Zooplankton community structure of the lower Xingu River ( PA ) related to the hydrological cycle

The zooplankton community of the lower Xingu River shows strong fluctuations in species richness and number of organisms during periods of water level fluctuation. Pulses of density and species richness are adapted to the pulses in water flows and water level. This is conected with reproductive strategies of some zooplankton groups. The spatial heterogeneity of the lower Xingu River consisting of braided channels, bedrocks, macrophyte stands, is probably a relevant factor for the species richness of the zooplankton communities, and may be a fundamental factor for the overall aquatic biodiversity of the lower Xingu River.


Introduction
It is well known that river discharges have influence in the aquatic biota of rivers due to changes in channel morphology, habitats, deposition of sediments, organisms drift and water level fluctuations.Benthic invertebrates, fishes, zooplankton, periphyton all respond to the temporal variability of the river system, that depends upon the regional hydrological cycle (Petts and Amoros, 1996;Dumont, 2009;Blettler et al., 2012, Abrial et al., 2014).
The lower Xingú River has strong water level fluctuations due to differential discharges throughout the year.Interactions between the zooplankton community and the river discharge in this ecosystem was studied by Brito (2008).
Carvalho (1983) demonstrated that the density and composition of zooplankton in a floodplain lake in Amamzonia is related to water level fluctuation.
The present study is framed into the field of ecological dynamics, in an attempt to provide an insight of the integration of hydrology, ecology and zooplankton community structure.
Zooplankton sampling and analysis were performed within an área of 10.000km 2 within the framework of the environmental impact assessment of Belo Monte reservoir, in the lower Xingu River carried out by Norte Energia S.A.The zooplankton studies are part of limnological, water quality and aquatic ecology research developed by Associação Instituto Internacional de Ecologia e Gerenciamento Ambiental (AIIEGA, 2015).
As defined by Bittencourt and Amadio (2007), the hydrologial cycle in the Amazon is composed by four hydrological periods: rising-water entering the river system and increasing water level from mid December to beggining of March, with change between 20 to 26 m above sea level; high -water level equal or above to 26m above sea level from beggining of March to the end of July; reciding -water leaving the river system and water levels falling from the end of July to the end of October between 26 and 20m above the sea level; low-water level equal or below 20m above sea level from the end of October to mid December.

Material and Methods
A general description of the overall sampling strategy adopted for limnological and ecological studies was described by Tundisi et al. (In press, this volume).Figure 1 shows the average monthly flow, total precipitation (monthly) and sampling periods.The four periods of hydrological cycle adopted was: rising (November, December, January) flood, (February, March, April), lowing (May, June, July) and dry (August, September, October) periods.
A total of 37 stations were distributed along the lower Xingu River including the main chanel, tributaries and igarapés.

Composition, density and richness of zooplankton related to the hydrological cycle
The most abundant group, numerically in all samples was Rotifera.Figure 2a,b show the relative abundance between Rotifera, Cladocera, Copepoda and Protozoa groups occurred in 2013 (Figure 2a) and in 2014 (Figure 2b).Generally the Rotifera independent of the hydrological cycle, represented 40-50% of the total zooplankton.Exception occurred during the high water in 2013 (Figure 2a) where Rotifera was substituted by Tecameba protozoan which dominated the zooplankton (47.4%) and Copepoda (35.3%) especially in this case in the phase of nauplii.Also in the dry period of 2014, Rotifera was outnumberd by Cladocera that represented 48.0% of the total zooplankton whereas Rotifera was 30,0%.The high percentage of Cladocera was due to the very high growth of Bosminopsis deitersi population.
Comparing the absolute density of planktonic organisms (ind/m 3 ) in 2013 and 2014 (Figure 3a, b) there were higher number of organisms in 2014 with 510,749 org/m 3 (Figure 3b) than in 2013 with 391,187org./m 3 (Figure 3a).In both cases high density of total zooplankton occurred during the dry period due to the increasing of number of Rotifera, Cladocera and Copepoda.
Tables 1, 2, 3 and 4 refer respectively to the list of species of Rotifera, Protozoa, Cladocera and Copepoda found in the samples of 2013 and 2014.
Table 5 shows zooplankton species richness recorded at the four phases of the hydrological cycle, showing more richness in the rising and flood periods than lowering and dry periods for both years 2013/2014.The decreasing of number of species during the dry period is clear for all the groups especifically for cladocerans and copepods.

Endemic species of the Amazonian region
Species that are restricted to one geographical or a hydrographic basin are considered endemic.In the lower Xingu River, Brachionus zahniseri, among rotifers, Bosminopsis brandorffi among cladocerans, Pseudodiaptomus gracilis, Notodiaptomus dahli, Notodiaptomus paraensis Table 1.List of Rotifera species recorded in the lower Xingu River.The subscribed species were the most abundant (more than 50.000org./m 3 ) and more frequent occorrence during the period of study (2013)(2014).

