds behind an earthen dam) and 'charcos' (ponds in natural depressions) . Charcos are also called `playas' (e .g ., Sublette & Sublette, 1967) . We were able to collect zooplankton on at least two dates from 5 reservoirs (presas ; four permanent, one, EL Llavero, that is annually drawn down to dryness), 10 artificial ponds (bordos) and 4 temporary ponds 2 (charcos) . Location and morphometrics of these water bodies are listed in Tables 1 and 2 . In addition to the 19 study ponds, ten reservoirs and ponds were sampled only once, in order to increase the chance of finding rare species . Zooplankton were collected using horizontal tows of a fine-mesh plankton net . The zooplankton were

165 Table 1 . Locations and dates for reservoirs and ponds sampled at least twice in Aguascalientes, Mexico

1. 2. 3. 4 5.

6. 7. 8. 9.

102° W Longitude

Latitude N

Dates sampled

PRESAS Presidente Calles

27'

Abelardo Rodriguez Malpaso Calvillo El Llavero (seasonally dry) El Taray

39' 39' 06' 29'40"

22°08' 21 0 51' 20"

8 Oct 80, Martinez (1981), 2 Sep 91 9 Apr 89, 7 Jun 89, 15 Jan 91 23 Jan 89, 15 Jan 91 3 Sep 88, 13 Nov 90

BORDOS Granja El Carinan ElSauz El Tunel de Potrerillos Cerro El Colorin

10 . 11 . 12.

Los Hoyos Carretera Calvillo Carretera Loreto

13 . 14. 15 .

Carretera Zacatecas Tepezala Los Arquitos

21'51'20" 22'06' 21 0 44'10"

22'30" 37'30"

21°00'53" 21 ° 52'30"

25'00" 29'00" 10'21"

22 ° 19' 21°00'58" 21 ° 44'20" 21°53'26"

23' 22" 07'45" 17'43" 11'45"

22°01'45" 21 0 55' 22 0 11'30"

22'50"

21 ° 55'20"

01'20"

22° 12'50"

10'21" 27'24" 27'24"

21 0 44'20" 21°53'26"

7 Jun 89, . 28 Aug 90

17 Jan 89 (limnetic, littoral), 7 Oct 89 22 Jun 89 (limnetic, littoral), 30 Sep 89 4 Aug 90 (limnetic, littoral), 14 Nov 92 10 Jun 89 (limnetic, littoral), 23 June 89 1 Jul 89, 9 Dec 89 19 Jul 88, 16 Sep 89 3 11 1 7

Sep 88, 12 Dec 89 Aug 87, 26 Aug 93 Sep 89, 2 Sep 89 Jul 88, 26 Jul 90

15 1 12 12

Jul Jul Mar Mar

CHARCOS 16. 17 . 18 . 19.

Rail Road, San Gil. Asientos Los Hoyos Carretera Calvillo Carretera Calvillo (Pequeflo)

preserved with formalin, and transferred to alcohol for identification . Species determinations were made after appropriate dissection . Specimens were mounted in Hoyer's medium and examined with phase contrast optical microscopy . These specimens are currently possessed by the authors, but will eventually be deposited in the Zoological Museum of the Zoology Department, University of Wisconsin . Species determinations were carried out using the keys in Edmondson (1959), supplemented with the following literature: Bosminidae : Deevey & Deevy (1971), Kof nek (1971), DeMelo & Hebert (1994) ; Ceriodaphnia : Berner (1986,1987) ; Chydoridae (genera) : Dodson & Frey (1991) ; Daphnia : Kraus (1986), Brandlova et al. (1972), Schwartz et al. (1985), Schwartz (1991) ; Diaphanosoma : Korinek (1981) ; Moina : Goulden (1968) ; Scapholeberinae : Dumont & Pensaert (1983) ; Cyclopoida: Dussart & Fernando (1990), Reid (1991, 1992), Reid & Reed (1994), Dodson (1994) ; Ergasilus: Galaviz-Silva et al. (1990) ; Anostraca, Notostraca: Belk (1975), Maeda-martinez (1991) .

21°53'26"

89, 25 Aug 90 89, 9 Dec 89 88, 7 Oct 89 88, 12 Aug 89, 7 Oct 89

The species-area relationship for pelagic crustacean zooplankton of Aguascalientes ponds and reservoirs was created by estimating the surface area from maps or from measurements of the smaller ponds . Pelagic crustacean zooplankton were counted using the criteria given in Dodson (1992) . The Aguascalientes species-area relationship was then compared to that of 71 northern hemisphere ponds, lakes and reservoirs between 28° and 64° north latitude (from the data set used in Dodson, 1992) . The bodies of water were chosen to cover the same range of surface areas found for the Aguascalientes sites : the temperate sites range from 12 m 2 to 1 .28 x 107 m 2 (further information on these ponds and lakes is available from Dodson upon request) . We chose the latitudinal range between 28° and 64° north latitude to allow for a test of the hypothesis that temperate ponds and lakes are more diverse than low-latitude ponds and lakes . The slopes of the two lines were tested for significant difference using a t-test. Pair-wise associations among the eleven most common microcrustaceans, Hyalella (an amphipod), Notonecta (an hemipterous insect) and fish were tested



1 66 Table 2 . Morphometric data and zooplankton species richness for reservoirs and ponds sampled at least twice in Aguascalientes, Mexico Area

1. 2.

