Phytoplankton from Duas Bocas Reservoir , Espírito Santo State , Brazil ( except diatoms )

(Phytoplankton from Duas Bocas Reservoir, Espírito Santo State, Brazil (except diatoms)). Duas Bocas is a small water supply reservoir (0.51 km2) located in a forested area (Duas Bocas Biological Reserve) (20°18’S and 40°28’W). Phytoplankton community was investigated by samplings weekly taken in the dry (July, 1998) and rainy (March 1999) seasons, at surface, euphotic/afotic boundary, and at the bottom of the water column. A total of 76 taxa was identified with the majority of Chlorophyceae (Chlorococcales) (37.3 %), Conjugatophyceae (Desmidiales) (26.7 %), and Cyanobacteria (Chroococcales, Nostocales e Oscillatoriales) (22.7 %). In addition, Euglenophyceae, Dinophyceae, Cryptophyceae, and Chrysophyceae totalized 13.3 %. The predominant genera were Staurastrum, Staurodesmus, Cosmarium, Desmodesmus, Ankistrodesmus, and Monoraphidium. Regarding to the frequency of occurrence, most of the rare and constant species were belong to Chlorophyceae (Chlorococcales) and Conjugatophyceae (Desmidiales) and Cyanobacteria, respectively. According to the literature, most of the species presented cosmopolitan distribution. Among the identified taxa, 44 species (57.9 %) are the first citation for the State of Espírito Santo.


Introduction
The state of Espírito Santo (Brazil) has a large potential for limnological studies related to phytoplanktonic community, because of its plentiful and diverse inland aquatic habitats.Nevertheless, not many studies have been carried out in these ecosystems.Among the very few studied there are a small number of lakes and coastal lagoons (Huszar et al. 1990, Dias Júnior & Barroso 1998) and one reservoir (Dias Júnior 1998).Lakes have been considered as a good alternative to supply water to attend increasing demands of Espírito Santo's urban and rural populations.Studies on phytoplankton communities have been shown a great importance to portray the limnological status of water supply reservoirs.Among the phytoplankton taxa, Cyanobacteria have been a key group because of its potential for cyanotoxins production in water supply reservoirs.Since there is a public health risks, Cyanobacteria monitoring has been mandatory according to recent regulations of the Brazilian Ministry of Health (Brazil 2000).In order to support a more effective monitoring program the floristic knowledge of phytoplankton communities is imperative.
The present article is the first report of a floristic survey on the phytoplankton from a water supply reservoir in the State of Espírito Santo.In addition to the floristic survey, the pattern of geographical distribution of the identified taxa is also presented.Diatoms were excluded from this study because they will be presented in another article.

