Diatom fossil records in closed-basin lake sediments are commonly used to reconstruct climate change based on the observed correlation between salinity and modern diatom assemblages. In the Northern Great Plains (NGP) of North America, diatom assemblages are strongly correlated not only with salinity but also anion composition, with certain taxa characteristic of bicarbonate systems and others sulfate-dominated waters. Although strong correlations exist, the actual mechanisms behind these correlations are unknown. To investigate the basis for these correlations, two approaches were employed. The fossil diatom record from Elk Lake (Grant County, Minnesota) was used to reconstruct salinity and brine type between 2640 to 4645 14C yr. B.P. to determine the extent to which diatoms respond to changes in ion dominance within a single system. This lake was selected for a brine-type reconstruction because a previous study using fossil-ostracode assemblages indicated shifts in anion composition over this period. In general, the diatom- and ostracode-inferred reconstructions showed the same trends. However, the diatom record provided less definitive information on anion proportions as compared to anion concentrations, and suggests that diatoms exhibit a strong response to bicarbonate concentrations as opposed to relative proportions of bicarbonate:sulfate.

In the second approach, a series of culturing experiments were performed to investigate whether nutrients may serve as a mechanistic link between diatom community structure and ionic concentration/composition. The growth rates of 4 saline-lake diatom taxa (Cyclotella quillensis Bailey, Chaetoceros elmorei Boyer, Cymbella pusilla Grunow, and Anomoeoneis costata (Kützing) Hustedt) were measured under varying conditions of salinity (5, 8, and 11 ppt), brine type (sulfate- versus bicarbonate-dominated), and nitrogen form (NH4+ versus NO3-), using a full factorial design. Brine type affected growth rates when nitrogen was supplied as NO3-, such that rates were consistently depressed in the sulfate versus bicarbonate media, but had no impact on growth rates on NH4+ . For the 2 taxa (C. pusilla and A. costata ) that are associated with bicarbonate lakes in the NGP, growth rates were consistently higher in the bicarbonate media, suggesting that these taxa may have higher carbon requirements. A direct physiological effect of brine type was not evident on the sulfate-associated taxon, C. elmorei. A series of resource competition experiments with these four taxa, in which N:P ratios were varied (6:1, 12:1, 24:1, 48:1) along with salinity and brine type in a factorial design, yielded opposite trends for C. pusilla and C. elmorei. In general, C. pusilla was more abundant in the bicarbonate media, whereas C. elmorei was more abundant in the sulfate media. Along with results from the growth rate experiments, this suggests that bicarbonate-associated taxa may be less abundant in sulfate systems due to the lower total carbon concentrations in these systems, while sulfate-associated taxa may be less abundant in bicarbonate systems due to competitive interactions. The combined results of the culturing experiments and paleolimnological reconstructions indicate that total carbon concentrations play an important role in determining diatom community structure in saline lakes of the NGP.