Comparative study of the zooplankton composition of two lacustrine ecosystems in Southwestern Amazonia

Comparative study of the zooplankton composition of two lacustrine ecosystems in Southwestern Amazonia. The zooplankton communities of two lacustrine ecosystems in southwestern Amazonia (Lago Amapá and Lago Pirapora) were studied based on samples collected over an 11-month period. The general aim of the present work was to contribute to the knowledge of the zooplankton fauna in southwestern Amazonia, by studying the occurrence of certain species; and to improve the knowledge of the Rio Acre tributaries. The total number of taxa included 38 species of rotifers, 6 cladocerans and 7 copepods. Most of the species were from the rotifer family Brachionidae. Jaccard ́s similarity index was similar for the two lakes at 0.6964 The constancy index defined the species Keratella cochlearis, Filinia cf. terminalis, Filinia opoliensis, Hexarthra intermedia braziliensis, Ceriodaphnia cornuta, Moina minuta, Diaphanosoma spinulosum, and immature forms (nauplii and copepodites) as the constant in these lakes. The presence of zooplankton with higher number of species occupying the middle depths, during morning and night periods.


Introduction
River-floodplain systems show marked temporal variation in physical, chemical, and biotic factors, associated with variations in the flood and dry phases, influenced by the hydrological regime of the principal river.According to Junk et al. (1989), most large or medium-sized rivers have adjacent areas subject to flooding and which, together with the main stream, constitute the river-floodplain systems.
Almost all Amazonian waterbodies and wetlands are subject to varying degrees of water-level fluctuation.Fluctuations are widest in the main rivers and their adjacent floodplains.These floods are regular annual events, resulting in a well defined high-water and low-water period each year (Junk and Howard-Williams, 1984).
Aquatic communities are influenced by these flood pulses.The zooplankton community has been investigated, mainly Cladocera, Copepoda and Rotifera, in order to better understand how these temporal changes influence the composition of these communities.
In Amazonian bodies of water, approximately 250 species of Rotifera (including sessile and illoricate forms) are known nowadays.The composition of the limnetic Cladocera of various lakes and rivers, excluding the Macrothricidae and Chydoridae, has a species number in the range of 20.The limnetic Copepoda found in these environments has a species number in the range of 40, the majority being calanoid copepods (Robertson and Hardy, 1984).
The general aim of the present work is to contribute to our knowledge of the zooplankton fauna in southwestern Amazonia, studying the occurrence of certain species; and to improve the knowledge of the Rio Acre tributaries.

