Evaluating water resource maintenance for lakes of the middle Doce river

The importance of water resources to mankind and the threat level these resources have withstood due to regional and worldwide human intervention are undeniable. In the middle stretch of the Doce river basin, there are close to 170 natural lakes, located between an environmental conservation area and monoculture eucalyptus plantations, with social, economic and biological importance. Our objective was to evaluate whether the water surface area in these lakes has remained constant in the last few decades, and make inferences on the result. To that end, we measured the water surface area of 16 lakes, from satellite images obtained between May and August in the years 1977, 1979, 1981, 2000 and 2005. Total water surface area was compared considering these dates the two different areas. We did not find any changes in water surface area for these lakes in the studied images. Likewise, we did not observe reductions in any single lake for any of the studied areas. These results indicate hydrological constancy of the system, as well as of the services this group of lakes provides to society and biodiversity in its vicinity, regardless of whether the environmental matrix consists of old-growth vegetation or eucalyptus plantations.


Introduction
Human activities represent a growing threat to the preservation of natural resources (BIKSEY et al., 2011;VACKAR et al., 2012).It is such an important factor that the main components of ecosystem changes worldwide are habitat fragmentation, climate change and invasion by exotic species (PERRINGS, 2010).These three components have direct effects on the maintenance of diversity and water resources (SÚAREZ; PETRERE JR., 2007;STAKHIV, 2011).
Habitat fragmentation leads to changes in soil surface and reduces its capacity to store water (SCHÄEFFER; PROCHNOW, 2002).The emission of fossil fuel gases alters the concentration of inorganic compounds in the atmosphere (PRATHER; HSU, 2010), affecting light and energy absorption (MEATYARD, 2003).Both factors are related to temporal and spatial rainfall distribution, as well as the availability of water resources (CARTER; PARKER, 2009;FERMÁN-ALMADA et al., 2008;HARRISON et al., 2008).
In the middle stretch of the Doce river basin, about 170 natural lakes were formed between four and ten thousand years ago (TUNDISI; MEIS, 1985).This lake group is unique in Brazil, and occurs in the Atlantic Forest biome, regarded as one of the world's most important areas for biodiversity conservation (MYERS, 2003).Therefore, these lakes have conservation importance, as already acknowledged in Minas Gerais state and nationwide in Brazil due to the presence of an important conservation unit (Doce river State Park -PERD) and its geological and environmental uniqueness.
The lakes are located over a 100,000 ha area and represent an important resource for the local population and regional businesses; its main uses are amateur fishing and water supply (LIMA et al., 2010).Thus, a reduction in the water surface area of these lakes could represent a serious socioeconomic issue for the region, as well as being yet another environmental problem in addition to the existing difficulties in preserving the fauna and flora present in the lakes (LIMA et al., 2010;GIACOMINI et al., 2011).
The study area of the project "Biological dynamics and conservation of Atlantic Forest biodiversity in the middle Doce river, Minas Gerais State" (PELD/CNPq) includes areas of monoculture eucalyptus plantations, mining and prospecting, and protected areas, such as Private Natural Heritage Reserves.The results of the project suggest there have been abrupt oscillations in regional rainfall in the last 15 years, with effects on the flow of the Doce river.In spite of this, there are no published studies that consider the hydrological balance of these lakes, even if we consider the possible consequences of global climate changes.Thus, our objective was to determine whether the water surface area of lakes in the middle Doce river area has changed in the last few years, which would represent an easily obtainable and highly correlated measurement with water volume in these lakes.We thereby set out to investigate whether: (i) there has been a reduction in water surface area in these lakes; (ii) water surface area reduction was greater in the most dendritic lakes; (iii) the reduction was greater in the lakes whose surroundings consisted of eucalyptus plantations as compared to lakes in areas with natural vegetation; and (iv) water surface area reduction was greater in smaller lakes.

Material and methods
The middle Doce river lake group is located in southeastern Minas Gerais State, in the municipalities of Marliéria, Timóteo and Dionísio.It includes an area between the 42 o 38'W and 48 o 28'W meridians and 19 o 41'S and 19 o 30'S parallels, with altitudes varying between 236 and 515 m.The local climate is warm semi-humid tropical, with rainy summers and dry for four to five months, in winter (NIMER, 1989).
In the study area, it is possible to identify two different landscapes adjacent to the lakes: one consisting of a eucalyptus plantation area (approximately 25,000 ha), with economic activity by the Companhia Agrícola e Florestal Santa Bárbara -CAF (Arcelor Mittal Group), and another consisting of about 36,000 ha of Atlantic Forest biome, preserved within Doce river State Park (PERD -IEF/MG).Considering the estimate of 149 lakes in the region obtained from the evaluation of satellite images (Landsat 7 ETM, 1988), we randomly selected 40 lakes in the left margin of the Doce river (20 lakes in the PERD area, and 20 in the CAF area) (Figure 1).
Satellite images from five different years (1977, 1979, 1981, 2000 and 2005) were obtained from the National Institute for Space Research (INPE; www.dgi.inpe.br/CDSR/).Of the 40 lakes selected initially, only 16 could be viewed in all five satellite images and were used in our analyses (7 from the CAF area and 9 from PERD, see Table 1).measured.Using the classification of images from Spring software (htttp://www.dpi.inpe.br/spring),we obtained three classification categories: vegetation (green color), water bodies (black) and others (white), which could be compared quantitatively.
To obtain the contour shape of the lakes, we used the perimeter development index, D p (HAKANSON, 1981;FANTIN-CRUZ et al., 2008), which produced values closer to 1 when lakes are more circular and lower values when they are more dendritic.To test the hypotheses, we used Repeated Measures Analysis of Variance, considering sample dependence among the response variables, obtained at different moments in time (ZAR, 1999).This analysis was used to test the occurrence or not of water surface area reduction in PERD and CAF areas, individually.Whenever necessary, the transformation 1999) was used to obtain homogeneity of variances and enable the use of repeated measures ANOVA parameter statistics.The level of significance used was 5%.

