Influence of land use changes on water chemistry in streams in the State of São Paulo , southeast Brazil

Streamwater is affected by several processes in the watershed including anthropogenic activities that result in changes in water quality as well as in the functioning of these stream ecosystems. Therefore, this work aims to evaluate the concentration of major ions (Ca2+, Mg2+, Na+, K+, NH4, NO3, NO2, Cl-, SO4, PO4, HCO3) in streams in the state of São Paulo (southeast Brazil). The sampling sites are located at undisturbed (ombrophilus dense forest, semideciduous forest and savanna – cerrado) and disturbed areas (pasture, urbanization and sugar cane crops). Streamwater chemistry varied according to land use change and, in general, was higher in disturbed sites. Streams located in undisturbed sites at Ribeira de Iguape/Alto Paranapanema watershed (streams 1, 2 and 3) seem to be regulated by soil characteristics, as the disturbed streams located at the same watershed covered by pasture (stream 7) showed high concentration for the most of the variables. Exception to streams located at Pontal do Paranapanema watershed where both disturbed (stream 8) and undisturbed streams (stream 4 and 5) presented similar patterns for almost all variables measured.


INTRODUCTION
Anthropogenic activities result in several physical and chemical changes that can alter the structure and functioning of stream ecosystems (Ohrui and Mitchell 1998, Caraco and Cole 1999, Niyogi et al. 2004).Small catchments represent important means of identifying changes in land use, mainly because they are most effective in processing and transporting elements, such as C and N, as well as, major cations and anions (Likens 2004, Thomas et al. 2004).Water chemistry analysis provides a measure of the health and integrity of the ecosystem since this streamwater represents local and regional hydrosphere and provide certain basic functions of ecosystems (Thomas et al. 2004, Niyogi et al. 2004).DANIELA M.L. SILVA et al.
Streamwater is affected by biotic and abiotic pro cesses in the watershed, including climate, hydrology, soil properties, geomorphology, topography and land use (Ometto et al. 2000, McKee et al. 2001, Likens 2004, Thomas et al. 2004).The effects of various processes on stream chemistry have been intensively studied in many forested watersheds in temperate regions.Changes in land use involve physical and chemical alterations such as increased light, nitrate, chloride and sulfate.These effects include reduced uptake by vegetation that change soil processes such as mineralization and nitrification (Wang et al. 2006).The effects of change in land use on the biogeochemistry have also been described in the literature for both major rivers and streams.It has been shown that these changes affect nitrate, ammonium, calcium, potassium and aluminum concentrations in streams located in temperate (Wang et al. 2006, Kamisako et al. 2008, Rai et al. 2010) and tropical regions (Neill et al. 2006, Andrade et al. 2011, Silva et al. 2011).
Several studies in tropical watersheds evaluated the impact of urbanization and agricultural practices on water quality (Ballester et al. 1999, Neill et al. 2001, Biggs et al. 2004, Martinelli et al. 2008, Salomão et al. 2008, Germer et al. 2009, Andrade et al. 2011).Studies in subcatchments in southeastern and southern Brazil have shown that small streams are severely affected by urbanization and agricultural practices such as pastures, considerably increasing dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) (Daniel et al. 2002, Silva et al. 2008) in addition to several ions such as nitrate, chloride and ammonium (Silva et al. 2008, Germer et al. 2009, Andrade et al. 2011).As a result, the structure of biota communities is affected by these activities, decreasing some species while urbanization increases (Ometto et al. 2000, Ramirez et al. 2009).It has been also recognized that urbanization represents a source of heavy metals (Tomazelli et al. 2003, Guimarães and Sígolo 2008, Campos et al. 2009) and organochlorine compounds (Miguel et al. 2003, Silva et al. 2008).
Since there are few studies of undisturbed streams in Brazil, mainly in the State of São Paulo, this work aims to evaluate the concentration of major ions in streams in this region (southeast Brazil).