Discussion
The zooplankton assemblage found in this research showed consistent results in the sampling periods of 2013 and 2014.In general species richness increased during the rising and flood periods being the main contributors Rotifera and Protozoa.On the other hand density increases during the dry period.This was due specially to the groups of Cladocera and Copepoda that have different mechanisms of reproduction than Rotifera and Protozoa.Cladocerans and copepods require an environment less turbulent for its reproduction and development of their populations and they could find this condition during the dry period.It is knowm that some species of cladocerans and copepods produce resting eggs that are deposited in the sediment and at favourable conditions they outburst increasing their density.This fact, probably occurred with Cladocera during the dry period of 2014, where the high density showed by Bosminopsis deitersi was responsible for a high density of total zooplankton.
As pointed out by Lodge (1987) comparative studies that are conducted in a variety of habitats are relevant to demonstrate the community dynamics.Therefore the strategy of sampling in several sites of the lower Xingu River was consistent in revealing the dynamics of zooplankton communities.
Greenwood and Richard-Coulet (1996) discussed how the distribution of species in a river ecosystem is related to adaptative strategies to physical parameters such as, water temperature, flow velocity, water level fluctuations, optiminization of food resources and available living space habitat.
The lower Xingu River is a highly heterogeneous environment consisting of several stretches with bedrocks, macrophyte vegetation and a extensive network of tributaries, (igarapés) wide and complex.A variety of aquatic environments occurs in the river created by several degrees of conectivity and mosaics.This is a factor that enhances diversity of communities, adapted to various conditions of hydraulic stresses, flows, thermal patterns and substrate.The use of a extensive series of measurements in hydrology, water flows, diversity of habitats and sampling of zooplanktonic community was fundamental to understand the spatial and temporal variation of the density/species richness problem.The spatial heterogeneity promotes a succession of communities which take advantage of food habitats.Braided channels such as those found in the lower Xingu river have dense stands of macrophytes (See Abe et al., In press, this volume) that provide protection from disturbances, surfaces for periphyton growth, and abundant food diversity.Macrophytes stands create an aquatic microclimate that affect distribution of organisms and species richness.The quality and quantity of detritus produced in the bedrocks and macrophyte stands and transported by the river offers a wide variety of food available to rotifers, protozoan and other zooplankton species.Open water habitats, dense macrophytes stands, regions of low flows and stagnant waters, are rich in diverse environments enhancing zooplankton diversity.Therefore the spatial heterogneity promotes a diversity of habitats and conditions that enhances zooplankton diversity and species richness.These species are distributed spatially during rising and flood periods, adapting their reproductive strategies to the hydraulic conditions, flows and spatial heterogeneity.This is conspicuous for Cladocera and Copepoda species.Rotifers and protozoan that have not resting eggs, for example shows continuous reproduction strategies during the whole hydrological cycle.
Density of zooplankton decreases during rising and flood period during to the higher dilution.Thus the river discharge, the extensive morphological and hydrological alterations that occur in the lower Xingu river are probably the main cause of the high species richness in this ecosystem.
Processes of long term with pulses that operate at different periods, are the main factor that enhances this diversity.Since the lower Xingu River has no floodplain, the morphology at the river bed the interaction of tributaries with the main river adds to the spatial heterogeneity, functioning as a substitute of the floodplain lakes in the várzea (Junk et al., 1989).The zooplankton community is only one example of an ample process that involves all the benthic, fish, periphyton and phytoplankton communities.Significant differences in the aquatic communities at the distinct stages of the hydrological regime occur.
Why there are several endemic species in the aquatic biota of the Amazon River?The hydrogeochemistry of the Amazon basin shows many rivers, lakes and freshwater ecosystems with low conductivity waters and a low degree of mineralization.It is well known from the literature that ionic composition, pH, conductivity of freshwaters are fundamental for the physiological functioning of species of fishes, and aquatic invertebrates in general.This could be one reason for the high degree of endemicity of the freshwater biota in Amazon.However the Amazon region is composed of several varied habitats with differing conditions (Furch, 1984).Several geographical barriers, such as the spatial heterogeneity of the region, the magnitude of water level fluctuations and the interactions terrestrial/aquatic ecosystem may be the cause of geographical isolation and therefore promote endemicity.

Future changes in zooplankton
This paper describes the ecological dynamics of the zooplankton communities in the river ecosystems.With the construction of the two reservoirs at Belo Monte power plant some changes will occur.These can be related to the loss of heterogeneity in habitats considering the flood of the bedrocks and macrophyte stands.Probably the reservoirs will favour the growth and development of Copepoda and Cladocera species that have more conditions to reproduce and growth in lentic environments.Changes in the foodchain can also be expected, since phytoplankton can predominate as food available for zooplankton in the reservoirs.Therefore the zooplankton can change from a lotic community to a lentic community with a predominance of pelagic species.Dumont (2009) reported changes in the diversity and species richness of Nile River zooplankton after the start of operation of the Aswan high dam.

Table 2 .
List of Protozoa species recorded in the sampling of the lower Xingu River during the period 2013-2014.The

Table 4 .
Copepoda species registered in sampling from lower Xingu River made in 2013 and 2014.

Table 5 .
Richness of zooplankton species in different phases of hidrological cycle.