Diatoms are the microscopic siliceous yellow-brown algae. There are a variety of shapes, but the two basic shapes are circular, referred to as centric, and more or less “football” shaped, referred to as pinnate. They possess silica frustules that have characteristic markings such as lines, striae, or holes, punctae. These morphologic features are unique to each species and are what makes diatoms identifiable under a microscope. Diatoms may be suspended (phytoplankton) in water or attached (periphyton) to a substrate such as bottom rocks or sediment. The diatom periphyton is what we are interested in for this project.

Diatoms have been used extensively as indicators of water quality. They are well accepted as indicators by the US EPA and other agencies, and have been used to evaluate aquatic stressors worldwide, including Europe and several areas of North America (including the Great Lakes). Diatom species respond with great fidelity to stressors associated with major “pressure” indicators in the Great Lakes (e.g., nutrient and salinity loading, sedimentation, and factors affecting water clarity, including exotic species). Diatom remains preserve well in sediment, so they also provide opportunities to reconstruct historical information at a site (paleolimnology). A number of other features make diatoms robust environmental indicators:

• ubiquitous, they occur in virtually any aquatic environment;

• diverse, they can provide a fine-grained assessment of environmental conditions;

• versatile, they are sensitive to a variety of stressors, particularly water chemistry;

• short turnover rate, they respond rapidly to changing conditions;

• they are more time-integrative than “snapshot” environmental measurements;

• they have narrow tolerances and specific optima to environmental conditions;