Diversity of Benthic Diatoms and Water Quality of the Mekong River Passing Through Ubon Ratchathani Province , Thailand

The study on the diversity of benthic diatoms and water quality in the Mekong River as it passes through Ubon Ratchathani, Thailand was carried out from February 2007 to January 2008. Samples were collected from 10 stations, which were situated in areas involving different geographical features and human activities. Three hundred and seven species of benthic diatoms were found. The majority of them were Gomphonema lagenula Kützing, Navicula rostellata Kützing, Nitzschia palea (Kützing) W. Smith, Luticola goeppertiana (Bleisch) D. G. Mann, Nitzschia liebetruthii Rabenhorst, Sellaphora pupula (Kützing) Mereschkowsky, Navicula erifuga Lange-Bertalot and Cymbella tumida (Brébisson) Van Heurck, respectively. The water quality was slightly different at each sampling site which was classified as moderate water quality (mesotrophic status). Ban Na Muang, Pak Seang Temple, Ban Ta Mui, Ban Na Sanarm, the location behind the Municipal Market and Wigittra Beach were all found to possess moderate water quality (mesotrophic status). Chang Mop Cataract, Ban Koom, Ban Dan and Ban Huay Phai were found to have moderate to polluted water quality (mesotrophic to eutrophic status). The Multivariate Statistical Package (MVSP) particularly Principal Correspondence Analysis (PCA), and the cluster analysis were used to determine the relationship between the water quality and the present of benthic diatoms, in terms of biomonitoring. It was found that Cymbella tumida (Brébisson) Van Heurck and Navicula erifuga Lange-Bertalot could be indicators of mesotrophic status, whereas Cymbella turgidula Grunow, Gomphonema clevei var. javanica Hustedt, Luticola goeppertiana (Bleisch) D. C. Mann, Nitzschia liebetruthii Rabenhorst and Nitzschia palea (Kützing) W. Smith could be indicator of the meso-eutrophic status.


INTRODUCTION
The Mekong River is the 12 th longest river in the world and flows from China through Myanmar, Thailand, Laos and Cambodia and into Vietnam.In Thailand, the Mekong River flows from the northern and north-eastern borders to Myanmar and Laos.Ubon Ratchathani is the eastern most provinces that this river passes through Thailand, and a high diversity of organisms, including benthic diatoms and macroalgae, were found to be present there.Generally, in lotic ecosystems, benthic diatoms are groups of organisms which could be used as bioindicators of water quality.The composition of benthic diatoms is dependent upon the river's characteristics and other specific environmental variables such as hydrology, substratum, light, temperature and water chemistry (Pfister, 1992).These factors could affect the species composition and the organisms were found to be species-specific according to the environmental factors.Spatial patterns of diatom species assemblages and environmental factors effecting diatom species communities have been targeted in several studies (Biggs, 1990;Pipp and Rott, 1994;Reavie and Smol, 1998;Rott et al., 1998;Pan et al., 1999;Potapova and Charles, 2003;Soininen, 2002).
Diatoms have been used routinely, worldwide to monitor water quality in rivers and streams (Schoeman, 1979;Coste et al., 1991;Whitton and Kelly, 1995;Jüttner et al., 2003), for example, in Europe (Kelly, 1998;Prygiel and Coste, 1993), North America (Lowe and Pan, 1996;Stevenson and Pan, 1999), Africa (Archibald, 1972), Australia (Chessman, 1986;John, 1998;Chessman et al., 1999) and Asia (Wu, 1986;Hirano, 1967;Atazadeh et al., 2007;Shibazaki et al., 2011).In Thailand, there have been few research studies involving the use of benthic diatoms as biomonitors in streams and rivers such as the Mae Sa Stream (Pekthong, 2002), the Ping and Nan Rivers (Kunpradid, 2005), the Mekong River (Suphan, 2009), as well as the Tha Chin, Chi, Chanthaburi, Kwai and Tapee Rivers in Thailand (Leelahakriengkrai and Peerapornpisal, 2011).The objective of this study was  to investigate the diversity of the benthic diatoms found to be present in the Mekong River as it leaves Thailand in Ubon Ratchathani Province and to study the relationship between the change of species composition and species-specific quantity found in this river, in relation to the physical and chemical properties used for monitoring the Mekong River.
Water quality analysis: Certain physico-chemical factors were measured at the sites by portable meters, i.e., temperature, pH, conductivity and velocity.The water samples were collected and preserved in a cool box (5-7°C) for chemical analysis in laboratories, i.e., DO BOD, soluble reactive phosphorus, nitrate-nitrogen and alkalinity by following Greenberg et al. (2005).The water quality was classified according to the trophic status using the methods of Lorraine and Vollenweider ( 1981), Wetzel (2001) and AARL-PC Score (Peerapornpisal et al., 2004).

Benthic diatoms diversity:
To sample the diatoms, a plastic sheet with a 3×3 cm 2 cutout was placed on the upper surface of the selected stone or other substratum.The benthic diatoms were then brushed and washed off, into a plastic bowl, until the cutout area was completely clear.Each sample was transferred to a plastic container (5-7°C) (Suphan, 2009).In the laboratory, the samples were cleaned and digested by concentrated nitric acid and hydrogen peroxide (Rott et al., 1997).A mounting agent, Naphrax, was added to make a permanent slide for diatom identification and counting under a compound light microscope and a Scanning Electron Microscope (SEM, JEOL model JSM-5410LV), respectively.