Results
The water surface area of the lakes did not undergo appreciable alteration in the analyzed years (n = 16; F = 1.702; p = 0.162; Figure 2), and there was no difference in water surface area retraction when considering the different areas of PERD and CAF (n = 16; F = 0.512; p = 0.485).Lastly, the interaction between the measurements obtained from the lakes in these two areas over time was not significant, either (n = 16; F = 0.133; p = 0.969).Thus, the study revealed an absence of total water surface area reduction in the lakes of the middle Doce river in recent years (hypothesis i) and an influence of shape (hypothesis ii); insertion area (hypothesis iii); and lake size (hypothesis iv) on this reduction, as it does not occur.
Through the visual exploration of total water surface area levels of the lakes in CAF and PERD areas, we can infer a similar pattern of variation for their water surface areas, but with no significant variation effects (Figure 3).

Discussion
Human actions cause impacts of varying intensity and scope, which currently threaten the maintenance of global biodiversity and natural resources (KASS et al., 2011).Traditionally, it has been reported that water is a renewable, non-finite natural resource.However, water distribution worldwide is inconstant, which frequently occurs due to human action (MACHADO, 2003).Rational maintenance of water resources is essential to guarantee the supply of water for the development of economic or subsistence activities, as well as being necessary to maintain biodiversity (LOPEZ-GUNN;LLAMAS, 2008).In order to guarantee the preservation of natural resources and aquatic biodiversity -in quantity and quality -it is necessary to control anthropic interference on riparian vegetation (MACHADO, 2003).Riparian vegetation affects the structure of habitats and refuges, and is important in preserving the diversity of aquatic fauna (MCLACHLAN, 1969;GOFORTH;BAIN, 2012) and in reducing silting in water bodies, which affects system biodiversity as well (FELFILI et al., 2000).As such, the composition of the landscape unit (with native vegetation or eucalyptus monoculture) must influence the quality of hydric resources and preservation of the aquatic habitat (GOFORTH; BAIN, 2012).The results found in this study do not indicate a temporal effect of change in total water surface area of the studied lakes, nor did they find any effect among lakes in different areas.Likewise, we did not find relationships to indicate perceptible changes through the analysis of the water surface area of conservation areas (PERD) and eucalyptus plantation areas (CAF).It is possible that the presence of vegetation in the immediate vicinity of the lakes in the region (including eucalyptus) plays an equivalent role in maintain water resources.
Nevertheless, even though it was not the original proposal of this work, after further data exploration we found a reduction in total water surface area between the years 2000 and 2005 (t = 4.274; n = 27; p < 0.001), which indicates that some lakes (Palmeiras, Boné and Chatinha) reduced their water surface area during that period.The causes of this retraction in some lakes may possibly be attributed to local phenomena, as all lakes have a similar geological origin (MEIS; MONTEIRO, 1979;PERÔNICO;CASTRO, 2008).Upon close examination of Figure 3, changes in total water surface area can be seen in individual lakes of the CAF and PERD areas, which may indicate natural changes due to varying yearly rainfall periods.Consequently, the changes found in the water surface area between 2000 and 2005 may be due simply to a return of the lakes to a normal water surface level condition that is above the mean for the study years, as a result of local factors.

Conclusion
Our study indicates that the water surface area of the lakes in the middle Doce river has kept constant in the last three decades, suggesting hydrological stability of these aquatic environments in the studied period, as well as the benefits they represent to society -either economically or in the form of leisure -and to the associated biodiversity.

Figure 1 .
Figure 1.Satellite image (Landsat 7 ETM, 1988) of the middle Doce river.The lakes in the region are highlighted in black, from which the lakes in the study were chosen at random (numbers 1 to 16).The dotted contour represents PERD (north) and CAF (south).In all PERD and CAF lakes, water surface area (m 2 ) and perimeter (m) of the lakes were

Figure 2 .
Figure 2. Water surface area (means and standard deviations) of the lakes (given in hectares) in different study years.

Figure 3 .
Figure 3. Converted values of total water surface area (in hectares) of CAF and PERD lakes in the different study years.

Table 1 .
Studied lakes and their regions.The numbers in the first column refer to lake positions as shown in Figure1.