Phytoplankton diversity in the middle Rio Doce lake system of southeastern Brazil

This study presents results of the inventory of algal flora conducted between August 2007 and May 2008 in 18 lakes of the middle Rio Doce lake system, most of which is in the state of Minas Gerais, Brazil. We recorded 481 taxa, increasing the known total phytoplankton diversity of the region (gamma diversity) by 80%. The following classes were represented: Zygnematophyceae (171 taxa), Cyanobacteria (101), Chlorophyceae (71), Bacillariophyceae (42), Euglenophyceae (43), Trebouxiophyceae (24), Dinophyceae (8), Xanthophyceae (8), Chrysophyceae (6), Cryptophyceae (6) and Oedogoniophyceae (1). We identified 221 taxa that were rare (restricted to one or two lakes), and 101 that were considered representative (present in at least nine lakes). Botryococcus braunii, Elakatothrix genevensis, Planktolyngbya limnetica, Peridinium pusillum, Trachelomonas volvocina, Cosmarium contractum, Staurastrum forficulatum, Staurastrum leptocladum, Staurastrum rotula, and Staurodesmus dejectus were present in all lakes. Richness varied from 95 taxa (in Lake Gambazinho) to 168 taxa (in Lake Palmeirinha). Jaccard indices were low, and the highest similarities between lakes were 53% (Ferrugem/Ferruginha), 47% (Central/Almécega) and 46% (Águas Claras/Palmeirinha), demonstrating high environmental and biotic dissimilarities between lakes. Geographic distance was not significantly associated with floristic similarity, suggesting that local factors are more important than are regional ones in shaping the phytoplankton composition of lakes.


Introduction
Species richness is considered one of the simplest measures to express and quantify biological complexity in a given region (Nabout et al. 2007).In aquatic environments, species richness is influenced by several factors, including water temperature, mixing patterns of the water column, light, nutrient availability, and herbivory (Reynolds, 1987).Therefore, knowledge of richness patterns is essential to proposals for monitoring strategies and for biodiversity conservation activities (Downing & Leibold 2002;Declerck et al. 2005;Nogueira et al. 2008).
Diversity can be assessed on different scales.Local, or alpha, diversity is given by the total number of species in each habitat.Regional, or gamma, diversity is the total number of species observed in a range of habitats (Magurran 2004).The term beta diversity was introduced by Whittaker in the 1960s.At first the term was used in order to describe changes in species composition along gradients of altitude and humidity through differences in rates of gain and loss of species.However, beta diversity is now defined as the taxonomic difference between samples, whether occurring along an environmental gradient or not (Veech et al. 2002).
The middle Rio Doce lake system is a large freshwater system in southeastern Brazil, formed as a result of a mass of sediments (from the original drainages of Rio Doce and its tributaries) that acted as a natural dam, giving rise to a dense network of lakes (Pflug 1969 cited in de Meis & Tundisi 1997).The biological and ecological importance of the system was recently demonstrated by its recognition as an international Ramsar site, making it the 11th site in Brazil to be added to the Ramsar list of wetlands of international importance (Ramsar 2010).
Limnological research in the middle Rio Doce lake system was initiated in the 1970s (Tundisi & Saijo 1997).Since then, various aspects have been investigated (Barbosa & Tundisi 1980;Henry & Barbosa 1989;Rocha et al. 1989;Tundisi & Saijo 1997), increasing knowledge of the geological, morphological, physical, chemical, and biological characteristics of the lakes.However, specific studies on phytoplankton were mainly focused on Lake Dom Helvécio and Lake Carioca (Hino et al. 1986;Taniguchi et al. 2003;Barros et al. 2006;Souza et al. 2008).However, a recent survey evaluated plankton diversity in a larger number of lakes (Maia- Barbosa et al. 2006).Therefore, there is still little information on phytoplankton diversity in this lake system, as shown by Barbosa et al. (1994).The present study aims to contribute to the knowledge of phytoplankton diversity of this lake complex by presenting the results of an inventory of the algal flora in 18 lakes.

