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Variation of frustular morphology within the Achnanthidium minutissimum (Kütz.) Czarn. species complex was studied in type populations of 12 described taxa and in 30 recent North American river samples. The SEM observations in this study and other publications showed that ultrastructural characters on their own do not discriminate among taxa within the A. minutissimum complex. Therefore, an attempt was made to use other characters, such as valve shape and striation pattern, to delineate morphological groups. The sliding-landmarks method was used to obtain valve-shape descriptors. These shape variables were combined with conventional morphological characters in multivariate analyses. It was shown that some historically recognized taxa are morphologically distinct, while others are difficult to differentiate. Morphological grouping of “old” taxa most similar to A. minutissimum did not correspond to their taxonomic hierarchy in contemporary diatom floras. Morphometric analysis of a data set of 728 specimens from North American rivers revealed six morphological groups, although it was impossible to draw clear boundaries among them. These morphological groups differed significantly in their ecological characteristics and could be recommended as indicators of water quality. Application of the discriminant function analysis based on shape variables and striation pattern showed that North American specimens could be more consistently classified into the six groups identified in our analysis than into historically recognized taxa. [ABSTRACT FROM AUTHOR]

Variation of frustular morphology within the Achnanthidium minutissimum (Kütz.) Czarn. species complex was studied in type populations of 12 described taxa and in 30 recent North American river samples. The SEM observations in this study and other publications showed that ultrastructural characters on their own do not discriminate among taxa within the A. minutissimum complex. Therefore, an attempt was made to use other characters, such as valve shape and striation pattern, to delineate morphological groups. The sliding-landmarks method was used to obtain valve-shape descriptors. These shape variables were combined with conventional morphological characters in multivariate analyses. It was shown that some historically recognized taxa are morphologically distinct, while others are difficult to differentiate. Morphological grouping of “old” taxa most similar to A. minutissimum did not correspond to their taxonomic hierarchy in contemporary diatom floras. Morphometric analysis of a data set of 728 specimens from North American rivers revealed six morphological groups, although it was impossible to draw clear boundaries among them. These morphological groups differed significantly in their ecological characteristics and could be recommended as indicators of water quality. Application of the discriminant function analysis based on shape variables and striation pattern showed that North American specimens could be more consistently classified into the six groups identified in our analysis than into historically recognized taxa. [ABSTRACT FROM AUTHOR]

The large variation in size and shape in diatoms is shown by morphometric measurements of 515 benthic and pelagic diatom species from the Baltic Sea area. The largest mean cell dimension (mostly the apical axis) varied between 4.2 and 653 μm, cell surface area between 55 and 344,000 μm2 , and cell volume between 21 and 14.2 × 106 μm3 . The shape-related index, length to width ratio, was between 1.0 and 63.3 and the shape- and size-related index, surface area to volume ratio, was between 0.02 and 3.13. Diatom community analysis by multivariate statistics is usually based on counts of a fixed number of diatom valves with species scores irrespective of cell size. This procedure underestimates the large species for two reasons. First, the importance of a species with higher cell volume is usually larger in a community. Second, larger species usually have lower abundances and their occurrence in the diatom counts is stochastic. This article shows that co-occurring small and large diatom species can respond very differently to environmental constraints. Large epiphytic diatoms responded most to macroalgal host species and small epiphytic diatoms most to environmental conditions at the sampling site. Large epilithic diatoms responded strongly to salinity, whereas small epilithic diatoms did so less clearly. The conclusion is that different scale-dependent responses are possible within one data set. The results from the test data also show that important ecological information from diatom data can be missed when the large species are neglected or underestimated. [ABSTRACT FROM AUTHOR]

We studied how size variation in populations of Diatoma moniliformis Kitz, was influenced by environmental effects on the diatom life cycle. One of the two populations sampled monthly in the northern Baltic Sea grew under natural conditions; the other population was in a cooling water discharge changed of a nuclear power plant, where the temperature and flow rate of the water were artificially higher. The life cycle was synchronous at the natural site, with sexual reproduction accuning in the winter; most of the initial cells were found in March-April. After this, a reduction in cell size occurred, and the vegetative life cycle consisted of two parts. During the first part, cell volume decreased, whereas the surface area to volume ratio increased, and during the second part of the cycle, both of these parameters decreased. No direct evidence was found for the existence of a supra-annual life cycle in D. moniliformis, as convincing modes for large cells were lacking in the size-frequency distributions. It was concluded from extrapolations of the data that the natural life cycle of D. moniliformis probably lasts 2 or 3 years. The changes in cell proportions during the life cycle fit well with included from extrapolations of the data that the natural life cycle of D. moniliformis probably lasts 2 or 3 years. The changes in cell proportions during the life cycle fit well with annual growth cycle of D. moniliformis at the natural site (i.e. the cells had high surface area to volume ratios during the period of optimal growth in late spring (May-June) At the site affected by cooling water discharge, the synchronization of the natural life cycle was disrupted, but some seasonal size variation did occur. Under natural conditions, auxosporulation is probably triggered by a combination of small cell size, low water temperature (0.3°C), and rapidly increasing light intensity or daylength in late winter to early spring. When these conditions were not met (e.g. at the heated site, the required low temperature was absent), auxosporulation did not occur simultaneously. This paper also presents scanning electron photomicrographs showing the typical shape and fine structure of the initial cell of D. moniliformis. These cells are semispherical in cross section, possess a pronounced curvature along the longitudinal axis, and are bent on the pervatuar plane. [ABSTRACT FROM AUTHOR]

The large variation in size and shape in diatoms is shown by morphometric measurements of 515 benthic and pelagic diatom species from the Baltic Sea area. The largest mean cell dimension (mostly the apical axis) varied between 4.2 and 653 μm, cell surface area between 55 and 344,000 μm2 , and cell volume between 21 and 14.2 × 106 μm3 . The shape-related index, length to width ratio, was between 1.0 and 63.3 and the shape- and size-related index, surface area to volume ratio, was between 0.02 and 3.13. Diatom community analysis by multivariate statistics is usually based on counts of a fixed number of diatom valves with species scores irrespective of cell size. This procedure underestimates the large species for two reasons. First, the importance of a species with higher cell volume is usually larger in a community. Second, larger species usually have lower abundances and their occurrence in the diatom counts is stochastic. This article shows that co-occurring small and large diatom species can respond very differently to environmental constraints. Large epiphytic diatoms responded most to macroalgal host species and small epiphytic diatoms most to environmental conditions at the sampling site. Large epilithic diatoms responded strongly to salinity, whereas small epilithic diatoms did so less clearly. The conclusion is that different scale-dependent responses are possible within one data set. The results from the test data also show that important ecological information from diatom data can be missed when the large species are neglected or underestimated. [ABSTRACT FROM AUTHOR]