The taxonomy of synurophytes, which relies extensively on the morphology of the siliceous components, is largely congruent with molecular analyses. Scales of extinct synurophytes were significantly larger than those of modern taxa and may have played a role in their demise. In contrast, many fossil species linked to modern lineages were smaller in the middle Eocene, possibly reflecting growth in the greenhouse climatic state that characterized this geologic interval.

The genus Mallomonas, the largest within the Class Synurophyceae, consists of motile, unicellular, freshwater heterokonts that are covered with an outer layer of overlapping siliceous scales. The scales are flat, circular to oval structures with taxonomically distinct designs. In addition to scales, the cell coverings of most Mallomonas species contain a second siliceous component known as a bristle. Bristles are composed of a long thin shaft and a flattened proximal foot. The foot of the bristle is fastened under the distal end of a scale in such a way as to allow the shaft to radiate out from the cell and rotate on its axis. For many species, the distal scale margin is distinctly raised, forming a hollow space that secures the bristle foot, and an inverted U-shaped opening from which the shaft emerges. This structure is called the dome. We describe three fossil species of Mallomonas from an Arctic Eocene maar lake that contain domes that are recessed to varying degrees back from the distal scale margin. This design is not known in the modern flora and we hypothesize that it represents an early attempt at securing bristles to the scale coat.

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Scaled chrysophytes and planktonic diatoms are used to infer changes in lake water pH, specific conductivity, trophic score, and total nitrogen in 23 Connecticut waterbodies over the last 100 yr, and the changes are correlated with quantified changes in land use in the surrounding watersheds. In general, there was good agreement between the changes inferred from both organismal groups in this suite of lakes. Significant correlations were observed between chemical conditions inferred from organisms in surface sediments and present-day land uses, especially the percentages of the watersheds that are forest or residential land cover types. Approximately 20% of the waterbodies have significantly increased in pH since 1890, and none of the lakes have significantly declined in pH despite the fact that this region receives significant amounts of acidic deposition. These findings support previous work, indicating that the pH of Connecticut lakes has not declined over the recent past. One fourth of the lakes have significantly increased in specific conductivity, especially those situated in watersheds that have become highly residential in nature. Inferred specific conductivity has more than doubled in six of the lakes during the last century. Six of the lakes have become significantly more eutrophic, while only one lake has become more oligotrophic since 1890. The majority of the lakes situated in watersheds that have remained over ϳ80% forested have not significantly changed, whereas those that have become over ϳ25% residential have experienced the greatest amount of change. The potential influences of watershed-based alkalinity generation, winter road salt use, and implications of these findings in lake management are discussed.

1. We assessed the distribution of scaled chrysophytes in fresh waters along 3200 km of the east coast of North America (29°to 48°N) to determine any biogeographic patterns in relation to chemical, physical, climatic and spatial variables. 2. Scaled chrysophytes were identified using scanning electron microscopy and counted from 264 waterbodies in nine regions (20 subregions). Eighteen chemical, physical and climate variables were determined for each waterbody. We used Sorensen's similarity index and analysis of similarity (ANOSIM) to evaluate whether the floras differed between regions, subregions, glaciated and non-glaciated areas, as well as within sets of waterbodies with similar chemical and physical characteristics but situated in different regions. Distance-based linear modelling (DISTLM) was used to evaluate the relative importance of the chemical, physical and climate factors in explaining the variability in the assemblages of scaled chrysophytes, and the resulting models were visualised using redundancy analysis (RDA). 3. Significant differences in the flora were found between all regions and most subregions, and between glaciated and non-glaciated areas. Significant differences were also recorded between waterbodies with similar chemical and physical characteristics but situated in different regions. Many species were abundant along specific sections of the latitudinal gradient, but lacking from others. A set of environmental variables explained significant and independent portions of the variation in scaled chrysophytes, with pH and mean minimum July temperature accounting for 20% of the total. 4. The distribution of scaled chrysophytes along the east coast of North America is not homogeneous and there are biogeographic patterns, despite apparent dispersal mechanisms (migratory birds and wind events) that might act to reduce differences between regions. Rather, differences exist even between neighbouring subregions containing sites with statistically similar chemical and physical attributes. Environmental variables clearly play a significant role in determining whether species will inhabit a given site. However, species were not always found in waterbodies likely to support growth, implicating inadequate dispersal, poor transportability or both.