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