Material and methods
Duas Bocas Reservoir (20°18'S e 40°28'W) is a small (0.51 km 2 ) and shallow (Z mean 4.3 m) water body, which supplies drinking water for part of the Great Vitória, Capital of the State of Espírito Santo.The entire reservoir watershed is protected by a state conservation unity.The Duas Bocas Biological Reserve (29.1 km 2 ) spreads over a low mountain relief (up to 800 m of altitude) covered by Atlantic forest.The reservoir lies in the lower part of the relief (200 m) where Naia-açu, Pau Amarelo and Panelas streams outflow.The discharge of the first two streams was estimated as 0.05 m 3 s -1 (Barroso & Marchioro 1999) and 0.14 m 3 s -1 (Perrone & Barroso1996), respectively.The summer is hot and wet and winter is cold and dry.Average annual air temperature is 22 ºC and rainfall is 1,500 mm.Even though there are no sources of agriculture, domestic or industrial pollution in the watershed, the tributaries streams input a considerable amount of allocthonous organic matter of forest detritus.Small stands of the free floating aquatic fern Salvinia minima Baker were observed in the riverine area of Pau Amarelo Stream.
Limnological studies in the Reservoir began in 1995 and were related to the physical and chemicals aspects (Delazari & Barroso 1998) and phytoplankton ecology (Delazari-Barroso 2000).Senna et al. (1999) described a new cyanobacteria species, Cyanodictyon tropicalis Senna, Delazari & Sant'Anna, in the Reservoir lacustrine zone.According to Delazari-Barroso (2000), Duas Bocas Reservoir shows a warm monomictic pattern with strong thermal stratification during summer and winter mixing.The reservoir was classified as meso-eutrophic based on mean annual concentrations of total phosphorus and chlorophyll (Vollenweider & Kerekes 1980).
Sampling campaigns were carried out in a single sampling station next to the dam and in the deepest part of the Reservoir (figure 1).Water samples were taken weekly in dry (July, 1998) and rainy (March, 1999) seasons, at surface, euphotic/afotic boundary (4 m depth), and at the bottom of the water column (9.5 m depth), in a 24 sample set.
Surface samples were taken with a plankton net (mesh size 20 mm).Euphotic/afotic boundary and bottom samples were taken with a Van Dorn bottle.All samples were preserved with 4% formaldehyde.Microscopic investigations were performed using a Zeiss Axioscop microscope equipped with phase contrast and Optovar lens.
Among the constant species of cyanobacterias registered in Duas Bocas Resevoir, we pointed out the occurrence of Cylindrospermopsis raciborskii, an invasive and toxic species.C. raciborskii has been reported in many temperate and tropical lakes (Padisák 1997, Briand et al. 2002), including Brazil (Branco & Senna 1994, Lagos et al. 1999, Tucci & Sant'Anna 2003), and is registered for the first time on a drinking water reservoir in the State of Espírito Santo.Two strains of this species were isolated from this reservoir and toxicological analyses are under development (Delazari-Barroso, unpublished data).
Four species of desmids belonging to the genus Staurastrum were recorded as constant species (table 1).Desmids are turbulence water tolerant species growing mainly in oligotrophic waters (Brook 1981).However, some species of Staurastrum can also grow in eutrophic waters (Reynolds 1987).
Regarding cell size, 25% of the desmids presented smaller size when compared with literature data (Prescott et al. 1981(Prescott et al. , 1982)).Small cell size, which increases the surface-area-to-volume ratio, and other morphological adaptations, such as complex shapes, enhance the capacity of nutrients uptake and reduce the sedimentation rates in the water column (Happey-Wood 1988, Reynolds 1984, 1988).The small size and the high surface-to-volume ratio of the microalgae registered by Huszar (1994) in Amazonian systems were considered as adaptations to low nutrient content waters.The majority of coccal green algae presented rare occurrence in this study.These microalgae are turbulence dependent to keep in suspension in water and hence have a disadvantage in stratified waters (Happey-Wood 1988).
Although the occurrence of microalgae is related to several environmental factors, light, temperature and nutrients conditions are the major limiting factors.The geographical distribution is governed by ecophysiological characteristics but also by the floristic knowledge of a region (Comas 1996).In an attempt to classify the life habit and geographical distribution of the species recorded in Duas Bocas Reservoir, we relied on the available literature.Based on this perspective, most of the Chlorococcales and Desmidiales was regard as cosmopolitan species, while the majority of the Cyanobacteria predominantly occur in tropical and temperate lakes and reservoirs.According to Komárek & Comas (1984), the application of cosmopolitism criterion is influenced by the use of North American and European identification keys on material from tropical areas, which can imply in erroneous identification and classification.To avoid such mistakes, Senna & Magrin (1999) pointed out the importance of a correct taxonomy, based on population samples, mainly when the floristic data are used on ecological characterization of aquatic systems.
Although most of the species is regarded as planktonic, some typically metaphytic taxa were recorded during rainy period (Cosmarium pseudoconnatum and C. pseudobroomei) suggesting a contribution of cells from riverine areas to the planktonic habitat.
Considering temporal occurrence, eighteen species, mainly coccal green, were registered only during dry period and fourteen species of almost all classes, were recorded only in rainy period (table 1).These data can show a slight trend to higher diversity in turbulence condition, probably because the increase in flux of nutrients from hypolimnion.However, to confirm this hypothesis it is necessary to develop a consistent monthly sampling in an annual basis, and ecological studies on phytoplankton community and nutrient concentrations.In respect of these issues another paper is being prepared.

Figure 1 .
Figure 1.Map of Duas Bocas Reservoir showing the sampling station (right) and the main tributaries.The left inset show the Espírito Santo State (below), in southeastern Brazil (above).

Table 1 .
Vertical distribution of the Phytoplankton species of the Reservoir Duas Bocas in dry and rainy season and frequency of occurrence.Sf = surface, E/A = euphotic/aphotic boundary, Bt = bottom; R = rare, C = common, Ct = constant.