PRESAS Presidente Calles Abelardo Rodriguez

3. 4. 5.

Malpaso Calvillo El Llavero El Taray

6. 7. 8.

BORDOS Granja El Carinan El Sauz El Tunel de Potrerillos

9. 10. 11 .

Cerro El Colorin Los Hoyos Carretera Calvillo

12. 13 . 14.

Carretera Loret Carretera Zacatecas Tepezala Los Arquitos

15 .

16. 17 . 18 . 19 .

CHARCOS Rail Road, San Gil . Asientos Los Hoyos Carretera Calvillo Carretera Calvlllo (pequeno)

Maximum depth (m)

Elevation (m)

Permanent

(m2 )

11,900,000 3,720,000

66 .4 18 .5

2000 1940

707,856 462,000 19,200

3 .0 7 .0 8 .0

1735 1950 1850

+ + +

6,000 3,000 1,500 600

3 .5 3 .0 2 .0 1 .5

1850 1835 1500 2080

+

1 .0 1 .0 0 .45

1855 1910 1940

+ + -

56 50 18

0 .2 0 .5 0 .6

1987 2010 1900

-

375 35 35 12

3 .0 0 .4 0 .4 0 .8

2010

+ + -

600 150 120

for significance using the 2 x 2 Contingency Table method (Grieg-Smith, 1983), with the Model I GStatistic used as a test of significance (Sokal & Rohlf, 1981) . Taxa were treated as present or absent, and the number of co-occurrences for each pair of taxa provided a measure of the tendency for positive or negative association . Only taxa present between 5 and 11 of the 19 ponds were analyzed (the test does not give useful information for rare or nearly ubiquitous species) . Because of the small number of ponds, this test of association does not identify subtle associations .

Results Table 2 lists the number of planktonic crustacean taxa, mostly at the species level, found at each site . The occurrence of each taxon is listed in Table 3 . We recorded a total of 33 cladoceran taxa, 15 calanoid and cyclopoid copepods, 1 Ergasilus copepod, 2 anostracans, 1 notostracan and an amphipod in the 19 reser-

1855 1960 1960

Pelagic zooplankton species

+

+ + ?

14 14 8 10 10

10 5 8 5 11 5 7 4 4 4

8 7 7 5

voirs and ponds . The 10 samples from additional reservoirs and ponds (sampled only once) revealed three additional species . Two conchostracan species (Caenestheriella setosa and Leptestheria compleximanus) were found in the drawn-down reservoir (Presa San Bartolo, 4 July 1989) . A species of the ilyocryptid genus Ilyocryptus was found in a small pond near the town of Calvillo . Twenty cladoceran and ten copeopd taxa were pelagic (Tables 2 and 3) . For all taxa except the genus Daphnia, we found fewer species in Aguascalientes state than have been reported for all of Mexico (Table 4) . The number of pelagic zooplankton observed in a lake was significantly correlated with the size of the lake (when both variables are log-transformed : Table 5) . The correlation coefficient was 0.73 (onetailed Pearson product-moment correlation p < 0 .01) . The slope of the line for the Aguascalientes water bodies (Figure 1, Table 5) was less than that of the temperate lakes, but not significantly so (two tailed t-test,



167 Table 3 . Crustacean zooplankton, meiofauna, and various predators from the 19 reservoirs and ponds . Species counted as zooplankton are

marked with a 'P' in the first column after the scientific name Lake Number (see Table 1) CLADOCERA Alona spp . Bosmina longirostris Ceriodaphnia cf. dubia Ceriodaphnia lacustris Ceriodaphnia reticulate Ceriodaphnia rigaudi Chydorus spp . Daphnia ambigua Daphnia dubia Daphnia galeata mendotae Daphnia laevis Daphnia parvula Dapnia pulex Daphnia pulicaria Daphnia schodleri Daphnia similis Diaphanosoma birgei Disparalona cf. hamata Dunhevedia crassa Kurzia cf. latissima Macrothrix sp. cf. laticornis Macrothrix mexicana t Moina micrura Neobosmina hagmanni Neobosmina tubicen Pleuroxus sp. Pseudochydorus globosus

