ESTIMATING LACUSTRINE SPECIES DIVERSITY

There have been few studies comparing faunal diversity between modern lakes and those from the geologic record. Whether or not this comparison is even possible is dependent upon preservation potential of invertebrate faunal components. To establish a baseline for comparison, preservational bias of the fauna from modern Lake Tanganyika was estimated. At maximum, approximately 43.8% of species would be identifiable, 59.3% of genera, and 65.8% of families. Only exceptional preservation of Konservat Lagerstätten would more closely reflect ancient diversity in lakes.

With this taphonomic filter in place, unique factors controlling diversity within a lake system were analyzed. Such factors include lake surface area, lake longevity, productivity, and lake type. When comparing fish species diversity and lake area in the modern, no strong relationship was found. Faunal species diversity and lake longevity have a strong correlation in long-lived lakes, both in modern and geologic context. Modern Lake Tanganyika was comparable to lakes in the geologic record, after the taphonomic filter was applied.

Finally, to test the relationship between species and lake type/productivity, the underfilled, balanced-filled, and overfilled lake basin model of Bohacs et al. (2003) was used as a basis for comparison between fauna found in the lake deposits of the Green River Formation and those of modern lakes from East Africa. Type and depth of a lake can change through the lifespan of a basin. Only faunas within the same lake type can be compared. Green River fish fauna are analogous to those in the East African rift lakes in that they have similar familial diversity even though species diversity is much lower, due to preservational bias. Balanced-filled, long-lived modern lakes and comparable lake deposits had higher fish diversity than those in overfilled or underfilled lake types. This may be due to the productivity differences associated with differing lake types as well as ecologic stability and broader environmental conditions. Fish diversity would decrease during times of stress (elevated salinity) and speciation rate may increase with the return of an open hydrologic setting in a balanced-fill basin. Overfilled lakes would remain fresh through their lives and offer little physical disturbance of ecologic conditions through time.