STUDY SITES
The characterization of study sites is described in tables I and II.The sampling sites 1, 2, 3 and 7 are located in the Ribeira de Iguape watershed (A), sites 4, 5 and 8 in the Pontal do Paranapanema watershed, site 6 in the Mogi-Guaçú watershed and sites 9 and 10 in the Piracicaba watershed (Figure 1).The sites 1, 2, 3, 4 and 5 are located at undisturbed areas of the Atlantic forest under two different physiognomies (Ombrophilus Dense and Semi Deciduous forests) and site 6 is located at an area of savanna (Cerrado) in state parks of São Paulo, Brazil.The Ribeirão Grande (site 7), Lageado (site 8), Marins (site 9) and Palmeiras (site 10) streams are located at disturbed areas with distinct land use types (Table II).
Water samples in forested areas were sampled monthly from May 2002 to April 2003 and in deforested areas were sampled six times in this same period.

METHODS
Abiotic parameters (dissolved oxygen, conductivity, pH and temperature) were measured in the field.Conductivity was measured with an Amber Science 2052 meter, dissolved oxygen and temperature were measured with a Yellow Spring 58 meter and pH with an Orion 250A meter.For chemical analyses, water samples were collected using a Niskin bottle.Samples for major ions (Ca 2+ , Mg    -C10H14O).Major ions were analyzed with a DIONEX DX 500 and DIC were determined with a Shimadzu TOC 5000 and the HCO 3 -concentration calculated from measurements of the pH, temperature and DIC concentrations (Butler 1991).Soil samples were collected at undisturbed streams in the depth of 0-10 cm and the analyses of pH, Organic Matter (OM), P (phosphorus), K + , Na 2+ , Mg 2+ were realized at ESALQ/USP and N% C:N ratio at Laboratory of Isotopic Ecology at CENA/USP.

RESULTS
Streamwater chemistry varied according to soil characteristics or land use change.In general, the majority of ions in streams, as well as pH, were lower in stream 6 (undisturbed; P<0.05) and higher at stream 7 (disturbed; P<0.05) (Figure 2).Stream 6 also showed low ionic content, with conductivity values ranging from 5.3 to 6.3 µS cm -1 and pH from 5.2 to 5.4.Dissolved oxygen presented distinct values, decreasing according to the decrease of native forest percentage in the watershed.Higher values were found in streams 1, 2, 3 and 7, all located in the same region (Capão Bonito) (Table II) followed by streams 4, 5 with 6, 8, 9 and 10 presenting the lower values (Figure 3).Stream chemistry concentration was dominated by HCO 3 -, Na + , Mg 2+ , Ca 2+ and Cl -(Table III).Streams located in undisturbed areas presented low values for almost all ions compared to disturbed ones (Table III).The concentration of Ca 2+ and Mg 2+ were significantly higher in stream 2 (P<0.05) and 7 (Figure 4).As a result, the conductivity and HCO 3 -concentrations were also higher, both results also coincides with soil characteristics since that cambisoils showed the higher concentrations to K + , Ca 2+ and Mg 2+ (Table IV).

WATER CHEMISTRY VARIATION IN STREAMS IN BRAZIL
On the other hand, SO 4 2-, Na + and Cl -concentrations were significantly higher in sites 1, 2, 3 and 7 (P<0.05).Conductivity and HCO 3 -also presented higher values in streams 3, 7, 9 and 10 (P<0.05), the exception was observed in site 8, that presented values similar to undisturbed sites.

