A checklist for the zooplankton of the Middle Xingu – an Amazon River system

A zooplankton checklist is presented for the Middle Xingu River, based on surveys conducted at four sites in the main channel and two fluvial lakes. A total of 175 taxa are listed, including 141 rotifers, 20 cladocerans, and five copepods. Rapids presented the greatest species richness, with up to 124 taxa, while Ilha Grande lake had 70 taxa, the lowest number. Non-planktonic benthic larvae were recorded frequently in the samples.


Introduction
Many inventories of the zooplankton diversity of the rivers of the Amazon basin are available, especially for the principal, white-and black-water tributaries.The principal studies have focused on the marginal várzea lakes of the Amazon and Negro floodplains (see Robertson and Hardy, 1984;Koste and Robertson, 1990;Brandorff, 1973;Brandorff and Andrade, 1978;Koste and Robertson, 1983;Carvalho, 1983;Hardy, 1980;Hardy et al., 1984;Robertson and Hardy, 1984;Santos-Silva et al., 1989;Waichman et al., 2002).However, studies of the composition of the zooplankton of clear-water rivers are scarce.The available studies include those of Bozelli (1992Bozelli ( , 1994) ) and Bozelli et al. (2000) in the region of the Trombetas River, Koste (1972Koste ( , 1974Koste ( a,b, 1989) ) on the Tapajós, and Brandorff et al. (1982) on the lower Nhamundá River.Given this, the present study provides a checklist of the zooplankton of two fluvial lakes and four sites in the main channel of the middle Xingu, a clear-water river in the southeastern Amazon basin.

Study area
The present study was conducted along a 180 km stretch of the middle Xingu River in Pará, in the eastern Brazilian Amazon basin (Figure 1).This region is characterized by numerous waterfalls and rapids, as well as extensive areas of alluvial rainforest (Camargo et al., 2005).
The climate of the study area is of Köppen's A type, with variants Aw and Am (Critchfield, 1968).Mean annual temperatures in the study area oscillate between 17.5 °C and 24.5 °C, with relative humidity of 84-86%.Mean annual precipitation varies from 2066.8 mm to 2379.4 mm (Camargo et al., 2005).
Six environments were monitored in the present study.Two were located in lakes on river islands (Ilha Grande and Pimentel) and the other four (Boa Esperança, Arroz Cru, Caitucá and CNEC) in the main Xingu River.Ilha Grande lake (3°34'47"S, 52°23'42"W) is semi-circular, with a depth of 0.5-2.5 m, and a total area of approximately 15,612 m 2 .The bed of this lake is covered in slimy silt, sand, and leaf litter derived from the island's dense alluvial rainforest, and high concentrations of phytoplankton were associated with the reduced Secchi transparency (0.8-1.1 m) and high levels of dissolved oxygen (DO: 5.0-7.5 mg.l -1 ), and the slightly acid water (pH = 5.9-6.7) with low conductivity, of 30 uS.cm -1 (Estupiñan and Camargo, 2008).The second lake, Pimentel (3°25'46" S, 52º24'4" W), located on the river's Great Bend, is elliptical in shape, with a mean surface area of 1570 m 2 , depths of between 0.8 and 3.0 m, with low pH (5.0-5.4) and Secchi transparency (0.90 0.98 m), and low DO (2 mg.L -1 ) and reduced levels of chlorophyll, which determine its low levels of primary productivity (Estupiñan and Camargo, 2008).During the rainy season, the forest is flooded by the creeks that drain the area surrounding the lakes.

Results
This study recorded a total of 175 taxa, comprising 141 rotifers (80.6%), 20 cladocerans (11.4%), five copepods (2.9%), and nine (5.1%) bottom-dwelling protozoans, gastrotrichs and insects (Tables 1 and 2).In general, the fluvial habitats were the most diverse.The richness of taxa varied from 70 in Ilha Grande lake to 124 in the rapids.Despite being much smaller than Ilha Grande lake, Pimentel lake had a relatively high richness, with a total of 114 taxa.
The Rotifera was the most diverse group, accounting for approximately 80% of total taxon richness in each of the study environments (Table 1).Much less diverse were the Cladocera, with around 8% of the taxa, and the Calanoida and Copepoda, each with approximately 2% (Table 1).
Most of the rotifers (19.86%) belonged to the family Lecanidae.Brachionus calyciflorus, Pallas, 1866; Testudinela   (Hauer, 1956) NP X X X Testudinela ohlei (Koste, 1972) NP X X X Pl = planktonic, NP = not planktonic, NDT = non determined.filtered by the surrounding forest before draining into the lake (Estupiñan and Camargo, 2008).The considerable phytoplankton biomass and high primary productivity recorded in Ilha Grande lake (Costa et al., 2008) may have determined the low zooplankton richness recorded in this lake.The higher zooplankton diversity recorded in Pimentel lake appears to have been related to specific characteristics of this environment, such as the low current velocity, given that reproductive populations of planktonic organisms are restricted to the slow-flowing lower reaches of these areas (Ward, 1994).

Figure 1 .
Figure 1.Map of the region with the localities in the main channel and island lakes.

Table 2 .
Number of genera and taxa within zooplankton groups for the midd Xingu river habitats.