Morphological Variation in Stephanodiscus Niagarae (Bacillariophyceae).

This study defines the S. niagarae complex. Species in the complex are larger in diameter and have wider, deeper mantles than other Stephanodiscus species in North America. Besides S. niagarae var. niagarae and S. niagarae var. magnifica, two species new to the literature are identified from the complex: S. yellowstonensis and S. superiorensis. Stephanodiscus superiorensis has a flat or nearly flat valve face. All other species in the complex are strongly concentrically undulate. I found no single character diagnostic for S. yellowstonensis. Characters with discriminating power were intercorrelated; this interdependency was resolved with principal component analysis, clarifying differences between S. yellowstonensis and S. niagarae. In particular, for valves of the same diameter, S. yellowstonensis has fewer spines and wider labiate processes, mantle processes, and spines than S. niagarae var. niagarae. Character variation independent of a diameter effect was strongly associated with variation in areolar density in all species. In S. niagarae var. niagarae, diameter-independent variation was correlated with Secchi transparency, a general index of lake tropic status. Analysis of S. niagarae from a core of Lake Erie sediments also illustrates that its morphology is dependent on lake trophic status; in particular, valves grown under more eutrophic conditions were more finely areolate than valves from more oligotrophic waters. The greatest change in areolar density occurred after silica apparently became limiting to diatom abundance in Lake Erie. For recent populations, silicon:phosphorus ratios were correlated with diameter-independent variation in both areolar density and degree of valve silicification, suggesting that silica availability may be the specific environmental parameter controlling morphological variability in S. niagarae var. niagarae. Stephanodiscus niagarae var. magnifica remains "varietas inquirenda". No morphological character(s) studied could distinguish it from the nominate variety. However, a comparison of morphological to ecological data for the two varieties suggests that the developmental pattern of var. magnifica is uniquely integrated with environmental conditions and so this taxon deserves at least separate varietal status.PhDBotanyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/159467/1/8314368.pd