Phytoplankton diversity in Digboi oil refinery effluent receiving stream of Assam, India

A comprehensive study was carried out to investigate phytoplankton community and water quality in the Digboi oil refinery effluent stream. Taxonomic composition, abundance, spatial distribution, temporal dynamics of phytoplankton were studied along with physicochemical properties of water based on monthly data collected from seven selected sampling stations during April, 2011 March, 2012. Altogether 139 species (7 orders, 19 families, 67 genera) of phytoplankton were identified of which Bacillariophyceae was the dominant class with 45 species followed by Chlorophyceae 40, Cyanophyceae 34 and Euglenophyceae 20. Though distinct changes in community structure were reported, higher phytoplankton abundance revealed during the post monsoon months. Correlation analysis showed influence of phenol and total oil content (TOC) along with pH, inorganic phosphorus and nitrate content in distribution and abundance of the phytoplankton. Introduction Phytoplankton are microscopic plants that grow in water bodies. They are very sensitive to slightest changes in environmental conditions of their habitat (Palmer 1959). Being located at the base level of energy transfer or trophic structure phytoplankton provide more accurate information on changing habitat characteristics compared to other aquatic lives (McCormick and Cairns 1994). Therefore, phytoplankton observation has been used as a reliable tool for biomonitoring of pollution in any aquatic bodies (Mathivanan et al. 2007). Digboi is the place in India where for the first time crude oil was explored in Asia during late 19th century and Digboi refinery is the oldest petroleum refinery in the subcontinent established in the year 1901. The refinery has been regularly discharging hazardous chemicals such as oil, hydrocarbon, phenol etc. to a natural stream that created a stress condition for growth of aquatic flora and fauna including phytoplankton. A little work has so far been done to understand the effect of petroleum refinery effluent on fresh water algal community in the region (Singh and Gaur 1988, Baruah et al. 2009). The present investigation was planned to undertake a study on diversity, distribution and abundance of phytoplankton community of effluent receiving stream of Digboi oil refinery (Assam), India in relation to water quality. Correlation between different water parameters and phytoplankton data were also studied to evaluate their interactions. Materials and Methods The study was conducted during April 2011 to March 2012. The effluents of the refinery are pushed out through an open drain named Telnala flowing through Digboi municipal area and discharged into a natural stream that originates from Digboi reserve forest and flows through paddy fields. Seven sampling stations (Fig. 1) were selected and designated as S1, S2, S3, S4, S5, S6 and S7. S1 station was the effluent receiving point of the drain. S2, S3 and S4 were about 500 m, 1 km and 2 km away from S1, respectively. S5 was the confluence of the effluent carrying *Author for correspondence: < partha_ghy16@rediffmail.com>. 164 BORDOLOI AND BARUAH drain (Telnala) and the natural stream. S6 was at 500 metre upstream and S7 was 500 meter downstream on the natural stream from the confluence point (S5). From each station phytoplankton and water samples were collected in three replicates in monthly intervals using Nansen sampler. Identification of phytoplankton samples were done by morphological observations consulting literature and monographs of Fritsch (1935, 1961), Smith (1950), Desikachary (1959), Ramanathan (1964), Prescott (1975), Gandhi (1998), Perumal and Anand (2009) and Yamagishi (2010). The physicochemical parameters, such as water temperature, pH, conductivity, turbidity were measured on the spot using Systronics Digital Water Analyzer 371. DO, free CO2, BOD, COD, inorganic phosphorus, nitrate, phenol and total oil content (TOC) were measured following APHA (2012). The abundance values were calculated under Sedgwick rafter plankton counting cell. The species with more than 75% frequency were designated as common, followed by frequent (40 75%) and rare (< 40%), respectively. Pearson’s correlation coefficients among the variables of water quality along with phytoplankton abundance were computed and analyzed using XLstat version 2013.5.