STREAMWATER CHEMISTRY
In general, streamwater chemistry was higher in disturbed sites.Several studies reported the impact of land use effect on streams, both in temperate and tropical watersheds (Kamisako et al. 2008 , K + and Mg 2+ ) in pastures watersheds compared to forest watersheds.Same trend were found by Silva et al. (2007) that found higher concentrations of inorganic nitrogen and carbon in sugar cane crops compared to silviculture and savanna.
It seems that dissolved oxygen reflected the differences in environmental characteristics more than land use.However, dissolved oxygen presents positive correlation only with forests catchments (Spearman correlation r = 0.35; P<0,05 -Table V).Streams 1, 2 and 3 (Capão Bonito) present rapid currents due the presence of a rock river bed, although streams 4, 5 (Teodoro Sampaio) and 6 (Santa Rita do Passa Quatro) present a sandy river bed with a slow current.Therefore, the concentration of dissolved oxygen was higher in streams located in Capão Bonito followed by streams in Teodoro Sampaio and Piracicaba and Santa Rita do Passa Quatro (Figure 3).The values found in stream 6 (located at Santa Rita do Passa Quatro) were lower if compared to other streams in state parks since this area is located under savanna region with presents highly weathered soils and poor nutrient concentrations (Markewitz et al. 2006) that reflected in all parameters of water chemistry evaluated in this work (see above).Silva et al. (2007) evaluating undisturbed stream in this area reported that land use changes such as sugar cane and eucalyptus contribute to a decrease of physicochemical conditions in savanna watersheds.
Conductivity values differed for disturbed and undisturbed sites.It was expected that streams located in disturbed sites would present higher values of conductivity, mainly because of the higher concentrations of several ions.This is true only for streams 7, 9 and 10.Stream 8 did not follow this pattern, presenting similar values of stream 4 and 5, which are located in the same area (Teodoro Sampaio/SP).This watershed is covered mainly by pasture (90%, Table II) and is near an interstate highway.However, it seems that little differences in ion concentrations were affected by these disturbances, probably due the fact that the sites collects were located before the highway.
At the same time, stream 3 is an exception, but in this case, the higher values were found to be due to the higher concentrations of Ca 2+   these same streams.These cations were found in higher concentrations in soil samples collected in these watersheds (Table IV).Therefore, it could be inferred that soil characteristics are the main factor affecting the Ca 2+ , Mg 2+ and, consequently, the conductivity values in stream 3.
The concentration of sodium and chloride found in streams 1, 2 and 3 could be due precipitation inputs since this area is near the coast, and its dry and wet climate could be the main factor affecting these concentrations.These results coincide with ionic concentration evaluated in rainwater in the same study areas that found higher concentrations of these ions also in Capão Bonito (V.P.S. Almeida, unpublished data).
Since most cities in Brazil have no sewage treatment and untreated waste is dumped directly into rivers, high concentrations of several ions were expected.Stream 7 was characterized by high concentrations of several ions, such as Cl-, K + , Ca 2+ and Mg 2+ , and despite the fact that this stream is probably influenced by precipitation inputs since it is located in the same area as streams 1, 2 and 3, the higher concentrations area probably due to urbanization.This sampling site was located near a small village where the sewage was freely dumped in the stream; therefore, we hypotheses that this sewage significantly contributed with major ions such as Cl-, Na + and Mg 2+ (Table V and Figure 2) (P<0.05).It is well recognized that the concentration of these ions is severely affected by sewage.In the Piracicaba River basin, the sewage added to these river showed an increase in concentration of Na + , Ca 2+ , Mg 2+ , SO 4 2-and Cl- (Martinelli et al. 2008).At the same time, Ca 2+ , Mg 2+ and HCO 3 -presented positive correlations with pasture and perennial cultures in watersheds with different degrees of disturbances in the state of São Paulo, although Cl -were correlated with urbanization and sugar cane crops (Salomão et al. 2008).Urbanization is also the main land use change that drives the concentrations of almost all major ions in streams in the southeastern of São Paulo if compared to pristine streams at the same area.The higher concentrations of nutrients and major ions were found in streams located at urban areas (Andrade et al. 2011).
On the other hand, potassium presented different patterns for undisturbed streams, with higher concentrations found in streams 4 and 5 covered by semideciduous forest.Dry and wet deposition could be the main factor affecting the concentration of K + in semi deciduous seasonal forest.The same trend was observed in wet deposition in this area with values 1 to 3 times higher compared to the other watersheds.However, the higher values of potassium were found in disturbed sites, with stream 7 also presenting the highest value of 66 µM, probably due to the domestic sewage and pasture.Pasture represents one of the main land covers affecting the potassium concentration at Turvo, Aguapeí and Piracicaba Rivers in the state of São Paulo, with percentages varying from 44% to 70% in Piracicaba and Aguapeí, respectively (Martinelli et al 2008, Salomão et al. 2008).This vegetation cover decrease the soil permeability, increasing the ion fluxes to streams (Germer et al. 2009) and this seems to be the case of undisturbed streams 1, 2 and 3 and disturbed stream 7 where pasture represents 75% of the watershed.

