Diatom assemblages from the freshwater Miocene Quincy Diatomite along the western margin of the Quincy Basin, central Washington, show stratigraphic and geographic variation in taxon abundance interpreted to represent broad scale shifts in lake ecology.  Material was examined from the Frenchman Hills and the Quincy Ancient Lakes Park.  In the Frenchman Hills the diatomite is ~10 m thick and can be divided into three subunits based on floral assemblage and lithologic characteristics.  Each subunit is interpreted to have a distinct ecologic association.  The bottom subunit shows a comparatively rich pennate assemblage (30-40%) with Tetracyclus spp. dominating all pennates.  The middle subunit is dominated (>50%) by the benthic centric Ellerbeckia baileyi and is the only subunit where Aulacoseira species do not average at least 25-30% relative abundance.  The upper subunit consists mainly of Actinocyclus spp. grading upward into Aulacoseira spp., including Aulacoseira canadensis and Aulacoseira granulata.  Cymbella spp., Pinnularia spp. and Melosira undulata are all rare in the lower subunits but become more common in the upper 2 m of diatomite in the Frenchman Hills.  The Quincy Ancient Lakes Park, ~10 km north of the Frenchman Hills, exposes 3 m of section correlative with the upper subunit.  The Ancient Lakes locality has a more diverse and abundant pennate assemblage dominated by Tetracyclus spp. and Pinnularia spp.  Melosira undulata abundance increases to near dominant levels in localized horizons. 

Several preliminary interpretations about lake ecology are made.  In the Frenchman Hills, the araphid pennates to centric diatom ratio (A/C) indicates the uppermost subunit is the deepest interval.  Increasing abundance of Aulacoseira granulata up section, a species reported to prefer alkalic, eutrophic waters, may further indicate a general increase in trophic level.  The uppermost subunit at the Ancient Lakes locality differs from the Frenchman Hills in that the A/C ratio is higher, suggesting shallower depths.Quantification of taxon abundances, including another locality near the George Amphitheater, is underway and will be used to test these initial hypotheses regarding local and prevailing ecological conditions over the course of lake evolution along the Quincy Basin western margin.