1 2 3 4 5 6 7 8 9

* P P P

10

11

12

13$

14 15 16

albuquerquensis Mastigodiaptomus montezume Leptodiaptomus siciloides Leptodiaptomus cf. signicauda COPEPODA (CYCLOPOIDA) Acanthocyclops robustus Acanthocyclops exilis Ectocylops phaleratus Eucyclops speratus

18

19

*

* * * *

*

P P P P P

2 4 1 6

* * *

2 2 3 3 5

P P * P * * P P P P P

P P P

* *

* *

N

6 3 3

* *

1 2 8

*

2 7 2 2

* * *

* * *

3 1 1 15

*

2 1 5 1

Scapholeberis armata freyi Scapholeberis microcephala Simocephalus expinosus Simocephalus vetulus COPEPODA (CALANOIDA) Mastigodiaptomus

17

*

3 1 1 8

P P

8 *

15

P P

*

11

*

1

P P *

*

17

P

* *

1 1 7 Continued on p. 168

1 68 Table 3 . Continued Lake Number (see Table 1)

1 2 3 4 5 6 7 8 9

10

11

12 13

14

15

16

17

18

*

Macrocyclops albidus

19

N 4

Thermocyclops inversus

P

8

Thermocyclops tenuis

P

2

Mesocyclops edax

P

1

Microcyclops varicans

4

Paracyclops fimbriatus Tropocyclops extensus

1 P

3

COPEPODA Ergasilus sp.

I

ANOSTRACA Streptocephalus mackini

3

Thamnocephalus platyurus

1

NOTOSTRACA Triops longicaudatus

1

AMPHIPODA Hyalella sp .

5

PELAGIC PREDATORS Chaoborus sp .

*

4

Mosquitos

*

2

Notonecta sp.

6

Plea sp .

*

1

Fish species

*

9

t This species of Macrothrix is being described by J. Ciros, M . Silva Briano, and M . Elias Gutierrez. $ Ephemeroporus added to # 13 in proof.

p > 0 .05) . In fact, the data points for Aguascalientes lie among those from temperatue North America (Figure 1) . The pair-wise analysis of association of the common species (Table 6) shows there are two distinct sets of species that tend to be associated and that are mutually exclusive . The first group is composed of Daphnia parvula, Diaphanosoma birgei, Leptodiaptomus siciloides, Mastigodiaptomus albuquerquensis, Thermocyclops inversus, and fish . This group occurred in all the permanent reservoirs (presas) . The second group is characterized by Daphnia schodleri which is negatively associated with species of the first group . Daphnia schodleri was significantly positively associated with Notonecta . The D. schodleri - Notonecta association tended to occur in smaller ponds . In addition to the two sets of species, there were a number of rare species and three nearly ubiquitous species : Moina micrura, Mastigodiaptomus montezumae, and Acanthocyclops robustus . These species did

not show associations, but because of the small sample size (19 reservoirs and ponds) it is unclear whether the lack of evidence for association is a statistical artifact or of biological significance .

Discussion Species numbers and identification This is the first ever published list of zooplankton taxa of Aguascalientes State . The cladoceran list of Aguascalientes is quite similar to, although less rich than, the list reported by Frey (1982) for all of Mexico (Table 4) . The larger diversity in all of Mexico is probably due to the greater range of habitats and the larger area of the studies reported by Frey, compared to the range of habitats and the smaller area of Aguascalientes State . Bosminopsis deitersi and Moinodaphnia macleayi, two common cladoceran inhabitants of ponds in Mexi-



1 69 Table 4 . For Mexico, number of cladoceran taxa (usually species) reported by Frey (1982) and Villalobos-Hiriart (1993) ; calanoid and cyclopoid copepod species reported by Zamudio-Vald6z (1991) ; number of ergasilids reported by Galaviz-Silva et al. (1990), and number of taxa (at the same level) found in our samples of Aguascalientas State . Frey and we report 'sp.' for some genera to indicate an additional unidentified species which may be either the same as one of the reported species, or a new, un-named species . Subfamilies are listed for the family Cyclopidae Family Genus

Sididae Daphnidae Daphnia Ceriodaphnia Scapholeberis Simocephalus Moinidae Moina Moinodaphnia Bosminidae Bosmina Bosminopsis Ilyocryptidae Ilyocryptus Macrothricidae Macrothrix Chydoridae Diaptomidae Cyclopinae Eucyclopinae Ergasilidae

No . of species reported for Mexico

No . of species reported for Aguascalientes

1 (and 1 'sp .')

