Freshwater Algae: Identification and Use as Bioindicators

126 3 ALGAE AS BIOINDICATORS(undisturbed) conditions and relating to five differentecoregions (i.e. river types with similar altitude rangeand geological characteristics). The six other communitieswere typical of rivers that had been disturbedby human activity. Although natural variability occurredwithin individual ecoregions (similar to thatnoted in Finnish rivers, see above), this was greatlyexceeded by the effects of pollution at the differentsampling sites. The study aimed to identify diatomindicators for different types of anthropogenic disturbanceby comparing benthic diatom communities innatural and disturbed sites.Use of bioindicatorsAn alternative approach to analysing human impact interms of general disturbance is to use diatom indiceswith indicator species that directly reflect particularenvironmental changes resulting from human activities.Individual species can be numerically weightedto reflect their degree of environmental specificity.Bioindicators may relate to single parameters suchas total-P (trophic diatom index – TDI; e.g. Hofman,1996) or reflect more general aspects of water quality,combining organic loading and inorganic nutrientconcentration (IPS: index of pollution sensitivity,GDI: generic diatom index). Diatom indicator listsfor other variables such as salinity, trophy, nitrogenmetabolism types, pH and Oxygen requirements havealso been published (van Dam et al., 1994).3.4.5 Calculation of diatom indicesAs with lake phytoplankton indices (Section 3.2.3)the benefit of river diatom indices is that they reducecomplex biological communities (involving a rangeof species) to a single numerical value. This providesa very simple quantitative evaluation which can be appreciatedand used by biologists and non-biologistsalike. The disadvantage of an index is that potentiallyuseful information may be lost as counts of individualspecies are brought together and combinedinto a general evaluation. Diatom indices, based onbioindicator algae, are either single or multiple taxonassessments.Single taxon assessmentIndices based on a single species represent a verysimple environmental approach. Kothe’s Index. This is referred to as the ‘speciesdeficiency index’ (F) and relates the number ofcells (N 1 ) of a particular key species at site 1 to thenumber (N x ) at site x, where:F = N 1 − N x× 100 (3.2)N 1This very simple index is assessing environmental impacton a single species. It assumes that site 1 is thetypical one for that river and that changes in the numbersof the selected species will occur at other sites(for example a decline with downstream pollution).Multiple taxon indicesMost diatom indices are based on multiple taxa (generaor species). They are determined either in termsof the presence/absence of key indicator species (e.g.Palmer’s index) or are based on the weighted averageequation of Zelinka and Marvan (1961):n∑a j s j v jj=1Index =(3.3)n∑a j v jj=1Where:a j = is the relative abundance (proportion) ofspecies j in the samples j = the pollution sensitivity of species jv j = the indicator value of species jn = is the number of species counted in the sample.Different indices adapt this equation in differentways, and the performance of a particular index dependson which taxa are used, the number of taxa andthe values given for the constants s and v for eachtaxon. The values of individual indices based on thisequation vary from 1 to a maximum value equal to thehighest value of s. Commonly used indices includethe following.