Water quality assessment using diatoms

Water chemistry variables are meaningful proxies for human disturbance in some cases: for example, when nutrient enrichment results from agriculture (McCormick and O’Dell, 1996 and Pan et al., 1996). For other disturbances, measures of chemical parameters may fail to capture changes associated with loss of in stream or riparian vegetation, increased sunlight or altered flow regime (Barbour et al., 1995 and Karr et al., 2000). Multi-parameter water quality monitoring programs with a high sampling frequency are not cost effective; this necessitates the use of an alternative method for assessing the integrity of water.  Consequently, other studies have taken a broader view of human influence and evaluated algal response to more direct measures of human disturbance, such as catchment land cover ⁄use and riparian disturbance (Kutka and Richards, 1996; Chessman et al., 1999; Pan et al., 1999; Carpenter and Waite, 2000; Hill et al., 2000 and Leland and Porter, 2000).

Algal monitoring evolved from the early indices of saprobity developed for European stream (Lange-Bertalot, 1979; Reid et al., 1995 and Lowe and Pan, 1996). Tolerance indices typically summarise the relative abundances of species weighted by their sensitivity to specific stressors (Prygiel and Coste, 1993; Kelly and Whitton, 1998 and Stevenson and Pan, 1999). Many studies have linked changes in algal assemblages, particularly diatoms, to changes in water chemistry such as pH, phosphorus and nitrogen (Carrick et al., 1998; Pan et al., 1996 and Winter and Duthie, 2000). The use of diatom tolerance values in water quality monitoring traces its history to Europe, where it has been used for a century (Kolkwitz and Marsson, 1908) and is currently considered, across the world, as important for Biomonitoring (De la Rey et al., 2004 and Schoeman, and Haworth, 1986). Diatoms have been shown to be reliable indicators of specific water quality problems such as organic pollution, eutrophication, acidification and metal pollution (Rott, 1991; Tilman et al., 1982; Dixit et al., 1982 and Cattaneo et al., 2004), as well as for general water quality (AFNOR, 2000). In India the taxonomy of diatom flora has been studied since 18^th centuary (Gandhi, 1998; Sarode and Kamat, 1984; Prasad and Misra, 1992 and Karthick et al., 2009), however a study of the ecology and application of diatoms in water quality monitoring has never been attempted. Although this paper is not intended as a motivation for the use of diatoms as bioindicators, it is perhaps important to mention the reasons why diatoms are useful organisms for Biomonitoring (Round, 1993).

Universal occurrence in lotic and lentic ecosystems;

• Field sampling is rapid and easy;
• Microscopic techniques are reliable;
• The ecological requirements of diatoms are better known compared to other groups of aquatic organisms;
• Shortest life cycle (~2 weeks) of all bioindicator organisms - primarily photoautotrophic organisms, reproduce profusely and respond to environmental changes and provide early indications of both pollution impacts and habitat restoration;
• Sensitive to changes in nutrient concentrations - growth response is directly affected by changes in prevailing nutrient concentrations and light availability. Each taxon has a specific optimum and tolerance for nutrients such as phosphate and nitrogen, and this is usually quantifiable;
• Their assemblages are typically species-rich. Since large number of taxa provides redundancies of information and important internal checks in datasets, increasing the confidence of environmental inferences;
• Their rapid immigration rates and the lack of physical dispersal barriers ensure that there is little lag-time between perturbation and response;
• Diatom frustules have a lasting permanence in sediments, such that sediment cores provide details of changes in the quality of the overlying water and also the past climatic changes;
• The taxonomy of diatoms is comprehensively documented largely based on frustule morphology – an attribute readily identifiable with modern light microscopy and image analysis techniques, and not, in most cases, dependent on electron microscopic techniques;
• Diatoms can be found on substrata and even in dry streambeds, enabling sampling throughout the year (Lane et al., 2009);
• Availability of ecologically associative information world-wide (e.g. http://craticula.ncl.ac.uk/Eddi/);
• Permanent records can be made from every sample by means of strewn slides;
• Unlike invertebrates, diatoms do not have specific food requirements, specialised habitat niches, and are not governed to a major extent by stream flow;
• The availability of interpretive software packages (e.g. OMNIDIA).

