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Length-Weight Relationships for Five Cladoceran Species in an Amazonian Lake

This work shows the length-weight regressions established for the five most frequent and abundant cladoceran species (Bosmina hagmanni, Bosminopsis deitersi, Ceriodaphnia cornuta, Diaphanosoma birgei, and Moina minuta) in two stations at Lake Batata. This lake has received for 10 years very fine bauxite processing tailings, which is responsible for the thick layer over the natural sediment still observed in its western section (impacted station). The individuals were separated into size classes, the dry weight values were determined for each class and were used for establishing weight-length equations. Bosminopsis deitersi and Ceriodaphnia cornuta were the species showing the smallest size in the two sampled areas (0.152 to 0.322 mm) and Diaphanosoma birgei was the largest (0.284 to 0.722 mm). In the natural station, Moina minuta and Bosminopsis deitersi were the lightest species (0.174 to 0.334 and 0.074 to 0.278 m g, respectively) and in the impacted station, Bosmina hagmanni and Moina minuta were the lightest (0.248 to 1.555 and 0.210 to 0.566 m g, respectively). In general, individuals from the impacted station exhibited smaller body sizes (0.152- 0.684 mm) and higher weight (0.208 – 1.983 mm), which can be explained by the ingestion of tailing particles. The equations established were shown to be adequate for weight estimates of the analyzed species since the differences found between the observed and estimated weights values were not statistically significant .


INTRODUCTION
The basis for construction of quantitative theories about the structure and dynamics of communities are formed by estimates of biomass associated to information on density and productivity (Bird and Prairie, 1985). The zooplanktonic biomass can be measured directly (fresh or dry weight) or estimated from gravimetric and volumetric methods. For Copepoda and Cladocera, the most frequently used technique involves determination of regressions that correlate weight and length; these regressions became common after the publications of Dumond et al. (1975); Bottrell et al. (1976) and McCauley (in Dowing, 1984). Although this information is fundamental for productivity and ecosystem modeling studies, this approach has not been thoroughly explored in tropical regions. In Brazil, few studies have shown biomass results (Rocha and Matsumura-Tundisi, 1984;Matsumura-Tundisi and Tundisi, 1986;Matsumura-Tundisi et al.1989;Okano, 1994;Rocha et al. 1995;Melão,1997) and all of them were estimated from length-weight equations previously established in the literature. Lake Batata, a typical floodplain lake, is distinct from other lakes of the region for the presence of clay material from bauxite processing that has been ejected during 10 years in its west section.
Although there is a substantial amount of information on zooplanktonic community at Lake Batata (Bozelli and Esteves, 1991;Bozelli and Garrido, 1999;Bozelli, 1992Bozelli, , 1994Bozelli, , 1996Bozelli, , 1998aBozelli, , 1998b, aspects related to productivity of this community have not been explored. Taking into account the peculiarities exhibited by Lake Batata, this work had as the objective the establishment of length-weight regression for the most representative cladoceran species in the natural and impacted areas. This information will allow the quantification of the zooplanktonic production, to evaluate the role of this community in the trophic structuring and to understand the importance of the bauxite tailings on the productivity of this system.

Study Area
Lake Batata is situated in the right margin of the  Brazil). The lake has a dendritic shape and total surface area between 18 and 30 Km 2 with high fluctuation of the water level. It is a well oxygenated lake showing low values for electric conductivity (c. 8 µS cm -1 in the filling phase and 13 µS cm -1 in the drawdown), pH varying from 6 to 8 and total alkalinity between 40 and 70 µEq l -1 in the dry season (Melo and Huszar, 2000). It is a lateral dam lake that has continuous communication with the Trombetas River. Depending on the intensity of the inundation pulse the river waters can flow over the dike interlinking the two systems. The lake has received bauxite tailings for 10 years (1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989). The liquid effluent is composed of fine solid particles containing 21% aluminum oxide, 47% silicates and 21% iron oxides. According to Lapa and Cardoso (1988) this material, which still covers the natural sediment of the lake forming a thick layer, silts up 30% of the lake area.

