Perturbations to environmental conditions experienced by any one species, can invoke a physiological and/or behavioural response that may be expressed morphologically and ecologically. This has important implications for diatom taxonomy and the use of diatoms as environmental indicators. The reliable recognition of a taxon with a particular life history, morphology, ecology and physiology may necessitate the adoption of narrower species concepts than those in current floras. This would have the advantage of clarifying the ecological ranges of taxa used in biological monitoring programmes. Gomphonema parvulum has long been a taxonomic problem. The species exhibit considerable is found over a wide range of environmental conditions. lt has also been used as an indicator of "pollution" in diatom indices of water quality. Clones of G. parvulum, exhibiting a range of morphologies, were isolated from different sites into unialgal culture. Experimental investigations on the clones, tested against different environmental variables to determine ecological tolerances of different isolates, occasionally produced auxospores. This rare opportunity allowed the morphology of particular clones to be studied over their full size range (initial cell through to mother cells). Additionally, aspects of diatom behaviour, including sexual reproduction could be observed and recorded. Results indicate that not only are there differences in cell shape and behaviour with size and environmental condition, but in some G. parvulum clones, heteropolarity is not determined in the auxospore or initial cell, but becomes established after a series of vegetative divisions. This increases the likelihood that specimens may have been incorrectly identified as different species or varieties (G. parvulum complex encompasses two previously described species, G. gracile and G. hebridense), underlining the need for experimental studies and culturing. These observations argue strongly against reliance on diatom valve morphology as the sole criterion on which to delimit taxa, and provide a compelling argument for the benefits of algal culturing and observation of live material. There are serious issues for the sampling of waters and the use of diatom indices of water quality. Community analyses, water quality indices and taxonomic studies will be invalid, especially if different parts of the life cycle are shown to be ecologically as well as morphological variable.

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