Non-marine (live in water that is not seawater) diatoms are algae that require sunlight to photosynthesize. Some diatoms are planktonic (live near the water surface) while others are benthic (bottom dwelling, attached to rocks and gravel). Gwen, a graduate student, and her undergraduate assistant, Lance, cored a closed basin lake (no outlet) in eastern North Dakota to study how diatom floral assemblage changed over time and to infer how the climate or the environment of the region changed. Look at the simplified diatom diagram shown below and suggest how the climate, the environment, or some other factor(s) might have changed that are consistent with the data presented..