Study area
The Rio Doce basin is located at in southeastern Brazil, between the state of Minas Gerais (86% of the total area) and the state of Espírito Santo (14% of the total area), encompassing 83,400 km 2 (Marques & Barbosa, 2002).Two lake systems compose this basin: one, at the middle course, comprising ca.250 lakes, distinct in their trophic status (Maillard et al. 2011), and another, at the lower course, comprising ca.70 lakes (Cavati & Fernandes 2008).Approximately 50 lakes in the middle course are protected within Rio Doce State Park, a conservation unit created in 1939 and representing the largest contiguous remnant of the Atlantic Forest in Minas Gerais (359.76 km 2 ).Lakes located in the surrounding area are affected mainly by hardwood (Eucalyptus spp.) plantations and pastureland, among several municipalities.
For the purposes of this study, 18 lakes were selected: eight located inside the Rio Doce State Park limits and ten in the surrounding areas (Fig. 1; Tab. 1).In selecting the lakes, we took into account their greatest physiographic differences and their accessibility, mainly during the rainy (summer) season.The climate of the region is classified as tropical semi-humid with 4-5 months of dry weather, exhibiting mesothermal characteristics (Nimer, 1989) with temperatures of approximately 25°C.According to Tundisi (1997), the monthly precipitation is highest in December (350 mm) and lowest in July and August (10 mm).

Samplings
Field work was conducted quarterly, in August 2007, November 2007, February 2008, and May 2008.Samples were collected from a fixed point in the limnetic region of each lake.Samplings were authorized by the Minas Gerais State Forestry Institute (permit no.005/07).Water transparency was estimated in situ by Secchi disk measurements (Cole 1983).Samples were collected for total phosphorus quantification (Mackereth et al. 1978).

Data analysis
For each lake, species richness was assessed on the basis of the number of taxa identified, considering the qualitative and quantitative data.Species richness for the sampled region (gamma diversity) was estimated with the first-order jackknife estimator (Nabout et al. 2007), using Stimate S software (Colwell 2006).Beta diversity was estimated by the difference between gamma diversity (total species recorded for the set of lakes) and average alpha diversity (mean species richness per lake), as suggested by Crist et al. (2003): where β is beta diversity, γ is gamma diversity, and α is alpha diversity.
Differences in relation to water transparency and total phosphorus levels were determined using ANOVA.Spearman's correlation coefficient was used in order to test for relationships between the morphometric features (area, depth, and margin development index) and physico--chemical variables.Hierarchical cluster analysis using Jaccard distance and Ward's method (Ward 1963) were performed in order to assess similarity between lakes in terms of the phytoplankton species composition.These statistical analyses were conducted using Past 1.90 software (Hammer et al. 2001).