RESULTS AND DISCUSSION
Water quality of the Mekong river: The study of some physico-chemical parameters of water quality of the Mekong River as it passed through Ubon Ratchathani Province, the results showed that the water quality varied at each sampling site (Table 2).
The trophic level and water quality were classified by Saprobic Index (Rott et al., 1997) and AARL-PC Score (Peerapornpisal et al., 2007) and were classified in two groups.In the first group, Sites 2, 3, 4, 5 and 6, were classified as having moderate water quality or mesotrophic status.In the second group, Sites 1, 7, 8, 9 and 10, were classified as having moderate to polluted water quality or mesotrophic to eutrophic status.The conductivity was found to be the highest in Site 1, 2, 3 and 4 during the summer season, followed by the cool dry and rainy seasons.In addition, it was found that the conductivity had a positively correlated with an abundance of benthic diatoms, such as Nitzschia palea (Kützing) W. Smith, Cymbella turgidula Grunow and Navicula erifuga Lange-Bertalot (Fig. 2).In the latter part of the rainy season, soluble reactive phosphorus and nitrate-nitrogen were recorded at high levels in Site 4, 5, 6, 7, 8 and 9, because the manure that is used in agriculture, leached into the river.In the late rainy season up to the early cool dry season, benthic diatoms were in gradually high abundance because of the present of suitable nutrients.(Table 2, 3 and Fig. 2) The similar result was reported by Patrick (1939).The results showed that Melosira varians Agardh, Synedra ulna (Nitzsch) Ehrenberg and Navicula viridula (Kützing) Ehrenberg were in high abundance in high nitrate-nitrogen (2.00-3.00mg/L) water.

Diatom diversity:
The study of the diversity of benthic diatoms in the Mekong River as it passed through Ubon Ratchathani, Thailand, from February 2007 to   3 and Fig. 3).
The highest diversity index figures were Cymbella turgidula Grunow, Luticola goeppertiana (Bleisch) D.G. Mann, Cocconeis placentula Ehrenberg, Nitzschia  The results of this study of the dominant species of benthic diatoms and the water quality were found that Cymbella tumida (Brébisson) Van Heurck, Gomphonema clevei var.javanica Hustedt, Luticola goeppertiana (Bleisch) D.G. Mann, Gomphonema lagenula Kützing, Nitzschia palea (Kützing) W. Smith and Nitzschia liebetruthii Rabenhorst had positively correlated with nitrate-nitrogen and soluble reactive phosphorus.Furthermore, it was found that Navicula erifuga Lange-Bertalot had positively correlated with BOD, alkalinity and conductivity.Navicula rostellata Kützing and Sellaphora pupula (Kützing) Mereschkowsky had positively correlated with DO, percentage of oxygen saturation, and the air and water temperature (Fig. 6).Nitzschia palea (Kützing) W. Smith and Cymbella turgidula Grunow were abundant at the sites depending on the amounts of nitrate-nitrogen and soluble reactive phosphorus found in the river and the community zone location.Navicula erifuga Lange-Bertalot was abundant at the sites, depending on the conductivity (Fig. 6 and 7).The abundance of Nitzschia palea (Kützing) W. Smith in each site was associated with nitrate-nitrogen and soluble reactive phosphorus, while Navicula erifuga Lange-Bertalot was associated with conductivity of the river (Fig. 6).Additionally, Nitzschia and Navicula are usually associated with polluted ecosystem (Rumeau and Coste, 1988).
The number of benthic diatoms species found in the Mekong River, as it passed through Ubon Ratchathani Province was found to be similar to other big rivers, both within the country and upcountry such as, 310 species in the Sundays and Great Fish Rivers, South Africa (Archibald, 1972), 186 species in the Mekong River (2004)(2005) (Peerapornpisal et al., 2005), 168 species in the Mekong River in 5 provinces of Thailand (Pruetiworanan, 2 008), 79 species in the Mekong river passing Chiang Rai Province, Thailand (Leelahakriengkrai and Pruetiworanan, 2009) and 252 species in the Mekong River passing from Myanmar to Thailand and Laos (Suphan, 2009).Since previously mentioned studies had collected data during specific seasons, not every month, the benthic diatoms were found to be less abundant than those recorded in this research, in which data was collected once a month over a single year from 10 sampling sites.

Fig. 1 :
Fig. 1: (1) Map of the Mekong river and Ubon ratchathani province; (2) the 10 sampling sites of the Mekong river as it passed through Ubon ratchathani province

Table 1 :
Latitude-longitude and elevation of the Mekong river in each sampling site

Table 2 :
Maximum and minimum levels of water quality in the Mekong river as it passed through Ubon Ratchathani province from February 2007 to January 2008

Table 3 :
Dominant species, frequency, diversity index and evenness of benthic diatoms in the Mekong river as it passed through Ubon Ratchathani province from February 2007 to January 2008