1

6 (and 1 'sp .') 5 (and 1 'sp .') 2 3

9

3 1

1 0

3 (and 1 'sp .') 1

3 0

1

1

2

2

15 11 14 12

7 4 6 5 1

1

4 2 2 0

I

i

i

1

2

3

I

4

5

I

6

7

8

log (area m2) Figure 1 . The species-area relationships as linear cor elations for a

set of temperate North American ponds and lakes (solid line and triangles) and the Aguascalientes ponds and lakes (dotted line and circles) .

calientes collections reported here add no new species to the list . We sampled too few temporary ponds to make a significant comparison between our list of the larger Branchiopods (listed in Table 3) and lists of other studies such as Maeda-Martinez, 1991 . Species-area relationship

co and the southwestern US (Edmondson, 1959 ; Dodson & Frey, 1991), were not found in Aguascalientes . These species are found in warm tropical or subtropical habitats . Their absence from Aquascalientes ponds is probably due to the temperate-like climate . For the same reason, there are more Daphnia species (9 in Aguascalinetes) that is typical of such a small region with a tropical climate (Dumont, 1994) . Reid (199) and Zamudio-Valdez (1991) review surveys of calanoid and copepods of Mexico . ZamudioValdez (1991) listed 11 calanoid species and 28 cyclopoid species or subspecies that have been reported by studies of free-living copepods in Mexico (Table 4) . He does not mention the common Leptodiaptomus siciloides, which has been collected in Mexico (Martinez-Martinez, 1981 ; Reid, 1990) . The Aguas-

We found no indication that there are fewer species in the low-latitude Aguascalientes sites compared to a higher-latitude sites in North America. It is true that our sites had relatively few species compared to temperate lakes in general, but when size is taken into account (most of the Aguascalientes sites were small), then there is no indication of a difference (Figure 1, Table 4) . This result is consistent with the conclusions of Reid (1994) and Dumont (1994) that tropical freshwater zooplankton faunas are as diverse as temperate faunas . Species associations Size selective predation is a major factor that determines the composition of zooplankton species asso-



1 70 Table 5 . Results of the linear regression of log number of pelagic crustacean species (Y) vs . log area (m 2 ) of the pond or lake . Coefficients are given with the standard error in parentheses . The temperate bodies of water are from Europe and North America . See text for further details Source of ponds X coefficient (SE) or lakes

Constant (SE)

r2

Number of ponds or lakes

Temperate Aguascalientes

0.4914 (0 .2061) 0.6178 (0 .1176)

0 .33 0 .58

71 19

0 .0938 (0.0161) 0 .0727 (0.0149)

Table 6. Associations of aquatic organisms that occurred in at least 5 of the ponds and reservoirs . For positive association,

p < 0 .05 ;

`++' = p < 0 .01 ; for negative association, `-' = p < 0 .05 ; `- -' = p < 0.01 Alona Alona sp. Chydorus sp. Daphnia parvula

Chyd.

D.par.

D .sch .

Diaph .

-

++

Daphnia schodleri Diaphanosoma birgei Pleuroxus sp. Simocephalus vetulus Mastigodiaptomus albuquerquensis

Pleu .

Simo .

M.alb .

L .sic .

++

++

E.ag.

T.inv.

Hya.

Not .

++

fish

++ +

+

+

++

Leptodiaptomus siciloides Eucyclops speratus Thermocyclops inversus Hyalella sp . Notonecta sp. Fish

ciations (Brooks & Dodson, 1965 ; Dodson, 1979) . Plankton-eating fish tend to remove the larger species (Brooks & Dodson, 1965) . Invertebrate predators tend to remove the smaller species (Dodson & Havel, 1988) . The two main Aguascalientes assemblages (Table 6) fit the pattern found elsewhere . Several of the Aguascalientes zooplankton species that occur with fish (Daphnia parvula, Diaphanosoma sp ., and Leptodiaptomus siciloides) are also reported in North American lakes with fish. The temperate lakes span a wide range of surface area and latitude, including Lake Superior (8 .2 x 1010 m 2 surface area at 47 .5° N latitude ; Balcer et al ., 1984), Lake Winnipeg (2 .4 x 10 10 m2 at 52° N latitude ; Patalas, 1981), Lake Texoma (3 .8 x 105 m2 at 33 .9° N latitude, Orcutt & Pace, 1984) . The association of a large daphniid (Daphnia schodleri) and a notonectid predator (such as Notonec-

+

to or Buenoa) is common in pounds in temperate North

America, having been reported from Wisconsin ponds (Dodson & Havel, 1988), in Texas and Oklahoma ponds (Schwartz, 1991), and in Utah stock ponds (Dodson, 1987) . Thus, the species associations of Aguascalientes are similar to associations seen in higher-latitude ponds and reservoirs . Their similarity to associations further north probably reflects the temperature climate of the state .

Acknowledgements We thank especially Dr Janet Reid for her careful reading of an earlier draft and for suggesting additional literature . Also thanks to an anonymous reviewer .

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