Methodology
The specimens used in this study were obtained from samples collected at fixed stations set in natural and impacted areas during the dry phase (October to January). Zooplankton were sampled by vertical hauls using a plankton net (68µm mesh size) and preserved with neutral formaldehyde (4%) for at least 12 months before analysis. No correction for losses due to preservation was made in this study. Aluminum weighting boats were dried at 60 0 C for 24 h, cooled in a desiccator for at least two hours and weighted with a Mettler microbalance (model UMT2). The specimens were washed in running tap water, rinsed at least three times in distilled water; eggs and embryos were removed from adult females. Body lengths were measured, excluding shell spines, under a microscope fitted with an eyepiece micrometer. Individuals of different species were separated into size (length) classes, which varied according to the species, and were transferred to the pre-weighted aluminum boats. Each weighting boat containing between three and sixty-three individuals of a known size class were dried at 60 0 C for 24 h, cooled in a desiccator and re-weighted. Mean dry weight values determined for each size class were used to derive length-weight regression after natural logarithmic transformation of both length (L) and weight (W) values, which are often assumed to be of the form: Y= a x b (Winberg, 1971), The results were compared by ANOVA followed by Mann-Whitney tests using a statistic software (Instat ®). The present results were compared with published ones.

RESULTS AND DISCUSSION
The variation range in length and dry weight are shown in Table 1. In general, individuals from the natural station showed higher body size and lower weight (except for Diaphanosoma birgei in relation to weight), although the differences were not always considered statistically significant. A comparison with literature data obtained for the same species or morpho-species showed that at Lake Batata cladocerans were smaller and in the impacted area they showed higher weights. These results could be explained by the ingestion of tailing particles and for the retention of mineral material in the internal part of the valves, which was difficult to remove during the preparation of the specimens for weighting. In the impacted station, most of the individuals analyzed exhibited the digestive tube full of tailings. The ingestion of tailings was also observed in the laboratory in individuals submitted to a solution of bauxite tailing and food. of tropical regions and has been explained by the influence of temperature and food limitation. According to Saint-Jean and Bonu (1994), the permanent high temperature in the tropics.
Smaller body size seemed to be a common characteristic for zooplanktonic species would accelerate metabolic processes (development, reproduction, respiration, and ingestion) creating higher food requirements. Permanent high food requirements are not always accompanied by high ingestion/assimilation. At Lake Batata, three factors would explain the weight and length values observed in this study: persistent higher temperatures (> 27 0 C); periodic water level fluctuations which determined substantial alterations in food offer, and the presence of bauxite tailings in the impacted station, that, although interferes in phytoplankton production, could be used as a nonconventional food source due to the bacterial aggregation in the particles (Bozelli, 1994b). The regression equations of the relationships between the values for body length and dry weight obtained in the present study (Table 2) could be considered adequate for weight estimates of the five cladoceran species since the differences between the observed and estimated weights were not significant (Mann-Whitney, p 0.8).
Although the equations obtained could be applied to biomass estimates in other environments it would be important to stress that the length-weight relationship of zooplankton could vary with temperature, quantity and quality of food, in addition to the physiological state of the animals (Geller and Müller, 1981;Kankaala and Johansson, 1986;Matsumura-Tundisi et al., 1989). Moreover, Giguère et al. (1989) observed 37 to 43% dry weight losses (for the large and small species, respectively) due to chemical preservation, independent of preservation methods. Table 2 -Relationship between length in millimeters (L) and dry weight in µg (W) in five cladocerans species from Lake Batata (PA,Brazil).

ACKNOWLEDGMENTS
We thank Mineração Rio do Norte S/A and Department of Ecology of Universidade Federal do Rio de Janeiro for the logistical support. This work was supported by CAPES fellowship.