Results and discussion
Morphometric features, such as depth, margin development index, and area, varied among lakes (Table 1).Secchi depth and total phosphorus concentrations also differed among lakes (F=996.888;p=0.000 and F=730.533;p=0.000, respectively) and correlated with depth (p<0.05;r=0.630 for Secchi disk measurements and r=0.596 for total phosphorus concentrations).Lakes that were shallower (<5 m: Pimenta, Central, Amarela, Ferrugem, and Ferruginha) showed lower Secchi disappearance depths (>1.2 m) and higher levels of total phosphorus (>30 μg/L; especially Lakes Amarela and Ferrugem, in which total phosphorus was >50 μg/L), than did the lakes that were deeper (>7 m: Dom Helvécio, Águas Claras, Almécega, Gambá, and Verde), which showed greater water transparency (down to 2.2 m) and lower values of the trophy indicator (below 21 μg/L of phosphorus).
Richness extrapolation indices, such as the jackknife, although not usually used for phytoplankton (Nabout et al. 2007;Nogueira et al. 2008), can be important tools to assess the representativeness of the sampling effort.In the present study, a total of 481 taxa were recorded (Fig. 2), corresponding to 77% of the expected richness, estimated using first-order jackknife (jackknife 1 = 624).The relationship between the observed and estimated values for richness indicated that our methods were appropriate for a diversity survey.In addition, the known phytoplankton diversity in the middle Rio Doce lake system was high in comparison with the 267 species reported for seven lakes in the system by Maia- Barbosa et al. (2006).Gamma diversity increased by 80% with the expansion of the number of studied environments, reinforcing the observed environmental and biotic heterogeneity.
Eleven classes were identified: Zygnematophyceae (171 taxa), Cyanobacteria (101), Chlorophyceae (71), Bacillario-   (1997), who attributed the dominance of this group to oligotrophic conditions and good preservation of the lakes.It has also been suggested that the thermal stratification pattern known as atelomixis (characterized by unusual, irregular circulation periods of short duration) is a key factor for desmid prevalence (Barbosa & Padisák 2002;Souza et al. 2008), because it allows these species, which have relatively high specific density (Padisák et al. 2003), to remain within the upper layers of the water column.
The fact that the incidence of exclusive species was highest in Lakes Amarela and Gambazinho indicates the great importance of the shoreline regions, given that, in the case of Lake Gambazinho, the main representative species were diatoms of the order Pennales (6 taxa) and large desmids (5 taxa), the latter also prevailing in Lake Amarela (12 taxa), together with Euglenophyceae (7 taxa).Those two lakes are quite distinct from the other lakes of the region and from those presented here.In Lake Amarela, high amounts of organic matter reflect its advanced state of eutrophication, as evidenced by the total phosphorous and transparency values.Lake Gambazinho has a polymictic pattern of thermal stratification, in contrast to the more common warm-monomictic pattern of thermal stratification observed in the majority of the middle Rio Doce lakes.Such characteristics seem to have been responsible for the higher numbers of typically periphytic species in the samples collected from these lakes.
We obtained low Jaccard indices.The highest similarities were 53% for Lake Ferrugem versus Lake Ferruginha, followed by 47% for Lake Central versus Lake Almécega, and 46% for Lake Águas Claras versus Lake Palmeirinha.Lake Amarela showed the lowest similarity with the other lakes, ranging from 14% to 25%.
With the exception of a few pairs of lakes that are geographically proximal and were all included in the same cluster-Gambá and Gambazinho (500 m apart); Ferrugem and Ferruginha (1200 m apart); and Águas Claras and Palmeirinha (1500 m apart)-it should be noted that the clusters were composed of lakes that were distant from one another, some located within the Rio Doce State Park and others located in the surrounding areas.In addition, the difference between the gamma diversity and the mean alpha diversity, considered here as an estimate of beta diversity, was high: 347 species.This suggests that the phytoplankton species composition of such lakes is more dependent on local factors than on regional factors.
Considering that diversity and rarity are important criteria for assessing the conservation value of a given region (Coesel 2001), the possibility of protecting the middle Rio Doce lake system as a whole should be considered.That will require specific strategies for the environments surrounding the state park, with the primary objective of maintaining phytoplankton diversity at the regional level.

Figure 1 .
Figure 1.Middle Rio Doce lake system, showing the outline of Rio Doce State Park, sampled lakes (1 to 18), and limnetic fixed points of sampling (+).Digital base (shapefiles) provided by the Minas Gerais State Forestry Institute.

Figure 2 .
Figure 2. Species accumulation curve of 18 lakes sampled in the middle Rio Doce lake system, between August 2007 and May 2008.

Table 1 .
Geographic coordinates, depth, margin development index, area, transparency, and total phosphorous concentration of 18 lakes sampled in the middle Rio Doce lake system, between August 2007 and May 2008.

Table 2 .
List of the most common species in the 18 lakes sampled in the middle Rio Doce lake system between August 2007 and May 2008