Diatom-based Indices

Diatom-based indices are used for a variety of practical purposes in ecological assessment throughout Europe, North America and Africa. In each case it is important that surveys are designed in such a way that data collected can be translated into information useful for management purposes. Autecological indices use the relative abundance of species in assemblages and their ecological preferences, sensitivities or tolerances to infer environmental conditions in an ecosystem. The sensitivity and tolerance of diatoms to a number of environmental characteristics, such as eutrophication, organic pollution, heavy metals, salinity, pH and pesticides, are known to differ among species (Stevenson, 1996). These species-specific sensitivities and tolerances can be used to infer environmental conditions in a habitat (Lange-Bertalot, 1978). Many diatom autecological indices of water pollution in rivers have been developed and are in widespread use (Table 1). Diatom autecological indices can infer specific or general environmental conditions. Most are indicators of organic pollution of water (Palmer, 1969; Sládeček, 1986; Descy, 1979; Lange-Bertalot, 1979 and Watanabe et al., 1986) and are reviewed by Coste et al., (1991) and in Whitton and Kelly (1995). The indices can be based on the detailed characterization of assemblages with many species (Prygiel, (1991) used 1550 species), or they can be simplified to only identify genera or a few species for use by non-specialists (Rumeau and Coste, 1988 and Round, 1993).

Table 1 : List of diatom autecological Indices for water pollution monitoring

Indicator lists for some of the important ecological variables such as salinity, trophy, nitrogen metabolism types, pH, oxygen requirements have been published e.g. by Van Dam et al., (1994). Diatom indices have gained considerable popularity throughout the world during the last two decades as a tool to provide an integrated reflection of water quality, which can form the basis of management decisions. Once the sample has been counted in the correct manner the data can be entered into a computer data-base OMNlDlA (Lecointe et al., 1993) from which several calculations can be made using a specific equation or using the sum of the water quality optima of all the species in the sample. The design of OMNlDlA (http://clci.club.fr/omnidia_english.htm) for computing diatom indices has further facilitated the use of diatom based bio-monitoring. This software has an inbuilt large number of data set for each species, which comprise complete name, references, family, type, sensibility and indicative values for the calculation of the indices. A variety of diatom indices based on comprehensive database in OMNIDIA have been adopted and validated by many countries across hemispheres. In most of the indices diatom species used in the calculation/equation is assigned two values; the first value reflects the tolerance or affinity of the diatom to a certain water quality (good or bad) while the second value indicates how strong (or weak) the relationship is. These values are then, in addition, weighted by the abundance of the diatom in the sample i.e. how many of the particular diatom in the sample occurs in relation to the total number counted. Most of the diatom indices are based on formula of Zelinka and Marvan (1961):

Where aj = abundance (proportion) of species j in sample, vj = indicator value and sj =pollution sensitivity of species j. The performance of the indices depends on the values given to the constants s and v for each taxon and the values of the index ranges from 1 to an upper limit equal to the highest value of s.