NITRATE AND MAJOR CATIONS CONCENTRATIONS
Higher concentrations of NO 3 -were found in streams 1, 2 and 3 located in ombrophilus dense forests when compared to the other streams located in disturbed areas.This is probably due to the soil characteristics and the oxygen availability in these streams (Fig. 4).These sites showed the higher concentration of total nitrogen (three times higher than the other watersheds) and inorganic nitrogen (Table IV).It is associated with higher nitrification and mineralization rates that contributed to an increase of nitrogen leaching.This positive correlation has been also demonstrated in watersheds located in southeast Brazil, with the higher concentrations of nitrate found in sites located in ombrophilus dense forests when compared to sites located in a wetland (Marques et al. 2003).
Nitrate concentration has been reported in several works associated with major cations.According McDowell (2001), tropical forests that are not N limited have high gaseous losses from nitrification, and denitrification and high losses of nitrate in surface run off, resulting in indirect effects such as loss of major cations and decrease in P availability (due to the acidification associated primarily to increased nitrification).
The cations concentration, such as Ca 2+ and Mg 2+ that were higher in streams 1 and 2 could be associated with the higher NO 3 -(Figure 5).This association was also confirmed with the spearman analyses that showed positive correlation (P<0.05) of NO 3 -with Na + , Mg 2+ , Ca 2+ and HCO 3 -(Table V).The leaching of this ion from the soil surface mobilizes a cation.As the base cation supply is progressively depleted, the leached cation will either be a proton or mobilized aluminum ion.As this ion is prevalent in tropical soils, aluminum rather than H + is the dominant cation associated with soil acidity in tropical forests (Jordan 1985, Fenn et al. 1998, Matson et al. 1999).and disturbed sites (7, 8, 9, 10).
The higher concentrations of aluminum, calcium and magnesium were found in these soils (streams 1 and 2) (Table IV), and nitrate leaching could represent a link between the N cycle and cation cycles in forest ecosystems, and its concentration could be correlated with Ca 2+ and Al in stream water (Brujinzel 1992).Losses of Ca 2+ and Al also have been reported for sites located on inceptisols, which is the predominant soil in 1 and 2. In contrast, sites located in spodsol and oxisol groups (e.g.Amazonian, and streams 4, 5 and 6) exhibit lower losses of calcium and magnesium (Currie et al. 1999).According to Christopher et al. (2006) the relationship between NO 3 -and Ca 2+ can be explained by differences in tree species and soil properties, and similar to our results they found a positive relationship between these ions.Some streams draining forested watersheds in Tennessee, USA showed a high export of Ca 2+, Mg 2+ and HCO 3 -.However in this case, the area is underlain with dolomites characterized by a high concentration of CaCO 3 - (Mulholland 1992).In our case, the bedrock composition is characterized by phyllites recognized to increase the nitrate from streams.According to Holloway et al. (1998), the nitrogen flux in streams through areas dominated by phyllites had an export 100 times higher (19.9 kg.N.ha -1 .yr - ) than streams located in areas dominated by intrusive DANIELA M.L. SILVA et al. rocks (0.12 kg.N.ha -1 .yr - ).Despite the fact that N concentration of the rocks in our areas has not been documented, we can infer that this rock probably could contribute to increased concentrations of nitrogen in streams located in ombrophilus forest.
The presence of aluminum and iron in Latosols (Ultisols or Oxisols) could also explain the lower concentration of phosphate in all watersheds, since the former can cause the immobilization of this ion in soil.There is some evidence that phosphorus limits productivity in late-successional tropical forests (Vitousek et al. 1993).The concentration of inorganic phosphorus in streams, in general, were above the detection limits reflecting the low mobility of this ion.However, semi deciduous forests showed values ranging from 1 to 3 µM, which coincides with streams located in pasture in Rondônia, Brazil (Neill et al. 2001) with values ranging from 0 to 2.5 µM.In this same area, they found an N:P ratio > 80 to forested sites showing the P limitation in tropical ecosystems (Seitzinger 1988).
of land use and variables analyzed.Coefficients in bold are significant in P<0.05.WATER CHEMISTRY VARIATION IN STREAMS IN BRAZIL

TABLE III Comparision of mean concentration of conductivity (Cond); dissolved oxygen (DO), HCO 3 -, Na
+, Mg2+, K + , Ca Rai et al. 2010, Andrade et al. 20111, Silva et al. 2011).Streams located in undisturbed sites were reported to be more affected by soil characteristics and precipitation, although the streams located in disturbed areas were affected by different sources according to the land use.
and Mg 2+ in