What have diatoms revealed about the ecological history of Lake Superior?

Prior to 1970, little was known of Lake Superior's primary producers. Since then phytoplankton have been studied to track anthropogenic influences that have clearly impacted the other Great Lakes. Most of this work has focused on assessments of diatom assemblages because of their well-known relationships to environmental conditions and because they leave diagnostic fossils in sedimentary records. Some problems with these assessments have included temporally sporadic collections and poor diatom preservation in areas with low sediment accumulation rates, but diatom indicators have nevertheless revealed a long-term record of human influences on Lake Superior's pelagic condition. Trends in diatom assemblages indicate nutrient enrichment, especially during the 20th century, but these impacts appear to be less significant than those observed in the lower lakes. Shifts in the phytoplanktonic species assemblages in recent decades suggest that nutrient reductions have resulted in water quality and biological improvements. However, as has been observed in the adjacent Lake Huron and Lake Michigan, algal abundance has dropped to very low levels, possibly in response to climate influences. It is clear that ongoing monitoring and new paleoecological investigations of diatoms in Lake Superior are needed to provide the necessary detail to explain long-term trends and put recent, rapid changes in a context of natural variability for the lake.