Study area
In the areas of the Amazonia lowlands, the character of the Andean rivers, instead of showing depth erosion, develop strong lateral incisions, resulting in meandering.After passing their cones of coarse rubble, deposited when leaving the mountainous section of their courses, the rivers carry a bottom-load of sand, which forms sandbanks, and their water becomes turbid, because of their load of suspended fine particles.The meanders migrate down the rivers, old ones are cut off and transformed into oxbow-lakes, new ones are formed, and the whole land adjacent to the rivers is in a constant and rapid process of dismantling and rebuilding, a process which can be observed on a time-scale of a few decades (Sioli, 1984).The lakes studied are environments situated in the Amazon Basin.Lago Amapá is situated in the Municipality of Rio Branco, and Lago Pirapora is located in Porto Acre, in the State of Amazonas, with the geographic co-ordinates 10°02'36''S; 67°50'24'' W and 9°27'21"S; 67°31'39"W , respectively (Figure 1).Both meander lakes are affluents of the Rio Acre, characterized according Sioli (1984) as whitewater rivers, of turbid, more or less ochre-colored water.This type of river is known in the pre-Andean zone as southwestern.One of the main characteristics of the lakes studied is the fluctuation of the water level, which is distinguished by two phases: high water and low water.They are directly influenced by the hydrological regime of the River Acre, especially during high water periods.These lakes are abandoned meanders.The minimum and maximum depths for Lago Amapá are respectively 2.7 and 4.5m and for Pirapora 4.5 and 13m.
According to Salati et al. (1978), the Amazon basin hydrological complex is formed by thousands of small rivers and probably has been, together with the exuberant forest, in dynamic equilibrium for millenia.The lakes studied are situated south of the equator, beyond parallel 65°W, with a minimum amount of precipitation in June/July (Sioli, 1984).
Being in the hydrographic basin of the Rio Acre, to which they are permanently linked, Lago Amapá and Lago Pirapora are considered white-water lakes according to Sioli (1968).The lakes are surrounded by forest, which is inundated during high water and, as a consequence, there is a great input of organic matter from the forest into the lake.
Zooplankton was collected from both lakes.Sampling was carried out monthly, for a period of 11 months.Complete characterization of the community was carried out by two methods.The first was by horizontal and vertical hauls using a net of 55-µm mesh size.In the second method, water was collected with a Van Dorn bottle at various depths, and then filtered through a 55-µm mesh plankton net, during the morning and at night, in the littoral zone at the surface and bottom, and in the pelagic zone at the surface, middle and bottom depths.Details of the vertical distribution and abundance density in that stratum as diurnal variations have been studied by Keppeler (2003).The samples were preserved with formalin solutin (4%).(1978) andAPHA (1992).Turbidity was measured with turbidimeter.
The constancy index of each zooplankton group at the sampling stations was determined using the following expression (Dajoz, 1972): where n = number of collections containing the group or species; and N = total number of collections.
The species groups were considered constant when they were recorded as more than 50% of the samples, accessory when present as 25 to 50% of the samples; and accidental when recorded as less than 25% of the samples.
The qualitative similarity between the populations of the different collection locations was calculated using Jaccard's Index (Krebs, 1989), as: Cj = a/a+b+c where C is the index, a is the number of species present in both locations, b is the number of species present only in location A, and c is the number of species present only in location B. The presence of a species in a collection location was scored as 1, and its absence was scored as 0.

Results
Figure 2 shows the rainfall during the period studied.Figure 3 presents data for certain limnological characteristics of the lakes (Marques-Lopes, 2001).The minimum temperature was 22.65°C in June, and the maximum was 30.40°C, in Lago Amapá.The mean temperature for both lakes was 27°C.Higher transparency levels occurred during the low water period.The transparency levels ranged, during this period, from 0.30m in August to 1.40m in September, with an average of 0.63m in Lago Amapá.For Lago Pirapora the average transparency level for the low water period was 0.39m, ranging from 0.30m in September/ October to 0.60 in May.For dissolved oxygen, during the low water period, the minimum was 3.15mg.L -1 (Lago Amapá) in March, while the maximum was 11.96 mg.L -1 , also for Lago Amapá, resulting in an overall average of 6.80 mg.L -1 for Lago Amapá and 4.74 for Lago Pirapora.The values for electrical conductivity oscillated for Lago Amapá between 28.83 and 56.73µS.cm - , and for Lago Pirapora 35.90-140µS.cm - .The minimum value for turbidity was 13.40NTU during the low water period (Lago Amapá) and the maximum 88.30NTU (Lago Pirapora).
Table 1 shows the species and/or main groups of zooplankton.The main taxonomic groups were Rotifera and Crustacea (Cladocera and Copepoda).The group that included most of the taxa was Rotifera, with 38 species.Brachionidae had most species, followed by Filinidae.Most species were found in both lakes: Anuraeopsis sp.In Lago Pirapora, the constant species were Brachionus falcatus, Keratella cochlearis, Polyarthra vulgaris, Filinia cf.terminalis, F. opoliensis, F. longiseta, Hexarthra intermedia braziliensis, Diaphanosoma spinulosum, Ceriodaphnia cornuta, copepodites, and nauplii.In Lago Amapá, K. cochlearis, Filinia cf.terminalis, F. opoliensis, Hexarthra intermedia braziliensis, D. spinulosum, Moina minuta, C. cornuta, copepodites, and nauplii were constant.(Figure 4).Jaccard´s similarity index was similar for the two lakes at 0.6964.Table 2 shows the species present in the surface, middle and bottom depths.Moina minuta was found to be common in all the compartments of the water column throughout the period of study.
Cephalodella gibba X C o l u r e l l i d a e Lepadella s p .The large species number of this group is considered to be opportunistic species in different environments.The lakes studied are subject to the successive changes that occur in floodplain areas, with the result that r-strategists, which reproduce rapidly under conditions of stress, are common.Birky and Gilbert (1971) suggested that there is a series of advantages in the rotifers system of reproduction, which could favor the participation of most of these animals in an opportunistic, colonizing lifestyle.Opportunistic organisms show wide fluctuations in population, being adapted to reproduce in relatively limiting conditions, but extremely rapidly under favorable circumstances.