Assessment approaches based on diatom indices were developed in the lacustrine environment, and have since been extended to encompass the riverine systems (Round, 1991a; Round, 1991b; Stevenson and Pan, 1991 and Eloranta and Soininen, 2002). Diatoms can be collected not only from natural surfaces (sediments, stones and vegetation) but also from other substrate or surface types in an aquatic environment. The living component can also be collected in a controlled fashion using the simple expedient of artificial substrates (Gold et al., 2002). They collectively show a broad range of tolerances along a gradient of aquatic productivity, with individual species having specific water chemistry requirements. They respond directly and rapidly to many environmental parameters as geology (Stevenson, 1997 and Pan et al., 2000), current velocity (Peterson and Stevenson, 1990), nutrients (Potapova and Charles, 2007), etc. These might vary according to species physiology and the species-specific sensitivity to parameters which leads to a large panel of assemblage composition according to the river ecological conditions.

Although diatom-based water quality monitoring has many advantages, problems are encountered such as rapid changes in diatom taxonomy, with the re-assignment of many taxa to new genera. Despite these problems, diatom based indices of aquatic pollution have gained considerable popularity throughout the world. Much of the development  and testing of diatom indices has been carried out in France, where that country’s size and typological diversity enabled a more general application in Europe (Prygiel and Coste, 1999). Design and validation of OMNIDIA for the computation of diatom indices has further enabled diatom-based Biomonitoring (Lecointe et al., 1993). Research of diatom species optima for nutrients and trophic status (Van Dam et al., 1994), as well as diatom tolerance to acidification (Van Dam et al., 1994), organic pollution (Lange-Bertalot, 1979 and Palmer, 1969), and sedimentation (Stevenson and Bahls, 1999) has helped in supporting the use of these indices. These along with other measures of assemblage attributes (such as diversity and biomass) may yield a multimetric index that is both responsive to general environmental degradation and diagnostic of specific causes (Karr, 1993).

Many European countries including Finland (Eloranta and Andersson, 1998), France (Prygiel et al., 2002) and Poland (Kwandrans et al., 1998) adopted and tested a variety of diatom indices. In recent years, diatom-based techniques have been incorporated in water quality monitoring in many countries including Europe (Kelly et al., 1998 and Prygiel et al., 2002), Taiwan (Wu, 1999), South Africa (Taylor et al., 2007a), Malaysia (Wan Maznah and Mansor, 2002), Argentina (Gomez, 1999), Australia (John, 2000), Switzerland (Hürlimann et al., 1999), Austria (Maier and Rott, 1988) and the United States of America (Stevenson and Pan, 1999). These countries are now either using diatoms as part of their routine monitoring programs or are in the process of developing the techniques necessary to do so.

European diatom indices were applied successfully in temperate regions, but there is little information regarding their application in the tropics and subtropics (Wu and Kow, 2002 and Taylor et al., 2007b). This necessitates evaluation of these indices before they are adopted in warmer climates. Measurable relationships between water quality variables such as pH, electrical conductivity, phosphorus and nitrogen, and the structure of diatom communities as reflected by diatom index scores in South Africa showed that the diatom-based pollution indices may be good bioindicators of water quality in riverine ecosystems (De la Rey et al., 2004). However, it was found that the technique needed further testing with standard field and laboratory protocols across the country (Taylor, 2004). Such testing entails the standardization of techniques for collection, preparation and enumeration of diatom samples. Such standard methodology also aids in the evaluation and refinement of diatom-based water quality indices based on the deviations between reference and observed communities.

Kelly et al., (1998) strongly recommended the standardisation of methods used for the sampling of benthic diatoms for water quality studies in Europe. Taylor et al., (2005) provided protocol for the collection, preparation and enumeration of diatoms from riverine habitats for water quality monitoring in South Africa. They emphasized the need for basic data collected in a robust and systematic manner to facilitate the evaluation of indices in different geographical areas and to enable individuals developing and refining indices to draw upon data from other regions in order to get a better idea of the environmental preferences of taxa. Recent studies, (Karthick et al., 2009) as well as studies in progress have identified diatoms as useful organisms to include in the suite of biomonitoring tools currently used in India both for assessments of current water quality and for establishing historical conditions in rivers in India.

The focus of this paper is to present a set of standardised protocols based on methodological information from Indian, South African and European studies for field collection of samples and the preparation and enumeration of these samples in a manner yielding the most reproducible data. This protocol will aid those wishing to use diatoms in water quality monitoring studies in India.