Materials and method

Study area
The study area falls in the Outer Himalaya Zone. Geologically, rocks belonging to Siwaliks Group arepresent in and around the study area. The Siwaliks are overlain by Doon Gravels. Towards the north of the study area, a major tectonics feature, known as Main Boundary Thrust (MBT), is present. The MBT separates the rocks of Siwaliks Group from the rocks of Krol Group (Lesser Himalayan Zone) (Valdiya 1980). Sampling location is at Nalota stream (lat. 30° 22′ 18″ N, long. 78° 04′ 51″ E; altitude 483 m above mean sea level (msl), located in the middle of Doon Valley, along its northern fringe. This stream is located close to Dehradun-Mussorie highway (State Highway-1) near Malsi, Deer Park. Locally this stream is called Nalota Nala. The stream channel is narrow, shallow and transparent, with streambed consisting of hard substrate with prismatic edges, including rocks, large boulders, cobbles and pebbles. Overhanging vegetation formspartial canopy over the stream.

Collection and Cleaning
Diatom samples were collected in December 2007 from 20 to 30 cm depth by a scraping 3 × 3 cm2 area from the cobble surface with the help of razor and brush. Samples were preserved in 4 % formaldehyde, digested using concentrated nitric acid, centrifuged, and rinsed several times with deionized water in order to remove the acid. Cleaned material was mounted onto glass slides with Naphrax® mounting medium andobserved with Olympus BX-51 light microscopes (LM) equipped with Differential Interference Contrast (DIC) and 1.4 NA (Numerical Aperture) objectives. Digital images were taken with an Olympus DP-71 digital camera. Scanning electron microscope (SEM) examination was performed using a high vacuum Jeol JSM-7401 F Field Emission SEM at the University of Colorado Nanomaterials Characterization Facility. For SEM examination, cleaned specimens were air dried onto cover glasses, attached to aluminum stubsand sputter-coated with 10 nm of Au-Pd. Specimens were examined at 3 kV accelerating voltage, with a working distance of 8 mm. The terminology and character descriptions of Kociolek & Stoermer (1993a,1993b) were followed for features found in gomphonemoid diatoms. Physical and chemical parametersof water were assessed as per standard protocol for water and wastewater analysis of the American Public Health Association (APHA, 2005). Water quality data of the type locality is presented in Table 1.