X S y n c h a e t i d a e
The rotifer family Brachionidae showed the highest number of species (12), mainly in the genera Brachionus and Keratella with 8 and 2 species, respectively, plus 1 variety of Keratella, followed by the Filinidae (6 species), with 5 species of the genus Filinia.The high species richness of these families was also found by Sendacz and Melo-Costa (1991), Lansac-Tôha et al. (1992), andFrutos (1998).The family Brachionidae predominates in tropical environments, and was also dominant in the study of Carvalho (1983) in Central Amazonia.
Microcrustaceans, especially calanoids, were poorly represented.Cyclopoids were represented by four genera: Thermocyclops, Mesocyclops, Microcyclops, and Neutrocyclops.The species of Thermocyclops is new to science, according to Sendacz and Melo-Costa (1991).Of this genus, the species Thermocyclops minutus and T. decipiens were common in two lakes, Fechada and Pousada das Garças, in Paraná (Lansac-Tôha et al., 1993).Neutrocyclops brevifurca was present, especially in Amapá Lake.According to Reid (1993), this species also occurs in the Cerrado region of Brazil.Mesocyclops meridianus was infrequent during the entire study period.
The cladocerans Bosminopsis deitersi, Ceriodaphnia cornuta, Diaphanosoma spinulosum, and Moina minuta were recorded.These species were also found either in great abundance or frequently in the Rio Paraná in Argentina (Paggi, 1979;Paggi and José de Paggi, 1990), and in Amazonia (Robertson and Hardy, 1984;Bozelli, 1992).Most of the cladoceran species recorded in the lakes were found in Lago Castanho, central Amazonia: Diaphanosoma spinulosum, Daphnia gessneri, Ceriodaphnia cornuta, Moina minuta, and Moina reticulata.Moina minuta was very frequent in both Amapá and Pirapora lakes.This genus is common in South America, particularly in floodplain lakes, but its distribution is poorly known (Hardy and Duncan, 1994;Lima, et al., 1998).Daphnia gessneri was infrequent during the entire study, occurring only in two collections in Lago Amapá.The cladocerans Bosminopsis deitersi, Ceriodaphnia cornuta and Moina minuta were very frequent in Lago Batata, an Amazon lake impacted by bauxite tailings, with low transparency of 0.5 to 2.5m (Bozelli, 1992), similar to the transparencies of 0.30 to 1.40m in Amapá and Pirapora.

Figure 1 .
Figure 1.Location of the Amapá and Pirapora lakes.
Water depth was measured by means of a marked and weighted rope.Transparency was determined using a Secchi disc.Water samples for physical and chemical analyses were collected with a Van Dorn bottle.Temperature was measured with a mercury thermometer.Electrical conductivity and pH were determined with conductivimeter and pH meters.Dissolved oxygen was determined according to the Winkler method modified according to Golterman et al.

Figure 2 .Figure 3 .
Figure 2. Rainfall during the period of April, 2000 to February, 2001.Source: Federal University of Acre, Department of Agrarian Sciences, Meteorological Station, Bulletin of Meteorological Data of the Municipality of Rio Branco.

Figure 4 .
Figure 4. Constancy indices of the different zooplankton groups during an 11-month period in Lago Amapá and Lago Pirapora.

Table 1 .
Zooplankton taxa recorded in the Amapá and Pirapora lakes over an 11-month period (A* = Amapá, P* = Pirapora, X = occurrence of taxon on collection date).